Article

Differential remineralization of major and trace elements in sinking diatoms

Authors:
To read the full-text of this research, you can request a copy directly from the authors.

Abstract

Macronutrients in sinking phytoplankton are typically remineralized at different rates, but less is known about the fate of micronutrient metals associated with sinking cells. Scavenging, the presence of co-occurring abiotic particles, and inadvertent contamination limit the utility of bulk analytical approaches to study remineralization of trace metals in sinking phytoplankton. We used synchrotron x-ray fluorescence mapping to measure macronutrients (P, S, and Si) and trace metals (Fe, Ni, and Zn) in individual cells of the diatom Asterionellopsis glacialis during a spring bloom in subtropical waters off New Zealand. P, S, Zn, and Ni were released significantly faster than Fe and Si from sinking cells in the upper 200 m. Bulk particulate element fluxes to sediment traps indicated similar trends, but biogenic silica flux was attenuated much faster than Si was lost from intact sinking cells collected in the traps. The metals were spatially co-located with P and S in upper ocean cells, but this association with P and S (based on a spatial resolution of 450 nm) was largely absent in sinking cells. In contrast, Fe retained a weak spatial association with Si, suggesting that remineralized Fe may be re-scavenged onto cell surfaces. As a result, dissolved Fe : macronutrient stoichiometries in the water column likely underestimate stoichiometries in sinking cells. We propose linkages between the selective loss of diatom cellular components (e.g., ribosomes or phospholipid membranes, Zn-finger proteins, and urease) and the observed recycling of specific elements (P, Zn, and Ni, respectively), which set the stoichiometry of macro-and micronutrient supply to surface waters.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

... The timing and speciation of the regeneration of trace metals, however, remain poorly understood. In particular, despite their role as important micronutrients, depth profiles of essential trace metals often deviate from the classic nutrient-type profiles of macronutrients (Johnson et al. 1997;Twining et al. 2014;Boyd et al. 2017;Tagliabue et al. 2017), reflecting the combination of abiotic and biotic factors that influence trace metal regeneration and scavenging in seawater. ...
... Recent dark incubation studies have examined the regeneration of Fe from mesopelagic particles collected in situ (Bressac et al. 2019), and of Fe and Mn from benthic sediments (Cheize et al. 2019). Substantial progress has also been made in modeling trace metal regeneration throughout the water column from particle flux studies (Twining et al. 2014;Boyd et al. 2017;Bressac et al. 2019). Nevertheless, a gap still exists in understanding the timeline of trace metal regeneration over the course of phytoplankton decay, a process central to ocean water column distributions. ...
... Taken together, these experimental results were consistent with the well-known decoupling of some trace metals and macronutrients in the water column, such as Fe and nitrate (Johnson et al. 1997;Twining et al. 2014;Boyd et al. 2017;Tagliabue et al. 2017). However, our experiments do not indicate that differences in regeneration rates are the cause of this decoupling. ...
Article
Full-text available
Macronutrients and trace metals are incorporated into phytoplankton during growth and regenerated back into the water column when phytoplankton decay, a process that contributes to the distributions of dissolved trace metals and macronutrients in depth profiles. To study this, we incubated mixed Gulf of Mexico phytoplankton assemblages and monocultures of the diatom Pseudo‐nitzschia dolorosa and the dinoflagellate Karenia brevis in the dark. Over 6 months, macronutrients (phosphate, silicic acid, nitrate + nitrite, nitrite, ammonium), chlorophyll‐a, particulate organic carbon and nitrogen, and prokaryotes were monitored alongside dissolved manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), cadmium (Cd), and lead (Pb). Results were compared to depth profiles to evaluate the role of regeneration in trace metal cycling. In contrast to water‐column distributions, silicic acid and phosphate were closely coupled in experiments containing diatoms, indicating a shared regeneration pathway. Nitrification and nitrifying prokaryotes were only observed near the end of a subset of the experiments. Of the trace metals, Cd was most tightly coupled with phosphate. Regeneration of Mn was followed by rapid drawdown, consistent with Mn‐oxide formation. Iron (Fe), Cu, and Pb typically remained low until Mn was depleted, suggesting either scavenging to Mn‐oxides or otherwise delayed regeneration of these elements. Cobalt (Co) and Ni were largely conservative, but behaved like nutrients in the experiment using more offshore water low in Cd and Zn. Although experimental conditions were limited in their representation of the water column, these incubations provide novel insight into macronutrient and trace metal regeneration in the oceans.
... These cruises were part of an iron cycling project and are known as FeCycle II and FeCycle III, respectively. Data from the 2008 cruise (FeCycle II) have been reported elsewhere (Chiswell, 2011;Twining et al., 2014). ...
... In 2008, the diatom species were analysed by using a combination of light microscopy and the abundance of Asterionellopsis16S rDNA sequences as a proportion of diatom sequences and all photoautotrophic sequences (e.g. Twining et al., 2014). ...
... Chiswell (2011) interpreted the 2008 data to suggest that spring blooms in surface chlorophyll initiate with the onset of shallow weak stratification that forms during periods of low winds and that chlorophyll can be stratified in the mixed layer defined by MLD 1 . The 2008 spring bloom was dominated by the diatom Asterionellopsis glacialis (Twining et al., 2014) and was likely terminated by iron limitation, even though diatoms consumed less than onethird of the mixed-layer dFe inventory (Boyd et al., 2012). ...
Article
Full-text available
Abstract Observations from two research cruises made in 2008 and 2012 to east of New Zealand are put into context with satellite data to contrast and compare surface chlorophyll a evolution in the two years in order to explore mechanisms of phytoplankton bloom development in the southwest Pacific Ocean. In 2008, surface chlorophyll a largely followed the long-term climatological cycle, and 2008 can be considered a canonical year, where the autumn bloom is triggered by increasing vertical mixing at the end of summer and the spring bloom is triggered by decreasing vertical mixing at the end of winter. In contrast, 2012 was anomalous in that there was no autumn bloom, and in early spring there were several periods of sustained increase in surface chlorophyll a that did not become fully developed spring blooms. (In this region, we consider spring blooms to occur when surface chlorophyll a exceeds 0.5 mg m-3). These events can be related to alternating episodes of increased or decreased vertical mixing. The eventual spring bloom in October was driven by increased ocean cooling and wind stress (i.e. increased mixing) and paradoxically was driven by mechanisms considered more appropriate for autumn rather than spring blooms.
... Lithogenic particles are supplied to the ocean mainly via atmospheric deposition (Duce et al., 1991) and resuspension of marine sediments (Ho et al., 2011). Biogenic particles are produced in the surface ocean; they concentrate trace metals through biological uptake (Twining and Baines, 2013;Twining et al., 2014) and/or adsorption onto their surface from seawater (Balistrieri et al., 1981;John and Conway, 2014;Weber et al., 2018), and transport the trace metals to the deep ocean. Authigenic FeeMn oxides are suggested to be responsible for the adsorption of trace metals (Sherrell and Boyle, 1992). ...
... In the Pacific, intracellular Ni/P ratios in phytoplankton are reported to be 0.2-1.2 mmol/mol by single-cell analyses (Twining et al., 2011;Twining et al., 2014). For sinking particles at the SEATS station, ratios of Ni/OC and Ni/P in biogenic particles are estimated to be 0.09 and 8 mmol/mol, respectively. ...
... In the water column, concentrations of dissolved Ni correlate with those of dissolved P. Assuming that concentration gradients of dissolved Ni and P are produced by biological uptake and subsequent remineralization, the Ni/P ratio in phytoplankton is estimated from the slope of a regression line of dissolved Ni concentrations against dissolved P. This slope in the upper water column (< 800 m) is reported to be 1.0 to 1.8 mmol/mol in the North Pacific, the North Atlantic, and the Southern Ocean (Bruland, 1980;Sclater et al., 1976), which is similar to the intracellular Ni/P ratios estimated from the analyses of single cells of phytoplankton (0.2-1.2 mmol/mol; Twining et al., 2011Twining et al., , 2014, but lower than the Ni/ P ratios in particulate organic matter estimated from the analyses of suspended particles in the oxygen minimum zone (~5 mmol/mol; Ohnemus et al., 2017), organic-rich marine sediments (4-8 mmol/mol; Ciscato et al., 2018), and sinking particles in this study (~8 mmol/mol). In addition, the linear relationship between dissolved concentrations of Ni and P is often broken in deep water (Bruland, 1980;Sclater et al., 1976), implying different behavior between particulate Ni and P in deep water. ...
Article
We present isotope ratios of Ni (δ⁶⁰Ni) and Cu (δ⁶⁵Cu) in sinking particles, aerosols, and seawater collected from the northern South China Sea to identify sources and transformation processes of the two metals. In aerosols, δ⁶⁰Ni values are in the range +0.05‰ to +0.56‰, and δ⁶⁵Cu values are in the range −0.33‰ to +0.83‰. The isotope ratios are different from those of lithogenic materials, indicating that the aerosols are anthropogenic in origin. In sinking particles collected at depths of 2000 and 3500 m, δ⁶⁰Ni values are in the range +0.01‰ to +0.54‰ at 2000 m and −0.18‰ to +0.54‰ at 3500 m, and the values exhibit similar temporal variation pattern between 2000 and 3500 m. Based on the significant correlation between δ⁶⁰Ni and the ratio of P/Ni or organic-C/Ni, we hypothesize that the main sources of Ni in the sinking particles originate from both resuspended marine sediments off southwest Taiwan, and biogenic organic particles. The δ⁶⁰Ni in biogenic particles is estimated to be +0.6‰ to +1.0‰, which is 0.3–0.7‰ lighter than that of dissolved Ni in seawater. The isotope ratios of Cu in sinking particles are fairly constant (+0.13‰ to +0.36‰), and the range is between those of marine sediments and labile fractions of marine particles. Thus, Cu in sinking particles is likely to be from marine sediments and biogenic organic particles. Compared with Ni, the correlation between Cu and P or organic-C is weaker, suggesting that the Cu/P and Cu/organic-C ratios are not constant in organic matter or there are additional sources of particulate Cu, such as Fe-Mn oxides and anthropogenic aerosols.
... Larger celled (>50 µm) diatoms, particularly in the Southern Ocean, can have low Fe use (Strzepek et al., 2011) but are bloom formers with substantial biomass. These large cells are more difficult to break down, both in the surface waters and in the mesopelagic by bacteria so the Fe is biologically retained and exported to depth along with the C (Twining et al., 2014). ...
... Trace metal remineralisation rates are harder to determine compared with macronutrient rates in the ocean as these nutrients occur at low dissolved and particulate concentrations, can be contaminated during sampling, and particulates often occur in heterogeneous aggregates in sediment traps making it hard to study individual trace metals (Twining et al., 2014). Most past research has focused on new Fe (e.g. from dust) rather than recycled Fe and even less focus has been placed on the remineralisation of Fe in mesopelagic waters . ...
... Other studies have investigated multiple trace metal remineralisation length scales, for example Twining et al. (2014) used trace metal clean techniques to study sinking diatoms and measured the remineralisation of trace elements in the suspended and sinking diatoms at 100 m and 200 m (upper mesopelagic) during a spring bloom event in subtropical waters off New Zealand. The study by Twining et al. (2014) found that Fe has longer remineralisation length scales and was retained within sinking diatom cells. ...
Thesis
Marine microbes are an important control on carbon (C) sequestration depth and biogeochemical cycling of nutrients and trace metals in the global ocean. The biological carbon pump (BCP) is the set of processes by which inorganic carbon (CO<sub>2</sub>) (along with nutrients and trace metals) is fixed into organic matter via photosynthesis by autotrophic phytoplankton and the C, nutrients and trace metals sequestered away from the atmosphere generally by transport into the deep ocean. Most (~80 %) of the organic C produced by autotrophic phytoplankton is remineralised (returned to the dissolved inorganic inventory from the particulate organic form) in the surface ocean and the inorganic CO<sub>2</sub> is available for release back into the atmosphere. The depth at which remineralisation occurs is important, as the deeper the remineralisation depth of the C the increased likelihood of long term storage in the deep water and sediment. The sequestration of C is primarily dependent on flux attenuation and remineralisation of organic matter in the mesopelagic or ‘twilight’ zone (100-1000 m), where much of the downward particle flux is attenuated via zooplankton and bacterial respiration, replenishing dissolved nutrients and trace metals back into the water column. Understanding the controls on the BCP in the twilight zone is important to understand the transfer efficiency of C sequestration and the regulation of atmospheric CO<sub>2</sub>. Oceanic regions such as the Southern Ocean have inefficient BCPs as the phytoplankton are unable to fully utilise available nutrients, restricting their growth and drawdown of C due to limited access to micronutrients such as iron (Fe). Iron is a scare resource in these regions and low concentrations of bioavailable Fe exert significant controls on global phytoplankton productivity, species composition and therefore ecosystem structure and the C cycle. Iron is not only an important micronutrient for phytoplankton growth but also for heterotrophic bacteria, limiting bacterial secondary production and abundance. Two focused and inter-related processes which influence Fe cycling and consequently C cycling in the mesopelagic were investigated. Firstly, differentiating the biotic and abiotic factors on Fe cycling in the twilight zone and the (de-) coupling of Fe and macronutrients at depth. Secondly, to investigate Fe and C (co-) limitation of mesopelagic bacteria. This researched performed shipboard experiments and subsequent laboratory work to evaluate the relative remineralisation rates of C, Fe and silica (Si) from live and detrital phytoplankton cells resuspended in upper mesopelagic waters. Iron consistently transferred from the particulate fraction into the dissolved fraction from both live and detrital cells, this transfer was dominated by the abiotic movement of extracellular adsorbed particulate iron into the dissolved fraction (de- absorption). The live phytoplankton cells remained viable throughout the incubations and continued to respire C whilst the detrital cells potentially leaked dissolved organic C which was subsequently taken up and respired by bacteria with minimal secondary bacterial production. Limited dissolution of Si occurred from the live viable cells with the detrital cells showing more Si dissolution potential. The remineralisation length scales of Fe, C and Si were thus decoupled in the upper mesopelagic as Fe resulted in the shortest remineralisation length scale due the abiotic transfer of extracellular Fe into the dissolved pool, which could resupply biota potentially alleviating Fe limitation. Intracellular pools of Fe (along with C and Si) would be exported to deeper depths with a slow remineralisation rate if processes such as grazing or cell lysis do not act to break cells up and speed up remineralisation processes. Heterotrophic bacterial production was Fe and C (co-) limited in the mesopelagic above the ferricline. An increase in cell abundance of very large high nucleic bacteria when combined Fe and C were added to mesopelagic waters from 150 and 500 m supported a large (1-2 order of magnitude) increase in bacterial production indicating the (co-) limitation of a sub-population of the free-living bacteria at depth. The controls on ferricline depth and mesopelagic standing stocks of Fe (from winter mixing, scavenging, Fe associated with sinking material and the de-absorption of Fe into the water column) will be important in determining the extent of ocean Fe C (co-) limitation of mesopelagic bacterial growth and production and will be a driver in bacterial community composition at depth. Nutrient limitation in the mesopelagic bacteria has potentially important consequences if it also reduces the overall rate of remineralisation and thus both generates a potential reinforcing feedback on the maintenance of a deep ferricline and increases the remineralisation depth and hence long-term storage of carbon in the ocean.
... A positive linear relationship (that is, similar biogenic flux and regenerative stoichiometries) should be observed if bacterial solubilization exerts a first-order control on DFe resupply. Here the absence of such a relationship, along with systematically lower DFe replenishment rates relative to those of P and C (Supplementary Table 2), confirm that the DFe resupply results from a combination of biotic and abiotic transformations of sinking PFe 15,17 . As highlighted in Fig. 3c, the dissolution of lithogenic Fe (that is, DFe release without O 2 consumption) increases R Fe=O2 I and hence R Fe/C , whereas Fe scavenging (that is, DFe removal without O 2 consumption) has the opposite effect. ...
... Saharan dust-derived Fe dissolution rates are reported to remain constant for several days in laboratory studies 30 , which argues for a relatively constant dissolution rate over this 115-195 m stratum that particles sink through on this timescale 7,9 . Although changes in the bacterial solubilization rate of PFe over this depth range cannot be assessed directly, the increasing proportion of C respired with depth (Supplementary Table 2) is not consistent with a decreased bacterial solubilization rate of PFe (by assuming a constant or increasing Fe bio /C ratio 15 ). Thus, increased scavenging ( Fig. 4) is the most likely mechanism to account for the trend in R Fe/C with depth. ...
... We report an about 1,000-fold range in the DFe replenishment rate, whereas only modest changes in P and C replenishment rates occurred at all the sites (Supplementary Table 2). At SAZ, the decoupling between Fe and C remineralization was relatively low and comparable to that reported for sinking diatoms in subtropical waters 15 . In contrast, a pronounced decoupling between Fe and both C and P remineralization was observed at ALG and ION. ...
Article
Full-text available
The dissolved iron supply controls half of the oceans’ primary productivity. Resupply by the remineralization of sinking particles, and subsequent vertical mixing, largely sustains this productivity. However, our understanding of the drivers of dissolved iron resupply, and their influence on its vertical distribution across the oceans, is still limited due to sparse observations. There is a lack of empirical evidence as to what controls the subsurface iron remineralization due to difficulties in studying mesopelagic biogeochemistry. Here we present estimates of particulate transformations to dissolved iron, concurrent oxygen consumption and iron-binding ligand replenishment based on in situ mesopelagic experiments. Dissolved iron regeneration efficiencies (that is, replenishment over oxygen consumption) were 10- to 100-fold higher in low-dust subantarctic waters relative to higher-dust Mediterranean sites. Regeneration efficiencies are heavily influenced by particle composition. Their make-up dictates ligand release, controls scavenging, modulates ballasting and may lead to the differential remineralization of biogenic versus lithogenic iron. At high-dust sites, these processes together increase the iron remineralization length scale. Modelling reveals that in oceanic regions near deserts, enhanced lithogenic fluxes deepen the ferricline, which alter the vertical patterns of dissolved iron replenishment, and set its redistribution at the global scale. Such wide-ranging regeneration efficiencies drive different vertical patterns in dissolved iron replenishment across oceanic provinces.
... where Cu is flux of Cu of the euphotic zone, is the depth in water column, is the euphotic zone depth (around 73 m), and is the flux attenuation exponent. The power-law model (i.e., Martin-curve) is widely applied for investigating the biological cycling of nutrients and trace metals (e.g., Twining et al., 2014;Weber et al., 2018). Because of the linear nature of the model, the spatial pattern of Cu is determined by b and the spatial pattern of (Cu:P)up while the magnitude of the (Cu:P)up ratios determines the amplitude of this pattern. ...
... Although the estimated b value for Cu from the model is small, it is within the range estimated for other elements in literature. For example, Twining et al. (2014) found that the estimated b value was ∼0.01-0.23 for Si and that the estimated b value for Fe was similar to that for Si. Because the estimated b value for P (∼0.35-0.92; ...
... Because the estimated b value for P (∼0.35-0.92; Twining et al., 2014;Weber et al., 2018) or S (∼0.5-1.7; Twining et al., 2014) is much higher than that for Si (∼0.01-0.23; ...
Article
Full-text available
Plain Language Summary While copper is known to be used as a trace nutrient by phytoplankton, its distribution in the ocean is very different from nutrients like phosphate and silicate. The material produced by phytoplankton in surface waters falls into the deep and regenerates phosphate and silicate. Both phosphate and silicate have mid‐depth maxima. They also accumulate as waters move from the Atlantic into the Pacific. In comparison, the concentration of copper increases steadily with depth compared to phosphate and silicate. Copper also shows accumulation in the Pacific relative to the Atlantic. We show that these features of the copper distribution can be explained by allowing copper to be taken up by phytoplankton at the surface if the resulting material is allowed to fall all the way to the bottom and return to solution there. Furthermore, in order to fit the marine distribution of copper, its uptake in the Southern Ocean must be substantially higher than that in other oceanic regions. This fact, in addition to the fact that copper is better correlated with silicate than phosphate, leads us to hypothesize that diatoms in the Southern Ocean play a particularly important role in marine copper distributions.
... Earlier studies have revealed that particulate phosphorus (pP), nickel (pNi), zinc (pZn), and organic carbon (POC) are remineralized faster than particulate iron (pFe) because of a stronger scavenging of dissolved Fe onto particles. 23,24 Thus, particle-solute interactions determine the residence times of TEs in the water column. However, these residence times are presently not well-known because of scarce estimates of TE inventories and fluxes into or out of the water column. ...
... Trace elements are known to be scavenged onto particles with depth 12,108 (see previous sections), and it is usual to observe an increase of the bulk ratios compared to cellular ratios. 24,109,110 This can be caused by the presence of detrital, authigenic, and lithogenic particles on the filters leading to an increase of the pTE/POC, as observed in Figure 5. Moreover, the intracellular TE/C changes considerably between taxa and regions depending on the environmental conditions (e.g., nutrients, sunlight limitations 3 ): Fe/C quota usually range between 10 and 1700 μg/g for diatoms and coccolithophorids in natural and culture experiments, 3,111,112 but Sunda and Huntsman have measured a Fe/C ratio equaling 8230 μg/ g, 113 highlighting the high variability of the ratio and the difficulty to generalize the intracellular phytoplankton TE/C ratios. ...
... These differences in the regeneration length scale reflect the release of these metals and phosphorous from suspended and sinking cells and detrital matter. For example, zinc is used in zinc-finger proteins associated with RNA and DNA expression, carbonic anhydrase and alkaline phosphatase, all of which have different affinities for zinc (Twining et al. 2014). By contrast, phosphorous is a significant component of RNA and DNA, membrane phospholipids, low molecular weight water-soluble phosphate esters and metabolically active inorganic orthophosphate (Raven 2013). ...
... A plot of the zinc : phosphorus ratio v. depth revealed an increase in the ratio with increasing depth (Fig. 8). This contrasts with the results of Twining et al. (2014), who observed no significant change in the zinc : phosphorus ratio v. depth for material sinking from a diatom-dominated phytoplankton bloom from subtropical waters. Because the sampling at the SOTS, CCE and SAZ sites occurred late in the growing season, the phytoplankton community was primarily dominated by small prokaryote and eukaryote phytoplankton, which may explain the difference between these two studies. ...
Article
Full-text available
In this study we investigated the distribution of dissolved and particulate zinc (dZn and pZn respectively) and its isotopes in the Subantarctic Zone as part of a Geotraces Process voyage. dZn and pZn depth profiles contrasted each other, with dZn showing depletion within the euphotic zone while pZn profiles showed enrichment. Fitting a power law equation to the pZn profiles produced an attenuation factor of 0.82, which contrasted values for particulate phosphorus, cadmium and copper. The results indicate that zinc has a longer regeneration length scale than phosphorus and cadmium, but shorter than copper. The differential regeneration of pZn relative to that of particulate phosphorus likely explains why dZn appears to have a deeper regeneration profile than that of phosphate. The dZn isotope (d66Zndissolved) profiles collected across the Subantarctic Zone showed differing profile structures. For one station collected within an isolated cold-core eddy (CCE), d66Zndissolved showed surface enrichment relative to deep waters. The corresponding pZn isotope profiles within the CCE did not show enrichment; rather, they were subtly depleted in surface waters and then converged to similar values at depth. Zinc isotope fractionation can be explained through a combination of fractionation processes associated with uptake by phytoplankton, zinc complexation by natural organic ligands and zinc regeneration from particulate matter.
... In the case of a scarce nutrient, such as Fe, individual organisms (e.g., Saito et al., 2011) and even entire ecosystems (e.g., Rafter et al., 2017) have evolved mechanisms to retain certain nutrients. Likewise, macro-and micronutrients may be regenerated by heterotrophic organisms at different rates (e.g., Ohnemus et al., 2019;Twining et al., 2014), which could affect the Me:C stoichiometry of exported POC and thus the accuracy of Me-derived C export fluxes. ...
... Although exceptions exist (see Section 4.2.2), generally, the oceanic [Zn]:[Si] correlation persists despite shallower remineralization of Zn relative to Si(Twining et al., 2014), and is especially clear in the South Atlantic, underlining that the mixing of water masses acts as the dominant control on [Zn](de Souza et al., 2018;Middag et al., 2019;Vance et al., 2017). WhileWeber et al. (2018) concur with these other studies about the overall importance of ocean circulation on setting the distribution of [Zn] and [Zn]:[Si] ...
... Benitez-Nelson et al. (2007) hypothesized that particulate C and N were preferentially remineralized (particulate N was even more labile than particulate C) relative to biogenic Si during sinking due to enhanced grazing by microzooplankton. Multiple studies have also found preferential particulate C remineralization and lower transfer efficiencies relative to biogenic Si in response to the overlying phytoplankton composition, zooplankton grazing strategy and microbial degradation (Karl et al. 1999;Reinfelder and Fisher 1999;Twining et al. 2014). For example, Prochlorococcus spp. ...
... are numerically abundant photoautotrophs in the gyre, and are more likely to be degraded relative to other cyanobacterial groups due to their semi-permeable proteinaceous membrane (Partensky and Blanchot 1999). Furthermore, elements, such as carbon and nitrogen that are incorporated into the algal cytoplasm, are more likely to be assimilated by zooplankton and recycled to the dissolved phase relative to structural elements, such as silica (Reinfelder and Fisher 1999;Twining et al. 2014). Nitrate and chlorophyll a concentrations in the upper 200 m were observed to be enhanced in cyclonic eddies relative to the . ...
Article
Mesoscale eddies may enhance nutrient injection into the photic zone and ultimately the magnitude and composition of particle export to depth. Using satellite altimetry, we identified 38 cyclonic eddies that passed in close proximity to the Hawaii Ocean Time‐series (HOT) Station ALOHA, located in the North Pacific Subtropical Gyre, from 1993 to 2018. Particulate carbon (C), nitrogen (N), and biogenic silica (Si) export rates, measured using free floating sediment traps deployed at 150 m as part of HOT, were then associated with either the eddy core or edge based on distance to the eddy center and time of eddy evolution. Elemental fluxes varied significantly within and among individual eddies depending on season and eddy age. Spatially, biogenic Si fluxes were enhanced relative to particulate C and N fluxes at both the cores and edges, with temporally highest particulate C, N and biogenic Si fluxes occurring during the mature stage (3–8 weeks). On average, biogenic Si fluxes were 200 ± 80% (30–270% increase) higher relative to non‐eddy and during non‐bloom periods, with modest enhanced particulate C (10–30% increase) and N (10–20% increase) fluxes. In contrast, during the bloom season (July and August), elemental fluxes were all reduced by 20% relative to non‐eddy references, suggesting that cyclonic eddies depress export during the bloom period. Our results indicate that cyclonic eddies not only increase, but differentially impact the sinking export of critical biological elements, thereby contributing to long term ecological changes in foodwebs that rely on silica as well as carbon for growth.
... Particulate samples for bulk ICP-MS trace-metal analysis were processed using a complete acid digestion protocol as described by Twining et al. (2014). Sample and blank filters were placed in digestion bottles and refluxed in 750 mL ultrapure HCl for 30 min in a 100 C water bath, then 250 mL ultrapure HNO 3 was added and refluxed for an additional 30 min, then finally 50 mL ultrapure HF was added and refluxed for 1 h. ...
... Selective loss of cellular components (e.g., Zn-finger proteins and urease) could release Zn and Ni significantly faster than macronutrient Si from sinking cells, producing the stoichiometry of macro-and micro-nutrients reached into sediment larger. Nevertheless, it has been observed that Fe is retained within sinking phytoplankton, to a greater extent than P and N (Twining et al., 2014), the reason may be the remineralized Fe rescavenged onto cell surfaces. These were further examined below from two perspectives of Fe and C org . ...
Article
Full-text available
To constrain the resuspension influence to the biogeochemical behavior of trace metals (TMs) in settlingmaterials, the concentrations and chemical speciations of macro-elements (Al, Fe, Mn) and selected particulate TMs (V, Cr, Co, Cu, Zn, Ga, Sr, Cd, Ba, Tl, Pb, U) in trap-collected particles (TCPs), surface sediments (SS) and core sediment samples (CS5) of the Jiaozhou Bay were compared. Two approaches, mass conservation method and vertical two end-members mixing model, both calculated a resuspension ratio of more than 90%. Greater TM concentrations and Al-normalization levels than SS/CS5 determined the TCPs an important TM-sink, predominantly owing to grain-size effects and TCP-specific characteristics, i.e., structural capacity of organic-Fe associations for TMs’ scavenging, preferential remineralization of TM than biogenic elements in autochthonous microorganisms. Comparison revealed distinct, Fe mineral controls on TM sequestration patterns: higher metal sequestration associated with amorphous Fe oxyhydroxides, while less reactive crystalline Fe oxides hold less metal. Nevertheless, turbulent hydrodynamics muted the wide TM retention divergences between TCP and SS, which should have happened based on different Fe minerals distribution for TCP/SS. The net effect of TM release by the organic carrier phase and then adsorption principally onto Mn/Fe oxyhydroxide phase for raised overall TCP-TM concentrations was also identified.
... In the case of a scarce nutrient, such as Fe, individual organisms (e.g., Saito et al., 2011) and even entire ecosystems (e.g., Rafter et al., 2017) have evolved mechanisms to retain certain nutrients. Likewise, macro-and micronutrients may be regenerated by heterotrophic organisms at different rates (e.g., Ohnemus et al., 2019;Twining et al., 2014), which could affect the Me:C stoichiometry of exported POC and thus the accuracy of Me-derived C export fluxes. ...
... Although exceptions exist (see Section 4.2.2), generally, the oceanic [Zn]:[Si] correlation persists despite shallower remineralization of Zn relative to Si(Twining et al., 2014), and is especially clear in the South Atlantic, underlining that the mixing of water masses acts as the dominant control on [Zn](de Souza et al., 2018;Middag et al., 2019;Vance et al., 2017). WhileWeber et al. (2018) concur with these other studies about the overall importance of ocean circulation on setting the distribution of [Zn] and [Zn]:[Si] ...
... This would imply that the oceanic cycle of Zn is dominated by uptake into and regeneration from diatom siliceous tests, which are regenerated more slowly during sinking through the water column than intracellular organic matter (Zhao et al., 2014). However, the vast majority of Zn in diatoms is associated with N and P in organic tissues (Twining & Baines, 2013;Twining et al., 2004Twining et al., , 2015 and not diatom opal (1-3% of total cellular Zn inventory; Ellwood & Hunter, 2000;Jaccard et al., 2009), and thus, Zn should be regenerated from this organic matter in the upper ocean alongside P rather than opal-derived Si (Twining et al., 2014). A more recent hypothesis suggests that the strong Zn-Si correlation across the major oceans is instead explained by extreme drawdown of Zn and Si relative to P by diatoms in the surface Southern Ocean, and it is the lateral transport and modification of these Zn-and Si-depleted waters that sets the unusual Zn-Si-P stoichiometry in global nutricline waters (Ellwood, 2008;Vance et al., 2017;Weber et al., 2018;Wyatt et al., 2014). ...
... Critically for the Arctic dZn cycle, we hypothesize that once the shelf phytoplankton deposit their elevated Zn:Si and Zn:C as phytodetritus in the shallow shelf sediments, sedimentary remineralization of the organic tissue of those cells would supply a Zn-rich dissolved nutrient flux back into shelf waters, effectively increasing the Zn:macronutrient ratios observed in the water column. This sedimentary dZn flux might be accentuated compared to Si due to the preferential remineralization of "soft" tissues containing Zn and C over the "hard" Si tests, as demonstrated by decreasing cellular Zn:Si ratios with depth in the subtropics (Twining et al., 2014) and by the 1,000-fold lower Zn:Si ratio of sedimentary frustules (Andersen et al., 2011;Ellwood & Hunter, 1999;Hendry & Rickaby, 2008) compared with the water column phytoplankton in Arctic shelf waters. (Bruland et al., 1991). ...
Article
Full-text available
The biogeochemical cycling of dissolved zinc (dZn) was investigated in the Western Arctic along the U.S. GEOTRACES GN01 section. Vertical profiles of dZn in the Arctic are strikingly different than the classic “nutrient-type” profile commonly seen in the Atlantic and Pacific Oceans, instead exhibiting higher surface concentrations (~1.1 nmol/kg), a shallow subsurface absolute maximum (~4–6 nmol/kg) at 200 m coincident with a macronutrient maximum, and low deep water concentrations (~1.3 nmol/kg) that are homogeneous (sp.) with depth. In contrast to other ocean basins, typical inputs such as rivers, atmospheric inputs, and especially deep remineralization are insignificant in the Arctic. Instead, we demonstrate that dZn distributions in the Arctic are controlled primarily by (1) shelf fluxes following the sediment remineralization of high Zn:C and Zn:Si cells and the seaward advection of those fluxes and (2) mixing of dZn from source waters such as the Atlantic and Pacific Oceans rather than vertical biological regeneration of dZn. This results in both the unique profile shapes and the largely decoupled relationship between dZn and Si found in the Arctic. We found a weak dZn:Si regression in the full water column (0.077 nmol/μmol, r ² = 0.58) that is higher than the global slope (0.059 nmol/μmol, r ² = 0.94) because of the shelf-derived halocline dZn enrichments. We hypothesize that the decoupling of Zn:Si in Western Arctic deep waters results primarily from a past ventilation event with unique preformed Zn:Si stoichiometries.
... Moreover, much like observations from the Southern Ocean 18 , the sub-surface NO 3 − and dFe pools were decoupled. Both dFe and NO 3 − increased immediately below the seasonal pycnocline, but dFe concentrations increased most significantly at greater depths, presumably due to longer remineralization length scales 60 and/or mid depth scavenging of dFe 63 . This did not occur as a distinct ferricline but nevertheless drove an increase in the dFe:NO 3 − ratio with depth (Fig. 7). ...
Article
Full-text available
The availability of iron (Fe) can seasonally limit phytoplankton growth in the High Latitude North Atlantic (HLNA), greatly reducing the efficiency of the biological carbon pump. However, the spatial extent of seasonal iron limitation is not yet known. We present autumn nutrient and dissolved Fe measurements, combined with microphytoplankton distribution, of waters overlying the Hebridean (Scottish) shelf break. A distinct biogeochemical divide was observed, with Fe deficient surface waters present beyond the shelf break, much further eastwards than previously recognised. Due to along and on-shelf circulation, the Hebridean shelf represents a much-localised source of Fe, which does not fertilise the wider HLNA. Shelf sediments are generally thought to supply large quantities of Fe to overlying waters. However, for this Fe to influence upper-ocean biogeochemical cycling, efficient offshelf transport mechanisms are required. This work challenges the view that the oceanic surface waters in close proximity to continental margins are iron replete with respect to marine primary production demands.
... The shape of resource profiles is influenced by biogeochemical and physical processes. Biological consumption removes resources from the upper water column, organic particles then sink and resources are released back into the water column at depth through resupply processes such as remineralization (Twining et al., 2014). The combination of removal in surface waters and resupply at depths forms gradients in the vertical profiles of key resources, with the strongest vertical gradient known as the nutricline (Omand & Mahadevan, 2015;Tagliabue et al., 2014). ...
Article
Full-text available
While phytoplankton play a key role in ocean biogeochemical cycles, the availability and supply pathways of resources that support their growth remain poorly constrained. Here, we show that the availability of various resources varies over several orders of magnitude throughout the Atlantic Ocean, causing regional contrasts in resource deficiency. Regional variations in the relative availability of nitrogen, phosphorous, silicon, iron, zinc, manganese, cobalt, and cadmium are important and result from the contrasts between winter mixing depths and differences in vertical profiles of the different resources. The winter‐time thickening of the mixed layer may replenish or deplete resources via entrainment, depending on the vertical nutrient profile. For nutrients like nitrate, phosphate, and cadmium, entrainment is a consistent source term. While for others, such as manganese and iron, entrainment can reduce ocean resource availability, particularly in subtropical regions. Any future change to the depth of winter‐time mixing will cause region‐specific changes in relative availability of different resources that may have important ecological consequences.
... Moreover, much like observations from the Southern Ocean 18 , the sub-surface NO 3 − and dFe pools were decoupled. Both dFe and NO 3 − increased immediately below the seasonal pycnocline, but dFe concentrations increased most significantly at greater depths, presumably due to longer remineralization length scales 60 and/or mid depth scavenging of dFe 63 . This did not occur as a distinct ferricline but nevertheless drove an increase in the dFe:NO 3 − ratio with depth (Fig. 7). ...
Article
Full-text available
The availability of iron (Fe) can seasonally limit phytoplankton growth in the High Latitude North Atlantic (HLNA), greatly reducing the efficiency of the biological carbon pump. However, the spatial extent of seasonal iron limitation is not yet known. We present autumn nutrient and dissolved Fe measurements, combined with microphytoplankton distribution, of waters overlying the Hebridean (Scottish) shelf break. A distinct biogeochemical divide was observed, with Fe deficient surface waters present beyond the shelf break, much further eastwards than previously recognised. Due to along and on-shelf circulation, the Hebridean shelf represents a much-localised source of Fe, which does not fertilise the wider HLNA. Shelf sediments are generally thought to supply large quantities of Fe to overlying waters. However, for this Fe to influence upper-ocean biogeochemical cycling, efficient off-shelf transport mechanisms are required. This work challenges the view that the oceanic surface waters in close proximity to continental margins are iron replete with respect to marine primary production demands.
... This would reconcile the low rates of dFe accumulation within intermediate waters with the significant export of non-lithogenic Fe and large rates of dFe solubilisation from sinking particles. The potential role of the removal of regenerated algal biogenic Fe has been previously observed using synchrotron-mapping of particles derived from sediment traps 49 and would also explain observations of an increasing association of sinking non-lithogenic particulate Fe with authigenic phases in deep-moored sediment traps (between 500, 1500 and 3200 m) in the Atlantic 50 . For the Pacific, we calculate that 20-40% of the particulate Fe within the intermediate water in the western portion of the GP16 Pacific section cannot be accounted for by the sum of lithogenic and algal biogenic components. ...
Article
Full-text available
Despite recent advances in observational data coverage, quantitative constraints on how different physical and biogeochemical processes shape dissolved iron distributions remain elusive, lowering confidence in future projections for iron-limited regions. Here we show that dissolved iron is cycled rapidly in Pacific mode and intermediate water and accumulates at a rate controlled by the strongly opposing fluxes of regeneration and scavenging. Combining new data sets within a watermass framework shows that the multidecadal dissolved iron accumulation is much lower than expected from a meta-analysis of iron regeneration fluxes. This mismatch can only be reconciled by invoking significant rates of iron removal to balance iron regeneration, which imply generation of authigenic particulate iron pools. Consequently, rapid internal cycling of iron, rather than its physical transport, is the main control on observed iron stocks within intermediate waters globally and upper ocean iron limitation will be strongly sensitive to subtle changes to the internal cycling balance. Iron is crucial for marine photosynthesis, but observational constraints on the magnitude of key iron cycle processes are lacking. Here the authors use a range of observational data sets to demonstrate that the balance between iron re-supply and removal in the subsurface controls upper ocean iron limitation.
... Arsenic concentrations in phytoplankton peak just prior to periods of high arsenic sedimentation (Fig. 6). Although average phytoplankton arsenic concentrations were lower than those observed in sediment trap material (Supporting Information Fig. S3) (26%, July-October 2016 and 2017), we hypothesize that arsenic was concentrated during plankton settling and decay through the preferential loss of carbon and other macronutrients, as has been observed with other particlereactive trace elements (Twining et al. 2014). Zooplankton grazing on phytoplankton may also concentrate arsenic in settling fecal pellets (Davies 1978). ...
Article
Arsenic contamination of lakebed sediments is widespread due to a range of human activities, including herbicide application, waste disposal, mining, and smelter operations. The threat to aquatic ecosystems and human health is dependent on the degree of mobilization from sediments into overlying water columns and exposure of aquatic organisms. We undertook a mechanistic investigation of arsenic cycling in two impacted lakes within the Puget Sound region, a shallow weakly stratified lake and a deep seasonally stratified lake, with similar levels of lakebed arsenic contamination. We found that the processes that cycle arsenic between sediments and the water column differed greatly in shallow and deep lakes. In the shallow lake, seasonal temperature increases at the lakebed surface resulted in high porewater arsenic concentrations that drove larger diffusive fluxes of arsenic across the sediment–water interface compared to the deep, stratified lake where the lakebed remained ~10°C cooler. Plankton in the shallow lake accumulated up to an order of magnitude more arsenic than plankton in the deep lake due to elevated aqueous arsenic concentrations in oxygenated waters and low phosphate : arsenate ratios in the shallow lake. As a result, strong arsenic mobilization from sediments in the shallow lake was countered by large arsenic sedimentation rates out of the water column driven by plankton settling.
... These flux reconstructions indicate that biogenic Zn in the Southern Ocean is associated with plankton soft tissues containing P, rather than the Si-rich frustules of diatoms, a finding that agrees with the small percentage of Zn (1-3%) associated with frustules in laboratory diatom cultures (Ellwood & Hunter, 2000). A sediment trap study in the Southern Ocean (~40°S) also found similar b values for PP and PZn, which were much higher than that derived for PSi (Twining et al., 2014). ...
Article
Full-text available
The internal cycling of zinc (Zn) in the ocean has been a longstanding mystery. Particularly, puzzling is the strong correlation between Zn and silicate (SiO4⁴⁻), but not phosphate (PO4³⁻), even though Zn is involved with cell functions that regulate PO4³⁻ uptake and are unrelated to SiO4⁴⁻ uptake. To help solve this mystery, we use an artificial neural network to produce global maps of dissolved Zn, and then use a diagnostic model to infer rates of uptake and regeneration for Zn, SiO4⁴⁻, and PO4³⁻. We find that plankton in the Southern Ocean account for 62 (±32)% of global Zn uptake. The plankton Zn:PO4³⁻ uptake ratio increases by more than tenfold from the low latitudes to the Southern Ocean, a much larger range than expected from culture studies, suggesting controls from factors such as iron availability. Reconstruction of particulate Zn (PZn), phosphorus (PP), and biogenic silica (PSi) fluxes reveals that PZn remineralizes like PP, and not like PSi. However, a small flux of PZn into the deep ocean is not matched by an equivalent flux of PP, which is likely due to the combined effects of desorption of scavenged Zn and the input of hydrothermal Zn in the deep ocean. This small difference in the remineralization of PZn and PP, combined with the patterns of surface uptake, eliminates the correlation between Zn and PO4³⁻ in the deep ocean and causes a tight correlation between Zn and SiO4⁴⁻. This coincidental correlation cannot be expected to hold for past and future states of the ocean.
... Underlying reasons behind this paradox have previously been attributed to an absolute biological control. Twining et al. (2014) proposed the simultaneous release of Zn and Si from sinking diatom cells packaged with other detrital material. Alternatively, scavenging of dissolved Zn onto marine organic matter and subsequent deeper dissolution may generate depth profiles similar to Si (John and Conway, 2014). ...
Article
First measurements of labile dissolved copper (LdCu) and dissolved zinc (dZn) and nickel (dNi) in the Atlantic sector of the Southern Ocean in winter are compared with summer data at reoccupied stations in order to better understand the winter reset state and supply of these trace metals to support productivity. In summer, vertical profiles of zinc behaved similarly to silicate (Si) and increased from sub-nanomolar surface concentrations to 8 nmol kg⁻¹ in bottom waters. Copper profiles also resembled Si and were typically 1 nmol kg⁻¹ in surface waters and increased to 3 nmol kg⁻¹ at depth. First summer nickel data reported from this transect displayed comparatively higher surface concentrations of ~4.6 nmol kg⁻¹ increasing more rapidly to local intermediate depth maximums between 6.5 and 7.0 nmol kg⁻¹, similar to phosphate (PO4). Trace metal seasonality was most apparent in the mixed layer where the average of winter concentrations within the mixed layer exceeded summer values by approximately 0.2 nmol kg⁻¹ for LdCu, 1.2 nmol kg⁻¹ for dZn and 0.3 nmol kg⁻¹ for dNi owing to low utilization under unfavourable growth conditions for phytoplankton. In an effort to estimate the winter reserve, two scenarios were considered. Scenario 1 accounted for in-situ mixed layer depths (MLD) where the winter reserve inventory was calculated by subtracting the summer depth integrated metal inventory (surface to summer MLD) from the winter equivalent (surface to winter MLD). Scenario 2 assumed a constant mixed layer (taken as the depth of the maximum winter mixed layer) where summer depth integrated metal inventories (surface to maximum winter MLD) were subtracted from the winter equivalents (surface to maximum winter MLD). Results for scenario 1 were predominantly dependent on the mixed layer depth which varied spatially and seasonally. Scenario 2 showed a southwards increase in the winter reserve inventory suggesting a greater role for entrainment at higher latitudes in this region. This is however heavily dependent on other physical processes controlling vertical trace metal supply e.g. diapycnal diffusion, Ekman upwelling/downwelling. Zinc (R² > 0.75) and copper (R² > 0.73) were strongly correlated with Si throughout the study implicating diatoms as strong controllers of their biogeochemical cycling. Nickel was more strongly correlated with PO4 in the upper water column (R² > 0.75), as compared to the whole water column (R² > 0.52), while in the deep ocean nickel appears to correlate with Si although more deep ocean data is needed to confirm this. Trace metal to major nutrient ratios were higher in winter suggesting reduced micronutrient requirement relative to macronutrients under stressed but low productivity conditions. This article is part of a special issue entitled: “Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond” - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.
... For the data here included (<200 m), it can be assumed that processes such as remineralization are overall more relevant below the surface-mixed layer (Boyd and Ellwood, 2010). Nevertheless, depending on the phytoplankton community structure, it can be observed faster remineralization for phosphorus in relation to Fe in the upper 200 m, which could lead to the potential underestimation of the Fe:P ratio (Twining et al., 2014). Additionally, abiotic factors such as physical mixing by different water masses can also affect the relation between the nutrient distributions. ...
Article
Full-text available
An oceanographic survey around the South Shetland Islands (SSI) and the South Orkney Islands (SOI) was conducted during January 2007 and February 2008, respectively, as part of the United States Antarctic Marine Living Resources (AMLR) program ecosystems surveys. At 27 stations, concentrations of dissolved labile Fe (DFe) and total acid leachable (unfiltered, TaLFe) iron (Fe) were measured in the upper 200 m (including coastal and oceanic waters) to better resolve the factors limiting primary production in these regions. Northwest of the SSI, a region influenced by Drake Passage (DP) waters, mean DFe (∼0.26 nM) and TaLFe (∼1.02 nM) concentrations were the lowest, whereas intermediate concentrations for both DFe and TaLFe were measured in the Bransfield Strait (BS). Around Elephant Island (EI), over and off the continental shelf, Fe concentrations differed between the west and the east margins. DFe and TaLFe concentrations further support the argument that the effect of the Shackleton Transverse Ridge (STR) is a crucial structure affecting both the Fe and the chlorophyll distributions in this region. The waters around the SOI had DFe concentrations higher than those in the SSI, with the area north of the South Scotia Ridge (SSR) (60°S), having the highest DFe (0.54 nM) concentrations and the waters in Powell Basin (PB) having the lowest DFe (1.17 nM) and TaLFe (4.51 nM) and concentrations. These spatial patterns of Fe suggest that there are different Fe inputs from shelf waters near the Antarctic Slope Front (ASF). The overall TaLFe:DFe ratios, used as indicator for understanding the relative distance of Fe sources, were lower around the SOI compared to those in the SSI, suggesting that the Fe source for SOI waters was more distant. The spatial patterns between Fe and chlorophyll-a (Chl-a) concentrations in relation to the hydrography highlight the complexity and variability of the oceanographic processes in the region. These results improve the knowledge on the Fe sources and inputs in the less known SOI waters during the austral summer, and they further support the importance of advective processes from the Fe-rich waters that flow from the eastern margin of the Antarctic Peninsula (AP) into the Weddell-Scotia Confluence (WSC).
... Iron is also highly associated with biogenic flux to the deep ocean; it is a micronutrient for most plankton so is taken up at the surface. Because it is particle reactive, only a portion is released back to the dissolved iron pool during remineralization of sinking biogenic particles, and the rest is scavenged back onto sinking particles (Twining et al., 2014). The EUZ, specifically, is a region of high productivity and has a highly efficient biological pump with particles raining at a high rate to the deep ocean (Kiko et al., 2017). ...
... Furthermore, the model remineralizes iron with the same Fe:P stoichiometric ratio with which it was utilized, i.e., the vertical profiles of iron and phosphate remineralization have identical shapes. However, measurements by Twining et al. (2014) show that, at least for some phytoplankton species, iron is remineralized more slowly than phosphate, suggesting that our remineralization profile for iron could be too shallow. Because the model is optimized to fit the DFe observations, with an emphasis on deep profiles relative to surface measurements (Pasquier and Holzer, 2017), a potentially too shallow remineralization of iron would be compensated by an increased strength of the biological pump. ...
Article
Full-text available
Iron fertilization is explored by tracking dissolved iron (DFe) through its life cycle from injection by external sources (birth) to burial in the sediments (death). We develop new diagnostic equations that count iron and phosphate regenerations with each passage through the biological pump and partition the ocean's DFe concentration according to the number of its past and future regenerations. We apply these diagnostics to a family of data-constrained estimates of the iron cycle with sources σ in the range 1.9–41Gmolyr−1. We find that for states with σ>7Gmolyr−1, 50% of the DFe inventory has not been regenerated in the past and 85% will not be regenerated in the future. The globally averaged mean number of past and future regenerations scale with the bulk iron lifetime τ~σ−1tot and have ranges of 0.05–2.2 and 0.01–1.4, respectively. Memory of birth location fades rapidly with each regeneration, and DFe regenerated more than ~5 times is found in a pattern shaped by Southern Ocean nutrient trapping. We quantify the natural fertilization efficiency at any point r in the ocean as the global export production resulting from the DFe at r, per iron molecule. We show that this efficiency is closely related to the mean number of future regenerations that the iron will experience. At the surface, the natural fertilization efficiency has a global mean in the range 0.7–7(molP)(mmolFe)−1 across our family of state estimates and is largest in the central tropical Pacific, with the Southern Ocean having comparable importance only for high iron-source scenarios.
... While surface waters in the Mertz Glacier polynya have dissolved Zn and PO 4 concentrations in the range where a lowslope ZnPO 4 trend is observed in the global ocean, it is the high-slope section (5.08 ± 0.34 mmol mol -1 ) which better corresponds to the Zn:PO 4 uptake ratio we infer (3.1 ± 0.4 mmol mol -1 ) (Figure 4, Table 2). This may reflect coupling between Zn and PO 4 uptake, export and regeneration in these samples at high Zn:PO 4 ratios, as previously described in the Southern Ocean (Twining et al., 2014;Vance et al., 2017;de Souza et al., 2018;Roshan et al., 2018). The Cd:PO 4 uptake we infer agrees well with the CdPO 4 trend observed in our data. ...
Article
Full-text available
The Southern Ocean is the largest high-nutrient low-chlorophyll environment in the global ocean, and represents an important source of intermediate and deep waters to lower latitudes. Constraining Southern Ocean trace metal biogeochemical cycling is therefore important not just for understanding biological productivity and carbon cycling regionally, but also for understanding trace metal distributions throughout the lower latitude oceans. We present dissolved Fe, Ni, Cu, Zn, Cd, Pb and macronutrient concentrations in the Indian and Pacific sectors of the Southern Ocean from the Antarctic Circumnavigation Expedition (austral summer 2016-17), which included the first opportunities to study trace metal cycling at the Mertz Glacier Polynya and the Balleny Islands, as well as two meridional cross-frontal transects. Dissolved Ni, Cu, Zn, Cd and macronutrient concentrations show similar or greater variability latitudinally within surface waters than vertically through the water column, reflecting the combined influence of circulation and biological drawdown in shaping the distributions of nutrient-type elements in the Southern Ocean. Slopes of Cu-Si(OH)4 and Cd-PO4 increase from the Polar Frontal Zone to south of the Southern ACC Boundary (Cu-Si(OH)4) and from the Subantarctic Zone to the Antarctic Zone (Cd-PO4). Latitudinal differences are also observed for Ni-Si(OH)4 and Zn-PO4, with distinct Subantarctic Zone trends relative to those south of the Polar Front. Similarities between our Zn-Si(OH)4 and Cd-PO4 correlations and global compilations reflect the importance of exported Southern Ocean waters in setting these metal-macronutrient couples globally. Distinct Ni-macronutrient correlations are observed in this dataset relative to the global ocean, which supports a distinct cycling of Ni in the Southern Ocean compared to other basins. Concentrations of Pb are among the lowest observed in the global ocean; however, a local maximum is seen along the density level corresponding with Antarctic Intermediate Water. Concentrations within this isopycnal decrease with increasing latitude, which can be explained by decreasing atmospheric Pb input to more recently subducted waters. Substantial biological uptake of metals and macronutrients is observed at the Mertz Glacier Polynya. Here, inferred metal:macronutrient uptake ratios are comparable to those found in the Amundsen Sea Polynya, in Southern Ocean phytoplankton, and to metal-macronutrient correlations in our data set as a whole, highlighting the potential of Southern Ocean polynyas as natural systems for trace metal uptake and export studies. The Balleny Islands are a source of Fe to surface waters and the islands also appear to influence distributions of Zn, Cu and macronutrients, which may reflect the combined impact of Fe supply on biological uptake, mixing, and scavenging in deeper waters. The Kerguelen Plateau is also a source of Fe, as previously identified. Throughout our dataset, the ferricline is found deeper than the nitricline, in agreement with existing data and indicating that Fe is less easily entrained into the surface ocean than NO3. Additionally, Fe:NO3 ratios in most samples throughout the water column are Fe-limiting (<0.01 mmol mol⁻¹). Therefore deep mixing, identified previously as the main Fe source to much of the Southern Ocean, would ultimately act to maintain Fe limitation.
... Differential remineralization length scales of bio-essential elements through the water column, attributed to variable lability of respectively associated (intra-)cellular components, have been described using synchrotron X-ray fluorescence mapping by Twining et al. (2014), focused on diatoms. Given the similarity of bio-essential T-pTM:T-pP ratios to plankton reference sources in surface waters along the transect (Table 1), but variable L-pP/T-pP fractions ( Figure 5), we postulate that it is plausible that the lability of biogenic pP is also variable between different phytoplankton taxa. ...
Article
Full-text available
We present labile (L‐pTM) and refractory (R‐pTM) particulate trace metal distributions of Fe, Mn, Al, Ti, Co, Zn, Cd, Ni, Pb, Cu, and P for a transect along the southwest African shelf and an off‐shore section at 3°S of the GEOTRACES GA08 section cruise. Particle sources and biogeochemical cycling processes are inferred using particle‐type proxies and elemental ratios. Enhanced concentrations of bio‐essential L‐pTMs (Zn, Cu, Ni, Cd, Co, and P) were observed in the Benguela upwelling region, attributed to enhanced primary production. Bio‐essential pTM stoichiometric ratios (normalized to pP) were consistent with phytoplankton biomass across the transect, except for Fe and Mn, which included adsorbed and labile oxide phases. Low pP lability (∼41%) suggests a potential refractory biogenic source on the Benguela shelf. Variable labilities observed between stations along the transect indicated potentially different biogenic pP labilities among different plankton groups. Benthic resuspension was prevalent in (near‐)bottom waters along the transect and formed an important source of Fe and Mn oxides. Lithogenic particles along the entire shelf were Mn deficient and particles on the Benguela shelf were enriched in Fe, consistent with regional sediment compositions. Enhanced available‐Fe (dissolved + labile particulate Fe) concentrations (up to 39.6 nM) were observed in oxygen‐deficient (near‐)bottom waters of the Benguela shelf coinciding with low L‐pMn. This was attributed to the faster oxidation kinetics of Fe, allowing Fe‐oxide precipitation and retention on the shelf, while Mn oxidation was slower. Enhanced L‐pFe in the Congo River plume, which comprised as much as 93% of the available‐Fe pool, was attributed to increased scavenging and formation of Fe oxides. Increased scavenging of other particle‐reactive trace metals (TMs) (Mn, Al, and Pb) was also apparent in Congo‐influenced waters. However, particles did not play a significant role in transporting TMs off‐shelf within Congo plume waters.
... Fe regeneration could account for half of the Fe demand in fertilized regions and up to 90% in HNLC areas (Poorvin et al., 2004). It has been demonstrated that in the upper 200 m, Ni from sinking diatoms can be remineralized faster than Fe (Twining et al., 2014). In surface waters, krill and whales faeces can act as local Fe fertilizers to the Southern Ocean, by successive bioaccumulation in the food chain (from diatom to krill and from krill to whales) and eventually contribute to enhance local primary productivity (Tovar-Sanchez et al., 2007;Nicol et al., 2010;Smith et al., 2013;Ratnarajah et al., 2014). ...
Thesis
Préparée à l’Institut Universitaire Européen de la Mer (IUEM) en cotutelle avec l’University of Tasmania (UTAS) Au sein du Laboratoire des sciences de l’Environnement MARin (LEMAR), Unité Mixte de Recherche (UMR 6539), Institute for Marine and Antarctic Studies (IMAS) et l’Antarctic Climate & Ecosystems Cooperative Research Centre (ACE CRC).
... This was particularly the case near coastal Greenland (0.835 ± 0.124 nM/μM, R 2 0.50 for stations 11, 19, 21 and 23) (Figures S2 and S3 in Supporting Information S1) were dZn/dSi was ∼7-10-fold elevated relative to observations on the Bering and Chukchi Shelf Sea (0.076 ± 0.012 nM/μM, Jensen et al., 2019). This suggests a strong influence of sedimentary Zn sources and a decoupling of dZn from dSi inputs, perhaps as a consequence of preferential dissolution of Zn from biogenic particles in the water column Twining et al., 2014) on the NE Greenland Shelf. Regression slopes of dZn/dSi in the EGC (0.155 ± 0.032 nM/μM, R 2 0.13) were significantly lower and at the upper end of observations from the Central Arctic Ocean (range: −0.17 to 0.17 nM/μM, including Arctic Ocean Polar Surface Water (0.129 nM/μM, R 2 0.72, and Arctic Atlantic Water (0.133 nM/μM, R 2 0.32, , suggesting comparatively little influence of NE Greenland shelf sources on the general distribution of dZn beyond the shelf break. ...
Article
Full-text available
The Arctic Ocean is considered a source of micronutrients to the Nordic Seas and the North Atlantic Ocean through the gateway of Fram Strait. However, there is a paucity of trace element data from across the Arctic Ocean gateways, and so it remains unclear how Arctic and North Atlantic exchange shapes micronutrient availability in the two ocean basins. In 2015 and 2016, GEOTRACES cruises sampled the Barents Sea Opening (GN04, 2015) and Fram Strait (GN05, 2016) for dissolved iron (dFe), manganese (dMn), cobalt (dCo), nickel (dNi), copper (dCu) and zinc (dZn). Together with the most recent synopsis of Arctic-Atlantic volume fluxes, the observed trace element distributions suggest that Fram Strait is the most important gateway for Arctic-Atlantic dissolved micronutrient exchange as a consequence of Intermediate and Deep Water transport. Combining fluxes from Fram Strait and the Barents Sea Opening with estimates for Davis Strait (GN02, 2015) suggests an annual net southward flux of 2.7 ± 2.4 Gg·a-1 dFe, 0.3 ± 0.3 Gg·a-1 dCo, 15.0 ± 12.5 Gg·a-1 dNi and 14.2 ± 6.9 Gg·a-1 dCu from the Arctic towards the North Atlantic Ocean. Arctic-Atlantic exchange of dMn and dZn were more balanced, with a net southbound flux of 2.8 ± 4.7 Gg·a-1 dMn and a net northbound flux of 3.0 ± 7.3 Gg·a-1 dZn. Our results suggest that ongoing changes to shelf inputs and sea ice dynamics in the Arctic, especially in Siberian shelf regions, affect micronutrient availability in Fram Strait and the high latitude North Atlantic Ocean.
... Among the factors responsible, fluvial input can be discarded because practically there is no river runoff to the gulf (Carriquiry et al., 2001), which is associated with almost null rainfall and high evaporation conditions throughout the year. Remineralization of organic matter could also be disregarded owing to Fe is retained within sinking phytoplankton to a greater extent than macronutrients such as P and N (Rapp et al., 2018;Twining et al., 2014). In contrast, benthic fluxes of dFe and their subsequent advection from the GC margins could explain the high concentration of dFe in the surface waters of the gulf. ...
Article
Iron (Fe) is an essential micronutrient for all living organisms and its atmospheric supply to the surface waters of marginal seas remains poorly understood. Here we report the seasonal and spatial variability of atmospheric mineral dust deposition and Fe fluxes along the west coast of the Gulf of California (GC). Meteorological data and dust samples were collected from June 2010 to October 2013 at three sites of the eastern side of Baja California Peninsula (BCP). Evidence of an across-BCP wind component (W-E) during the warm season (May–September) was found, suggesting that winds crossing the BCP from west to east were an important source of variability. Coincidently, dust deposition at the northern and center sites were significantly (p < .05) higher during the warm season, revealing that the BCP could be an important source of dust and Fe to the GC during the warm months. An analysis of the total Fe concentration and Fe/Al ratios also suggest that the BCP and the Mojave Desert are potential sources of the dust arriving to the northern and central region of the GC. Our total averages of dust and Fe fluxes were comparable to those reported for other marginal seas (e,g., Mediterranean, Aqaba) similarly influenced by inputs of mineral dust from the surrounding deserts. A comparison between fluxes show that atmospheric soluble Fe inputs are equivalent to between 9 ± 5% (cool season) and 39 ± 21% (warm season) of the dissolved Fe supplied by upwelling. Thereby, during the warm season, when the GC is warmer and strongly stratified, atmospheric deposition represents a significant source of soluble Fe. We estimate that this supply of Fe would be enough to meet the requirements of N2-fixing primary producers that reside in the oligotrophic surface waters of summer.
... Heavily silicified diatoms sink faster than less silicified forms and may lead to increased bSiO 2 sequestration in the sediments (Dugdale et al. 1995;Liu et al. 2013;Liu and Glibert 2018). Increased nutrient sequestration as a result of faster sinking rates can slow down the rate of bSiO 2 dissolution and may impact the subsequent availability of Si relative to other essential nutrients in the water column, especially because Si remineralization occurs at a slower rate than N, P, and C remineralization (Twinning et al. 2014). Such changes in the relative availability of Si over time may promote the growth of non-silicious and potentially harmful phytoplankton species (Anderson et al. 2002;Liu and Glibert 2018). ...
Article
Full-text available
Diatom cells utilize a variety of metabolic pathways to cope with internal energy imbalances caused by stressful environmental conditions. In this study, the model diatom species, Thalassiosira pseudonana, was grown in nutrient replete and nitrate (NO3−)- and dissolved silicate (Si)-depleted media at three growth temperatures (4, 17, 28 °C) to determine how nutrient enrichment and temperature affects diatom growth, photosynthetic efficiency, nitrate reductase (NR) enzyme activity, biogenic silica (bSiO2) deposition, and NR gene expression. Growth rates for nutrient-replete cultures were highest at 17 °C. Across all nutrient treatments, the cells grown at 17 °C had an average Fv/Fm of 0.44 ± 0.006, while the cells were grown at 4 °C and 28 °C had an average Fv/Fm of 0.37 ± 0.004 and 0.38 ± 0.01, respectively. Activity of NR was variable across treatments with no significant effect of temperature. The relative expression of the targeted NR gene was, on average, ~ 10 times higher in the 4 °C cultures and ~ 4 times higher in the 28 °C than in the 17 °C cultures, while the activity of the NR enzyme was generally highest in the cultures grown at 17 °C that were enriched with NO3−. Cells grown under nutrient-replete conditions had significantly higher bSiO2 deposition rates at 4 °C than cells grown at 17 and 28 °C. These data support the notion that cold, nutrient-replete conditions lead to increases in diatom silicification and that NR activity may be regulated downstream of mRNA transcription under specific environmental conditions.
... However, similar to the dissolution of sinking CaCO 3 particles (Lam et al., 2015a), McManus et al. (1995) reported opal dissolution in the equatorial Pacific. Such dissolution releases the adsorbed trace elements to the water column, including Fe, Mn, and Al (Gieskes, 1981;Murray et al., 2000;Twining et al., 2014). Nevertheless, the dissolution rate of biogenic silica decreased by Fe and Al coating (Michalopoulos & Aller, 2004), the level of dissolved Al within the interstitial spaces of the sediment (McManus et al., 1995;Murray et al., 2000;Van Cappellen & Qiu, 1997a, 1997b and the incorporation of Al within the biogenic silica (Van Bennekom, Berger, Van der Gaast, & de Vries, 1988;Van Bennekom, Buma, & Nolting, 1991). ...
Article
Sequential extraction of core samples collected from Pulicat Lake, east coast of India, showed that on average, 59% of labile Cr and 28% of labile Al remain adsorbed onto Fe–Mn oxides. Similarly, 61% of labile Al and 36% of labile Cr are associated with organic matter. Cross‐correlation analysis revealed that in the Fe–Mn oxides phase, Cr primarily exists with Mn‐oxides, while Al remains mainly adsorbed onto Fe‐oxides. Factor analysis revealed four factors that could explain 82% of the variance in the data. Changes in water column ionic composition, salinity, dissolution of Fe–Mn oxides during microbial oxidation of organic matter, changes occurring to the organic matter and calcium carbonate during early diagenesis are the key factors influencing the Al and Cr concentrations in the labile fractions. The results of this study are useful to interpret palaeoenvironmental conditions.
... More important for the stoichiometry of sinking particles are interelement differences in the ways in which Fe and C exchange with the environment. The release of organic C from fecal pellets has been shown to be fast due to the presence of soluble carbon compounds (Jumars et al. 1989) and more importantly, it is faster than the release of Fe in fecal pellets and sinking particles (Hutchins et al. 1995;Frew et al. 2006;Twining et al. 2014), which should contribute to increasing Fe:C in fecal pellets over time and with depth. In addition, fecal pellets may scavenge dissolved Fe from the water column as they sink to depth, which increases their Fe:C with time, as evidenced by Geesey et al. (1984), whose work on fish fecal pellets showed that old feces contained more Fe than fresh ones (Fig. 2). ...
Article
Full-text available
The impact of marine animals on the iron (Fe) cycle has mostly been considered in terms of their role in supplying dissolved Fe to phytoplankton at the ocean surface. However, little attention has been paid to how the transformation of ingested food into fecal matter by animals alters the relative Fe-richness of particles, which could have consequences for Fe cycling in the water column and for the food quality of suspended and sinking particles. Here, we compile observations to show that the Fe to carbon (C) ratio (Fe:C) of fecal pellets of various marine animals is consistently enriched compared to their food, often by more than an order of magnitude. We explain this consistent enrichment by the low assimilation rates that have been measured for Fe in animals, together with the respiratory conversion of dietary organic C to excreted dissolved inorganic C. Furthermore, we calculate that this enrichment should cause animal fecal matter to constitute a major fraction of the global sinking flux of biogenic Fe, a component of the marine iron cycle that has been previously unappreciated. We also estimate that this fecal iron pump provides an important source of Fe to marine animals via coprophagy, particularly in the mesopelagic, given that fecal matter Fe:C can be many-fold higher than the Fe:C of local phy-toplankton. Our results imply that the fecal iron pump is important both for global Fe cycling and for the iron nutrition of pelagic and mesopelagic communities.
... remineralization of Zn relative to Si (Twining et al., 2014), and is especially clear in the South Atlantic, underlining that the mixing of water masses acts as the dominant control on [Zn] de Souza et al., 2018;Middag et al., 2019). While Weber et al. (2018) ...
Article
Full-text available
Phytoplankton productivity and export sequester climatically significant quantities of atmospheric carbon dioxide as particulate organic carbon through a suite of processes termed the biological pump. Constraining how the biological pump operated in the past is important for understanding past atmospheric carbon dioxide concentrations and Earth's climate history. However, reconstructing the history of the biological pump requires proxies. Due to their intimate association with biological processes, several bioactive trace metals and their isotopes are potential proxies for past phytoplankton productivity, including iron, zinc, copper, cadmium, molybdenum, barium, nickel, chromium, and silver. Here, we review the oceanic distributions, driving processes, and depositional archives for these nine metals and their isotopes based on GEOTRACES‐era datasets. We offer an assessment of the overall maturity of each isotope system to serve as a proxy for diagnosing aspects of past ocean productivity and identify priorities for future research. This assessment reveals that cadmium, barium, nickel, and chromium isotopes offer the most promise as tracers of paleoproductivity, whereas iron, zinc, copper, and molybdenum do not. Too little is known about silver to make a confident determination. Intriguingly, the trace metals that are least sensitive to productivity may be used to track other aspects of ocean chemistry, such as nutrient sources, particle scavenging, organic complexation, and ocean redox state. These complementary sensitivities suggest new opportunities for combining perspectives from multiple proxies that will ultimately enable painting a more complete picture of marine paleoproductivity, biogeochemical cycles, and Earth's climate history.
... Through a variety of techniques, previous studies have suggested substantial remineralization of P. antarctica in the upper 100-150 m, consistent with our findings (Asper & Smith, 2019;Jones & Smith, 2017;Reigstad & Wassmann, 2007). Conversely, diatom-associated carbon is thought to be remineralized less rapidly, since the carbon associated with the siliceous tests may sink more rapidly than does non-silica associated organic material and therefore reach greater depths (Twinning et al., 2014), although this may not be universally true (Yasuda et al., 2016). ...
Article
Full-text available
To assess the temporal biological and hydrographic features of the southwestern Ross Sea, we deployed a glider in a spatially restricted, ice‐free area during the austral summer (1 December–6 February), and quantified from sensor measurements the particulate organic carbon (POC; via particulate backscatter) concentrations, their changes through time, and net community production (NCP; via dissolved O2 concentrations). The POC levels could be divided into three distinct phases (I, II, and III, respectively) characterized by changes in NCP, surface‐layer POC concentrations, remineralization, and export. Surface POC concentrations increased from 215 mg C m⁻³ in early December to a peak of >400 mg C m⁻³ by mid‐December, before decreasing to 227 mg C m⁻³ in late January–early February. NCP was highly variable throughout the summer, becoming maximal in mid‐December. By constructing a carbon budget, we estimated rates of change of POC and export potential to the mesopelagic in each phase. Changes in euphotic zone POC concentrations and NCP suggested that the system is slightly net autotrophic during the observational period (average NCP is 0.05 g C m⁻² d⁻¹), and POC removal from the top 240 m of the water column averaged 0.22 g C m⁻² d⁻¹. Our data confirm that the southern Ross Sea during the ice‐free season is a high productivity, low export system while providing high‐resolution POC dynamics that had not been previously observed. Although the Ross Sea is a site of substantial carbon fixation, there remains an incomplete understanding both of the processes involved in export and the rates and controls of remineralization.
... Such strong ligands which promote mineral dissolution (Kalinowski et al. 2000) at high dissolution rates (Akafia et al. 2014) could be released to scavenge Fe from different types of minerals, such as ilmenite as observed in the particles used here (van der Merwe et al. 2019). For metals other than Fe, knowledge on trace metal release linked to prokaryotic degradation of particles is limited (Twining et al. 2014). Our results provide clues that these processes should be significant and require further attention and quantification. ...
Article
As marine microorganisms and environmental conditions coevolved over geological timescales, metals have been incorporated into all essential metabolic processes. In the modern ocean, metals are present from trace amounts limiting microbial growth to toxic concentrations. Dissolved trace metals are a major bioavailable reservoir. However, the acquisition of metals from marine particles remains largely unexplored. Here, we combined chemical characterization and a comparative metatranscriptomics approach to investigate the availability of nine metals of biological importance on particles collected in the region of Heard Island (Indian sector of the Southern Ocean). Elemental ratios identified particulate matter as a potential source of metals for prokaryotes. The expression of genes for the uptake of metals through various mechanisms demonstrated that particles are a bioavailable reservoir. But genes involved in the control of resistance to metal toxicity, storage, sensing, and regulation were also highly expressed. Our observations suggest that homeostasis associated with a diverse prokaryotic community is the overarching mechanism that enhances the trace element processing on particles. These results provide clues that microbial activity on particles is critical in the redistribution of trace elements between different fractions and chemical forms in the ocean.
... North (STF) and South Subtropical Front (S-STF) designations adapted from Smith et al. (2013) around southern New Zealand and on the Chatham Rise (Sutton, 2001), east of Aotearoa New Zealand. only little information available on the taxonomic composition of phytoplankton communities prevailing in open-ocean waters away from the STFZ over the Chatham Rise region Twining et al., 2014;Chang and Northcote, 2016;Chiswell et al., 2019). Further east of this region (170 • W), phytoplankton community composition from polar to equatorial waters have been characterized using pigment analysis (DiTullio et al., 2003) whereas a more recent study applied DNA metabarcoding analysis to investigate microbial diversity patterns in relation to physico-chemical gradients and oceanographic features (Raes et al., 2018). ...
Article
Planktonic protists are an essential component of marine pelagic ecosystems where they mediate important trophic and biogeochemical functions. Although these functions are largely influenced by their taxonomic affiliation, the composition and spatial variability of planktonic protist communities remain poorly characterized in vast areas of the ocean. Here, we investigated the diversity of these communities in contrasting oceanographic conditions of the southwest Pacific (33–58 °S) using DNA metabarcoding of the 18S rRNA gene. Seawater samples collected during twelve cruises (n = 482, 0–3100 m) conducted east of New Zealand were used to characterize protist communities in Subtropical (STW) and Subantarctic (SAW) surface water masses and the Subtropical Front (STF) that separates them. Diversity decreased with increasing latitude and increasing temperature but tended to be lowest in the STF. Sample ordination resulting from the abundance of amplicon single variants (ASVs) corresponded to the different water masses. Overall, Dinoflagellata (Syndiniales, 27%; Dinophyceae, 24% of standardized number of reads) dominated the euphotic zone followed by Chlorophyta (20%), but their relative abundance and composition at class and lower taxonomic levels varied consistently between water masses. Among Chlorophyta, several picoplanktonic algae species of the Mamiellophyceae class including Ostreococcus lucimarinus dominated in STW, while the Chloropicophyceae species Chloroparvula pacifica was most abundant in SAW. Bacillariophyta (5%), Prymnesiophyceae (5%), and Pelagophyceae (2%) classes were less abundant but showed analogous water mass specificity at class and finer taxonomic levels. Protist community composition in the STF had mixed characteristics and showed regional differences with the southern STF (50 °S) having more resemblance with subantarctic communities than the STF over the Chatham Rise region (42–44 °S). Below the euphotic zone, Syndiniales sequences (40%) dominated the dataset followed by Radiolaria (31%), Dinophyceae (14%) and other heterotrophic groups like Marine Stramenopiles and ciliates (1–1.5%). Among Radiolaria, several unidentified ASVs assigned to Spumellaria were most abundant, but showed significantly different distributions between STW and SAW highlighting the need to further investigate the taxonomy and ecology of this group. The present study represents a significant step forward towards characterizing protistan communities composition in relation to major physical oceanographic features in the southwest Pacific providing new insights about the biogeography and ecological preferences of different planktonic protist taxa from class to species and genotypic level.
Article
Silver is a non-nutrient element that is readily taken up by diatoms. It exists in the ocean in pM concentrations and is known to accumulate in marine sediments underlying surface waters that support diatom blooms. Laboratory studies and open-ocean water column profiles of Ag versus Si have led to the hypothesis that diatoms accumulate Ag within their silica frustules, subsequently delivering Ag to underlying sediments as the organisms die and sink to the seafloor. Through this delivery mechanism, sedimentary Ag concentrations might serve as a useful record of past export production. However, the actual partitioning of Ag between diatom organic and frustule parts is unknown, and as such represents a considerable source of uncertainty in the development of Ag as a proxy for diatom paleoproductivity and in fully describing Ag biogeochemical cycling. In this study, we use a synchrotron XRF microprobe to map the location of Ag within the laboratory-grown diatom Thalassiosira pseudonana and show that diatoms primarily store Ag in their soft tissue, not in their frustules. Intracellular Ag appears to be concentrated in vacuoles, although the majority of Ag is widely distributed and may be associated with the cell membrane and/or cytoplasm. These results imply an alternate mechanism for Ag delivery to sediments whereby Ag is associated with decomposing organic particles rather than skeletal remains. Silver therefore continues to show promise as a qualitative paleoproductivity proxy. Quantitative use is unlikely, however, given variable uptake rates by diatoms in response to environmental conditions and the significant potential for remineralization with organic matter in the water column.
Article
Full-text available
Nickel is a biologically essential element for marine life, with the potential to influence diverse processes, including methanogenesis, nitrogen uptake and coral health, in both modern and past oceans. However, an incomplete view of oceanic Ni cycling has stymied understanding of how Ni may impact marine life in these modern and ancient oceans. Here we combine data-constrained global biogeochemical circulation modelling with culture experiments and find that Ni in oligotrophic gyres is both chemically and biologically labile and only minimally incorporated into diatom frustules. We then develop a framework for understanding oceanic Ni distributions, and in particular the two dominant features of the global marine Ni distribution: the deep concentration maximum and the residual pool of approximately 2 nM Ni in subtropical gyres. We suggest that slow depletion of Ni relative to macronutrients in upwelling regions can explain the residual Ni pool, and reversible scavenging or slower regeneration of Ni compared with macronutrients contributes to the distinct Ni vertical distribution. The strength of these controls may have varied in the past ocean, impacting Ni bioavailability and setting a fine balance between Ni feast and famine for phytoplankton, with implications for both ocean chemistry and climate state.
Article
Full-text available
Iron fertilization is explored by tracking dissolved iron (DFe) through its life cycle from injection by aeolian, sedimentary, and hydrothermal sources (birth) to burial in the sediments (death). We develop new diagnostic equations that count iron and phosphate regenerations with each passage through the biological pump and partition the ocean's DFe concentration according to the number of its past or future regenerations. We apply these diagnostics to a family of data-constrained estimates of the iron cycle with sources σtot in the range 1.9–41Gmol yr⁻¹. We find that for states with σtot > 7Gmol yr⁻¹, 50% or more of the DFe inventory has not been regenerated in the past and 85% or more will not be regenerated in the future. The globally averaged mean number of past or future regenerations scales with the bulk iron lifetime τ ∼ σtot⁻¹ and has a range of 0.05–2.2 for past and 0.01–1.4 for future regenerations. Memory of birth location fades rapidly with each regeneration, and DFe regenerated more than approximately five times is found in a pattern shaped by Southern Ocean nutrient trapping. We quantify the intrinsic fertilization efficiency of the unperturbed system at any point r in the ocean as the global export production resulting from the DFe at r per iron molecule. We show that this efficiency is closely related to the mean number of future regenerations that the iron will experience. At the surface, the intrinsic fertilization efficiency has a global mean in the range 0.7–7mol P (mmol Fe)⁻¹ across our family of state estimates and is largest in the central tropical Pacific, with the Southern Ocean having comparable importance only for high-iron-source scenarios.
Article
Full-text available
The Southern Ocean (SO) is of global importance to the carbon cycle, and processes such as mesopelagic remineralisation that impact the efficiency of the biological carbon pump in this region need to be better constrained. During this study early austral winter barium excess (Baxs) concentrations were measured for the first time, along 30∘ E in the southern Indian Ocean. Winter Baxs concentrations of 59 to 684 pmol L−1 were comparable to those observed throughout other seasons. The expected decline of the mesopelagic Baxs signal to background values during winter was not observed, supporting the hypothesis that this remineralisation proxy likely has a longer timescale than previously reported. A compilation of available SO mesopelagic Baxs data, including data from this study, shows an accumulation rate of ∼0.9 µmol m−2 d−1 from September to July that correlates with temporally integrated remotely sensed primary productivity (PP) throughout the SO from data spanning ∼20 years, advocating for a possible annual timescale of this proxy. The percentage of mesopelagic particulate organic carbon (POC) remineralisation as calculated from estimated POC remineralisation fluxes over integrated remotely sensed PP was ∼2-fold higher south of the polar front (19 ± 15 %, n=39) than north of the polar front (10 ± 10 %, n=29), revealing the higher surface carbon export efficiency further south. By linking integrated remotely sensed PP to mesopelagic Baxs stock, we could obtain better estimates of carbon export and remineralisation signals within the SO on annual and basin scales.
Article
The seas are acidifying as a result of carbon dioxide emissions. It now emerges that this will alter the solubility of the shells of marine organisms called diatoms — and thereby change the distribution of nutrients and plankton in the ocean. Reduced solubility of diatom shells could alter marine silicon fluxes.
Article
First winter measurements of dissolved zinc (dZn) and particulate zinc (pZn) are presented from seven stations, between 41 and 58°S, occupied in July 2017 along the 30°E longitude in the Indian Sector of the Southern Ocean. This unique spatial and seasonal dataset provided the opportunity to investigate Zn biogeochemical cycling in a region which is extremely data scarce and during a period when conditions are unfavourable for phytoplankton growth. Surface comparisons of our winter dZn and pZn to previous measurements during spring and summer revealed that Zn seasonality is most pronounced at the higher latitudes where higher dZn (and higher ratios of dZn to phosphate; dZn:PO4) and lower pZn in winter reflect decreased biological uptake and preferential dZn resupply (relative to PO4) to surface waters through deep winter mixing. The composition of pZn was majorly biogenic however localised lithogenic inputs were attributed to potential hydrothermal activity and transport of continental sediment via Agulhas waters. Calculated vertical attenuation factors (b values) for pZn (0.31) and phosphorus (P; 0.41) suggest that Zn has a longer remineralisation length scale than P, providing a mechanism as to why dZn appears to be remineralised deeper in the water column than PO4. Ratios of pZn to P (pZn:P) in surface waters increased with latitude from 1.12 to 8.28 mmol mol⁻¹ due to increased dZn availability and the dominance of diatoms (with high cellular Zn quotas) in the high latitude Antarctic Zone (AAZ). Interestingly, the high surface pZn:P ratios in the AAZ did not change significantly with depth (in contrast to the northern stations where pZn:P increased with depth) suggesting the export of diatom cells below the winter mixed layer where remineralisation and rigorous mixing may resolve the linear dZn to silicic acid (dZn:Si(OH)4) correlation (dZn (nmol kg⁻¹) = 0.064 Si(OH)4 (μmol kg⁻¹) + 0.690; r² = 0.93; n = 120) despite these elements being located in separate components of the diatom cell. Additionally, elevated concentrations of dZn and Si(OH)4 below 3000 m in the AAZ may reflect nutrient accumulation in bottom waters where northward flow is inhibited by the Indian mid-Ocean ridge.
Preprint
Full-text available
Planktonic protists are an essential component of marine pelagic ecosystems where they mediate important trophic and biogeochemical functions. Although these functions are largely influenced by their taxonomic affiliation, the composition and spatial variability of planktonic protist communities remain poorly characterized in vast areas of the ocean. Here, we investigated the diversity of these communities in contrasting oceanographic conditions of the southwest Pacific sector (33-58°S) using DNA metabarcoding of the 18S rRNA gene. Seawater samples collected during twelve cruises (n = 482, 0-2000 m) conducted east of New Zealand were used to characterize protist communities in Subtropical (STW) and Subantarctic (SAW) water masses and the Subtropical Front (STF) that separates them. Diversity decreased with latitude and temperature but tended to be lowest in the STF. Sample ordination resulting from the abundance of amplicon single variants (ASVs) corresponded to the different water masses. Overall, Dinophyceae (34% of standardized total number of reads) and Chlorophyta (27%) co-dominated the euphotic zone, but their relative abundance and composition at class and lower taxonomic levels varied consistently between water masses. Among Chlorophyta, several picoplanktonic algae species of the Mamiellophyceae class including Ostreococcus lucimarinus dominated in STW, while the Chloropicophyceae species Chloroparvula pacifica was most abundant in SAW. Bacillariophyta (7%), Prymnesiophyceae (5%), and Pelagophyceae (3%) classes were less abundant but showed analogous water mass specificity at class and finer taxonomic levels. Protist community composition in the STF had mixed characteristics and showed regional differences with the southern STF (50°S) having more resemblance with subantarctic communities than the STF over the Chatham Rise region (42-44°S). Below the euphotic zone, Radiolaria sequences dominated the dataset (52%) followed by Dinophyceae (27%) and other heterotrophic groups like Marine Stramenopiles and ciliates (3%). Among Radiolaria, several unidentified ASVs assigned to Spumellarida were most abundant, but showed significantly different distribution between STW and SAW highlighting the need to further investigate the taxonomy and ecology of this group. This study represents a significant step forward towards characterizing protistan communities composition in relation to major water masses and fronts in the South Pacific providing new insights about the biogeography and ecological preferences of different taxa from class to species and genotypic level. Highlights Water-mass preference of different taxa emerged at class, species and genotypic level. Mamiellophyceae green algae dominated in subtropical waters. Dinophyceae and Chloropicophyceae green algae dominated in subantarctic waters. A diverse assemblage of Radiolaria dominated the mesopelagic zone. Small rather than large taxa dominated phytoplankton blooms in subtropical waters.
Article
Full-text available
The GEOTRACES program has greatly increased basin‐scale concentration measurements for a large number of elements in the ocean, both constraining external sources and internal sinks and exposing complex internal cycles of trace elements. Our conceptual frameworks for marine trace element cycling, however, often remain simplified as the production and remineralization of phytoplankton biomass. Despite their complexity, or perhaps because of it, trace element cycles are often predominantly considered as an extension of traditional Redfield macronutrient ratios to C or P. Here we utilize extensive data sets of particulate trace element concentrations from GEOTRACES section cruises in the South Pacific and North Atlantic Oceans to look for evidence of the internal cycles of multiple trace elements without requiring normalization to phytoplankton biomass. Using both traditional and expanded power law regression analyses and multi‐element factor analysis, we expose the internal distributions of six authigenic, biogenic, and lithogenic particulate phases and their multi‐element associations. Critically, no particulate trace element is observed to behave identically to P. Observations include a scavenged Fe phase with a slight surface maximum, which increases linearly with depth below ~ 300 m and which appears to co‐scavenge Cu, V, and La. Particulate Co is found to be associated with phytoplankton, Mn‐biooxides just below the mixed layer, and with a putative heterotrophic phase observed in the surface and at depth. We present an expanded conceptual framework for particulate trace element cycling that has explicit roles for these multiple particulate phases.
Article
Full-text available
As mass loss from the Greenland Ice Sheet accelerates, this modeling study considers how meltwater inputs to the ocean can impact marine ecosystems using a simplified fjord scenario. At marine-terminating glaciers in Greenland fjords, meltwater can be delivered far below the sea surface, both as subglacial runoff (from atmosphere-driven surface melt) and as basal melt (from ocean heat). Such delivery can result in buoyancy-driven upwelling and the upward entrainment of nutrient-rich deep water, which can support phytoplankton growth in fjord surface waters. For this study, we use an idealized fjord-scale model to investigate which properties of glaciers and fjords govern the transport of buoyantly upwelled nutrients from fjords. We model the influence of fjord geometry, hydrology, wind, tides, and phytoplankton growth within the fjord on meltwater-driven nutrient export to the ocean. We use the Regional Ocean Modeling System (ROMS) coupled to a buoyant plume model and a biogeochemical model to simulate physical and biogeochemical processes within an idealized tidewater glacial fjord. Results show that meltwater-driven nutrient export increases with larger subglacial discharge rates and deeper grounding lines, features that are both likely to change with continued ice sheet melting. Nutrient export decreases with longer residence times, allowing greater biological drawdown. While the absence of a coastal current in the model setup prevents the downstream advection of exported nutrients, results suggest that shelf-forced flows could influence nutrient residence time within fjords. This simplified model highlights key uncertainties requiring further observation to understand ecological impacts of Greenland mass loss.
Article
The biogeochemical cycles of trace elements and their isotopes (TEIs) constitute an active area of oceanographic research due to their role as essential nutrients for marine organisms and their use as tracers of oceanographic processes. Selected TEIs also provide diagnostic information about the physical, geological, and chemical processes that supply or remove solutes in the ocean. Many of these same TEIs provide information about ocean conditions in the past, as their imprint on marine sediments can be interpreted to reflect changes in ocean circulation, biological productivity, the ocean carbon cycle, and more. Other TEIs have been introduced as the result of human activities and are considered contaminants. The development and implementation of contamination-free methods for collecting and analyzing samples for TEIs revolutionized marine chemistry, revealing trace element distributions with oceanographically consistent features and new insights about the processes regulating them. Despite these advances, the volume and geographic coverage of high-quality TEI data by the end of the twentieth century were insufficient to constrain their global biogeochemical cycles. To accelerate progress in this field of research, marine geochemists developed a coordinated international effort to systematically study the marine biogeochemical cycles of TEIs—the GEOTRACES program. Following a decade of planning and implementation, GEOTRACES launched its main field effort in 2010. This review, roughly midway through the field program, summarizes the steps involved in designing the program, its management structure, and selected findings. Expected final online publication date for the Annual Review of Marine Science Volume 12 is January 3, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
Article
Full-text available
Marine oxygen deficient zones (ODZs) promote unique plankton communities and redox environments which impact the cycling of biologically essential trace metals in the ocean. Here we use measurements of dissolved and particulate Ni concentrations and isotopes to investigate the biotic and abiotic processes controlling Ni cycling in the world’s largest ODZ, located in the Eastern Tropical North Pacific (ETNP). We observed a negative correlation between dissolved Ni concentrations and isotopic composition (60Ni) throughout the water column, such that Ni concentrations increased from roughly 3 nmol kg-1 to 8 nmol kg-1 over the upper 1000 m, while 60Ni values decreased by 0.2‰ from about +1.6‰ to +1.4‰. These vertical patterns are characteristic of both the subtropical North and South Pacific, and can be explained by a combination of physical mixing of water masses and biological uptake and export, either with all of the Ni being bioavailable or with separate bioavailable and non-bioavailable Ni pools. Although evidence for additional Ni cycling processes such as sulfide precipitation or Ni sorption/desorption through Fe/Mn redox chemistry have been observed in other ODZs and euxinic waters, we found no clear evidence for these in either the redoxcline or low oxygen waters of the ETNP. Indeed, the relationship between dissolved [Ni] and 60Ni observed in the ETNP is similar to results reported elsewhere in the subtropical North and South Pacific, falling generally on a mixing line between a surface water endmember (dissolved [Ni] = 2 nmol kg-1 and 60Ni = +1.7‰) and a deep-water endmember (dissolved [Ni] = 6–10 nmol kg-1 and 60Ni = ~+1.4‰). While this surface water endmember is similar to that of the Atlantic, the deep endmember in the Pacific is approximately 0.1‰ heavier than deep Atlantic Ni. This subtle isotopic difference suggests gradual accumulation of isotopically heavy Ni isotopes in the deep ocean, consistent with recent evidence of heavy Ni remobilization during early diagenesis. Lastly, in the ETNP, particulate 60Ni is generally ~0.5‰ lighter than the dissolved Ni pool, and this pattern is consistent across both the euphotic zone and redoxcline, suggesting that biological export from the euphotic zone is the primary source of particulate Ni to the deep ocean.
Article
Full-text available
Fluxes of major bioelements associated with sinking particles were quantified in late summer 2018 as part of the EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) field campaign near Ocean Station Papa in the subarctic northeast Pacific. The thorium-234 method was used in conjunction with size-fractionated (1–5, 5–51, and >51 μm) concentrations of particulate nitrogen (PN), total particulate phosphorus (TPP), biogenic silica (bSi), and particulate inorganic carbon (PIC) collected using large volume filtration via in situ pumps. We build upon recent work quantifying POC fluxes during EXPORTS. Similar remineralization length scales were observed for both POC and PN across all particle size classes from depths of 50–500 m. Unlike bSi and PIC, the soft tissue–associated POC, PN, and TPP fluxes strongly attenuated from 50 m to the base of the euphotic zone (approximately 120 m). Cruise-average thorium-234-derived fluxes (mmol m–2 d–1) at 120 m were 1.7 ± 0.6 for POC, 0.22 ± 0.07 for PN, 0.019 ± 0.007 for TPP, 0.69 ± 0.26 for bSi, and 0.055 ± 0.022 for PIC. These bioelement fluxes were similar to previous observations at this site, with the exception of PIC, which was 1 to 2 orders of magnitude lower. Transfer efficiencies within the upper twilight zone (flux 220 m/flux 120 m) were highest for PIC (84%) and bSi (79%), followed by POC (61%), PN (58%), and TPP (49%). These differences indicate preferential remineralization of TPP relative to POC or PN and larger losses of soft tissue relative to biominerals in sinking particles below the euphotic zone. Comprehensive characterization of the particulate bioelement fluxes obtained here will support future efforts linking phytoplankton community composition and food-web dynamics to the composition, magnitude, and attenuation of material that sinks to deeper waters.
Article
p>Trace metal micronutrients play key roles in photosynthesis by oceanic phytoplankton. Though they are required in much smaller amounts than the major nutrients (P, N, Si), their bioavailable forms are also present in the seawater solution at much lower levels. Relationships between the dissolved chemistry of the nutrient-type trace metals, their stable isotope variations, as well those of the major nutrients, have highlighted the importance of biological and physical processes in the Southern Ocean in controlling their oceanic biogeochemistry. However, the first-order Southern Ocean processes are overprinted by vertical cycling in other parts of the ocean, particularly upwelling regions remote from the Southern Ocean, with the North Pacific standing out in particular. Here we present new zinc (Zn) concentration and isotope, as well as major nutrient data for the NE Pacific, and couple these new data with a compilation of published data from across the region, with the objective of better understanding the impact of this important region on oceanic biogeochemical cycles. The new Zn isotope data for two stations along Line P (P04 and P26) show a large range in δ<sup>66</sup>Zn in the upper ocean (−0.4‰ up to >1‰), associated with a very small isotope fractionation but extreme depletion of the dissolved pool during photic zone biological uptake, and the regeneration of this cellular Zn at very shallow depths (50 m). Beneath this, the two profiles approach the δ<sup>66</sup>Zn value of +0.5‰, seen throughout the deep ocean, by about 500 m. The minimum δ<sup>66</sup>Zn resulting from regeneration is associated with very high Zn concentrations, particularly at the marginal P04 station where diatoms dominate the phytoplankton ecology. Combining the new data with published Zn and major nutrient concentrations from across the North Pacific emphasises the role of vertical biological cycling in controlling regional biogeochemistry in the North Pacific, resulting in the partial overprinting of biogeochemical signatures transported out of the Southern Ocean by the ocean circulation. Zinc isotope data document the uptake of this metal into diatoms and the co-regeneration of Zn with phosphate in the upper water column. Silica in contrast is regenerated at greater depth, resulting in a decoupling of the Zn–Si correlation that is set in the Southern Ocean and that dominates the Atlantic. Previous work has suggested that the decoupling of Zn and Si in the subarctic North Pacific results from removal of Zn (and other metals) to water column particulate sulphide. In our dataset, and in the compilation of data documenting relationships between Zn and the major nutrients across the North Pacific, this decoupling is clearly due to the different lengthscales of regeneration for organic matter (Zn and P) and diatom opal (Si).</p
Article
Controlling zinc in the oceans Zinc, a key micronutrient for marine phytoplankton, has a global distribution remarkably similar to that of silicic acid, even though Zn and Si have very different biogeochemical cycles. Weber et al. investigated why this is so by combining model calculations and observations. They found that biological uptake in the Southern Ocean and reversible scavenging of Zn onto sinking particles both affect the distribution of Zn in the ocean. Thus, Zn and Si distributions will be affected differently by future changes in ocean temperature, pH, and carbon fluxes. Science , this issue p. 72
Article
Full-text available
Biogenic Fe quotas were determined using three distinct techniques on samples collected concurrently in the subtropical Pacific Ocean east of New Zealand. Fe quotas were measured using radioisotope uptake experiments (24 h incubation), bulk filtration and analysis by inductively-coupled plasma mass spectrometer (ICPMS), and single-cell synchrotron x-ray fluorescence (SXRF) analysis over a sixteen-day period (year days 263 to 278 of 2008) during a quasi-Lagrangian drifter experiment that tracked the evolution of the annual spring diatom bloom within a counter-clockwise open-ocean eddy. Overall, radioisotope uptake-determined Fe quotas (washed with oxalate reagent to remove extracellular Fe) were the lowest (0.5–1.0 mmol Fe:mol P; 4–8 μmol Fe:mol C), followed by single-cell Fe quotas (2.3–7.5 mmol Fe:mol P; 17–57 μmol Fe:mol C), and the highest and most variable quotas were from the bulk filtration ICPMS approach that used the oxalate reagent wash, corrected for lithogenic Fe using Al (0.8–21 mmol Fe:mol P; 4–136 μmol Fe:mol C). During the evolution of the spring bloom within the eddy (year days 263 to 272), the surface mixed layer inventories of particulate biogenic elements (C, N, P, Si) and chlorophyll increased while Fe quotas estimated from all three approaches exhibited a general decline. After the onset of the bloom decline, the drogued buoys exited the eddy center (days 273 to 277). Fe quotas returned to pre-bloom values during this part of the study. Our standardized and coordinated sampling protocols reveal the general observed trend in Fe quotas: ICPMS > SXRF > radioisotope uptake. We discuss the inherent differences between the techniques and argue that each technique has its individual merits and uniquely contributes to the characterization of the oceanic particulate Fe pool.
Article
Full-text available
A global three-dimensional marine ecosystem model with several key phytoplankton functional groups, multiple limiting nutrients, explicit iron cycling, and a mineral ballast/organic matter parameterization is run within a global ocean circulation model. The coupled biogeochemistry/ ecosystem/circulation (BEC) model reproduces known basin-scale patterns of primary and export production, biogenic silica production, calcification, chlorophyll, macronutrient and dissolved iron concentrations. The model captures observed high nitrate, low chlorophyll (HNLC) conditions in the Southern Ocean, subarctic and equatorial Pacific. Spatial distributions of nitrogen fixation are in general agreement with field data, with total N-fixation of 55 Tg N. Diazotrophs directly account for a small fraction of primary production (0.5%) but indirectly support 10% of primary production and 8% of sinking particulate organic carbon (POC) export. Diatoms disproportionately contribute to export of POC out of surface waters, but CaCO3 from the coccolithophores is the key driver of POC flux to the deep ocean in the model. An iron source from shallow ocean sediments is found critical in preventing iron limitation in shelf regions, most notably in the Arctic Ocean, but has a relatively localized impact. In contrast, global-scale primary production, export production, and nitrogen fixation are all sensitive to variations in atmospheric mineral dust inputs. The residence time for dissolved iron in the upper ocean is estimated to be a few years to a decade. Most of the iron utilized by phytoplankton is from subsurface sources supplied by mixing, entrainment, and ocean circulation. However, owing to the short residence time of iron in the upper ocean, this subsurface iron pool is critically dependent on continual replenishment from atmospheric dust deposition and, to a lesser extent, lateral transport from shelf regions.
Article
Full-text available
An open flow reactor is used to simulate the dissolution process of mineral aerosol particles in atmospheric water droplets. Data on dissolution kinetic and solubility are provided for the major trace metals from two kinds of matrix: alumino-silicated and carbonaceous sample. The results emphasise that the metals contained in the carbonaceous aerosols are easier dissolved than in the alumino-silicated particles. The released concentrations are not related to the total metal composition or the origin of particles, but are directly associated with the type of liaisons whereby the metals are bound in the solid matrix. Thus, the metals coming from carbonaceous particles are adsorbed impurities or salts and hence are very soluble and with a dissolution hardly dependent on pH, whereas the metals dissolved from alumino-silicated particles are less soluble, notably the ones constitutive of the matrix network (Fe, Mn), and with a dissolution highly influenced by pH. Consequently, in the regions with an anthropogenic influence, the dissolved concentrations of metals found in the atmospheric waters are mainly governed by the elemental carbon content. Moreover, it appears that the dissolution kinetic of metals is not constant as a function of time. The dissolution rates are very rapid in the first 20 min of leaching and then they are stabilised to lower values in comparison to initial rates. By consequence, the total dissolved metal content is provided after the first 20 min of the droplet lifetime. For this reason, the effects of trace metals on the atmospheric aqueous chemistry and as atmospheric wet input to the marine biota are maximal for ''aged'' droplets.
Article
Full-text available
Dissolved nickel (Ni) typically displays a `nutrient-like' vertical profile in the ocean, with lower concentrations in surface waters and higher concentrations in deep waters, similar to other micronutrient metals such as iron and zinc. Vertical profiles of Ni show particular similarities to profiles of the macronutrients phosphate and silicic acid, suggesting that diatoms play an important role in mediating the vertical distribution of this metal. We performed synchrotron x-ray fluorescence (SXRF) analysis on individual phytoplankton cells collected from stations in the equatorial Pacific Ocean and from nutrient-addition incubation experiments conducted on the same cruise. Diatoms were enriched in Ni twofold to fivefold relative to picoplankton and flagellated cells. Changes in cellular quotas of Si, P and Ni observed in diatoms growing in response to Fe and Si additions were used to estimate the Ni:P (0.52 ± 0.10 mmol/mol) and Ni:Si (28 ± 13 μmol/mol) ratios of internal biomass and the frustule, respectively. Elevated internal Ni:P suggests a heightened role for urease or the Ni isoform of superoxide dismutase in diatoms (similar to cyanobacteria), while Ni associated with the frustule appears to contribute an additional 50% of cellular Ni found in the diatoms. The derived Ni:Si ratio for frustule material is comparable to Ni:Si ratios in published nutrient profiles, confirming the dominant role that diatoms play in ocean Ni biogeochemistry. While a molecular explanation for the association of Ni with frustules remains to be determined, this study demonstrates the unique biogeochemical insight that can be gained from microanalytical element analysis.
Article
Full-text available
Advances in iron biogeochemistry have transformed our understanding of the oceanic iron cycle over the past three decades: multiple sources of iron to the ocean were discovered, including dust, coastal and shallow sediments, sea ice and hydrothermal fluids. This new iron is rapidly recycled in the upper ocean by a range of organisms; up to 50% of the total soluble iron pool is turned over weekly in this way in some ocean regions. For example, bacteria dissolve particulate iron and at the same time release compounds - iron-binding ligands - that complex with iron and therefore help to keep it in solution. Sinking particles, on the other hand, also scavenge iron from solution. The balance between these supply and removal processes determines the concentration of dissolved iron in the ocean. Whether this balance, and many other facets of the biogeochemical cycle, will change as the climate warms remains to be seen.
Article
Full-text available
The global marine distributions of Cd and phosphate are closely correlated, which has led to Cd being considered as a marine micronutrient, despite its toxicity to life. The explanation for this nutrient-like behavior is unknown because there is only one identified biochemical function for Cd, an unusual Cd/Zn carbonic anhydrase. Recent developments in Cd isotope mass spectrometry have revealed that Cd uptake by phytoplankton causes isotopic fractionation in the open ocean and in culture. Here we investigate the physiochemical pathways that fractionate Cd isotopes by performing subcellular Cd isotope analysis on genetically modified microorganisms. We find that expression of the Cd/Zn carbonic anhydrase makes no difference to the Cd isotope composition of whole cells. Instead, a large proportion of the Cd is partitioned into cell membranes with a similar direction and magnitude of Cd isotopic fractionation to that seen in surface seawater. This observation is well explained if Cd is mistakenly imported with other divalent metals and subsequently managed by binding within the cell to avoid toxicity. This process may apply to other divalent metals, whereby nonspecific uptake and subsequent homeostasis may contribute to elemental and isotopic distributions in seawater, even for elements commonly considered as micronutrients.
Article
Full-text available
A common strategy among microbes living in iron-limited environments is the secretion of siderophores, which can bind poorly soluble iron and make it available to cells via active transport mechanisms. Such siderophore-iron complexes can be thought of as public goods that can be exploited by local communities and drive diversification, for example by the evolution of "cheating." However, it is unclear whether bacterial populations in the environment form stable enough communities such that social interactions significantly impact evolutionary dynamics. Here we show that public good games drive the evolution of iron acquisition strategies in wild populations of marine bacteria. We found that within nonclonal but ecologically cohesive genotypic clusters of closely related Vibrionaceae, only an intermediate percentage of genotypes are able to produce siderophores. Nonproducers within these clusters exhibited selective loss of siderophore biosynthetic pathways, whereas siderophore transport mechanisms were retained, suggesting that these nonproducers can act as cheaters that benefit from siderophore producers in their local environment. In support of this hypothesis, these nonproducers in iron-limited media suffer a significant decrease in growth, which can be alleviated by siderophores, presumably owing to the retention of transport mechanisms. Moreover, using ecological data of resource partitioning, we found that cheating coevolves with the ecological specialization toward association with larger particles in the water column, suggesting that these can harbor stable enough communities for dependencies among organisms to evolve.
Article
Full-text available
The release rates of Ag, Am, Cd, Ce, Co, Pb, Se and Zn from decomposing diatom cells were determined using gamma-emitting radiotracers; rates were compared with C and protein loss rates over time. Additionally, experiments were designed to evaluate various artifacts involved in the experimental use of radioisotopes, handling of biogenic debris, and the use of poisons. The release rates of C at 18°C exponentially decreased with time from 17.5% d-1 at 1 d to 2.7% d-1 at 6 d; those of protein slowed from 9.2% d-1 at 1 d to 2.0% d-1 at 6 d. Rates at 18°C were 2–4 times faster than rates at 4°C. Rate changes at both temperatures were much less pronounced from 6–25 d. Retention half-times (tr1/2s) of Ag, Am, Ce, Co and Pb in diatom debris were significantly greater than those of Cd, Se and Zn under the same conditions; tr1/2 values decreased inversely with temperature. The tr1/2 values of C and protein were generally comparable to those of Cd, Se and Zn, whereas the ratios of the other metals to C and protein increased significantly over time. Microbial activity very strongly enhanced Co scavenging onto decaying particles in the dark. The elemental loss rate data suggest that Cd, Se and Zn should generally follow the fate of organic C and protein in decomposing planktonic debris. These elements should be biologically recycled and have longer residence times in surface waters than the other metals which are more particle-reactive and which do not follow organic C and protein release.
Article
Full-text available
1] Diatom blooms play a central role in supporting food-webs and sequestering biogenic carbon to depth. Oceanic conditions set bloom initiation, whereas both environmental and ecological factors determine bloom magnitude and lon-gevity. Our study reveals another fundamental determinant of bloom dynamics. A diatom spring bloom in offshore New Zealand waters was likely terminated by iron limitation, even though diatoms consumed <1/3 of the mixed-layer dis-solved iron inventory. Thus, bloom duration and magnitude were primarily set by competition for dissolved iron between microbes and small phytoplankton versus diatoms. Signifi-cantly, such a microbial mode of control probably relies both upon out-competing diatoms for iron (i.e., K-strategy), and having high iron requirements (i.e., r-strategy). Such resource competition for iron has implications for carbon biogeo-chemistry, as, blooming diatoms fixed three-fold more car-bon per unit iron than resident non-blooming microbes. Microbial sequestration of iron has major ramifications for determining the biogeochemical imprint of oceanic diatom blooms. Citation: Boyd, P. W., et al. (2012), Microbial control of diatom bloom dynamics in the open ocean, Geophys. Res. Lett., 39, L18601, doi:10.1029/2012GL053448.
Article
Full-text available
Biogenic Fe quotas were determined using three distinct techniques on samples collected concurrently in the subtropical Pacific Ocean east of New Zealand. Fe quotas were measured using radioisotope uptake experiments (24 h incubation), bulk filtration and analysis by inductively-coupled plasma mass spectrometer (ICPMS), and single-cell synchrotron x-ray fluorescence (SXRF) analysis over a sixteen-day period (year days 263 to 278 of 2008) during a quasi-Lagrangian drifter experiment that tracked the evolution of the annual spring diatom bloom within a counter-clockwise open-ocean eddy. Overall, radioisotope uptake-determined Fe quotas (washed with oxalate reagent to remove extracellular Fe) were the lowest (0.5-1.0 mmol Fe:mol P; 4-8 mumol Fe:mol C), followed by single-cell Fe quotas (2.3-7.5 mmol Fe:mol P; 17-57 mumol Fe:mol C), and the highest and most variable quotas were from the bulk filtration ICPMS approach that used the oxalate reagent wash, corrected for lithogenic Fe using Al (0.8-21 mmol Fe:mol P; 4-136 mumol Fe:mol C). During the evolution of the spring bloom within the eddy (year days 263 to 272), the surface mixed layer inventories of particulate organic elements (C, N, P, Si) and chlorophyll increased while Fe quotas estimated from all three approaches exhibited a general decline. After the onset of the bloom decline, the drogued buoys exited the eddy center (days 273 to 277). Fe quotas returned to pre-bloom values during this part of the study. Our standardized and coordinated sampling protocols reveal the general observed trend in Fe quotas: ICPMS > SXRF > radioisotope uptake. We discuss the inherent differences between the techniques and argue that each technique has its individual merits and uniquely contributes to the characterization of the oceanic particulate Fe pool.
Article
Full-text available
Over the past few decades, we have realized that the silica cycle is strongly intertwined with other major biogeochemical cycles, like those of carbon and nitrogen, and as such is intimately related to marine primary production, the efficiency of carbon export to the deep sea, and the inventory of carbon dioxide in the atmosphere. For nearly 20 years, the marine silica budget compiled by Tréguer et al. (1995), with its exploration of reservoirs, processes, sources, and sinks in the silica cycle, has provided context and information fundamental to study of the silica cycle. Today, the budget needs revisiting to incorporate advances that have notably changed estimates of river and groundwater inputs to the ocean of dissolved silicon and easily dissolvable amorphous silica, inputs from the dissolution of terrestrial lithogenic silica in ocean margin sediments, reverse weathering removal fluxes, and outputs of biogenic silica (especially on ocean margins and in the form of nondiatomaceous biogenic silica). The resulting budget recognizes significantly higher input and output fluxes and notes that the recycling of silicon occurs mostly at the sediment-water interface and not during the sinking of silica particles through deep waters. Expected final online publication date for the Annual Review of Marine Science Volume 5 is December 05, 2012. Please see http://www.annualreviews.org/catalog/pubdates.aspx for revised estimates.
Article
Full-text available
1] We investigate the functioning of the ocean's biological pump by analyzing the vertical transfer efficiency of particulate organic carbon (POC). Data evaluated include globally distributed time series of sediment trap POC flux, and remotely sensed estimates of net primary production (NPP) and sea surface temperature (SST). Mathematical techniques are developed to compare these temporally discordant time series using NPP and POC flux climatologies. The seasonal variation of NPP is mapped and shows regional-and basin-scale biogeographic patterns reflecting solar, climatic, and oceanographic controls. Patterns of flux are similar, with more high-frequency variability and a subtropical-subpolar pattern of maximum flux delayed by about 5 days per degree latitude increase, coherent across multiple sediment trap time series. Seasonal production-to-flux analyses indicate during intervals of bloom production, the sinking fraction of NPP is typically half that of other seasons. This globally synchronous pattern may result from seasonally varying biodegradability or multiseasonal retention of POC. The relationship between NPP variability and flux variability reverses with latitude, and may reflect dominance by the large-amplitude seasonal NPP signal at higher latitudes. We construct algorithms describing labile and refractory flux components as a function of remotely sensed NPP rates, NPP variability, and SST, which predict POC flux with accuracies greater than equations typically employed by global climate models. Globally mapped predictions of POC export, flux to depth, and sedimentation are supplied. Results indicate improved ocean carbon cycle forecasts may be obtained by combining satellite-based observations and more mechanistic representations taking into account factors such as mineral ballasting and ecosystem structure.
Article
Full-text available
The physical and biogeochemical components of nutrients and inorganic carbon distributions along WOCE line A14 are objectively separated by means of a constrained least-squares regression analysis of the mixing of eastern South Atlantic water masses. Contrary to previous approaches, essentially devoted to the intricate South Atlantic circulation, this work is focused on the effects of circulation on nutrients and carbon biogeochemistry, with special emphasis on the stoichiometry and the rate of mineralization processes. Combination of nutrient and apparent CFC-age anomalies, derived from the mixing analysis, indicate faster mineralization rates in the equatorial (12 × 10−2 μmol P kg−1 yr−1) and subequatorial (5.3 × 10−2 μmol P kg−1 yr−1) than in the subtropical (4.3 × 10−2 μmol P kg−1 yr−1) regime at the South Atlantic Central Water (SACW) depth range. Lower rates are obtained in the Antarctic Intermediate Water (AAIW) domain (3.0 × 10−2 μmol P kg−1 yr−1). Significant variation with depth of O2/C/N/P anomalies indicates preferential mineralization of proteins in thermocline waters, as compared with the reference Redfield composition.
Article
Full-text available
1] For the determination of the elemental composition of particulate organic material (POM) and its impact on the marine carbon cycle, we assembled C:N data for POM from many different sources into a single data collection for joint evaluation. The data set contains 10,200 C:N values, encompassing all major oceans and trophic levels, showing that C:N ratios are highly variable with values below the traditional Redfield ratio (C:N = 6.6) to values greatly exceeding it. On a global mean, C:N ratios of marine sinking particles from the surface water amount to 7.1 ± 0.1, and there is a systematic increase of C:N ratios with depth of 0.2 ± 0.1 units per 1000 m. The discrepancy with results from analyses of dissolved nutrient fields, yielding constant C:N ratios close to the Redfield value, can be explained by the implicit depth averaging caused by depth variations of the surfaces under consideration. Additionally, due to preferential remineralization of nitrogen compared to carbon, the C:N ratio of the dissolving component, which is seen on dissolved nutrient fields, is smaller than the C:N ratio of the remaining particles. For carbon flux estimations, elevated and depth dependent C:N ratios should be implemented in biogeochemical models to correctly represent relative strengths of downward carbon and nitrogen fluxes.
Article
Full-text available
Concurrent distributions of dissolved and suspended particulate organic carbon (DOC and POCsusp), nitrogen (DON and PONsusp) and phosphorus (DOP and POPsusp), and of suspended particulate inorganic phosphorus (PIPsusp), are presented for the open ocean water column. Samples were collected along a three-station transect from the upper continental slope to the abyssal plain in the eastern North Pacific and from a single station in the Southern Ocean. The elemental composition of surface sedimentary organic matter (SOM) was also measured at each location, and sinking particulate organic matter (POMsink) was measured with moored sediment traps over a 110-d period at the abyssal site in the eastern North Pacific only. In addition to elemental compositions, C : N, C : P and N : P ratios were also calculated. Surface and deep ocean concentrations of dissolved organic matter (DOM) and inorganic nutrients between the two sites displayed distinct differences, although suspended POM (POMsusp) concentrations were similar. Concentrations of DOM and POMsusp displayed unique C, N and P distributions, with POMsusp concentrations generally about 1–2 orders of magnitude less than the corresponding DOM concentrations. These differences were likely influenced by different biogeochemical factors: whereas the dissolved constituents may have been influenced more by the physical regime of the study site, suspended particulate matter may have been controlled to a greater extent by biological and chemical alteration. Up to 80% of total particulate P in POMsusp, POMsink and SOM consisted of PIP. For all organic matter pools measured, elemental ratios reveal that organic P is preferentially remineralized over organic C and organic N at both sites. Increases in C : P and N : P ratios with depth were also observed for DOM at both sites, suggesting that DOP is also preferentially degraded over C and N as a function of depth. A simple one-dimensional vertical eddy diffusion model was applied to estimate the contributions of dissolved and suspended particulate organic C, N and P fluxes from the upper mixed layer into the permanent thermocline. Estimated vertical DOM fluxes were 28–63% of the total organic matter fluxes; POMsusp and POMsink fluxes were 8–20 and 28–52% of the total.
Article
A multi-disciplinary examination of the drivers of dissolved methane was carried out during a phytoplankton bloom located in a subtropical mesoscale eddy. This investigation related temporal signals in methane concentrations with other biophysical and biogeochemical parameters in the upper waters (<300 m) of the southwest Pacific Ocean. In the surface mixed layer, methane supersaturation increased and δ13CCH4 became more depleted coincident with increases in particulate dimethylsulfoniopropionate (DMSPp) and succession from the diatom Asterionellopsis glacialis to the nanoflagellate Phaeocystis globosa and the cyanobacterium Synechococcus sp. In situ methane production was calculated in a surface mixed layer methane budget that incorporated sea-to-air exchange and vertical diffusion. Methane concentrations increased in and below the mixed layer when the export of biogenic particles increased. Increased grazing of microbes by microzooplankton may have contributed to particle recycling (rich in organic carbon and DMSP) and increased the potential for methanogenesis. Phytoplankton species composition and biomass in different bloom phases, and eddy dynamics, were important determinants of methane saturation and emission, and the potential implications for methane are considered for the future surface ocean.
Article
Plankton samples have been carefully collected from a variety of marine environments for major and trace-chemical analysis. The samples were collected and handled under the rigorous conditions necessary to prevent contamination of the trace elements. Immediately after collection, the samples were subjected to a series of physical and chemical leaching-decomposition experiments designed to identify the major and trace element composition of the biogenic particulate matter. Emphasis was placed on the determination of the trace element/major element ratios in the various biogenic phases important in biogeochemical cycling.
Article
Bacteria (and possibly archaea) accelerate silica dissolution in the sea by colonizing and enzymatically degrading the organic matrix of diatom frustules. We tested whether colonizer species composition and ectohydrolase profiles critically control silicon regeneration by allowing diatom ( Thalassiosira weissflogii and Chaetoceros simplex) de- tritus to be colonized by natural bacterial assemblages and 12 phylogenetically characterized marine isolates. We characterized the colonizers' ectohydrolase profiles and rates of silicon regeneration. The colonizers' cell-specific protease activity was consistently the dominant ectohydrolase, and it strongly correlated with silica dissolution rates. Cell-specific glucosidase, lipase, and chitinase activities showed no correlation with silicon regeneration. Denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S rRNA genes was used to monitor colonization of detritus by natural microbial assemblages and to identify colonizing phylotypes. Representatives from gammaproteobacteria and sphingobacteria-flavobacteria classes dominated colonizer populations by comprising 65% and 25% of detected phylotypes, respectively. Archaea were not detected among colonizer populations. All bacterial isolates accelerated silica dissolution, but individual rates varied by .300%. Significant variability was observed within the Altero- monadaceae, which indicates different abilities to process diatom organic matter. Isolates that displayed enhanced colonization and protease activities were the most effective at regenerating silicon. The most effective isolate belonged to the sphingobacteria-flavobacteria, a group specialized in colonizing marine particles. Other effective isolates grouped with Pseudoalteromonas, Alteromonas,and Vibrio genera. One isolate caused intense aggregation of diatom detritus, significantly reducing silicon regeneration. Our results indicate that bacterial species identity strongly controlled silicon regeneration by influencing the colonization potential and ectohydrolytic profiles of bacteria as well as aggregate formation. Mechanistic models of oceanic silica cycling should incorporate species composition and ectohydrolase profiles of bacteria involved in silicon regeneration.
Article
As particulate organic carbon rains down from the surface ocean it is respired back to carbon dioxide and released into the ocean's interior. The depth at which this sinking carbon is converted back to carbon dioxide-known as the remineralization depth-depends on the balance between particle sinking speeds and their rate of decay. A host of climate-sensitive factors can affect this balance, including temperature, oxygen concentration, stratification, community composition and the mineral content of the sinking particles. Here we use a three-dimensional global ocean biogeochemistry model to show that a modest change in remineralization depth can have a substantial impact on atmospheric carbon dioxide concentrations. For example, when the depth at which 63% of sinking carbon is respired increases by 24m globally, atmospheric carbon dioxide concentrations fall by 10-27ppm. This reduction in atmospheric carbon dioxide concentration results from the redistribution of remineralized carbon from intermediate waters to bottom waters. As a consequence of the reduced concentration of respired carbon in upper ocean waters, atmospheric carbon dioxide is preferentially stored in newly formed North Atlantic Deep Water. We suggest that atmospheric carbon dioxide concentrations are highly sensitive to the potential changes in remineralization depth that may be caused by climate change.
Article
An automated nitrate determination is described in which nitrate is reduced to nitrite with hydrazine sulphate under alkaline conditions in the presence of Cu2+ and Zn2+. Interferances encountered in natural water samples were eliminated by the addition of Zn2+ to the Cu2+ catalyst solution.The method is suitable for the determination of low NO3−N concentrations and compares favourably with the manual copperised cadmium technique for freshwater samples containing 10–800 mg m−3 NO3−N. The method is also linear at nitrate concentrations below 10 mg N m−3. The standard deviations (S.D.) of blanks and of samples containing 2 mg NO3−N m−3 were 0.013 and 0.06 mg N m−3 respectively at an analysis rate of 30 samples h−1.
Article
During consecutive transects at the 6°W meridian including the Polar Frontal region (PFr), the southern Antarctic Circumpolar Current area (sACC area) and the Weddell Gyre Boundary Front, the spatial and temporal distribution of copper (Cu), nickel (Ni), zinc (Zn) and dissolved silica (Si) was related to phytoplankton activity and hydrography. In the PFr, a diatom spring bloom coincided with reduced trace metal and Si concentrations. The trace metal/Si ratios increased during bloom development due to preferential Si net uptake. Within the surface water of the sACC area, a continuous increase in trace metal and Si concentration towards the south was observed. The increase in concentration towards the south is attributed either to a constant flux of trace metals and Si mediated by sinking biogenic particles out of the AASW, or by a combination of the southward increasing upwelling of Upper Circumpolar Deep Water (UCDW) and the continuous downward particle flux. The observed subsurface maxima in the sACC area are probably caused by leaching of sea ice diatoms sedimenting after sea ice melting. Minima in transmission above the pycnocline point to the formation, sinking and dissolution of marine snow responsible for the concentration maxima of trace metals. At the deep sampling stations, Cu correlated strongest with Si among the trace metal/major nutrient correlations, whereby the Cu/Si slopes were significantly lower at the stations in the relatively high productive PFr (0.013–0.018 nM/μM) than at the stations in the relatively low productive sACC area (0.020–0.022 nM/μM). The relatively low Cu/Si slopes at the stations in the PFr are probably due to preferential Si uptake by diatoms in the upper water column and Cu scavenging in the deeper water column. A longer retention of Cu compared to Si during the dissolution of the diatom frustules in the sediment may have contributed to the relatively low Cu/Si slopes as well. Within the UCDW, the trace metals showed maxima in concentration similar to those of phosphate and nitrate, indicating the core of the UCDW. At one station near the Cape Basin, the concentrations of trace metals, phosphate and nitrate reflect the input of North Atlantic Deep Water into the Antarctic Circumpolar Current. In the Low Circumpolar Deep Water (LCDW), trace metal maxima were ascribed to hydrothermal input.