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Leachable particulate iron in the Columbia River, estuary, and near-field plume
Abstract
This study examines the distribution of leachable particulate iron (Fe) in the Columbia River, estuary, and near-field plume. Surface samples were collected during late spring and summer of 2004–2006 as part of four River Influence on Shelf Ecosystems (RISE) cruises. Tidal amplitude and river flow are the primary factors influencing the estuary leachable particulate Fe concentrations, with greater values during high flow and/or spring tides. Near the mouth of the estuary, leachable particulate Fe [defined as the particulate Fe solubilized with a 25% acetic acid (pH 2) leach containing a weak reducing agent to reduce Fe oxyhydroxides and a short heating step to access intracellular Fe] averaged 770 nM during either spring tide or high flow, compared to 320 nM during neap tide, low flow conditions. In the near-field Columbia River plume, elevated leachable particulate Fe concentrations occur during spring tides and/or higher river flow, with resuspended shelf sediment as an additional source to the plume during periods of coastal upwelling and spring tides. Near-field plume concentrations of leachable particulate Fe (at a salinity of 20) averaged 660 nM during either spring tide or high flow, compared to 300 nM during neap tide, low flow conditions. Regardless of tidal amplitude and river flow, leachable particulate Fe concentrations in both the river/estuary and near-field plume are consistently one to two orders of magnitude greater than dissolved Fe concentrations. The Columbia River is an important source of reactive Fe to the productive coastal waters off Oregon and Washington, and leachable particulate Fe is available for solubilization following biological drawdown of the dissolved phase. Elevated leachable Fe concentrations allow coastal waters influenced by the Columbia River plume to remain Fe-replete and support phytoplankton production during the spring and summer seasons.
... Reactive Fe -The fraction that is potentially available to biology in short timescales (days to weeks). (DFeþ LPFe; Lippiatt et al. (2010aLippiatt et al. ( , 2010b). ...
... Glacially derived sediment is produced by physical weathering, and in coastal waters of the GoA, glacially derived suspended particles have been shown to contain on average a lower percentage of LPFe (11% of suspended particulate Fe) as compared to suspended sediment from coastal waters influenced by watersheds where chemical weathering dominates (e.g. Columbia River plume with 26% LPFe) (Lippiatt et al., 2010b). Although glacially-derived suspended particles contain less labile Fe, the massive input of particulate material along the GoA coast can result in highly elevated LPFe and [TDFe] that reach micromolar (μM) levels in low salinity river plumes (Lippiatt et al., 2010a;Schroth et al., 2014). ...
... Station samples from 20 m depth contained relatively low concentrations of LPFe ( $ 19 nM or less; median 1.06 nM). Although the fraction of the suspended particulate Fe that was leachable varied (6-42% LPFe), on average ($ 25%) it was more akin to the average %LPFe found in suspended sediment of waters influenced by the Columbia River plume (Lippiatt et al., 2010b). The contribution of DFe to the reactive Fe pool tended to be greater during spring 2011 as compared to the late summer, with an average of 24% in surface transects (1 m) and 41% in station profiles (20 m). ...
... Raiswell et al. (2006) estimate a "highly reactive" Fe (oxyhydr)oxide flux from ice sheets and glaciers to the coastal ocean (accounting for estuarine loss) of 0 to 2 Tg yr − 1 , compared to estimates of riverine dissolved Fe input ranging from 0.2 to 2 Tg yr − 1 (Wollast and Mackenzie, 1983;Haese, 2000;de Baar and de Jong, 2001). Thus, particularly in coastal regions dominated by glacial input, it is essential to measure the reactive (leachable) particulate Fe phase in addition to dissolved Fe, as the solubilization of even a fraction of the reactive particulate Fe phase can dominate potentially bioavailable Fe concentrations ( Lippiatt et al., 2010). ...
... The portion of the particulate Fe phase solubilized by this leach is herein referred to as "leachable particulate Fe", and the sum of the dissolved and leachable particulate Fe concentrations is defined as "reactive Fe." Leached filters were subsequently microwave bomb-digested to solubilize the remaining refractory metals. The bomb digestion method is described elsewhere ( Lippiatt et al., 2010). ...
... Leach and bomb digestion solutions were analyzed using a Thermo-Finnigan high-resolution inductively coupled plasma-mass spectrometer ( Lippiatt et al., 2010). Fe standards were made from a 1000 ppm stock solution (SPEX, Edison, NJ; Fisher Scientific) in 1 N TMG HNO 3 (with 1 ppb Ga as an internal standard). ...
Coastal waters in the northern Gulf of Alaska (GoA) are considered iron-rich and nitrate-poor, in contrast to the iron-poor, high-nitrate, low chlorophyll (HNLC) waters of the central GoA. The degree of mixing between these two regimes, enhanced by mesoscale eddies, is essential to the high productivity observed in the region. As part of a study on iron delivery to the central GoA via mesoscale eddies, extensive work was focused on characterizing the coastal endmember, the Alaska Coastal Current. In surface Alaskan coastal waters between Yakutat and the Kenai Peninsula, dissolved iron concentrations ranged from 0.5 to 4.1 nM with an average of ∼ 2 nM. In contrast, leachable particulate iron concentrations were much higher and more variable, ranging from over 1 μM in the Alsek River plume to less than 5 nM at the base of Cook Inlet. Cross-shelf transport of both surface and subsurface dissolved iron and leachable particulate iron was observed. Throughout the study area, leachable particulate iron values were at least an order of magnitude higher than dissolved values, suggesting that the system's ability to solubilize this large concentration of leachable particulate iron is overwhelmed by the massive input of glacial-derived particulate iron. Nevertheless, suspended leachable particulate iron remains available for exchange to the dissolved phase and is suggested to maintain a relatively constant (∼ 2 nM) source of dissolved iron in the coastal GoA.
... Raiswell et al. (2006) estimate a "highly reactive" Fe (oxyhydr)oxide flux from ice sheets and glaciers to the coastal ocean (accounting for estuarine loss) of 0 to 2 Tg yr − 1 , compared to estimates of riverine dissolved Fe input ranging from 0.2 to 2 Tg yr − 1 (Wollast and Mackenzie, 1983;Haese, 2000;de Baar and de Jong, 2001). Thus, particularly in coastal regions dominated by glacial input, it is essential to measure the reactive (leachable) particulate Fe phase in addition to dissolved Fe, as the solubilization of even a fraction of the reactive particulate Fe phase can dominate potentially bioavailable Fe concentrations (Lippiatt et al., 2010). ...
... The portion of the particulate Fe phase solubilized by this leach is herein referred to as "leachable particulate Fe", and the sum of the dissolved and leachable particulate Fe concentrations is defined as "reactive Fe." Leached filters were subsequently microwave bomb-digested to solubilize the remaining refractory metals. The bomb digestion method is described elsewhere (Lippiatt et al., 2010). ...
... Leach and bomb digestion solutions were analyzed using a Thermo-Finnigan high-resolution inductively coupled plasma-mass spectrometer (Lippiatt et al., 2010). Fe standards were made from a 1000 ppm stock solution (SPEX, Edison, NJ; Fisher Scientific) in 1 N TMG HNO 3 (with 1 ppb Ga as an internal standard). ...
Coastal waters in the northern Gulf of Alaska (GoA) are considered iron-rich and nitrate-poor, in contrast to the iron-poor, high-nitrate, low chlorophyll (HNLC) waters of the central GoA. The degree of mixing between these two regimes, enhanced by mesoscale eddies, is essential to the high productivity observed in the region. As part of a study on iron delivery to the central GoA via mesoscale eddies, extensive work was focused on characterizing the coastal endmember, the Alaska Coastal Current. In surface Alaskan coastal waters between Yakutat and the Kenai Peninsula, dissolved iron concentrations ranged from 0.5 to 4.1 nM with an average of ∼ 2 nM. In contrast, leachable particulate iron concentrations were much higher and more variable, ranging from over 1 μM in the Alsek River plume to less than 5 nM at the base of Cook Inlet. Cross-shelf transport of both surface and subsurface dissolved iron and leachable particulate iron was observed. Throughout the study area, leachable particulate iron values were at least an order of magnitude higher than dissolved values, suggesting that the system's ability to solubilize this large concentration of leachable particulate iron is overwhelmed by the massive input of glacial-derived particulate iron. Nevertheless, suspended leachable particulate iron remains available for exchange to the dissolved phase and is suggested to maintain a relatively constant (∼ 2 nM) source of dissolved iron in the coastal GoA.
... The dissolved phase (dFe) is considered the most biologically available fraction [Wells et al., 1995]; however, the main flux of Fe to the ocean is in the particulate form (i.e., dust deposition, river transport, sediment resuspension, off-shelf transport, and ice-rafted debris). The oceanic Fe inventory in shelf systems is dominated by the particulate phase (pFe) [Hong and Kester, 1986;Lippiatt et al., 2010], yet the cycling of this fraction in shelf or open ocean environments is not well constrained. In oxygenated seawater, the predominant Fe species, Fe(III), is highly insoluble and precipitates to form particulate phases, [Sunda, 2001;Wu and Luther, 1994]. ...
... Scavenging and precipitation of Fe to particulate phases result in losses of dFe. However, a surface-bound labile-pFe (L-pFe) fraction is considered to be involved in adsorption/desorption processes [Homoky et al., 2012] with Fe becoming available to phytoplankton following dissolution and solubilization [Hurst et al., 2010;Lippiatt et al., 2010]. This fraction can include acid-labile hydroxides and biogenic particles as well as surface-bound forms of Fe. ...
The supply and bioavailability of iron (Fe) controls primary productivity and N2-fixation in large parts of the global ocean. An important, yet poorly quantified, source to the ocean is particulate Fe (pFe). Here we present the first combined dataset of particulate, labile-particulate (L-pFe) and dissolved Fe (dFe) from the (sub)-tropical North Atlantic. We show a strong relationship between L-pFe and dFe, indicating a dynamic equilibrium between these two phases whereby particles ‘buffer’ dFe and maintain the elevated concentrations observed. Moreover, L-pFe can increase the overall ‘available’ (L-pFe + dFe) Fe pool by up to 55%. The lateral shelf flux of this available Fe was similar in magnitude to observed soluble aerosol-Fe deposition, a comparison that has not been previously considered. These findings demonstrate that L-pFe is integral to Fe cycling and hence plays a role in regulating carbon cycling, warranting its’ inclusion in Fe budgets and biogeochemical models.
... Coastal systems are a dynamic subset of this DOC pool with carbon inputs from terrestrial sources adding to in-situ production. Rivers contribute up to 0.2 Pg/C DOC to coastal systems per year with most having had little exposure to sunlight before its arrival in coastal waters (Ludwig et al., 1996). Carbon of terrestrial origin is generally refractory to microbial oxidation and strongly absorbs solar radiation in blue and ultraviolet (UV) wavelengths (Bricaud et al., 1981;Carlson, 2002). ...
... It is also documented that estuarine iron is usually positively correlated with river flow and particulate loading (e.g. Lippiatt et al., 2010) and that its complexation with DOM can greatly affect photochemical reactions in estuarine systems (Gao and Zepp, 1998;White et al., 2003). These potential variations in the chemical nature of CDOM would certainly add to the variation of AQY in coastal systems and may not be directly related to salinity. ...
The photochemical oxidation of oceanic dissolved organic carbon (DOC) to
carbon monoxide (CO) and carbon dioxide (CO2) has been
estimated to be a significant process with global photoproduction
transforming petagrams of DOC to inorganic carbon annually. To further
quantify the importance of these two photoproducts in coastal DOC
cycling, 38 paired apparent quantum yield (AQY) spectra for CO and
CO2 were determined at three locations along the coast of
Georgia, USA over the course of one year. The AQY spectra for
CO2 were considerably more varied than CO. CO AQY spectra
exhibited a seasonal shift in spectrally integrated (260 nm-490 nm) AQY
from higher efficiencies in the autumn to less efficient photoproduction
in the summer. While full-spectrum photoproduction rates for both
products showed positive correlation with pre-irradiation UV-B sample
absorption (i.e. chromophoric dissolved organic matter, CDOM) as
expected, we found no correlation between AQY and CDOM for either
product at any site. Molecular size, approximated with pre-irradiation
spectral slope coefficients, and aromatic content, approximated by the
specific ultraviolet absorption of the pre-irradiated samples, were also
not correlated with AQY in either data set. The ratios of CO2
to CO photoproduction determined using both an AQY model and direct
production comparisons were 23.2 ± 12.5 and 22.5 ± 9.0,
respectively. Combined, both products represent a loss of 2.9 to 3.2% of
the DOC delivered to the estuaries and inner shelf of the South Atlantic
Bight yearly, and 6.4 to 7.3% of the total annual degassing of
CO2 to the atmosphere. This result suggests that direct
photochemical production of CO and CO2 is a small, yet
significant contributor to both DOC cycling and CO2 gas
exchange in this coastal system.
... The observations and conclusions in this work contribute to a growing body of literature (e.g., Lee et al., 2018;Lippiatt, Brown, et al., 2010;Milne et al., 2017;Twining et al., 2019;Xiang & Lam, 2020) demonstrating the importance of particles for TM cycling and transport within the ocean and emphasize the importance of selective leach procedures for understanding TM cycling by particles. ...
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.
... Quant au fer particulaire labile, il est présent à des concentrations systématiquement supérieures d'un à deux ordres de grandeur à celles du DFe. Par sa dissolution il peut prétendre à la solubilisation à la suite d'une diminution du DFe par son assimilation par les organismes (Lippiatt et al., 2010). Cependant, la proportion de fer soluble provenant des eaux fluviales transportées vers l'océan ouvert reste inconnue, ce qui rend difficile une estimation de l'apport réel (Ussher et al., 2004). ...
Il existe encore des incertitudes importantes concernant le cycle biogéochimique du fer, sa nature et la quantification de ses sources. Ce fer dissous (dFe) est considéré comme étant la forme la plus biodisponible ce qui a induit la sous-évaluation du rôle du fer particulaire (pFe) comme une source potentielle de dFe. Pourtant, la remise en suspension des sédiments libère davantage de pFe que de dFe. Dans ce contexte, ma thèse remet en question la vision traditionnelle du rôle du fer particulaire inorganique sédimentaire (pFeinorg) et propose la première modélisation de ce dernier comme source externe de dFe. Le modèle numérique PISCES a donc été adapté pour tenir compte d’un flux supplémentaire de fer en s’appuyant sur une climatologie de la dynamique à partir de la configuration NEMOPISCES globale à 2 degrés de résolution. Les simulations mettent en exergue la sensibilité de la biomasse phytoplanctonique à la forme de fer provenant des sédiments ; les limitations en macronutriments et celles en fer sont considérablement modifiées, ainsi que les gradients côte–large de chlorophylle. Le transport plus efficace du fer en tant que pFeinorg permet d’atteindre des régions éloignées de sa source. Son accumulation et sa dissolution dans les zones de convergences induisent via downwelling l’enrichissement de la surbsurface ; à ceci s’ajoute le processus de chute de la particule. Cependant, ces processus demeurent peu étudiés. Les tests de sensibilité ont montré que le gain (absence de chute) ou la perte (chute rapide) en fer dans l’océan, ou encore la prépondérance du pFe sur le dFe seraient modulés par le taux de dissolution. En revanche, la distribution de la chlorophylle est mieux représentée dans la mesure où les processus qui régissent la distribution du PFeinorg et du dFe qui en dérive sont, de concert, pris en compte. Une manière de mieux représenter les répercussions du fer sur les cycles biogéochimiques marins, serait de mieux contraindre les processus liés au PFeinorg.
... Homoky et al. (2013) observed the non-reductive dissolution of iron in sediments deposited on South Africa margin, where diagenesis occurs under oxic conditions, confirming the Radic et al. (2011) hypothesis. Brown and Bruland (2009) and Lippiatt et al. (2010) concluded that much of the Al and Fe bound in the particulate material in the Columbia River are available for solubilization into the seawater. The Fe released by these particles allows the coastal waters, influenced by the Columbia River Plume, to remain Fe-replete and support phytoplankton production during the spring and summer seasons. ...
... [21] The utilization of Fe sources by phytoplankton depends on factors such as lability of DFe and species of Febinding ligands [e.g., Hutchins et al., 1999], hence not all released DFe may contribute to primary production. Conversely, there is evidence that a significant fraction of particulate Fe may become available to phytoplankton via, e.g., ligand-assisted dissolution and photochemical processes [Lippiatt et al., 2010, and references herein]; a possible source of Fe that we have not accounted for in this study. ...
Low dissolved iron (DFe) concentrations limit primary production in most
high-nutrient low-chlorophyll (HNLC) regions. Increased recycling of
iron (Fe) relative to nitrogen (N) by zooplankton may help to sustain
phytoplankton production in these conditions. We concurrently determined
rates of DFe and ammonium (NH4+) recycling by
natural mesozooplankton communities in HNLC conditions of the Northeast
Atlantic. NH4+ excretion remained constant and
ranged between 14.2-54.1 nmol NH4+ mg
dry weight-1 h-1. Fe recycling
ranged between 6-138 pmol DFe mg dry
weight-1 h-1 during the first
hour and decreased thereafter, reflecting the transition from the loss
of phytoplankton-derived Fe to basal DFe excretion.
Mesozooplankton-driven nutrient recycling was estimated to support
6-59% and <1-13% of the respective phytoplankton
requirements for DFe and N; DFe:N regeneration ratios were 5-26
times larger than those required by phytoplankton. Our data suggest that
Fe recycling by grazing organisms has the potential to reduce the
intensity of HNLC conditions.
... It is generally accepted that terrestrial sources via fluvial routes contribute to the majority of inputs of Fe, a micronutrient, into the continental shelf waters, while aeolian transport of mineral dust is the predominant pathway for Fe inputs to the open ocean (de Jong et al., 2007;Moore and Braucher, 2008). In some cases, riverine Fe can also be the major source of Fe to the open ocean (Wetz et al., 2006;Lippiatt et al., 2010). Furthermore, lateral transport from the shelf, where benthic fluxes may be the source of Fe, to the interior ocean may also play an important role (Nedelec et al., 2007;Hurst et al., 2010). ...
More than eighty surface water samples from marginal seas in southeastern Asia, including the South China Sea, East China Sea, Western Philippine Sea, and the Taiwan Strait, were collected in order to assess the sources and transport of iron in this region. Dissolved Fe (Chelex-labile and total dissolved) concentrations were determined. Additionally, samples were collected from near-shore waters along the west coast of Taiwan during two different seasons and analyzed for potential lability of Fe. Although total dissolved Fe concentrations were, in general, replete in coastal waters and decreasing seaward, the proportions of the dissolved labile (Chelex extractable) and non-labile Fe fractions showed substantial spatial variability. Results from dissolved Fe distributions indicate that Fe in estuarine and coastal regimes forms highly stable non-labile complexes, which are removed quickly, resulting in moderate to low dissolved concentrations offshore. Most of the Fe-replete river plumes contain a large proportion of dissolved non-labile Fe species that also show a low potential ‘bio-reactivity’ and seasonally variable concentrations, making biological production in the coastal and adjacent marginal seas temporally dependent on the potential chemical reactivity of the different Fe fractions in the source waters. Regional flow patterns in the study area led to localized supplies of Fe-enriched waters in the northern Taiwan Strait and southern East China Sea waters.
... Upwelling and vertical mixing over the continental shelf fuels a narrow (~100 km wide during summer, normally east of 125°W) band of high PP, roughly parallel to the coast (Fig. 2c). High productivity in the region also reflects input of nutrients (nitrate, phosphate, and for diatoms silica) and micronutrients (such as leachable particulate iron; Lippiatt et al., 2010) from the Columbia River (Figs. 1 and 4). The annual PP composite (Fig. 2e) reflects the more or less continuously high PP off the Columbia River mouth and the coastal upwelling between 40°and 42°N. ...
Non-linear, unimodal techniques are suitable for estimation of environmental properties in the northeast (NE) Pacific based on fossil diatom taxa (species and/or species groups) found in modern (core-top) sediments. Canonical Correspondence Analysis (CCA), a constrained ordination technique, discerned best-fit relationships among two multivariate datasets (the floral and the environmental) and thus yielded insight into the environmental variables that best explain the species variance within diatom populations. Based on these insights, we developed predictive functions for annual Primary Productivity (PP) and seasonal range of sea Sea-Surface Temperature using unimodal models and cross validation techniques. Estimates of annual PP (r2jack = 0.92; RMSEP = 91.94 gC/m2/y) explained the highest percentage of variance in the core-top diatom record (22.1%).
Mesoscale anticyclonic eddies in the Gulf of Alaska are an important mechanism for cross-shelf exchange of high iron, low nitrate coastal waters and low iron, high nitrate offshore waters. A Kenai eddy was sampled in September 2007, 8 months after formation. The subsurface eddy core layer contained reactive iron concentrations more than eight times greater than waters at the same depths outside the eddy. The subsurface core of the Kenai eddy (25.4≤σθ≤25.8) is suggested to be seasonally important as these waters can be brought to the surface with storm events and deep winter mixing. The deeper core layer (25.8≤σθ≤27.0) is suggested to be a source of iron to HNLC waters on a longer timescale, due to isopycnal mixing and eventual eddy relaxation. The subsurface and deeper core layers are important reservoirs of iron that can promote and sustain primary productivity over the lifetime of the Kenai eddy. In addition, dissolved and leachable particulate manganese are shown to be excellent tracers of eddy surface and subsurface waters, respectively.
1] River Influences on Shelf Ecosystems (RISE) is the first comprehensive interdisciplinary study of the rates and dynamics governing the mixing of river and coastal waters in an eastern boundary current system, as well as the effects of the resultant plume on phytoplankton standing stocks, growth and grazing rates, and community structure. The RISE Special Volume presents results deduced from four field studies and two different numerical model applications, including an ecosystem model, on the buoyant plume originating from the Columbia River. This introductory paper provides background information on variability during RISE field efforts as well as a synthesis of results, with particular attention to the questions and hypotheses that motivated this research. RISE studies have shown that the maximum mixing of Columbia River and ocean water occurs primarily near plume liftoff inside the estuary and in the near field of the plume. Most plume nitrate originates from upwelled shelf water, and plume phytoplankton species are typically the same as those found in the adjacent coastal ocean. River-supplied nitrate can help maintain the ecosystem during periods of delayed upwelling. The plume inhibits iron limitation, but nitrate limitation is observed in aging plumes. The plume also has significant effects on rates of primary productivity and growth (higher in new plume water) and microzooplankton grazing (lower in the plume near field and north of the river mouth); macrozooplankton concentration (enhanced at plume fronts); offshelf chlorophyll export; as well as the development of a chlorophyll ''shadow zone'' off northern Oregon.
Iron is hypothesized to be a limiting micronutrient for ocean primary production. This paper presents an analysis of the iron budget in the upper ocean. The global distribution of annual iron assimilation by phytoplankton was estimated from distributions of satellite-derived oceanic primary production and measured (Fe:C)cellular ratios. The distributions of iron supply by upwelling/mixing and aeolian deposition were obtained by applying (Fe:NO3)dissolved ratios to the nitrate supply and by assuming the soluble fraction of mineral aerosols. A lower bound on the rate of iron recycling in the photic zone was estimated as the difference between iron assimilation and supply. Global iron assimilation by phytoplankton for the open ocean was estimated to be 12×109molFeyr-1. Atmospheric deposition of total Fe is estimated to be 96×109molFeyr-1 in the open ocean, with the soluble Fe fraction ranging between 1 and 10% (or 1-10×109molFeyr-1). By comparison, the upwelling/entrainment supply of dissolved Fe to the upper ocean is small, ~0.7×109molFeyr-1. Uncertainties in the aeolian flux and assimilation may be as large as a factor of 5-10 but remain difficult to quantify, as information is limited about the form and transformation of iron from the soil to phytoplankton incorporation. An iron stress index, relating the (Fe:N) demand to the (Fe:N) supply, confirms the production in the high-nitrate low-chlorophyll regions is indeed limited by iron availability.
The supply of iron, relative to that of the macronutrients nitrate, phosphate, and silicic acid, plays a critical role in allowing extensive diatom blooms to develop in coastal upwelling regimes. The presence or absence of a broad continental shelf influences the supply of iron. The iron input to central California upwelling waters varies spatially and can be characterized by two end-member regimes. One end member, which includes Monterey Bay and ex- tending north to Pt. Reyes, is an iron-replete regime where upwelling occurs over a relatively broad continental shelf that results in waters with high concentrations of dissolved and particulate iron ( .10 nM) entrained together with high concentrations of nitrate and silicic acid. In these iron-replete regions, extensive blooms of large diatoms deplete macronutrient concentrations, which results in correspondingly high chlorophyll a concentrations. The other end member, located to the south of Monterey Bay off the Big Sur coast, is an iron-deplete regime where upwelling is focused offshore of a narrow continental shelf. Upwelled waters in the Big Sur region are characterized by low dissolved and particulate iron concentrations (,1 nM), together with high concentrations of nitrate and silicic acid. Extremely low iron concentrations, unused nitrate and silicic acid, and a low abundance of large diatoms characterize surface waters in these iron-deplete regions, and thus represent coastal upwelling, high-nutrient, low-chlorophyll systems limited by the micronutrient iron.
Approximately half of the world's net annual photosynthesis occurs in the oceans ( similar to 48 Pg C y(-1)). Areas bordering continents ( bottom < 2000 m) support 10 - 15% of this production. We used satellite data to compute annual global net primary production ( 1998 - 2001), and derived the global particulate organic carbon ( POC) flux settling below the permanent thermocline and to the seafloor using an empirical model of POC remineralization. Approximately 0.68 Pg C y(-1) sink below the thermocline on continental margins, compared to 1.01 Pg C y(-1) in the deep ocean. Over 0.62 Pg C y(-1) settles to the seafloor on margins, compared to 0.31 Pg C y(-1) to deep ocean sediments. At least 0.06 Pg C y(-1) may be buried in sediments on margins. Therefore, margins may be responsible for > 40% of the carbon sequestration in the ocean. These regions must be accounted for in realistic models of the global carbon cycle and its linkages to climate change.
1] We examined high-resolution cross-shelf distributions of particulate organic carbon (POC) and dissolved O 2 during the upwelling season off the Oregon coast. Oxygen concentrations were supersaturated in surface waters, and hypoxic in near-bottom waters, with greatly expanded hypoxic conditions late in the season. Simplified time-dependent mass balances on cross-shelf integrated concentrations of these two parameters, found the following: (1) The average net rate of photosynthesis generated 2.1 mmol O 2 m À3 d À1 and (2) essentially none of the corresponding net carbon fixation of 1.4 mmol m À3 d À1 could be accounted for in the observed standing stocks of POC. After examining other possible sinks for carbon, we conclude that most of the net production is being exported to the adjacent deep ocean. A simplified POC budget suggests that about a quarter of the export is via alongshore advection, and the remainder is due to some other process. We propose a simplistic conceptual model of across-shelf transport in which POC sinks to the bottom boundary layer where it comes into contact with mineral ballast material but is kept in suspension by high turbulence. When upwelling conditions ease, the BBL waters move seaward, carrying the suspended, ballasted POC with it where it sinks rapidly into the deep ocean at the shelf break. This suggests a mechanism whereby the duration and frequency of upwelling events and relaxations can determine the extent to which new carbon produced by photosynthesis in the coastal ocean is exported to depth rather than being respired on the shelf.
1] A biological pump for transferring atmospheric CO 2 to deep ocean regimes has been identified in the upwelling zone of the U.S. Pacific coast off Oregon using high-resolution measurements of P CO2 and nutrient concentrations that were made in May through August 2001. Surface water over most of the shelf was a strong sink for atmospheric CO 2 , while a narrow nearshore strip was an intense source. The dominance of the low-CO 2 waters over the shelf area makes the region a net sink during upwelling season. This is due to (1) upwelled water that carries abundant preformed nutrients, (2) complete photosynthetic uptake of these excess nutrients and a stoichiometric proportion of CO 2 , and (3) moderate warming of upwelled waters. If the remaining North Pacific's eastern boundary area is assumed to have similar conditions, this area should represent a sink of atmospheric CO 2 that is 5% of the annual North Pacific CO 2 uptake, and roughly equivalent to the North Pacific's uptake in the summer season. By mid-August, P CO2 in subsurface waters increased 20–60%, corresponding to a 1.0–2.3% TCO 2 increase, due to respiration of settling biogenic debris. This water would be transported off the shelf to depth by winter downwelling flow, providing an important mechanism for sequestering atmospheric CO 2 into the oceans' interior. Citation: Hales, B., T. Takahashi, and L. Bandstra (2005), Atmospheric CO 2 uptake by a coastal upwelling system, Global Biogeochem. Cycles, 19, GB1009, doi:10.1029/2004GB002295.
1] Aeolian dust input may be a critical source of dissolved iron for phytoplankton growth in some oceanic regions. We used an atmospheric general circulation model (GCM) to simulate dust transport and removal by dry and wet deposition. Model results show extremely low dust concentrations over the equatorial Pacific and Southern Ocean. We find that wet deposition through precipitation scavenging accounts for 60% over the open ocean. Our estimates suggest that the annual input of dissolved Fe by precipitation scavenging ranges from 0.5 to 4 Â 10 12 g yr À1 , which is 4–30% of the total aeolian Fe fluxes. Dissolved Fe input through dry deposition is significantly lower than that by wet deposition, accounting for only 0.6–2.4 % of the total Fe deposition. Our upper limit estimate on the fraction of dissolved Fe in the total atmospheric deposition is thus more than three times higher than the value of 10% currently considered as an upper limit for dissolved Fe in Aeolian fluxes. As iron input through precipitation may promote episodic phytoplankton growth in the ocean, measurements of dissolved iron in rainwater over the oceans are needed for the study of oceanic biogeochemical cycles. INDEX TERMS: 0312 Atmospheric Composition and Structure: Air/sea constituent fluxes (3339, 4504); 1615 Global Change: Biogeochemical processes (4805); 3210 Mathematical Geophysics: Modeling; 4875 Oceanography: Biological and Chemical: Trace elements; KEYWORDS: Aeolian iron, dust modeling, wet and dry deposition, iron fertilization, precipitation Citation: Gao, Y., S.-M. Fan, and J. L. Sarmiento, Aeolian iron input to the ocean through precipitation scavenging: A modeling perspective and its implication for natural iron fertilization in the ocean, J. Geophys. Res., 108(D7), 4221, doi:10.1029/2002JD002420, 2003.
1] The flux of dissolved iron from sediments to the water column was measured with flux chambers along the California coast over a five-year period. High fluxes were observed from sediments on the continental shelf. The measured fluxes were an average of 75 times larger than flux values derived from pore-water iron gradients. The iron flux was significantly correlated with the oxidation of organic matter, which allows an extrapolation to the global shelf. The input from shelf sediments is at least as significant as the global input of dissolved iron from aerosols, which has been presumed to be the dominant external iron source. Evidence of this input is seen 100's of kilometers offshore where it can enable the high productivity of broad coastal regions seen in satellite images. (2004), The flux of iron from continental shelf sediments: A missing source for global budgets, Geophys. Res. Lett., 31, L12307, doi:10.1029/2004GL020216.
1] The initial composition of a river plume depends on the cumulative turbulent entrainment within the estuary and how this dilutes the supplied freshwater. Here we examine the relative roles of turbulence and freshwater input using observations from the Columbia River estuary and plume during two periods with contrasting river flow. Within the estuary, intense turbulence observed on flood and ebb stages is controlled by the bottom stress and scales with tidally dominated near-bottom velocity as u tidal 3 . Shear associated with the estuarine circulation is found to have a much weaker influence on turbulence dissipation rates. On the basis of these observations, we suggest that properties of the Columbia River tidal plume should be controlled by the ratio of horizontal advection to turbulent mixing within the estuary. This ratio depends on the magnitude of freshwater river input (characterized by its volumetric flow rate Q f) as compared to turbulent fluxes due to tidal mixing. This is summarized in terms of the estuary Richardson number Ri E , a nondimensional ratio between Q f and u tidal 3 . From 17 tidally resolving offshore surveys during spring/neap tides and low/high river flows, we find that the plume's median salinity, thickness, and turbulent mixing are each predicted through Ri E . It is hoped that these simple formulations will provide guidance in assessing critical properties of river plumes and their influence on coastal circulation.
Particulate trace metal (Cu, Cr, Ni, Pb and Zn) and major element (Fe, Mn and Al) concentrations have been determined following intensive sampling over two consecutive spring tidal cycles in the 'turbidity maximum zone' (TMZ) of the Port Jackson estuary, Australia. Salinity, temperature, pH, dissolved oxygen, suspended particulate matter (SPM) and chlorophyll a were also determined. A three-factor analysis of variance was used to test temporal variability in concentrations of particulate trace metals and major elements as a result of tidal oscillation. Estuarine master variables, such as temperature and pH, varied within a narrow range; nevertheless, the tidal signal was clear for surface and bottom waters. In surface water, no variance was detected in SPM concentrations between consecutive tidal cycles or between tidal stages (i.e. flood, ebb and slack water). In bottom water, however, SPM concentrations were significantly higher (PА.05) at flood tide than at slack high water and ebb tide. Concentrations of particulate trace metals and major elements in surface water do not display significant variability between tidal cycles or stages. Nevertheless, differences within each tidal stage were significant (PА.05) for all elements. In bottom water, only particulate Fe and Al exhibited significant differences (PА.05) between tidal cycles, whereas particulate Ni was the only trace element that presented significant differences (PА.05) between tidal stages, following the distribution of SPM, with highest concentrations at flood tide. Among the metals studied, significant variation was found at all three temporal scales examined (i.e. from hours to consecutive tidal cycles), although the patterns of variation were different for each metal. The semi-diurnal fluctuation of SPM and particulate trace metal concentrations during spring tides is interpreted as a resuspension-deposition cycle caused by cyclical oscillations of bottom currents. The results are discussed in the context of the implications of tidal cycle influence on the geochemistry and cycling of particulate trace metals in the Port Jackson estuary.
Ocean processes are generally large scale on the U.S. Pacific Northwest coast; this is true of both seasonal variations and
event-scale upwelling-downwelling fluctuations., which are highly energetic. Coastal upwelling supplies most of the macronutrients
available for production, although the intensity of upwelling-favorable wind forcing increases southward while primary production
and chlorophyll are higher in the north, off the Washington coast. This discrepancy could be related to several mesoscale
features: the wider, more gently sloping shelf to the north, the existence of numerous submarine canyons to the north, the
availability of Columbia River plume water and sediment north of the river mouth, and the existence of a semi-permanent eddy
offshore of the Strait of Juan de Fuca. We suggest that these features have important effects on the magnitude and timing
of macronutrient or micronutrient delivery to the plankton. These features are potentially important as well to transport
pathways and residence times of planktonic larvae and to the development of harmful algal blooms. The coastal plain estuaries,
with the exception of the Columbia River, are relatively small, with large tidal forcing and highly seasonal direct river
inputs that are low to negligible during the growing season. Primary production in these estuaries is likely controlled not
by river-driven stratification but by coastal upwelling and exchange with the ocean. Both baroclinic mechanisms (the gravitational
circulation) and barotropic ones (lateral stirring by tide and, possibly, wind) contribute to this exchange. Because estuarine
hydrography and ecology are so dominated by ocean signals, the coastal estuaries, like the coastal ocean, are largely synchronous
on seasonal and event time scales, though, intrusions of the Columbia River plume can cause strong asymmetries between Washington
and Oregon estuaries especially during spring downwelling conditions. Water property correlation increases between spring
and summer as wind forcing becomes more spatially coherent along the coast. Estuarine habitat is structure not only, by large
scale forcing but also by fine scale processes in the extensive intertidal zone, such as by solar heating or differential
advection by tidal, curents.
Surface water transects and vertical profiles for dissolved iron, macronutrients, chlorophyll a (Chl a), and hydrographic data were obtained in the Peru upwelling regime during August and September 2000. The supply of the micronutrient iron, relative to that of the macronutrients nitrate, phosphate and silicic acid, is shown to play a critical role in allowing extensive diatom blooms to develop in the Peru upwelling system. The extremely high-chlorophyll “brown waters of Peru” (with Chl a concentrations between 20 and 45 μg/l) result from massive diatom blooms with maximal photochemical efficiencies (Fv/Fm >0.6) occurring in the iron-rich upwelling region observed over the broad continental shelf off northern and central Peru. The source of the upwelled water in this region is the nutrient-rich subsurface countercurrent in contact with the organic-rich shelf sediments. This subsurface shelf water is suboxic and has extremely high concentrations of dissolved Fe (>50 nM) in the near-bottom waters. In marked contrast, relatively low-chlorophyll “blue waters” (Chl a <2 μg/l) with low concentrations of dissolved Fe (<0.1 nM) and high unutilized macronutrient concentrations are observed in the coastal upwelled waters along the southern coast of Peru and in the offshore regions of the Peru Current. Southern Peru is a region without a wide shelf to serve as a source of iron and, as a result, dissolved Fe concentrations in the near-bottom suboxic waters of this region are an order-of-magnitude lower than observed off northern and central Peru. In addition, the offshore Peru Current is a broad, Fe-limited, high-nitrate, lower than expected chlorophyll region extending hundreds of kilometers offshore into the northeast region of the South Pacific subtropical gyre and northwestward into the South Equatorial Pacific.
The distributions of particulate elements (Al, P, Mn, Fe, Co, Cu, Zn, Cd, and Pb), dissolved trace metals (Mn, Fe, Co, Cu, Zn, and Cd), and dissolved nutrients (nitrate, phosphate, and silicic acid) were investigated in the Gulf of the Farallones, a region of high productivity that is driven by the dynamic mixing of the San Francisco Bay plume, upwelled waters, and California coastal surface waters. Particulate metals were separated into >10 and 0.4–10 μm size-fractions and further fractionated into leachable (operationally defined with a 25% acetic acid leach) and refractory particulate concentrations. Dissolved metals (< 0.4 μm pore-size filtrate) were separated into colloidal (0.03–0.4 μm) and soluble (<0.03 μm) fractions. The percent leachable particulate fractions ranged from 2% to 99% of the total particulate concentration for these metals with Mn and Cd being predominantly leachable and Fe and Al being predominantly refractory. The leachable particulate Pb concentration was associated primarily with suspended sediments from San Francisco Bay and was a tracer of the plume in coastal waters. The particulate trace metal data suggest that the leachable fraction was an available source of trace metal micronutrients to the primary productivity in coastal waters. The dissolved trace metals in the San Francisco Bay plume and freshly upwelled surface waters were similar in concentration, with the exception of Cu and Co, which exhibited relatively high concentrations in plume waters and served as tracers of this water mass. The dissolved data and estimates of the plume dynamics suggest that the impact of anthropogenic inputs of nutrients and trace metals in the San Francisco Bay plume contributes substantially to the concentrations found in the Gulf of the Farallones (10–50% of estimated upwelled flux values), but does not greatly disrupt the natural stoichiometric balance of trace metal and nutrient elements within coastal waters given the similarity in concentrations to sources in upwelled water. In all, the data from this study demonstrate that the flux of dissolved nutrients and bioactive trace metals from the San Francisco Bay plume contribute to the high and relatively constant phytoplankton biomass observed in the Gulf of the Farallones.
copyrighted by American Geophysical Union The northern California Current System is impacted by two primary freshwater sources: the Strait of Juan de Fuca and the Columbia River. The Columbia is frequently bidirectional in summer, with branches both north and south of the river mouth simultaneously. We describe the interaction of these two warm Columbia plumes with each other and with the colder plume originating from the strait. The interactions occurred when a period of strong downwelling-favorable winds and high Columbia River discharge was followed by persistent and strong upwelling-favorable winds. The northward plume that developed under the downwelling winds extended over 200 km along the coast to the Strait of Juan de Fuca and into the strait. The plume subsequently wrapped around Juan de Fuca Strait water in the counterclockwise seasonal eddy just offshore of the strait. Inspection for similar wind and outflow conditions (>0.15 N m¯² and 10⁴ m³ s¯¹, respectively) suggest that these events might have occurred in roughly half the years since 1994. Surface drifters deployed in the Columbia plume near its origin tracked this plume water northward along the coast, then reversed direction at the onset of upwelling-favorable winds, tracking plume water southward past the river mouth once again. ‘‘Recent’’ (~1–2 day old) and ‘‘Aged’’ (>14 day old) plume water folded around the newly emerging southwest tending Columbia plume, forming a distinctive ‘‘sock’’ shaped plume. This plume was a mixture of ~10% ‘‘New’’ (<1 day old) water and ~90% Recent and Aged water from prior north tending plumes.
Copyrighted by American Geophysical Union. The relationship between iron and nitrate concentrations was examined off the coast of Oregon during the upwelling season. Surface Fe and N (nitrate + nitrite) concentrations measured underway by flow injection analysis ranged from <0.3 to 20 nmol L¯¹ and <0.1 to 30 mmol L¯¹, respectively. Total dissolvable Fe concentrations, measured in unfiltered, acidified samples in surface waters and in vertical profiles, ranged from <0.3 to 300 nmol L¯¹. Surface water Fe and N concentrations were highly variable and uncoupled. Our observations indicate two dominant sources of Fe to Oregon coastal waters: Slope or shelf sediments and the Columbia River. Sedimentary iron, probably largely in the particulate form, appears to be added to surface waters through wind-induced vertical mixing during strong winds, through thickening of the bottom mixed layer during relaxation or downwelling favorable wind conditions, and through outcropping of shelf bottom waters during upwelling events. The existence of multiple iron sources and the generally high iron concentrations may explain why the distribution of phytoplankton, measured both remotely (by Sea-viewing Wide Field-of-view Sensor) and underway (by in vivo fluorescence), appeared to be driven primarily by physical dynamics and was not strongly linked to the distribution of iron. Nevertheless, at some offshore stations where underway Fe concentrations were <0.3 nmol L¯¹, underway measurements of the physiological state of phytoplankton by fast repetition rate fluorometry were consistent with mild iron stress, and cross-shelf nutrient distributions were consistent with iron regulation of the magnitude of phytoplankton blooms.
Copyrighted by American Geophysical Union. We measured iron concentrations off the Oregon coast in spring (May–June) and summer (August) of 2001 as part of the Coastal Ocean Advances in Shelf Transport (COAST) program. Dissolvable and total dissolvable iron levels in surface waters were generally higher in spring (mean of 2.1 and 33.9 nmol L¯¹, respectively) than in summer (means of 1.4 and 15.4 nmol L¯¹). In spring and summer, high iron concentrations in surface waters were associated with both cold and saline, recently upwelled waters, and with fresh, relatively warm water influenced by the Columbia River. Comparison of total dissolvable iron in 0.45 μm filtered and in unfiltered samples indicated a substantial contribution from particulate iron. Iron concentrations in summer were generally lower than in spring throughout the water column, with the exception of the near-bottom, where concentrations were generally higher in summer than spring. Optical backscatter data from moored sensors were used to infer the vertical and cross-shelf transport of iron-bearing particles during the upwelling season over a steep shelf. Cross-correlation analysis showed downslope movement of particles from the deep inner shelf to the deep midshelf. There was also evidence for sinking of biogenic particles at the midshelf and inner shelf, but we found no evidence of upslope transport of benthic particles. Sufficient iron is available in this system to meet the demands of the phytoplankton, which are able to make full use of available nitrate.
In order to determine the total concentration of bioavailable trace metals in seawater, measurement of both the dissolved and labile particulate fractions is necessary. Comparison of labile particulate metal concentrations from various researchers is limited because of differing definitions of the fraction that is potentially available to phytoplankton on a time frame of generations. A comparison experiment was conducted on coastal and riverine suspended particulate matter to determine the difference between several commonly used techniques that operationally define the labile particulate trace metal fraction. Furthermore, we compared two leach techniques for surface transect samples from within the Columbia River plume and water offshore of Oregon and Washington, United States. The particulate trace metal concentration in the leachate was determined by high-resolution inductively coupled plasma–mass spectrometry. From this comparison, one chemical leach was chosen to best define the labile particulate fraction of Al, Fe, and Mn: a weak acid leach (25% acetic acid at pH 2) with a mild reducing agent (0.02 M hydroxylamine hydrochloride) and a short heating step (10 min 90–95°C). This leach was applied to three surface transects within the Columbia River plume. These coastal waters were found to be rich in labile particulate trace metals that are directly delivered from the Columbia River and indirectly supplied via resuspension from upwelling over a broad continental shelf.
Three species of photosynthetic flagellates capable of phagotrophy (mixotrophic species) were tested for their abilities to use inorganic iron colloids for growth. Ochromonas sp., Chrysochromulina ericina (a coastal strain), and C. ericina (an oceanic strain) were grown in iron-free seawater supplemented with 1 M Fe-ferrihydrite (amor-phous hydrous ferric oxide), magnetite (Fe 3 O 4)/maghemite (-Fe 2 O 3), hematite (-Fe 2 O 3), or goethite (-FeOOH). Desferrioxamine B, an iron-binding siderophore, was used to reduce the concentration of dissolved iron in the colloid-amended media, and none of the flagellates were able to use its iron complex as an iron source under the conditions of the experiments. Both strains of Chrysochromulina grew at 35%-70% of their maximum rates with goethite, hematite, and magnetite/maghemite but were unable to use ferrihydrite. Ochromonas grew well with ferrihydrite but could not use any of the other forms. Thalassiosira oceanica (clone 1003) and Thalassiosira pseudonana (clone 3H), diatoms that could only take up dissolved forms of iron, were unable to use any of the colloids tested. The mechanism of iron acquisition by the flagellates appeared to involve ingestion of the iron colloids, because bacteria resident in the cultures were too iron poor to be a significant source of iron and were unable to use the iron contained in the colloids themselves. Variations in the sizes of the colloids were hypothesized to account for differences in their availability, independent of colloid chemical stability. The results provide the first strong evidence for direct use (i.e., without prior dissolution) of colloidal iron by mixotrophic phytoplankton and document a new pathway of iron acquisition that may be important for their survival in low-iron waters of the sea.
The supply of iron, relative to that of the macronutrients nitrate, phosphate, and silicic acid, plays a critical role in allowing extensive diatom blooms to develop in coastal upwelling regimes. The presence or absence of a broad continental shelf influences the supply of iron, The iron input to central California upwelling waters varies spatially and can be characterized by two end-member regimes. One end member, which includes Monterey Bay and extending north to Pt. Reyes, is an iron-replete regime where upwelling occurs over a relatively broad continental shelf that results in waters with high concentrations of dissolved and particulate iron (>10 nM) entrained together with high concentrations of nitrate and silicic acid. In these iron-replete regions, extensive blooms of large diatoms deplete macronutrient concentrations, which results in correspondingly high chlorophyll a concentrations. The other end member, located to the south of Monterey Bay off the Big Sur coast, is an iron-deplete regime where upwelling is focused offshore of a narrow continental shelf. Upwelled waters in the Big Sur region are characterized by low dissolved and particulate iron concentrations (<1 nM), together with high concentrations of nitrate and silicic acid. Extremely low iron concentrations, unused nitrate and silicic acid, and a low abundance of large diatoms characterize surface waters in these iron-deplete regions, and thus represent coastal upwelling, high-nutrient, low-chlorophyll systems limited by the micronutrient iron.
Surface waters are cold and nutrient-rich during winter months. Prevailing winds are from the south, resulting in general downwelling and causing the Columbia River effluent to lie to the north and along the Washington coast. Chlorophyll concentrations off Washington illustrate that the spring bloom develops during March and continues into April when it becomes evident in increasing oxygen concentration of surface waters and declining nutrient levels. Low nutrient concentrations persist in the euphotic zone from the later half of April through early May when wind conditions become favorable for coastal upwelling. The seasonal shift to upwelling (northerly) winds causes the Columbia River (maximum discharge during June) influence to migrate from the Washington nearshore to the Oregon offshore. Although periodic upwelling events affect the dynamics of both nearshore environments during summer, wind effects are strongest off Oregon. Maximum upwelling influences on nearshore temperature, salinity, oxygen, nutrient levels and chlorophyll concentration occur during June off Washington. A strong thermocline develops over the Washington shelf region during mid- to late summer and northern excursions of low-salinity, river water during periodic wind reversals further contribute to water-column stability in the nearshore environment. Phytoplankton standing stock, measured as chlorophyll, declines to moderate winter levels (about 1μg Chl a l-1) as the upwelling season ends in August or September. In the very nearshore, diatom communities thrive in the surf zone during winter. -from Authors
[1] Here we show that labile particulate iron and manganese concentrations in the upper 500 m of the Western Subarctic Pacific, an iron-limited High Nutrient Low Chlorophyll (HNLC) region, have prominent subsurface maxima between 100–200 m, reaching 3 nM and 600 pM, respectively. The subsurface concentration maxima in particulate Fe are characterized by a more reduced oxidation state, suggesting a source from primary volcagenic minerals such as from the Kuril/Kamchatka margin. The systematics of these profiles suggest a consistently strong lateral advection of labile Mn and Fe from redox-mobilized labile sources at the continental shelf supplemented by a more variable source of Fe from the upper continental slope. This subsurface supply of iron from the continental margin is shallow enough to be accessible to the surface through winter upwelling and vertical mixing, and is likely a key source of bioavailable Fe to the HNLC North Pacific.
Coastal waters in the northern Gulf of Alaska (GoA) are considered iron-rich and nitrate-poor, in contrast to the iron-poor, high-nitrate, low chlorophyll (HNLC) waters of the central GoA. The degree of mixing between these two regimes, enhanced by mesoscale eddies, is essential to the high productivity observed in the region. As part of a study on iron delivery to the central GoA via mesoscale eddies, extensive work was focused on characterizing the coastal endmember, the Alaska Coastal Current. In surface Alaskan coastal waters between Yakutat and the Kenai Peninsula, dissolved iron concentrations ranged from 0.5 to 4.1 nM with an average of ∼ 2 nM. In contrast, leachable particulate iron concentrations were much higher and more variable, ranging from over 1 μM in the Alsek River plume to less than 5 nM at the base of Cook Inlet. Cross-shelf transport of both surface and subsurface dissolved iron and leachable particulate iron was observed. Throughout the study area, leachable particulate iron values were at least an order of magnitude higher than dissolved values, suggesting that the system's ability to solubilize this large concentration of leachable particulate iron is overwhelmed by the massive input of glacial-derived particulate iron. Nevertheless, suspended leachable particulate iron remains available for exchange to the dissolved phase and is suggested to maintain a relatively constant (∼ 2 nM) source of dissolved iron in the coastal GoA.
Factors influencing concentrations of nitrate, silicic acid, dissolved
Fe, and dissolved Mn in the near-field Columbia River plume were
examined during late spring and summer from 2004 to 2006 as part of the
River Influences on Shelf Ecosystems program. Under upwelling-active
phases, cold, high-nitrate coastal seawater was entrained in the plume,
and nitrate concentrations of 16-19 μM were observed with as much as
90% from a coastal seawater origin. Under downwelling-relaxation phases,
warm, nutrient-depleted coastal seawater was entrained forming a
near-field plume with nitrate concentrations of 2.5-6 μM, with the
river as the only source. Elevated silicic acid in the river is the
dominant source, with concentrations of 60-80 μM in the near-field
plume. During upwelling-active phases, high concentrations of dissolved
Fe (as high as 40 nM) in the cold, low-oxygen, nutrient-rich coastal
seawater were entrained to form a near-field plume with 15-20 nM
dissolved Fe. During downwelling-relaxation phases, dissolved Fe in the
intruding underlying warm coastal seawater was 1-3 nM, producing plume
concentrations of 2-13 nM, with higher concentrations during the high
river flow of May 2006. Dissolved Mn in the near-field plume covaried
markedly as a function of increased tidal flushing in the estuary. The
use of CORIE (pilot environmental observation and forecasting system for
the Columbia River) time series conductivity-temperature-depth data
within the estuary, along with data presented in this study, allows
extrapolation of the near-field plume chemistry throughout the spring
and summer seasons to provide insight into this important source of
nutrients to the coastal waters in this region.
A sensitive flow injection method for determining iron in seawater developed by Measures et al. (1995) has been substantially modified to allow the direct preconcentration of dissolved iron in acidified seawater samples (pH 1.7) onto a nitrilotriacetic acid (NTA) chelating resin. This removes the need to adjust the pH and buffer samples before the preconcentration step, and the low pH eliminates potential interference from the presence of strong iron-binding organic ligands. As part of an international intercalibration exercise for the Sampling and Analysis of Fe (SAFe), we investigated at sea the precision and accuracy of this flow injection method with its preconcentration step plus catalytic spectrophotometric detection with N,N-dimethyl-p-phenylenediamine dihydrochloride (FI-NTA-DPD). Acidified seawater samples analyzed using FI-NTA-DPD were shown to be in excellent agreement with other ship- and lab-based methods. The acidification of seawater samples to pH 1.7 is an important protocol if total dissolved iron in seawater is to be determined within hours of collection. A ship- and lab-based analytical intercomparison of two flow injection methods (FI-NTA-DPD and FI-NTA-ICP-SFMS) for the determination of total dissolved iron in seawater was carried out on SAFe samples collected from surface waters and at 1000 m depth from the North Pacific Ocean. For the two methods, total dissolved iron concentrations in surface samples were 0.101 +/- 0.009 and 0.098 +/- 0.009 nM, respectively, and in samples from 1000 m, 0.93 +/- 0.04 and 0.92 +/- 0.08 nM. No statistical difference between the FI-NTA-DPD and FI-NTA-ICP-SFMS methods was observed (P = 0.05).
Seven years of observations of surface water iron concentrations in the
Monterey Bay region of central California reveal a consistent annual
cycle dominated by the injection of high concentrations of particulate
iron each spring. A companion study of the water column near the
upwelling center at the north end of the bay clearly indicates a
sedimentary source for the iron. Local river discharge, during winter
storm events, results in the deposition of a fine-grained sediment
"fluff" layer along the shelf. The initial rapid shoaling of isotherms
at the onset of upwelling in spring brings water from the fluff layer to
the surface. Concentrations of iron in both surface and source waters
are then quickly diminished although upwelling intensifies, bringing the
highest concentration of nitrate to the surface ˜3 months later.
The concentration of a number of constituents measured in bottom water
samples collected during the companion study strongly suggests that the
rapid decrease in surface water iron concentrations is due to the
depletion of the fluff layer following initial isotherm shoaling. The
decoupling of iron and nitrate supply sets up the potential for iron
limitation. However, on the basis of average Fe/NO3 ratios,
we conclude that the region is not often iron limited. Iron limitation
was apparent only one summer at the offshore station. Summer chlorophyll
concentrations are highly correlated to dissolvable (unfiltered) iron
concentrations, evidence in support of the role of particulate iron in
meeting the ecosystem's iron requirements.
Iron concentrations in open ocean are orders of magnitude lower than levels in coastal waters. Experiments with coastal and oceanic phytoplankton clones representing different algal groups and cell sizes indicate that cellular iron uptake rates are similar among the species when rates are normalized to cell surface area. This similarity in rates apparently is explained by evolutionary pressures that have pushed iron uptake in all species toward the maximum limits imposed by diffusion and ligand exchange kinetics. Because of these physical/chemical limits on uptake, oceanic species have been forced to decrease their cell size and/or to reduce their growth requirements for cellular iron by up to 8-fold. The biochemical mechanisms responsible for this reduction in metabolic requirements are unknown.
Over geological time, photosynthetic carbon fixation in the oceans has
exceeded respiratory oxidation of organic carbon. The imbalance between
the two processes has resulted in the simultaneous accumulation of
oxygen in, and drawdown of carbon dioxide from, the Earth's atmosphere,
and the burial of organic carbon in marine sediments1-3. It
is generally assumed that these processes are limited by the
availability of phosphorus4,5, which is supplied by
continental weathering and fluvial discharge5-7. Over the
past two million years, decreases in atmospheric carbon dioxide
concentrations during glacial periods correlate with increases in the
export of organic carbon from surface waters to the marine
sediments8-11, but variations in phosphorus fluxes appear to
have been too small to account for these changes12,13.
Consequently, it has been assumed that total oceanic primary
productivity remained relatively constant during glacial-to-interglacial
transitions, although the fraction of this productivity exported to the
sediments somehow increased during glacial periods12,14. Here
I present an analysis of the evolution of biogeochemical cycles which
suggests that fixed nitrogen, not phosphorus, limits primary
productivity on geological timescales. Small variations in the ratio of
nitrogen fixation to denitrification can significantly change
atmospheric carbon dioxide concentrations on glacial-to-interglacial
timescales. The ratio of these two processes appears to be determined by
the oxidation state of the ocean and the supply of trace elements,
especially iron.
Measurements of suspended sediment concentration, velocity, salinity, and turbulent microscale shear in the near-field region of the Columbia River plume are used to investigate the mechanisms of sediment resuspension and entrainment into the plume. An east-west transect was occupied during spring and neap tide periods in August 2005 and May 2006, corresponding to low and high river discharge conditions, respectively. During the high-discharge period the plume is decoupled from the bottom, and fine sediment resuspended from the bottom does not leave the benthic boundary layer. The primary modes of sediment transport associated with the plume are advection of sediment from the estuary and removal of sediment from the plume by gravitational settling and turbulent mixing. In contrast, the plume is much less stratified during low-discharge conditions, and large resuspension events are observed that entrained sediment through the water column and into the plume. Our measurements indicate that two factors control the magnitude and timing of sediment resuspension and entrainment: the supply of fine sediment on the seabed and the relative influence of tidal turbulence compared with buoyancy input from the river. The latter is quantified in terms of the estuary Richardson number RiE. The magnitude of vertical turbulent sediment flux is correlated with RiE during the low-flow period when there is a sufficient supply of bottom sediment in the near-field region. Such sediment resuspension may be an important mechanism for the delivery of bioavailable micronutrients to the plume during the summer.
Dissolved iron (Fe) speciation in the Columbia River plume, the San Francisco Bay plume, and the Columbia River estuary was investigated using competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) with the added ligand salicylaldoxime. A stronger L₁-type Fe-binding ligand class was measured in all surface samples, and in the Columbia River estuary. A weaker L₂-type ligand class was present in the far-field Columbia River plume and the San Francisco Bay plume but was not observed in the low-salinity (S = 1.4-22.5) waters of the near-field Columbia River plume or estuary. Concentrations of total dissolved Fe were correlated with the concentrations of the stronger L₁-type ligand in nonestuarine (S > 13) surface samples. Leachable particulate (>0.4 µm) Fe concentrations in the Columbia River plume were measured to supplement existing data from the San Francisco Bay plume. There is a large concentration of readily leachable particulate Fe in the two plumes, yet it is the concentration of ambient L₁-type ligands that appears to dictate the concentration of dissolved Fe in these waters and, consequently, the supply of dissolved Fe to neighboring coastal waters. The correlation between dissolved Fe and L₁ ligand concentrations in both plume waters, as well as in California Current and upwelled surface waters, suggests that this relationship will persist in other coastal environments and should be considered when evaluating and modeling coastal Fe cycling and supply.
Manganese is an essential micronutrient for all organisms. Its requirement by plants is particularly high because of its role in the oxidation of water in photosynthesis1–5. According to thermodynamic considerations, manganese should exist in oxic waters as MnO2 (ref. 6) which is insoluble and, therefore, not directly available for plant nutrition. In contrast to thermody-namic predictions, however, most of the manganese in near surface seawater exists as soluble reduced Mn(II) (ref. 7). Although slow oxidation kinetics are at least partially responsible for the presence of Mn(II) in oxic waters8,9, reduced manganese, nevertheless, should be converted to particulate manganese oxides (at rates that depend on several kinetic factors10,11) and be lost from the water column by sinking12. We report here experiments that demonstrate photoreduction of manganese oxides by dissolved organic substances (humic substances) in seawater. Such reactions appear to be important in maintaining manganese in a dissolved reduced form in photic waters, thereby enhancing its supply to phytoplankton.
Within the benthic boundary layer (BBL) and seabed of river-dominated ocean margins (RiOMars), the timing, kinetics and extent of important biogeochemical processes are greatly influenced by large riverine inputs of dissolved and particulate terrestrial materials. An examination of our current state of knowledge reveals that the rates of primary productivity, sediment deposition, remineralization and burial in these margins are among the highest of all marine systems. Transport and transformation processes within the benthic region of these RiOMar areas are highly variable (temporally and spatially). As a result, measurement and modeling of these processes are very challenging. A more quantitative understanding of these systems will require coordinated interdisciplinary studies that: (a) better define the quantity and composition of riverine inputs; (b) greatly improve our current knowledge of transport and transformation within the BBL of these systems; (c) focus on the sequential timing of physical forcings (riverine discharge, high energy events); (d) develop new nonclassical diagenetic models; (e) further characterize and delineate differences between sub-environments within a RiOMar and between RiOMar ''types''; and, (f) provide a better mechanistic understanding of what controls the net retention of terrestrial materials (diagenetic transformation vs. burial) within RiOMar systems. r 2004 Published by Elsevier Ltd.
This paper has two purposes. The first is to use tidal-monthly variations in the density and velocity fields and the salt and water transports as key to understanding the circulation of the Columbia River Estuary and other river estuaries. The Columbia River Estuary is a good natural laboratory in this regard, because the flushing time of the system (a few days) is short relative to the tidal month during all seasons. This allows the occurrence of distinct transitions from a strongly to a weakly stratified water column (and back) during the tidal month. Furthermore, because atmospheric processes are secondary to riverflow and tidal influence in determining the circulation, most of the energy in circulatory phenomena is confined to distinct tidal, tidal-monthly and seasonal frequency bands. Observations of salt transport and neap-spring transitions reported herein should provide important constraints on future theoretical studies of estuarine circulation.
A vertical mixing event was simulated in shipboard incubation experiments on the mid-continental shelf of the eastern Bering Sea to investigate Fe and Zn cycling between the soluble (< 0.03 μm or 200 kDa), colloidal (0.03–0.2 μm), and particulate (0.2–10 μm, > 10 μm) size-fractions. The particulate Fe and Zn were further separated into chemically labile (25% acetic acid-leachable) and refractory pools. The experiment employed 57Fe (+ 0.90 nM) and 68Zn (+ 0.99 nM) as stable, low-abundance isotope amendments to the soluble fraction, and the exchange of Fe and Zn between the different physico-chemical fractions was measured using high resolution-inductively coupled plasma-mass spectrometry (HR-ICP-MS). More than 50% of the added 57Fe partitioned to the colloidal fraction within 45 min of adding the tracer. Both the 57Fe and 56Fe colloidal fraction were removed from the dissolved phase at a faster rate than the soluble Fe fraction. In contrast, the colloidal 66Zn and 68Zn concentrations remained constant over the 5-day experiment, suggesting a unique removal mechanism for colloidal Fe. The net removal of dissolved 57Fe was observed to be 3 to 4 times more rapid than dissolved 56Fe, which can be attributed to the regeneration of particulate Fe. Using a simple first-order model, it was determined that the net removal of 2.0 nM of dissolved Fe during the experiment was a consequence of dynamic cycling, whereby 2.9 nM of particulate Fe was regenerated and contributed to an overall removal of 4.9 nM of Fe from the dissolved phase. The amended 68Zn tracer resided in the soluble fraction and was assimilated by the diatom biomass (> 10 μm size-fraction) at the same rate as 66Zn. This similarity in rates suggests that nearly all of the net removal of Zn was due to assimilation and that regeneration did not play a significant role in Zn cycling within the incubation experiment. This research demonstrates the advantage of using low-abundance isotopes as tracers and the importance of particulate and colloidal Fe in the overall biogeochemical cycling of Fe in ocean surface waters.
Iron, Mn, Cu, Pb and Zn have been determined in suspended particulate matter (SPM) collected in the estuarine plume regions of the Humber (during winter, spring and summer) and Thames (winter only). Metal concentrations (w/w) were found to increase with SPM concentration and could be defined in terms of the mixing of an ambient, slow settling population, with variable proportions of a diluent population. The end-members of the particle mixing series are fine material derived from coastal erosion, which is modified seasonally by biological production, and contaminated estuarine material which is contained within the estuarine discharge or derived from local resuspension of reworked deposits by tidal currents and wave activity. Iron-normalized metal concentrations exhibited an inverse relationship with SPM concentration in the Humber region and regression analyses enabled seasonal changes in end-member compositions to be evaluated. Since the metal:Fe ratios of the ambient population did not accord with those of local cliff samples, additional sources of metal were proposed whose importance to particle composition increases with a reduction in SPM concentration. Qualitatively, the seasonal variation of end-member compositions was consistent with (i) the coupling between redox processes occurring in the bed sediment and adsorption of metals (Mn, Cu, Zn) released from the pore waters onto ambient and diluent suspended particles in the overlying water column, and (ii) adsorption of metal (Pb) by ambient suspended particles from an extraneous (atmospheric) source. In the Thames plume, an increase in Fe-normalized metal concentrations with increasing particle concentration resulted from the mixing of end-member particles and the effects of additional metal from an internal or extraneous source were less clear, possibly because of metal desorption from suspended particles traversing the salinity gradient in the outer estuary. The processes described in this study regulate the internal cycling of trace metals in estuarine plume regions and the export of metals to neighbouring shelf sea environments.
Vertical and horizontal distributions of dissolved and suspended particulate Fe and Mn, and vertical fluxes of these metals (obtained with sediment traps) were determined throughout the Pacific Ocean. In general, dissolved Fe is low in surface and deep waters (0.1 to 0.7 nmol/kg), with maxima associated with the intermediate depth oxygen minimum zone (2.0 to 6.6 nmol/kg). Vertical distributions of dissolved Mn are similar to previous reports, exhibiting a surface maximum, a subsurface minimum, a Mn maximum layer coincident with the oxygen minimum zone, and lowest values in deep waters.Near-shore removal processes are more intense for dissolved Fe than for dissolved Mn. Dissolved Mn in the surface mixed layer remains elevated much farther offshore than dissolved Fe. Elevated near-surface dissolved Mn concentrations occur in the North Pacific Equatorial Current, suggesting transport from the eastern boundary. Near-surface mixed-layer dissolved Mn concentrations are higher in the North Pacific gyre reflecting enhanced northern hemisphere aeolian sources.Residence time estimates for the settling of refractory paniculate Fe and Mn from the upper water column are 62–220 days (Fe), and 105–235 days (Mn). In contrast, residence times for the scavenging of dissolved Fe and Mn are 2–13 years (Fe) and 3–74 years (Mn). Scavenging residence times for dissolved Mn based on horizontal mixing in the surface mixed layer of the northeast Pacific are 0.4 years (nearshore) to 19 years (1000 km offshore).There is no evidence for in situ Fe redox dissolution within sub-oxic waters in the eastern tropical North Pacific. Dissolved Fe appeared to be controlled by dissolution from sub-oxic sediments, with oxidative scavenging in the water column or upper sediment layers. However, in situ Mn dissolution within the oxygen minimum zone was evident.
A laboratory experiment was carried out in which the flocculation products, formed from the mixing of filtered (0.4 μm) river water and seawater, were analysed. This study established that Fe, Mn, Al, Cu, Ni, Cd and Co have resolvable and well-defined estuarine chemistries. Copper, Ni, Mn and Co have salinity dependences of removal which are similar to those of dissolved Fe and humic acids. The amount of removal of the above trace metals increases between 0 and 15–18‰, after which little additional removal occurs. The extents of removal from river water are very different: Fe, 95%; Al, 20%; Cu, Ni, 40%; Co, 10%; Cd, 5% and Mn, 25–45%. The basic removal mechanism appears to be the estuarine flocculation of trace metals which exist, in part, in river water as colloids in association with colloidal humic acids and hydrous iron oxides. A qualitative model, based on this mechanism, supports the observations of this flocculation study.The results of this study give the most complete and consistent set of data presently available, from which to postulate the most important processes controlling the estuarine chemistry of trace metals. The generality of their behaviours still needs to be determined by future investigations.
Siderophores play an important role in biological iron acquisition in iron-limited aquatic systems. While it is widely accepted that the solubilization of iron-bearing mineral phases is a key function of siderophores, the mechanism of siderophore-promoted mineral dissolution in aquatic systems is largely unknown. In this study, we investigated the effect of siderophores (desferrioxamine B (DFOB) and aerobactin) on light-induced dissolution of goethite and lepidocrocite in the presence or absence of oxalate in aerated and deaerated suspensions at pH 6. For the irradiated two-ligand system (oxalate/siderophore), the experimental results suggest that oxalate acts as the electron donor for the formation of surface Fe(II), and the siderophore acts as an efficient shuttle for the transfer of surface Fe(II) into solution. Furthermore, even in the absence of an electron donor such as oxalate, both DFOB and aerobactin accelerated the light-induced dissolution of lepidocrocite as compared to the thermal dissolution. Experiments with dissolved Fe(III)–DFOB and Fe(III)–aerobactin complexes suggest that this enhancing effect is not due to photolysis of corresponding surface complexes but to efficient transfer of reduced surface Fe(II) into solution, where surface Fe(II) may be formed, e.g., through photolysis of surface Fe(III)–hydroxo groups. Based on this study, we conclude that the interplay of light and siderophores may play a key role in the dissolution of colloidal iron(III) (hydr)oxides in marine systems, particularly in the presence of efficient electron donors.
A survey of U.S. east coast estuaries confirms that large-scale rapid removal of iron from river water is a general phenomenon during estuarine mixing. The river-borne ‘dissolved’ iron consists almost entirely of mixed iron oxide-organic matter colloids, of diameter less than 0.45 μm, stabilized by the dissolved organic matter. Precipitation occurs on mixing because the seawater cations neutralize the negatively charged iron-bearing colloids allowing flocculation. The process has been duplicated in laboratory experiments using both natural filtered and unfiltered river water and a synthetic colloidal goethite in 0.05 μm filtered water. The colloidal nature of the iron has been further confirmed by ultracentrifugation and ultrafiltration. A major consequence of the precipitation phenomena is to reduce the effective input of ‘dissolved’ iron to the ocean by about 90% of the primary river value, equivalent to a concentration of less than 1 μmol per liter of river water.
The rare earth pattern in sedimentary rocks is nearly constant and is considered to represent the relative abundances of these elements in the continental crust. This pattern is derived principally from mixing of basic and acidic igneous rock patterns. Preliminary calculations indicate that approximately equal contributions of basic and acid patterns are required to give the observed rare earth abundance pattern in sedimentary rocks.A table of element abundances in the continental crust, calculated on the basis of a 1:1 mixture of granite and basalt abundances, is presented. Attention is drawn to differences from previous estimates.
Six years (1998–2003) of SeaWiFS multispectral satellite data are used to document the seasonal and interannual variability of the Columbia River plume on the North American west coast. A supervised classification scheme using 5 channels of normalized water-leaving radiance (nLw at 412, 443, 490, 510 and 555 nm), with training pixels adjusted temporally to optimize the signature of plume core characteristics, quantifies the climatological seasonal location of 4 spectrally defined classes of surface water and provides estimates of variability in position as a probability. Winter plume orientation was northward and close to the shore, with infrequent adjustments to the south. Summer plume orientation was offshore and to the south, dissociated from the coast, with more frequent (> 20%) occurrences of plume water and peripheral plume water (> 50%) to the north. An effective characterization of interannual variability in plume dynamics is provided by time series of temporally averaged nLw at 555 nm, used as an estimate of suspended particulate material. Monthly means during maxima and minima in annual river discharge show the plume to be weakest both in spatial extent as well as absolute nLw values in 2001, a year of minimum river discharge. Time series of both (a) nLw 555 values at the river mouth and (b) Mode 2 of an empirical orthogonal function decomposition of the 6-year nLw 555 time series variance that isolates winter patterns are strongly correlated with river discharge. Interannual differences in monthly mean wind forcing are evident as changes in plume position during the winter, but at the 8-day and longer time scales examined here, summer interannual differences are dominated by differences in discharge volume.
The influence of the Columbia River plume on the distributions of nitrate and iron and their sources to coastal and shelf waters were examined. In contrast to other large estuaries, the Columbia River is a unique study area as it supplies very little nitrate (5 μM) and iron (14–30 nM) at salinities of 1–2 to coastal waters. Elevated nitrate and dissolved iron concentrations (as high as 20 μM and 20 nM) were observed, however, in the near field Columbia River plume at salinities of 20. Surface nitrate concentrations were higher than observed in the Columbia River itself and therefore must be added by entrainment of higher nitrate concentrations from subsurface coastal waters. Tidal flow was identified as an important factor in determining the chemical constituents of the Columbia River plume. During the rising flood tide, nitrate and iron were entrained into the plume waters resulting in concentrations of 15 μM and 6 nM, respectively. Conversely, during the ebb tide the concentrations of nitrate and total dissolved iron were reduced to 0.3–3 μM and 1–2 nM, respectively, with a concomitant increase in chlorophyll a concentrations. As these plume waters moved offshore the plume drifted directly westward, over a nitrate depleted water mass (< 0.2 μM). The plume water was also identified to move southwards and offshore during upwelling conditions and nitrate concentrations in this far field plume were also depleted. Iron concentrations in the near-field Columbia River plume are sufficient to meet the biological demand. However, due to the low nitrate in the Columbia River itself, nitrate in the plume is primarily dependent on mixing with nitrate rich, cold, high salinity subsurface waters. Without such an additional source the plume rapidly becomes nitrate limited.
The distribution of dissolved (soluble and total) and particulate (leachable and total) aluminum was examined in the Columbia River and estuary, in near-field and far-field river plumes, and in adjacent coastal waters of Washington and Oregon during the River Influence on Shelf Ecosystems (RISE) cruise of May/June 2006. Dissolved and particulate aluminum (Al) concentrations were significantly greater in the river than in the coastal waters that mixed to form the plume. Dissolved Al concentrations in the Columbia River (∼80 nM) were low relative to other major rivers. Leachable and total particulate Al concentrations within the river reached concentrations greater than 1000 nM and 18,000 nM, respectively. Dissolved Al within the Columbia River estuary showed a significant removal (∼60%) at salinities between 0 and 10 with salt-induced flocculation of colloidal Al complexes and enhanced particle scavenging being probable explanations for aluminum removal. Dissolved and particulate Al concentrations were significantly greater in near-field plumes relative to surrounding coastal waters. As the plume advected from near-field to far-field away from the river mouth, dilution of the plume with lower dissolved Al surface waters as well as particle scavenging along the flow path appeared to be controlling dissolved Al distributions. Particle settling as well as dilution with lower particle-load waters led to observed decreases in particulate Al as the plume moved from near-field to far-field. However, the percent-leachable particulate aluminum in both near-field and far-field plumes was remarkably constant at ∼7%. Dissolved and particulate Al in a far-field plume over 100 km southwest of the Columbia River mouth were over an order-of-magnitude greater than surrounding waters, illustrating the importance of the Columbia River plume as a mechanism for transporting Al offshore. Aluminum could be used to trace the input of biologically-required elements such as iron into waters off the shelf.
Freshwater plumes have important effects on marine ecosystems: in the presence of a plume, stratification, nutrient pathways, light and circulation patterns are significantly altered from patterns that occur under the influence of wind and ambient currents alone. The historical picture of the plume from the Columbia River is of a freshwater plume oriented southwest offshore of the Oregon shelf in summer and north or northwest along the Washington shelf in winter. Recent CTD data and new data from moored sensors support a picture quite different from the historical seasonal pattern. Specifically, the plume is frequently present up to 150 km north of the river mouth on the Washington shelf from spring to fall, even during periods of upwelling. The plume is frequently bi-directional, with branches both north and south of the river mouth. During a downwelling event, the southwest plume moves onshore over the Oregon shelf. At the same time, a new plume forms north of the river mouth over the Washington shelf, trapped within ∼20–30 km of the coast. This plume propagates and also is advected northward by inner shelf currents that reverse during downwelling. When winds return to upwelling-favorable, inner shelf currents reverse immediately to flow to the south and the shallow plume is advected offshore in the wind-driven Ekman layer. Once over the central shelf, the plume is advected farther south by the seasonal mean ambient flow. Overall, freshwater from the Columbia plume overlies the Washington shelf ∼50% or more of the summer. Capping of upwelling on the inner shelf by the Columbia freshwater plume is illustrated, where the “capping potential” is related to stratification and wind magnitude and duration. Evidence is also presented to suggest that the seaward front of the Columbia River plume may provide a barrier to the transport of harmful algal blooms to coastal beaches in summer and early fall.
An intensive Lagrangian particle-tracking analysis of the July 2004 upwelling period was conducted in a hindcast model of the US Pacific Northwest coast, in order to determine the effect of the Columbia River plume on the fate of upwelled water. The model, implemented using Regional Ocean Modeling System (ROMS), includes variable wind and atmospheric forcing, variable Columbia river flow, realistic boundary conditions from Navy Coastal Ocean Model (NCOM), and 10 tidal constituents. Model skill has been demonstrated in detail elsewhere [MacCready, P., Banas, N.S., Hickey, B.M., Dever, E.P., Liu, Y., 2008. A model study of tide- and wind-induced mixing in the Columbia River estuary and plume. Continental Shelf Research, this issue, doi:10.1016/j.csr.2008.03.015]. Particles were released in the Columbia estuary, along the Washington coastal wall, and along the model's northern boundary at 48°N. Particles were tracked in three dimensions, using both velocities from ROMS and a vertical random displacement representing turbulent mixing. When 25 h of upwelling flow is looped and particles tracked for 12 d, their trajectories highlight a field of transient eddies and recirculations on scales from 5 to 50 km both north and south of the Columbia. Not all of these features are caused by plume dynamics, but the presence of the plume increases the entrainment of inner-shelf water into them. The cumulative effect of the plume's interaction with these transient features is to increase cross-shelf dispersion: 25% more water is transported laterally past the 100 m isobath when river and estuarine effects are included than when they are omitted. This cross-shelf dispersion also disrupts the southward transport of water along the inner shelf that occurs in the model when the Columbia River is omitted. This second effect—increased retention of upwelled water on the Washington shelf—may be partly responsible for the regional-scale alongcoast gradient in chlorophyll biomass, although variations in shelf width, the Juan de Fuca Eddy to the north, and the intermittency of upwelling-favorable winds are likely also to play important roles.
The spatial distributions of dissolved manganese and nutrients were examined in the Columbia River plume off Oregon and Washington during the summer of 2004 and 2005 as part of the River Influence on Shelf Ecosystems (RISE) program. Factors influencing the hydrochemical characteristics of the freshly formed and aged Columbia River plume were investigated. Hydrographic data and nutrient concentrations were used to delineate three distinct water sources for the Columbia River Plume: California Current surface water, coastal upwelled water, and Columbia River water. The warm, intermediate salinity, nutrient poor California Current water contains low levels of dissolved manganese (< 5 nM) and silicic acid (< 5 μM), and is depleted in nitrate. The cold, high salinity, nutrient rich, freshly upwelled water is highly variable (2–20 nM) in dissolved manganese and can be as high as ∼ 45 μM in silicic acid and ∼ 30 μM nitrate. The variable Columbia River has summer temperatures ranging from ∼ 13 to 24 °C, high silicic acid concentrations (ranging from ∼ 120 to 200 μM), and lower nitrate concentrations (ranging from ∼ 2 to 20 μM). During the summer, the concentrations of silicic acid and dissolved manganese can exceed 100 μM and 200 nM, respectively, in near-field Columbia River plumes. These values are markedly greater than those of surface coastal waters (even during upwelling conditions). As the plume advects and mixes, the concentrations of these two constituents remain relatively high within plume waters. The concentrations of dissolved manganese in the near-field plume vary with tidal amplitude, exhibiting much higher concentrations for a given salinity during spring tides than during neap tides. For example, the Columbia River plume at a salinity of 20 has a concentration of dissolved manganese of ∼ 240 nM during spring tides, as compared to only ∼ 60 nM during low amplitude tides. Silicic acid concentrations in the near-field plume remain relatively constant throughout the tidal month. Calculations indicate there is roughly an equivalent yearly delivery of dissolved manganese and silicic acid to the coastal waters off Oregon and Washington by upwelled waters and by the Columbia River plume.