Cliff S. Law's research while affiliated with National Institute of Water and Atmospheric Research and other places

Publications (114)

Large-scale climatic forcing is impacting oceanic biogeochemical cycles and is expected to influence the water-column distribution of trace gases, including methane and nitrous oxide. Our ability as a scientific community to evaluate changes in the water-column inventories of methane and nitrous oxide depends largely on our capacity to obtain robust and accurate concentration measurements that can be validated across different laboratory groups. This study represents the first formal international intercomparison of oceanic methane and nitrous oxide measurements whereby participating laboratories received batches of seawater samples from the subtropical Pacific Ocean and the Baltic Sea. Additionally, compressed gas standards from the same calibration scale were distributed to the majority of participating laboratories to improve the analytical accuracy of the gas measurements. The computations used by each laboratory to derive the dissolved gas concentrations were also evaluated for inconsistencies (e.g., pressure and temperature corrections, solubility constants). The results from the intercomparison and intercalibration provided invaluable insights into methane and nitrous oxide measurements. It was observed that analyses of seawater samples with the lowest concentrations of methane and nitrous oxide had the lowest precisions. In comparison, while the analytical precision for samples with the highest concentrations of trace gases was better, the variability between the different laboratories was higher: 36 % for methane and 27 % for nitrous oxide. In addition, the comparison of different batches of seawater samples with methane and nitrous oxide concentrations that ranged over an order of magnitude revealed the ramifications of different calibration procedures for each trace gas. Finally, this study builds upon the intercomparison results to develop recommendations for improving oceanic methane and nitrous oxide measurements, with the aim of precluding future analytical discrepancies between laboratories.
Ocean acidification, arising from the influx of anthropogenically generated carbon, poses a massive threat to the ocean ecosystems. Our knowledge of the effects of elevated anthropogenic CO2 in marine waters and its effect on the performance of single species, trophic interactions and ecosystems is increasing rapidly. However, our understanding of the biogeochemical cycling of nutrients such as nitrogen is less advanced and lacks a comprehensive overview of how these processes may change under OA conditions. We conducted a systematic review and meta –analysis of eight major nitrogen transformation processes incorporating 49 publications to synthesize current scientific understanding of the effect of OA on nitrogen cycling in the future ocean. The following points were identified by our meta‐analysis: (1) diazotrophic nitrogen fixation is likely enhanced by 29 ± 4% under OA. (2) species and strain‐specific response of nitrogen fixers to OA was detectable, which may result in alterations in microbial community composition in the future ocean. (3) nitrification processes were reduced by a factor of 29 ± 10%. (4) declines in nitrification rates were not reflected by nitrifier abundance. (5) contrasting results in uni‐specific culture experiments versus natural communities were apparent for nitrogen fixation and denitrification. The net effect of the nitrogen cycle process responses also suggests there may be a shift in the relative nitrogen pools, with excess ammonium originating from CO2 fertilized diazotrophs. This regenerated inorganic nitrogen can recycle in the upper water column increasing the relative importance of the ammonium‐ fueled regenerated production. However, several feedback mechanisms with other chemical cycles, such as oxygen, and interaction with other climate change stressors may counteract these findings. Finally, our review highlights the shortcomings and gaps in current understanding of the potential changes in nitrogen cycling under future climate and emphasises the need for further ecosystem studies. This article is protected by copyright. All rights reserved.
Direct measurements of marine dimethylsulfide (DMS) fluxes are sparse, particularly in the Southern Ocean. The Surface Ocean Aerosol Production (SOAP) voyage in February–March 2012 examined the distribution and flux of DMS in a biologically active frontal system in the southwest Pacific Ocean. Three distinct phytoplankton blooms were studied with oceanic DMS concentrations as high as 25 nmol L⁻¹. Measurements of DMS fluxes were made using two independent methods: the eddy covariance (EC) technique using atmospheric pressure chemical ionization–mass spectrometry (API-CIMS) and the gradient flux (GF) technique from an autonomous catamaran platform. Catamaran flux measurements are relatively unaffected by airflow distortion and are made close to the water surface, where gas gradients are largest. Flux measurements were complemented by near-surface hydrographic measurements to elucidate physical factors influencing DMS emission. Individual DMS fluxes derived by EC showed significant scatter and, at times, consistent departures from the Coupled Ocean–Atmosphere Response Experiment gas transfer algorithm (COAREG). A direct comparison between the two flux methods was carried out to separate instrumental effects from environmental effects and showed good agreement with a regression slope of 0.96 (r² = 0.89). A period of abnormal downward atmospheric heat flux enhanced near-surface ocean stratification and reduced turbulent exchange, during which GF and EC transfer velocities showed good agreement but modelled COAREG values were significantly higher. The transfer velocity derived from near-surface ocean turbulence measurements on a spar buoy compared well with the COAREG model in general but showed less variation. This first direct comparison between EC and GF fluxes of DMS provides confidence in compilation of flux estimates from both techniques, as well as in the stable periods when the observations are not well predicted by the COAREG model.
Iron, phosphate and nitrate are essential nutrients for phytoplankton growth and hence their supply into the surface ocean controls oceanic primary production. Here, we present a GEOTRACES zonal section (GP13; 30-33oS, 153oE-150oW) extending eastwards from Australia to the oligotrophic South Pacific Ocean gyre outlining the concentrations of these key nutrients. Surface dissolved iron concentrations are elevated at >0.4 nmol L-1 near continental Australia (west of 165°E) and decreased eastward to ≤0.2 nmol L-1 (170oW-150oW). The supply of dissolved iron into the upper ocean (<100m) from the atmosphere and vertical diffusivity averaged 11 ±10 nmol m-2 d-1. In the remote South Pacific Ocean (170oW-150oW) atmospherically sourced iron is a significant contributor to the surface dissolved iron pool with average supply contribution of 23 ± 17% (range 3% to 55%). Surface-water nitrate concentrations averaged 5 ±4 nmol L-1 between 170oW and 150oW whilst surface-water phosphate concentrations averaged 58 ±30 nmol L-1. The supply of nitrogen into the upper ocean is primarily from deeper waters (24-1647 μmol m-2 d-1) with atmospheric deposition and nitrogen fixation contributing <1% to the overall flux, in remote South Pacific waters. The deep water N:P ratio averaged 16 ±3 but declined to <1 above the deep chlorophyll maximum (DCM) indicating a high N:P assimilation ratio by phytoplankton leading to almost quantitative removal of nitrate. The supply stoichiometry for iron and nitrogen relative to phosphate at and above the DCM declines eastward leading to two biogeographical provinces: one with diazotroph production and the other without diazotroph production.
Direct measurements of marine DMS fluxes are sparse, particularly in the Southern Ocean. The Surface Ocean Aerosol Production (SOAP) voyage in February–March 2012 examined the distribution and flux of dimethylsulfide (DMS) in a biologically-active frontal system in the southwest Pacific Ocean. Three distinct phytoplankton blooms were studied with oceanic DMS concentrations as high as 25 nmol L−1. Measurements of DMS fluxes were made using two independent methods: the eddy covariance (EC) technique using API-CIMS chemical ionization mass spectrometry, and the gradient flux technique (GF) from an autonomous catamaran platform. Catamaran flux measurements are relatively unaffected by air flow distortion and are made close to the water surface where gas gradients are largest. Flux measurements were complemented by near-surface hydrographic measurements to elucidate physical factors influencing DMS emission. Individual DMS fluxes derived by EC showed significant scatter and, at times, consistent departures from the COARE gas exchange parameterization. A direct comparison between the two flux methods was carried out to separate instrumental effects from environmental effects, and showed good agreement with a regression slope of 0.96 (r² = 0.89). A period of abnormal downward atmospheric heat flux enhanced near-surface ocean stratification and reduced turbulent exchange, during which GF and EC transfer velocities showed good agreement but modelled COAREG values were significantly higher. The transfer velocity derived from near surface ocean turbulence measurements on a spar buoy compared well with the COAREG model in general, but showed less variation. This first direct comparison between EC and GF fluxes of DMS provides confidence in compilation of flux estimates from both techniques, and also in the stable periods when the observations are not well-predicted by the COAREG model.
The oceanic frontal region above the Chatham Rise east of New Zealand was investigated during the late austral summer season in February and March 2012. Despite its potential importance as a source of marine-originating and climate-relevant compounds, such as dimethyl sulfide (DMS) and its algal precursor dimethylsulfoniopropionate (DMSP), little is known of the processes fuelling the reservoirs of these sulfur (S) compounds in the water masses bordering the subtropical front (STF). This study focused on two opposing short-term fates of DMSP-S following its uptake by microbial organisms (either its conversion into DMS or its assimilation into bacterial biomass) and has not considered dissolved non-volatile degradation products. Sampling took place in three phytoplankton blooms (B1, B2, and B3) with B1 and B3 occurring in relatively nitrate-rich, dinoflagellate-dominated subantarctic waters, and B2 occurring in nitrate-poor subtropical waters dominated by coccolithophores. Concentrations of total DMSP (DMSPt) and DMS were high across the region, up to 160 and 14.5 nmol L⁻¹, respectively. Pools of DMSPt showed a strong association with overall phytoplankton biomass proxied by chlorophyll a (rs = 0.83) likely because of the persistent dominance of dinoflagellates and coccolithophores, both DMSP-rich taxa. Heterotrophic microbes displayed low S assimilation from DMSP (less than 5 %) likely because their S requirements were fulfilled by high DMSP availability. Rates of bacterial protein synthesis were significantly correlated with concentrations of dissolved DMSP (DMSPd, rs = 0.86) as well as with the microbial conversion efficiency of DMSPd into DMS (DMS yield, rs = 0.84). Estimates of the potential contribution of microbially mediated rates of DMS production (0.1–27 nmol L⁻¹ day⁻¹) to the near-surface concentrations of DMS suggest that bacteria alone could not have sustained DMS pools at most stations, indicating an important role for phytoplankton-mediated DMS production. The findings from this study provide crucial information on the distribution and cycling of DMS and DMSP in a critically under-sampled area of the global ocean, and they highlight the importance of oceanic fronts as hotspots of the production of marine biogenic S compounds.
Establishing the relationship between marine boundary layer (MBL) aerosols and surface water biogeochemistry is required to understand aerosol and cloud production processes over the remote ocean and represent them more accurately in earth system models and global climate projections. This was addressed by the SOAP (Surface Ocean Aerosol Production) campaign, which examined air–sea interaction over biologically productive frontal waters east of New Zealand. This overview details the objectives, regional context, sampling strategy and provisional findings of a pilot study, PreSOAP, in austral summer 2011 and the following SOAP voyage in late austral summer 2012. Both voyages characterized surface water and MBL composition in three phytoplankton blooms of differing species composition and biogeochemistry, with significant regional correlation observed between chlorophyll a and DMSsw. Surface seawater dimethylsulfide (DMSsw) and associated air–sea DMS flux showed spatial variation during the SOAP voyage, with maxima of 25 nmol L−1 and 100 µmol m−2 d−1, respectively, recorded in a dinoflagellate bloom. Inclusion of SOAP data in a regional DMSsw compilation indicates that the current climatological mean is an underestimate for this region of the southwest Pacific. Estimation of the DMS gas transfer velocity (kDMS) by independent techniques of eddy covariance and gradient flux showed good agreement, although both exhibited periodic deviations from model estimates. Flux anomalies were related to surface warming and sea surface microlayer enrichment and also reflected the heterogeneous distribution of DMSsw and the associated flux footprint. Other aerosol precursors measured included the halides and various volatile organic carbon compounds, with first measurements of the short-lived gases glyoxal and methylglyoxal in pristine Southern Ocean marine air indicating an unidentified local source. The application of a real-time clean sector, contaminant markers and a common aerosol inlet facilitated multi-sensor measurement of uncontaminated air. Aerosol characterization identified variable Aitken mode and consistent submicron-sized accumulation and coarse modes. Submicron aerosol mass was dominated by secondary particles containing ammonium sulfate/bisulfate under light winds, with an increase in sea salt under higher wind speeds. MBL measurements and chamber experiments identified a significant organic component in primary and secondary aerosols. Comparison of SOAP aerosol number and size distributions reveals an underprediction in GLOMAP (GLObal Model of Aerosol Processes)-mode aerosol number in clean marine air masses, suggesting a missing marine aerosol source in the model. The SOAP data will be further examined for evidence of nucleation events and also to identify relationships between MBL composition and surface ocean biogeochemistry that may provide potential proxies for aerosol precursors and production.
The future status of the surface ocean around New Zealand was projected using two Earth System Models and four emission scenarios. By 2100 mean changes are largest under Representative Concentration Pathway 8.5 (RCP8.5), with a +2.5°C increase in sea surface temperature, and decreases in surface mixed layer depth (15%), macronutrients (7.5–20%), primary production (4.5%) and particle flux (12%). Largest macronutrient declines occur in the eastern Chatham Rise and subantarctic waters to the south, whereas dissolved iron increases in subtropical waters. Surface pH projections, validated against subantarctic time-series data, indicate a 0.335 decline to ∼7.77 by 2100. However, projected pH is sensitive to future CO2 emissions, remaining within the current range under RCP2.6, but decreasing below it by 2040 with all other scenarios. Sub-regions vulnerable to climate change include the Chatham Rise, polar waters south of 50°S, and subtropical waters north of New Zealand, whereas the central Tasman Sea is least affected.
A series of semi-continuous incubation experiments were conducted with the coccolithophore Emiliania huxleyi strain NIWA1108 (Southern Ocean isolate) to examine the effects of five environmental drivers (nitrate and phosphate concentrations, irradiance, temperature, and partial pressure of CO2 (pCO2)) on both the physiological rates and elemental composition of the coccolithophore. Here, we report the alteration of the elemental composition of E. huxleyi in response to the changes in these environmental drivers. A series of dose–response curves for the cellular elemental composition of E. huxleyi were fitted for each of the five drivers across an environmentally representative gradient. The importance of each driver in regulating the elemental composition of E. huxleyi was ranked using a semi-quantitative approach. The percentage variations in elemental composition arising from the change in each driver between present-day and model-projected conditions for the year 2100 were calculated. Temperature was the most important driver controlling both cellular particulate organic and inorganic carbon content, whereas nutrient concentrations were the most important regulator of cellular particulate nitrogen and phosphorus of E. huxleyi. In contrast, elevated pCO2 had the greatest influence on cellular particulate inorganic carbon to organic carbon ratio, resulting in a decrease in the ratio. Our results indicate that the different environmental drivers play specific roles in regulating the elemental composition of E. huxleyi with wide-reaching implications for coccolithophore-related marine biogeochemical cycles, as a consequence of the regulation of E. huxleyi physiological processes.
Establishing the relationship between marine boundary layer (MBL) aerosols and surface water biogeochemistry over the remote ocean is required to understand aerosol and cloud production processes, and also represent them accurately in Earth System Models and global climate projections. This was addressed in the SOAP (Surface Ocean Aerosol Production) campaign, which examined air-sea interaction over biologically-productive frontal waters east of New Zealand. This overview details the objectives, regional context, sampling strategy, and provisional findings of a pilot study, PreSOAP, in austral summer 2011, and the following SOAP voyage in late austral summer 2012. Both voyages characterised surface water and MBL composition in three phytoplankton blooms of differing species composition and biogeochemistry, with significant regional correlation observed between chlorophyll-a and DMSsw. Surface seawater dimethylsulfide (DMSsw) and associated air-sea DMS flux showed spatial variation during the SOAP voyage, with maxima of 25 nmol L−1 and 100 µmol m−2 d−1, respectively, recorded in a dinoflagellate bloom. Inclusion of SOAP data in a regional DMSsw compilation confirmed that the current climatological mean is an underestimate for this region of the South-west Pacific. Estimation of the DMS gas transfer velocity (kDMS) by independent techniques of eddy covariance and gradient flux showed good agreement, although both exhibited periodic deviations from model estimates. Flux anomalies were related to surface warming and sea surface microlayer enrichment, and also reflected the heterogeneous distribution of DMSsw and the associated flux footprint. Other aerosol precursors measured included the halides and various volatile organic carbon compounds, with the first measurements of the short-lived gases glyoxal and methylglyoxal in pristine Southern Ocean marine air indicating an unidentified local source. The application of a real-time clean-sector, contaminant markers, and a common aerosol inlet facilitated multi-sensor measurement of uncontaminated air. Aerosol characterisation identified variable Aitken mode, and consistent sub-micron sized accumulation and coarse modes. Sub-micron aerosol mass was dominated by secondary particles containing ammonium sulfate/bisulfate under light winds, with an increase in sea-salt under higher wind-speeds. MBL measurements and chamber experiments identified a significant organic component in primary and secondary aerosols. Comparison of SOAP aerosol number and size distributions reveals an underprediction in GLOMAP-mode aerosol number in clean marine air masses, suggesting a missing marine aerosol source in the model. The SOAP data will be further examined for evidence of nucleation events, and also for relationships between MBL composition and surface ocean biogeochemistry with the aim of identifying potential proxies for aerosol precursors and production.
The oceanic frontal region above the Chatham Rise east of New Zealand was investigated during the late austral summer season in February and March 2012. Despite its potential importance as a source of marine-originating and climate-relevant compounds, such as dimethylsulfide (DMS) and its algal precursor dimethylsulfoniopropionate (DMSP), little is known of the processes fuelling the reservoirs of these sulfur (S) compounds in the water masses bordering the Subtropical Front (STF). This study focused on the two opposing fates of DMSP-S following its uptake by microbial organisms: either its conversion into DMS, or its assimilation into bacterial biomass. Sampling took place in three phytoplankton blooms (B1, B2 and B3) with B1 and B3 occurring in relatively nitrate-rich, dinoflagellate-dominated Subantarctic waters, and B2 occurring in nitrate-poor Subtropical waters dominated by coccolithophores. Concentrations of total DMSP (DMSPt) and DMS were high across the region, up to 160 nmol L−1 and 14.5 nmol−1, respectively. Pools of DMSPt measured in this study showed a strong association with overall phytoplankton biomass proxied by chlorophyll a (rs = 0.83) likely because of the persistent dominance of dinoflagellates and coccolithophores, both DMSP-rich taxa. Heterotrophic microbes displayed low S assimilation from DMSP (less than 5 %) likely because their S requirements were fulfilled by high DMSP availability. Rates of bacterial protein synthesis were significantly correlated with concentrations of dissolved DMSP (DMSPd, rs = 0.86) as well as with the microbial conversion efficiency of DMSPd into DMS (DMS yield, rs = 0.84). Estimates of the potential contribution of microbially-mediated rates of DMS production (0.1–27 nmol L−1 d−1) to the near-surface concentrations of DMS suggest that bacteria alone could not have sustained DMS pools at most stations, indicating an important role for phytoplankton-mediated DMS production. The findings from this study provide crucial information on the distribution and cycling of DMS and DMSP in a critically under-sampled area of the global ocean, and they highlight the importance of oceanic fronts as hotspots of the production of marine biogenic S compounds and as potential sources of aerosols particularly in regions of low anthropogenic perturbations such as the frontal waters of the Southern Hemisphere.
Extracellular bacterial enzymes play an important role in the degradation of organic matter in the surface ocean but are sensitive to changes in pH and temperature. This study tested the individual and combined effects of lower pH (-0.3) and warming (+3°C) projected for the year 2100 on bacterial abundance, process rates and diversity in plankton communities of differing composition from 4 locations east of New Zealand. Variation was observed in magnitude and temporal response between the different communities during 5 to 6 day incubations. Leucine aminopeptidase activity showed the strongest response, with an increase in potential activity under low pH alone and in combination with elevated temperature in 3 of 4 incubations. Temperature had a greater effect on bacterial cell numbers and protein synthesis, with stronger responses in the elevated temperature and combined treatments. However, the most common interactive effect between temperature and pH was antagonistic, with lower bacterial secondary production in the combined treatment relative to elevated temperature, and lower leucine aminopeptidase activity in the combined treatment relative to low pH. These results highlight the variability of responses to and interactions of environmental drivers, and the importance of considering these in experimental studies and prognostic models of microbial responses to climate change.
The flux of dimethylsulfide (DMS) to the atmosphere is generally inferred using water sampled at or below 2μm depth, thereby excluding any concentration anomalies at the air-sea interface. Two independent techniques were used to assess the potential for near-surface DMS enrichment to influence DMS emissions and also identify the factors influencing enrichment. DMS measurements in productive frontal waters over the Chatham Rise, east of New Zealand, did not identify any significant gradients between 0.01 and 6μm in sub-surface seawater, whereas DMS enrichment in the sea-surface microlayer was variable, with a mean enrichment factor (EF; the concentration ratio between DMS in the sea-surface microlayer and in sub-surface water) of 1.7. Physical and biological factors influenced sea-surface microlayer DMS concentration, with high enrichment (EF>1.3) only recorded in a dinoflagellate-dominated bloom, and associated with low to medium wind speeds and near-surface temperature gradients. On occasion, high DMS enrichment preceded periods when the air-sea DMS flux, measured by eddy covariance, exceeded the flux calculated using National Oceanic and Atmospheric Administration (NOAA) Coupled-Ocean Atmospheric Response Experiment (COARE) parameterized gas transfer velocities and measured sub-surface seawater DMS concentrations. The results of these two independent approaches suggest that air-sea emissions may be influenced by near-surface DMS production under certain conditions, and highlight the need for further study to constrain the magnitude and mechanisms of DMS production in the sea-surface microlayer.
Bacterial extracellular enzymes play a significant role in the degradation of labile organic matter and nutrient availability in the open ocean. Although bacterial production and extracellular enzymes may be affected by ocean acidification, few studies to date have considered the methodology used to measure enzyme activity and bacterial processes. This study investigated the potential artefacts in determining the response of bacterial growth and extracellular glucosidase and aminopeptidase activity to ocean acidification as well as the relative effects of three different acidification techniques. Tests confirmed that the observed effect of pH on fluorescence of artificial fluorophores, and the influence of the MCA fluorescent substrate on seawater sample pH, were both overcome by the use of Tris buffer. In experiments testing different acidification methods, bubbling with CO2 gas mixtures resulted in higher β-glucosidase activity and 15–40 % higher bacterial abundance, relative to acidification via gas-permeable silicon tubing and acid addition (HCl). Bubbling may stimulate carbohydrate degradation and bacterial growth, leading to the incorrect interpretation of the impacts of ocean acidification on organic matter cycling.
The flux of dimethylsulfide (DMS) to the atmosphere is generally inferred using water sampled at or below 2 m depth, thereby excluding any concentration anomalies at the air–sea interface. Two independent techniques were used to assess the potential for near-surface DMS enrichment to influence DMS emissions and also identify the factors influencing enrichment. DMS measurements in productive frontal waters over the Chatham Rise, east of New Zealand, did not identify any significant DMS gradients between 0.01 and 6 m in sub-surface seawater, whereas DMS enrichment in the sea-surface microlayer was variable, with a mean enrichment factor (EF; the concentration ratio between DMS in the SSM and in sub-surface water) of 1.7. Physical and biological factors influenced sea-surface microlayer DMS concentration, with high enrichment (EF > 1.3) only recorded in a dinoflagellate-dominated bloom, and associated with low to medium wind speeds and near-surface temperature gradients. On occasion, high DMS enrichment preceded periods when the air–sea DMS flux, measured by eddy covariance, exceeded the flux calculated using COARE parameterised gas transfer velocities and measured sub-surface seawater DMS concentrations. The results of these two independent approaches suggest that air–sea emissions may be influenced by near-surface DMS production under certain conditions, and highlights the need for further study to constrain the magnitude and mechanisms of DMS production in the sea surface microlayer.
Bacterial extracellular enzymes play a significant role in the degradation of labile organic matter and nutrient availability in the open ocean. Although bacterial production and extracellular enzymes may be affected by ocean acidification, few studies to date have considered the methodology used to measure enzyme activity and bacterial processes. This study investigated the potential artefacts in determining the response of bacterial extracellular glucosidase and aminopeptidase to ocean acidification, and the relative effects of three different acidification techniques. Tests confirmed that the fluorescence of the artificial fluorophores was affected by pH, and that addition of MCA fluorescent substrate alters seawater pH. In experiments testing different acidification methods, bubbling with CO2 gas mixtures resulted in higher β-glucosidase activity relative to acidification by their introduction via gas-permeable silicon tubing, or by acid addition (HCl). In addition, bacterial numbers were 15–40 % higher with bubbling relative to seawater acidified with gas-permeable silicon tubing and HCl. Bubbling may lead to overestimation of carbohydrate degradation and bacterial abundance, and consequently incorrect interpretation of the impacts of ocean acidification on organic matter cycling.
The domain of the surface ocean and lower atmosphere is a complex, highly dynamic component of the Earth system. Better understanding of the physics and biogeochemistry of the air-sea interface and the processes that control the exchange of mass and energy across that boundary define the scope of the Surface Ocean-Lower Atmosphere Study (SOLAS) project. The scientific questions driving SOLAS research, as laid out in the SOLAS Science Plan and Implementation Strategy for the period 2004-2014, are highly challenging, inherently multidisciplinary and broad. During that decade, SOLAS has significantly advanced our knowledge. Discoveries related to the physics of exchange, global trace gas budgets and atmospheric chemistry, the CLAW hypothesis (named after its authors, Charlson, Lovelock, Andreae and Warren), and the influence of nutrients and ocean productivity on important biogeochemical cycles, have substantially changed our views of how the Earth system works and revealed knowledge gaps in our understanding. As such SOLAS has been instrumental in contributing to the International Geosphere Biosphere Programme (IGBP) mission of identification and assessment of risks posed to society and ecosystems by major changes in the Earth́s biological, chemical and physical cycles and processes during the Anthropocene epoch. SOLAS is a bottom-up organization, whose scientific priorities evolve in response to scientific developments and community needs, which has led to the launch of a new 10-year phase. SOLAS (2015–2025) will focus on five core science themes that will provide a scientific basis for understanding and projecting future environmental change and for developing tools to inform societal decision-making.
Shallow CO2 vents are used as natural laboratories to study biological responses to ocean acidification, and so it is important to determine whether pH is the primary driver of bacterial processes and community composition, or whether other variables associated with vent water have a significant influence. Water from a CO2 vent (46m, Bay of Plenty, New Zealand), was compared to reference water from an upstream control site, and also to control water acidified to the same pH as the vent water. After 84 hours, both vent and acidified water exhibited higher potential bulk water and cell-specific glucosidase activity relative to control water, whereas cell-specific protease activities were similar. However, bulk vent water glucosidase activity was double that of the acidified water in both experiments, so too was bacterial secondary production at selected sampling points in experiment 1, suggesting that pH was not the only factor affecting carbohydrate hydrolysis. In addition there were significant differences in bacterial community composition in the vent water relative to the control and acidified water after 84 hours, including the presence of extremophiles which may influence carbohydrate degradation. This highlights the importance of characterising microbial processes and community composition in CO2 vent emissions, to confirm that they represent robust analogues for the future acidified ocean.
To fully understand the impact of ocean acidification on biogeochemical cycles, the response of bacterial extracellular enzymes needs to be considered as they play a central role in the degradation and distribution of labile organic matter. This study investigates the methodology, and potential artefacts involved in determining the response of bacterial extracellular glucosidase and protease to ocean acidification. The effect of pH on artificial fluorophores and substrates was examined, as well as the impact of three different acidification methods. The results indicate that pH has a significant effect on the fluorescence of the artificial fluorophore 4-methylumbeliferone for glucosidase activity, and 7-amino-4-methylcoumarin for protease activity, while artificial aminopeptidase substrate alters the pH of seawater, confirming previous observations. Before use in ocean acidification research these enzyme assay components must be buffered in order to stabilise sample pH. Reduction of coastal seawater pH to 7.8 was shown to increase β-glucosidase activity rapidly (0.5 h), while no significant response was detected for leucine aminopeptidase, highlighting the need for short-term direct effects of pH on enzyme activities. Bubbling with CO2 gas resulted in higher β-glucosidase activity when compared to acidification using gas-permeable silicon tubing and acidification with HCl. Although bubbling showed variable effects between two experiments conducted at different times of the year. In addition, bacterial cell numbers were 15–40% higher with bubbling relative to seawater acidified with gas-permeable silicon tubing and HCl. Artefacts associated with bubbling may lead to the overestimation of extracellular enzyme activities, and interpretation of the impacts of ocean acidification on organic matter cycling.
Air-sea dimethylsulfide (DMS) fluxes and bulk air-sea gradients were measured over the Southern Ocean in February-March 2012 during the Surface Ocean Aerosol Production (SOAP) study. The cruise encountered three distinct phytoplankton bloom regions, consisting of two blooms with moderate DMS levels, and a high biomass, dinoflagellate-dominated bloom with high seawater DMS levels (> 15 nM). Gas transfer coefficients were considerably scattered at wind speeds above 5 m s(-1). Bin averaging the data resulted in a linear relationship between wind speed and mean gas transfer velocity consistent with that previously observed. However, the wind-speed-binned gas transfer data distribution at all wind speeds is positively skewed. The flux and seawater DMS distributions were also positively skewed, which suggests that eddy covariance-derived gas transfer velocities are consistently influenced by additional, log-normal noise. A flux footprint analysis was conducted during a transect into the prevailing wind and through elevated DMS levels in the dinoflagellate bloom. Accounting for the temporal/spatial separation between flux and seawater concentration significantly reduces the scatter in computed transfer velocity. The SOAP gas transfer velocity data show no obvious modification of the gas transfer-wind speed relationship by biological activity or waves. This study highlights the challenges associated with eddy covariance gas transfer measurements in biologically active and heterogeneous bloom environments.
In the vast Low Nutrient Low-Chlorophyll (LNLC) Ocean, the vertical nutrient supply from the subsurface to the sunlit surface waters is low and atmospheric contribution of nutrients may be one order of magnitude greater over short timescales. The short turnover time of atmospheric Fe and N supply (<1 month for nitrate) further supports deposition being an important source of nutrients in LNLC regions. Yet, the extent to which atmospheric inputs are impacting biological activity and modifying the carbon balance in oligotrophic environments has not been constrained. Here, we quantify and compare the biogeochemical impacts of atmospheric deposition in LNLC regions using both a compilation of experimental data and model outputs. A metadata-analysis of recently conducted field and laboratory bioassay experiments reveals complex responses, and the overall impact is not a simple “fertilization effect” as observed in HNLC regions. Although phytoplankton growth may be enhanced, increases in bacterial activity and respiration result in weakening of biological carbon sequestration. The application of models using climatological or time-averaged non-synoptic deposition rates produced responses that were generally much lower than observed in the bioassay experiments. We demonstrate that experimental data and model outputs show better agreement on short timescale (days to weeks) when strong synoptic pulse of aerosols deposition, similar in magnitude to those observed in the field and introduced in bioassay experiments, is superimposed over the mean atmospheric deposition fields. These results suggest that atmospheric impacts in LNLC regions have been underestimated by models, at least at daily to weekly timescales, as they typically overlook large synoptic variations in atmospheric deposition and associated nutrient and particle inputs. Inclusion of the large synoptic variability of atmospheric input, and improved representation and parameterization of key processes that respond to atmospheric deposition, is required to better constrain impacts in ocean biogeochemical models. This is critical for understanding and prediction of current and future functioning of LNLC regions and their contribution to the global carbon cycle.
Air/sea dimethylsulfide (DMS) fluxes and bulk air/sea gradients were measured over the Southern Ocean in February/March 2012 during the Surface Ocean Aerosol Production (SOAP) study. The cruise encountered three distinct phytoplankton bloom regions, consisting of two blooms with moderate DMS levels, and a high biomass, dinoflagellate-dominated bloom with high seawater DMS levels (>15 nM). Gas transfer coefficients were considerably scattered at wind speeds above 5 m s−1. Bin averaging the data resulted in a linear relationship between wind speed and mean gas transfer velocity consistent with that previously observed. However, the wind speed-binned gas transfer data distribution at all wind speeds is positively skewed. The flux and seawater DMS distributions were also positively skewed, which suggests that eddy covariance-derived gas transfer velocities are consistently influenced by additional, log-normal noise. A~flux footprint analysis was conducted during a transect into the prevailing wind and through elevated DMS levels in the dinoflagellate bloom. Accounting for the temporal/spatial separation between flux and seawater concentration significantly reduces the scatter in computed transfer velocity. The SOAP gas transfer velocity data shows no obvious modification of the gas transfer-wind speed relationship by biological activity or waves. This study highlights the challenges associated with eddy covariance gas transfer measurements in biologically active and heterogeneous bloom environments.
During the 13 day Southern Ocean Iron RE-lease Experiment (SOIREE), dissolved iron concentrations decreased rapidly following each of three iron-enrichments, but remained high (>1 nM, up to 80% as FeII) after the fourth and final enrichment on day 8. The former trend was mainly due to dilution (spreading of iron-fertilized waters) and particle scavenging. The latter may only be explained by a joint production-maintenance mechanism; photoreduction is the only candidate process able to produce sufficiently high FeII, but as such levels persisted overnight (8 hr dark period) -ten times the half-life for this species- a maintenance mechanism (complexation of FeII) is required, and is supported by evidence of increased ligand concentrations on day 12. The source of these ligands and their affinity for FeII is not known. This retention of iron probably permitted the longevity of this bloom raising fundamental questions about iron cycling in HNLC (High Nitrate Low Chlorophyll) Polar waters.
The Tasman Sea and the adjacent subantarctic zone (SAZ) are economically important regions, where the parameters controlling the phytoplankton community composition and carbon fixation are not yet fully resolved. Contrasting nutrient distributions, as well as phytoplankton biomass, biodiversity and productivity were observed between the North Tasman Sea and the SAZ. In situ photosynthetic efficiency (FV/FM), dissolved and particulate nutrients, iron biological uptake, and nitrogen and carbon fixation were used to determine the factor-limiting phytoplankton growth and productivity in the North Tasman Sea and the SAZ. Highly productive cyanobacteria dominated the North Tasman Sea. High atmospheric nitrogen fixation and low nitrate dissolved concentrations indicated that non-diazotroph phytoplankton are nitrogen limited. Deck-board incubations also suggested that, at depth, iron could limit eukaryotes, but not cyanobacteria in that region. In the SAZ, the phytoplankton community was dominated by a bloom of haptophytes. The low productivity in the SAZ was mainly explained by light limitation, but nitrogen, silicic acid as well as iron were all depleted to the extent that they could become co-limiting. This study illustrates the challenge associated with identification of the limiting nutrient, as it varied between phytoplankton groups, depths and sites.
These SOAP project Pacific Ocean measurements reveal that phytoplankton blooms with sunny conditions make possible secondary organic contribution to ultrafine particles size and composition, and thus on cloud formation ability, and finally on climate. This is in agreement with other biologically active region observations about the presence of secondary organics even the exact fraction is also depending on the local marine life (e.g. plankton blooms, seaweeds, corals). An organic contribution is clearly needed to add to CLAW hypothesis.
A multi-disciplinary examination of the drivers of dissolved methane was carried out during a phytoplankton bloom located in a subtropical mesoscale eddy. This investigation related temporal signals in methane concentrations with other biophysical and biogeochemical parameters in the upper waters (<300 m) of the southwest Pacific Ocean. In the surface mixed layer, methane supersaturation increased and δ13CCH4 became more depleted coincident with increases in particulate dimethylsulfoniopropionate (DMSPp) and succession from the diatom Asterionellopsis glacialis to the nanoflagellate Phaeocystis globosa and the cyanobacterium Synechococcus sp. In situ methane production was calculated in a surface mixed layer methane budget that incorporated sea-to-air exchange and vertical diffusion. Methane concentrations increased in and below the mixed layer when the export of biogenic particles increased. Increased grazing of microbes by microzooplankton may have contributed to particle recycling (rich in organic carbon and DMSP) and increased the potential for methanogenesis. Phytoplankton species composition and biomass in different bloom phases, and eddy dynamics, were important determinants of methane saturation and emission, and the potential implications for methane are considered for the future surface ocean.
Three ocean acidification experiments were conducted on water from the same location in the Ross Sea, Southern Ocean, to ascertain how surface-water mixed populations, including the microbial community, would respond to changes in pH (pH 7.80 and 7.65). Bacterial extracellular enzymes, abundances, thymidine uptake rate, the diversity of the active fraction of the bacterial community and phytoplankton diversity were measured in response to changes in pH. Bacterial abundance increased at lower pH, and the active fraction of the bacteria decreased, concurrently becoming less diverse within 8 d. However, as the active fraction of the bacterial community evolved, changes in bacterial extracellular enzyme rates occurred, with phosphatase, beta-glucosidase and lipase activity increasing up to 2-fold in the acidified incubations. These results suggest that carbohydrates and lipids may be hydrolysed faster with more rapid regeneration of nutrients at lower pH. The changes observed in our experiments indicate that the bacteria in the Ross Sea adapt quickly to lower pH but that bacterial diversity will be lost. However, this loss of diversity did not adversely affect bacterial activity and in fact enhanced their ability to break down carbohydrates and lipids and recycle phosphate. These changes will alter the rate of carbon and phosphate regeneration, potentially accelerating decomposition in surface waters and short-circuiting the biological pump.
The Surface Ocean Aerosol Production (SOAP) study was undertaken in February/ March 2012 in the biologically active waters of the Chatham Rise, NZ. Aerosol hygroscopicity and volatility were examined with a volatility hygroscopicity tandem differential mobility analyser. These observations confirm results from other hygroscopicity-based studies that the dominant fraction of the observed remote marine particles were non-sea salt sulfates. Further observations are required to clarify the influences of seawater composition, meteorology and analysis techniques seasonally across different ocean basins.
We develop a tool to assist in identifying a link between naturally occurring aeolian dust deposition and phytoplankton response in the ocean. Rather than examining a single, or small number of dust deposition events, we take a climatological approach to estimate the likelihood of observing a definitive link between dust deposition and a phytoplankton bloom for the oceans proximal to the Australian continent. We use a dust storm index (DSI) to determine dust entrainment in the Lake Eyre Basin (LEB) and an ensemble of modelled atmospheric trajectories of dust transport from the basin, the major dust source in Australia. Deposition into the ocean is computed as a function of distance from the LEB source and the local over-ocean precipitation. The upper ocean's receptivity to nutrients, including dust-borne iron, is defined in terms of time-dependent, monthly climatological fields for light, mixed layer depth and chlorophyll concentration relative to the climatological monthly maximum. The resultant likelihood of a dust-phytoplankton link being observed is then mapped as a function of space and time. Our results suggest that the Southern Ocean (north of 45°S), the North West Shelf, and Great Barrier Reef are ocean regions where a rapid biological response to dust inputs is most likely to be observed. Conversely, due to asynchrony between deposition and ocean receptivity, direct causal links appear unlikely to be observed in the Tasman Sea and Southern Ocean south of 45°S.
The emerging research field of ocean acidification studies has gained international attention during the past years and recently defined international standards in the Guide to best practices for ocean acidification research and data reporting. However, a combination of ocean acidification studies with trace metal research is very rare and possible trace metal side effects on marine phytoplankton in ocean acidification incubation studies are often not assessed. Here we describe a trace metal clean, pH-controlled incubator system for laboratory and seagoing ocean acidification research. Seawater pH adjustment is achieved via passing CO2 gas through diffusive silicone tubing to minimize the risk of contamination and to avoid the negative mechanical effects of gas bubbles on phytoplankton. The system measures pH automatically with an accuracy of 0.004 and a precision of 0.001 and includes a feedback regulation to adjust pH during the incubation if required. Mn, Fe, Co, Ni, Cd, and Pb measurements show that our system and the pH adjustment method do not contaminate the samples with any of these metals. We tested this system in laboratory studies as well as during the PINTS voyage in the Tasman Sea.
Author Posting. © American Geophysical Union, 2005. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 110 (2005): C09S16, doi:10.1029/2004JC002601. Comparison of eight iron experiments shows that maximum Chl a, the maximum DIC removal, and the overall DIC/Fe efficiency all scale inversely with depth of the wind mixed layer (WML) defining the light environment. Moreover, lateral patch dilution, sea surface irradiance, temperature, and grazing play additional roles. The Southern Ocean experiments were most influenced by very deep WMLs. In contrast, light conditions were most favorable during SEEDS and SERIES as well as during IronEx-2. The two extreme experiments, EisenEx and SEEDS, can be linked via EisenEx bottle incubations with shallower simulated WML depth. Large diatoms always benefit the most from Fe addition, where a remarkably small group of thriving diatom species is dominated by universal response of Pseudo-nitzschia spp. Significant response of these moderate (10–30 μm), medium (30–60 μm), and large (>60 μm) diatoms is consistent with growth physiology determined for single species in natural seawater. The minimum level of “dissolved” Fe (filtrate < 0.2 μm) maintained during an experiment determines the dominant diatom size class. However, this is further complicated by continuous transfer of original truly dissolved reduced Fe(II) into the colloidal pool, which may constitute some 75% of the “dissolved” pool. Depth integration of carbon inventory changes partly compensates the adverse effects of a deep WML due to its greater integration depths, decreasing the differences in responses between the eight experiments. About half of depth-integrated overall primary productivity is reflected in a decrease of DIC. The overall C/Fe efficiency of DIC uptake is DIC/Fe ∼ 5600 for all eight experiments. The increase of particulate organic carbon is about a quarter of the primary production, suggesting food web losses for the other three quarters. Replenishment of DIC by air/sea exchange tends to be a minor few percent of primary CO2 fixation but will continue well after observations have stopped. Export of carbon into deeper waters is difficult to assess and is until now firmly proven and quite modest in only two experiments. This research was supported by the European Union through programs CARUSO (1998– 2001), IRONAGES (1999 –2003), and COMET (2000–2003); the Netherlands- Bremen Oceanography program NEBROC-1; and the Netherlands Organization for Research NWO through the Netherlands Antarctic Program project FePath. Both the U.S. National Science Foundation and the U.S. Department of Energy provided significant support for the SOFeX program. M.R.L. acknowledges the U.S. National Science Foundation for support of IronEx and SOFeX projects and related studies (OCE-9912230, -9911765, and -0322074).
The first Southern Ocean Iron RElease Experiment (SOIREE) was performed during February 1999 in Antarctic waters south of Australia (61°S, 140°E), in order to verify whether iron supply controls the magnitude of phytoplankton production in this high nutrient low chlorophyll (HNLC) region. This paper describes iron distributions in the upper ocean during our 13-day site occupation, and presents a pelagic iron budget to account for the observed losses of dissolved and total iron from waters of the fertilised patch. Iron concentrations were measured underway during daily transects through the patch and in vertical profiles of the 65-m mixed layer. High internal consistency was noted between data obtained using contrasting sampling and analytical techniques. A pre-infusion survey confirmed the extremely low ambient dissolved (0.1 nM) and total (0.4 nM) iron concentrations. The initial enrichment elevated the dissolved iron concentration to 2.7 nM. Thereafter, dissolved iron was rapidly depleted inside the patch to 0.2–0.3 nM, necessitating three re-infusions.
[1] The effects of physical processes on the distribution, speciation, and sources/sinks for Fe in a high-nutrient low-chlorophyll (HNLC) region were assessed during FeCycle, a mesoscale SF6 tracer release during February 2003 ( austral summer) to the SE of New Zealand. Physical mixing processes were prevalent during FeCycle with rapid patch growth ( strain rate gamma = 0.17 - 0.20 d(-1)) from a circular shape (50 km(2)) into a long filament of similar to 400 km(2) by day 10. Slippage between layers saw the patch-head overlying noninfused waters while the tail was capped by adjacent surface waters resulting in a SF6 maximum at depth. As the patch developed it entrained adjacent waters containing higher chlorophyll concentrations, but similar dissolved iron (DFe) levels, than the initial infused patch. DFe was low similar to 60 pmol L-1 in surface waters during FeCycle and was dominated by organic complexation. Nighttime measurements of Fe(II) similar to 20 pmol L-1 suggest the presence of Fe( II) organic complexes in the absence of an identifiable fast Fe(III) reduction process. Combining residence times and phytoplankton uptake fluxes for DFe it is cycled through the biota 140 - 280 times before leaving the winter mixed layer (WML). This strong Fe demand throughout the euphotic zone coupled with the low Fe: NO3- (11.9 mu mol: mol) below the ferricline suggests that vertical diffusion of Fe is insufficient to relieve chronic iron limitation, indicating the importance of atmospheric inputs of Fe to this region.
Dangerous climate change is best avoided by drastically and rapidly reducing greenhouse gas emissions. Nevertheless, geoengineering options are receiving attention on the basis that additional approaches may also be necessary. Here we review the state of knowledge on large-scale ocean fertilization by adding iron or other nutrients, either from external sources or via enhanced ocean mixing. On the basis of small-scale field experiments carried out to date and associated modelling, the maximum benefits of ocean fertilization as a negative emissions technique are likely to be modest in relation to anthropogenic climate forcing. Furthermore, it would be extremely challenging to quantify with acceptable accuracy the carbon removed from circulation on a long term basis, and to adequately monitor unintended impacts over large space and time-scales. These and other technical issues are particularly problematic for the region with greatest theoretical potential for the application of ocean fertilization, the Southern Ocean. Arrangements for the international governance of further field-based research on ocean fertilization are currently being developed, primarily under the London Convention/London Protocol.
An 11-d quasi-Lagrangian surface layer experiment to the east of Norfolk Island tracked a 140-m-deep drifting vertical array (DVA) of instruments, including conductivity sensors, thermistors, and current meters. These are the first in situ data from the area to measure large-amplitude internal waves. The observations show isotherm peak-to-peak excursions reaching 50 m and were dominated by the semidiurnal forcing frequency. The DVA exhibited horizontal loops at the semidiurnal tidal frequency as it tracked water at depths of 80-140 m to within 10% of total horizontal displacement. The large vertical isotherm excursions generated substantial vertical shear. Temperature microstructure estimates of the vertical diffusivity of scalar properties at 70-m depth, around the center of the thermocline, were on the order of 10(-4) m(2) s(-1) and around an order of magnitude larger at shallower depths. When combined with nitrate data, this implies vertical fluxes of nitrate in the pycnocline of similar to 1 mmol m(-2) d(-1). The internal waves also potentially cause an interaction between the variability in tidal forcing and the diurnal radiation cycle that influences chlorophyll in the deep chlorophyll maximum. The average effect of the internal wave was to increase the light level for a particular isotherm over the static case. The internal wave-induced velocities were strong enough to dominate phytoplankton rise speeds and so potentially played a role in the formation and persistence of an observed Trichodesmium bloom.
Nitrogen fixation by diazotrophic cyanobacteria is a critical source of new nitrogen to the oligotrophic surface ocean. Research to date indicates that some diazotroph groups may increase nitrogen fixation under elevated pCO2. To test this in natural plankton communities, four manipulation experiments were carried out during two voyages in the South Pacific (30–35oS). High CO2 treatments, produced using 750 ppmv CO2 to adjust pH to 0.2 below ambient, and ‘Greenhouse’ treatments (0.2 below ambient pH and ambient temperature +3 °C), were compared with Controls in trace metal clean deckboard incubations in triplicate. No significant change was observed in nitrogen fixation in either the High CO2 or Greenhouse treatments over 5 day incubations. qPCR measurements and optical microscopy determined that the diazotroph community was dominated by Group A unicellular cyanobacteria (UCYN-A), which may account for the difference in response of nitrogen fixation under elevated CO2 to that reported previously for Trichodesmium. This may reflect physiological differences, in that the greater cell surface area:volume of UCYN-A and its lack of metabolic pathways involved in carbon fixation may confer no benefit under elevated CO2. However, multiple environmental controls may also be a factor, with the low dissolved iron concentrations in oligotrophic surface waters limiting the response to elevated CO2. If nitrogen fixation by UCYN-A is not stimulated by elevated pCO2, then future increases in CO2 and warming may alter the regional distribution and dominance of different diazotroph groups, with implications for dissolved iron availability and new nitrogen supply in oligotrophic regions.
Context and purpose of this Report Card Increasing atmospheric CO2 concentration is causing increased absorption of CO2 by the world’s oceans, in turn driving a decline in seawater pH and changes in ocean carbonate chemistry that are collectively referred to as ocean acidification. Evidence is accumulating to suggest ocean acidification may directly or indirectly affect many marine organisms and ecosystems, some of which may also hold significant social and economic value to the Australian community. This report card aims to provide a brief overview of the current state of scientific knowledge regarding the process of ocean acidification; current and future projected levels of ocean acidification; and, observed and projected impacts of current and future predicted levels of ocean acidification on marine organisms and ecosystems in the region. This Report Card also briefly discusses potential social and economic implications, policy challenges, and the key knowledge gaps needing to be addressed.
This review focuses on critical issues in ocean–atmosphere exchange that will be addressed by new research strategies developed by the international Surface Ocean–Lower Atmosphere Study (SOLAS) research community. Eastern boundary upwelling systems are important sites for CO2 and trace gas emission to the atmosphere, and the proposed research will examine how heterotrophic processes in the underlying oxygen-deficient waters interact with the climate system. The second regional research focus will examine the role of sea-ice biogeochemistry and its interaction with atmospheric chemistry. Marine aerosols are the focus of a research theme directed at understanding the processes that determine their abundance, chemistry and radiative properties. A further area of aerosol-related research examines atmospheric nutrient deposition in the surface ocean, and how differences in origin, atmospheric processing and composition influence surface ocean biogeochemistry. Ship emissions are an increasing source of aerosols, nutrients and toxins to the atmosphere and ocean surface, and an emerging area of research will examine their effect on ocean biogeochemistry and atmospheric chemistry. The primary role of SOLAS is to coordinate coupled multi-disciplinary research within research strategies that address these issues, to achieve robust representation of critical ocean–atmosphere exchange processes in Earth System models.
During a Lagrangian study in the Mauritanian upwelling (April 15 to May 27 2009), we investigated the biogeochemical cycling of carbon monoxide (CO) through a series of experiments, field observations and a simple 1-D model. We carried out ten photochemical experiments in a solar simulator and a further dark experiment in order to determine the magnitude of the photochemical source and microbial sink for CO. Parallel irradiation experiments using a long-pass filter (390nm cutoff) showed that CO photoproduction was dominated by UV-light. Assuming that the apparent quantum yield (AQY) of CO photoproduction followed an exponential decrease with increasing wavelength, we applied a non-linear fit to derive the AQY for each of 10 experiments. The average AQY at 350nm from our irradiation experiments was 1.4×10−5mol CO produced per mol photons absorbed. One of our photochemical experiments showed a distinct dissolved organic matter absorption ‘shoulder’ in the UV-B, consistent with the presence of mycosporine-like amino acids. Though this feature was rapidly photobleached during irradiation, CO AQY did not differ from other experiments. During the microbial CO oxidation experiment we found first-order loss of CO with a rate constant of 0.17/h. We further used our experimental data to parameterise a 1-D steady state model which included photochemical and dark production of CO, microbial oxidation, mixing and sea to air loss. Our sea–air flux estimates were in the range of 4.6–9.6μmol CO per square metre per day. The estimated annual flux of CO to the atmosphere from the Mauritanian upwelling was 0.04TgCOyr−1.
To examine the controls of new nitrogen supply in oligotrophic waters, nitrogen fixation and nutrients were monitored during a quasi-Lagrangian study in surface waters of the north Tasman Sea during March-April 2006. The study was initiated coincident with a tropical cyclone, with high rainfall and winds that eroded surface layer density structure and increased surface phosphate and silicate concentrations by 50%. However, there was no corresponding increase in nitrate, as the enhanced vertical mixing perturbed the phosphacline but not the deeper nitracline. Dissolved iron was highest in the upper 20 m, and declined to a minimum at 100-150 m, indicating wet deposition as the source, as also inferred from the salinity decrease. Nitrogen fixation increased significantly by an order of magnitude over the following 9 d. In deckboard perturbation experiments immediately after the cyclone, phosphate and iron addition did not elicit a response in nitrogen fixation, whereas 8 d later, when in situ dissolved iron had declined, the addition of iron stimulated nitrogen fixation by 1.5-fold. Conversely, the addition of dust from Australia and the Gobi desert immediately after the cyclone increased nitrogen fixation by 2- and 4.5-fold, respectively. The observed 10-fold increase in nitrogen fixation in situ exceeded that reported in other nutrient perturbation experiments despite comparatively low in situ dissolved iron, indicating that iron bioavailability may be a critical factor. Although stimulation by a cyclone was unexpected, it promoted favorable conditions for diazotrophy by enhancing phosphate availability in the absence of nitrate, and increased dissolved iron supply and availability via wet deposition. This suggests that future increases in tropical cyclones, combined with other climate-driven trends, may increase nitrogen fixation in the north Tasman Sea.
Carbon monoxide (CO) apparent quantum yields (AQYs) are reported for a suite of riverine, estuarine and sea water samples, spanning a range of coloured dissolved organic matter (CDOM) sources, diagenetic histories, and concentrations (absorption coefficients). CO AQYs were highest for high CDOM riverine samples and almost an order of magnitude lower for low CDOM coastal seawater samples. A conservative mixing model predicted only 4% decreases in CO AQYs between the head and mouth of the estuary, whereas measured reductions in CO AQYs were between 47 and 80%, indicating that a highly photoreactive pool of terrestrial CDOM is lost during estuarine transit. The CDOM absorption coefficient (a) at 412 nm was identified as a good proxy for CO AQYs (linear regression r>20.8; n=12) at all CO AQY wavelengths studied (285, 295, 305, 325, 345, 365, and 423 nm) and across environments (high CDOM river, low CDOM river, estuary and coastal sea). These regressions are presented as empirical proxies suitable for the remote sensing of CO AQYs in natural waters, including open ocean water and were used to estimate CO AQY spectra and CO photoproduction in the Tyne estuary based upon annually averaged estuarine CDOM absorption data. Annual CO photoproduction in the Tyne was estimated to be between 1.38 and 3.57 metric tons of carbon per year, or 0.005 to 0.014% of riverine dissolved organic carbon (DOC) inputs to the estuary. Extrapolation of CO photoproduction rates to estimate total DOC photomineralisation indicate that less than 1% of DOC inputs are removed via photochemical processes during transit through the Tyne estuary.
The SOLAS air–sea gas exchange experiment (SAGE) was a multiple-objective study investigating gas-transfer processes and the influence of iron fertilisation on biologically driven gas exchange in high-nitrate low-silicic acid low-chlorophyll (HNLSiLC) Sub-Antarctic waters characteristic of the expansive subpolar zone of the southern oceans. This paper provides a general introduction and summary of the main experimental findings. The release site was selected from a pre-voyage desktop study of environmental parameters to be in the south-west Bounty Trough (46.5°S 172.5°E) to the south-east of New Zealand and the experiment was conducted between mid-March and mid-April 2004. In common with other mesoscale iron addition experiments (FeAX’s), SAGE was designed as a Lagrangian study, quantifying key biological and physical drivers influencing the air–sea gas exchange processes of CO2, DMS and other biogenic gases associated with an iron-induced phytoplankton bloom. A dual tracer SF6/³He release enabled quantification of both the lateral evolution of a labelled volume (patch) of ocean and the air–sea tracer exchange at tenths of kilometer scale, in conjunction with the iron fertilisation. Estimates from the dual-tracer experiment found a quadratic dependency of the gas exchange coefficient on windspeed that is widely applicable and describe air–sea gas exchange in strong wind regimes. Within the patch, local and micrometeorological gas exchange process studies (100 m scale) and physical variables such as near-surface turbulence, temperature microstructure at the interface, wave properties and windspeed were quantified to further assist the development of gas exchange models for high-wind environments.
Vessel-based observations of the oceanic surface layer during the 14-day 2004 SAGE ocean fertilization experiment were conducted using ADCP, CTD and temperature microstructure in a frame of reference moving with a patch of injected SF6 tracer. During the experiment the mixed layer depth zmld ranged between 50 and 80 m, with several re-stratifying events that brought zmld up to less than 40 m. These re-stratifying events were not directly attributable to local surface-down development of stratification and were more likely associated with horizontal variation in density structure. Comparison between the CTD and a one-dimensional model confirmed that the SAGE experiment was governed by 3-d processes. A new method for estimating zmld was developed that incorporates a component that is proportional to density gradient. This highlighted the need for well-conditioned near-surface data which are not always available from vessel-based survey CTD profiles. A centred-displacement scale, Lc, equivalent to the Thorpe lengthscale, reached a maximum of 20 m, with the eddy-centroid located at around 40 m depth. Temperature gradient microstructure-derived estimates of the vertical turbulent eddy diffusivity of scalar (temperature) material yielded bin-averaged values around 10−3 m2 s−1 in the pycnocline rising to over 10−2 m2 s−1 higher in the surface layer. This suggests transport rates of nitrate and silicate at the base of the surface layer generate mixed layer increases of the order of 38 and 13 mmol/m2/day, respectively, during SAGE. However, the variability in measured vertical transport processes highlights the importance of transient events like wind mixing and horizontal intrusions.
A dual tracer experiment was carried out during the SAGE experiment using the inert tracers SF6 and 3He, in order to determine the gas transfer velocity, k, at high wind speeds in the Southern Ocean. Wind speed/gas exchange parameterization is characterised by significant variability and we examine the major measurement uncertainties that contribute to that scatter. Correction for the airflow distortion over the research vessel, as determined by computational fluid dynamics (CFD) modelling, had the effect of increasing the calculated value of k by 30%. On the short time scales of such experiments, the spatial variability of the wind field resulted in differences between ship and satellite QuikSCAT winds, which produced significant differences in transfer velocity. With such variability between wind estimates, the comparison between gas exchange parameterizations from diverse experiments should clearly be made on the basis of the same wind product. Uncertainty in mixed layer depth of ∼10% arose from mixed layer deepening at high wind speed and limited resolution of vertical sampling. However the assumption of equal mixing of the two tracers is borne out by the experiment. Two dual tracer releases were carried out during SAGE, and showed no significant difference in transfer velocities using QuikSCAT winds, despite the differences in wind history. In the SAGE experiment, duration limitation on the development of waves was shown to be an important factor for Southern Ocean waves, despite the presence of long fetches.
The SOLAS Air–Sea Gas Exchange (SAGE) experiment was conducted in Sub-Antarctic waters off the east coast of the South Island of New Zealand in the late summer of 2004. This mesoscale iron enrichment experiment was unique in that chlorophyll a (chl a) and primary productivity were only 2× OUT stations values toward the end of the experiment and this enhancement was due to increased activity of non-diatomaceous species. In addition, this enhancement in activity appeared to occur without a significant build up of particulate organic carbon. Picoeukaryotes (
Measurements of pCO2, pH and alkalinity in the surface waters of an iron fertilised patch of sub-Antarctic water were made during SAGE (SOLAS SAGE: Surface-Ocean Lower Atmosphere Studies Air–Sea Gas Experiment). The iron addition induced a minor phytoplankton bloom, however the patch dynamics were dominated by physical processes which suppressed and masked the biological effects. The Lagrangian nature of the experiment allowed the carbonate chemistry in the patch to be followed for 15.5 days, and the relative importance of the biological and physical factors influencing the surface water pCO2 was estimated. The pCO2 of the surface waters of the patch increased from 327 μatm prior to iron addition to 338 μatm on Day 14, effects of vertical and horizontal mixing offset the 15 μatm drawdown that would have occurred had the induced biological uptake been the sole factor to influence the pCO2. The air–sea carbon flux calculated using the measured skin temperature and a piston velocity parameterisation determined during SAGE (Ho et al., 2006) was 98.5% of the flux determined using conventional bulk temperature measurement and the Wanninkhof (1992) piston velocity parameterisation. The skin temperature alone contributed to an 8% increase in the flux compared with that determined using bulk temperature.
An in situ iron addition experiment (SAGE) was carried out in high-nitrate low-chlorophyll low-silicic acid (HNLCLSi) sub-Antarctic surface waters south-east of New Zealand. In contrast to other iron addition experiments, the phytoplankton response was minor, with a doubling of biomass relative to surrounding waters, with the temporal trends in dissolved iron and macronutrients instead dominated by physical factors such as mixing and dilution. The initial increase in patch surface area indicated a lateral dilution rate of 0.125 d−1, with a second estimate from a model of the decline in peak SF6 concentration yielding a higher lateral dilution rate of 0.16–0.25 d−1. The model was tested on the SOIREE SF6 dataset and provided a lateral dilution of 0.07 d−1, consistent with previous published estimates. MODIS ocean colour images showed elevated chlorophyll coincident with the SF6 patch on day 10 and 12, and an elevated chlorophyll filament at the SAGE experiment location 3–4 days after ship departure, which provided additional lateral dilution estimates of 0.19 and 0.128 d−1. Dissolved iron at the patch centre declined by 85% within two days of the initial infusion, of which dilution accounted for 50–65%; it also decreased rapidly after the 2nd and 3rd infusions but remained elevated after the fourth infusion. Despite decreases in nitrate and silicic acid from day 7 and 10, respectively, the final nutrient concentrations in the patch exceeded the initial concentrations due to supply from lateral intrusion and mixed-layer deepening. The low Si:N loss ratio suggested that the observed limited response to iron was primarily by non-siliceous phytoplankton. Algal growth rate exceeded the minimum dilution rate during two periods (days 3–6 and 10–14), and coincided with net chlorophyll accumulation. However, as the ratio of algal growth to dilution was the lowest reported for an iron addition experiment, dilution was clearly a significant factor in the SAGE experiment recording the lowest phytoplankton response to mesoscale iron addition.
Carbon monoxide (CO) apparent quantum yields (AQYs) are reported for a suite of riverine, estuarine and sea water samples, spanning a range of coloured dissolved organic matter (CDOM) sources, diagenetic histories, and concentrations (absorption coefficients). CO AQYs were highest for high CDOM riverine samples and almost an order of magnitude lower for low CDOM coastal seawater samples. CO AQYs were between 47 and 80% lower at the mouth of the estuary than at its head. Whereas, a conservative mixing model predicted only 8 to 14% decreases in CO AQYs between the head and mouth of the estuary, indicating that a highly photoreactive pool of terrestrial CDOM is lost during estuarine transit. The CDOM absorption coefficient ( a ) at 412 nm was identified as a good proxy for CO AQYs (linear regression r <sup>2</sup> > 0.8; n = 12) at all CO AQY wavelengths studied (285, 295, 305, 325, 345, 365, and 423 nm) and across environments (high CDOM river, low CDOM river, estuary and coastal sea). These regressions are presented as empirical proxies suitable for the remote sensing of CO AQYs in natural waters, including open ocean water, and were used to estimate CO AQY spectra and CO photoproduction in the Tyne estuary based upon annually averaged estuarine CDOM absorption data. A minimum estimate of annual CO production was determined assuming that only light absorbed by CDOM leads to the formation of CO and a maximum limit was estimated assuming that all light entering the water column is absorbed by CO producing photoreactants (i.e. that particles are also photoreactive). In this way, annual CO photoproduction in the Tyne was estimated to be between 0.99 and 3.57 metric tons of carbon per year, or 0.004 to 0.014% of riverine dissolved organic carbon (DOC) inputs to the estuary. Extrapolation of CO photoproduction rates to estimate total DOC photomineralisation indicate that less than 1% of DOC inputs are removed via photochemical processes during transit through the Tyne estuary.
Cold seeps are widely distributed on active and passive margins and display considerable temporal variability in terms of gas and fluid expulsion rates and volume, over scales of hours to days. To constrain this variability, two cold seeps, the North and South Tower, located at the Wairarapa Seep site on Opouawe Bank, Cook Strait, New Zealand, were monitored in an integrated geological and biogeochemical study during eight surveys between 2005 and 2008. To ensure sampling of the water column within the flare overlying the seeps the vessel was manoeuvred in response to the backscatter data from a hull-mounted 38 kHz single-beam echo-sounder. High methane concentrations (> 100 nmol l− 1) were found within 400–450 m-high flares at both sites, and within an 825 m-high flare at a newly identified seep, southeast of Pahaua Bank. Elevated bottom water methane was also found over newly identified mud volcanoes on the Campbell Bank, Cook Strait. The locations of the seeps appear to be structurally controlled with active methane vents occurring on anticlinal ridges and associated with a 30 m-high normal fault scarp on Opouawe Bank. Fluid flow is facilitated by sedimentary layer permeability properties, structural focusing and dilational fracturing on the crest of a large anticlinal ridge, cored by a seaward-verging reverse fault. The South Tower cold seep did not significantly influence bulk sediment and water column particulates and nutrients, although high bacterial biomass, productivity and methane oxidation rates were associated with elevated methane in the flare. The seep flares exhibited depleted δ13C–CH4 values (− 63 to − 70‰), characteristic of shallow biogenic gas sources. Incorporation of the δ13C–CH4 and CH4 concentration data in a mixing model, and comparison of the water residence time with methane oxidation rates, indicated that the primary fate of methane in the flare was dilution. The dissolved methane originating from the seeps was retained in the water column at depths > 700 m, with minimal contribution to atmospheric emissions, as confirmed by surface methane mapping. Nevertheless, the South Tower seep was a significant source of methane with a column-integrated burden below 700 m of 5.6 mmol m− 2, and an estimated annual emission of 0.5–1 × 106 mol CH4. The structural and tectonic setting of the Wairarapa Seep provides a useful analogue model for other convergent margin cold seeps on the Hikurangi Margin and elsewhere. This unique, three-year integrated study provided valuable insights into the linkage between geological structure and overlying water column biogeochemistry, and confirmed the value of this cold seep site for long-term monitoring.
Large-scale (>40 000 km2, >1 yr) ocean iron fertilization (OIF) is being considered as an option for mitigating the increase in atmospheric CO2 concentrations. However OIF will influence trace gas production and atmospheric emissions, with consequences over broad temporal and spatial scales. To illustrate this, the response of nitrous oxide (N 2O) and dimethylsulphide (DMS) in the mesoscale iron addition experiments (FeAXs) and model scenarios of large-scale OIF are examined. FeAXs have shown negligible to minor increases in N2O production, whereas models of long-term OIF suggest significant N2O production with the potential to offset the benefit gained by iron-mediated increases in CO 2 uptake. N2O production and emission will be influenced by the magnitude and rate of vertical particle export, and along-isopycnal N2O transport will necessitate monitoring over large spatial scales. The N2O-O2 relationship provides a monitoring option using oxygen as a proxy, with spatial coverage by Argo and glider-mounted oxygen optodes. Although the initial FeAXs exhibited similar increases (1.5- to 1.6-fold) in DMS, a subsequent sub-arctic Pacific experiment observed DMS consumption relative to unfertilized waters, highlighting regional variability as a complicating factor when predicting the effects of large-scale OIF. DMS cycling and its influence on atmospheric composition may be studied using naturally occurring blooms and be constrained prior to OIF by pre-fertilization spatial mapping and aerial sampling using new technologies. As trace gases may have positive or negative synergistic effects on atmospheric chemistry and climate forcing, the net effect of altered trace gas emissions needs to be considered in both models and monitoring of large-scale OIF.
We report the results of an experiment in the Northeast Atlantic in which sulphur hexafluoride (SF6) was released within an eddy and the behaviour of trace gases, nutrients and productivity followed within a Lagrangian framework over a period of 24 days. Measurements were also made in the air above the eddy in order to estimate air–sea exchange rates for some components. The physical, biological and biogeochemical properties of the eddy resemble those of other eddies studied in this area, suggesting that the results we report may be applicable beyond the specific eddy studied. During a period of low wind speed at the start of the experiment, we are able to quantitatively describe and balance the nutrient and carbon budgets for the eddy. We also report concentrations of various trace gases in the region which are similar to those observed in other studies and we estimate exchange rates for several trace gases. We show that the importance of gas exchange over other loss terms varies with time and also varies for the different gases. We show that the various trace gases considered (CO2, dimethyl sulphide (DMS), N2O, CH4, non-methane-hydrocarbons, methyl bromide, methyl iodide and volatile selenium species) are all influenced by physical and biological processes, but the overall distribution and temporal variability of individual gases are different to one another. A storm disrupted the stratification in the eddy during the experiment, resulting in enhanced nutrient supply to surface waters, enhanced gas exchange rates and a change in plankton community, which we quantify, although overall productivity was little changed. Emphasis is placed on the regularity of storms in the temperate ocean and the importance of these stochastic processes in such systems.
Aqueous solutions of humic substances (HSs) and pure monomeric aromatics were irradiated to investigate the chemical controls upon carbon monoxide (CO) photoproduction from dissolved organic matter (DOM). HSs were isolated from lakes, rivers, marsh, and ocean. Inclusion of humic, fulvic, hydrophobic organic, and hydrophilic organic acid fractions from these environments provided samples diverse in source and isolation protocol. In spite of these major differences, HS absorption coefficients (a) and photoreactivities (a bleaching and CO production) were strongly dependent upon HS aromaticity (r2 > 0.90; n = 11), implying aromatic moieties are the principal chromophores and photoreactants within HSs, and by extension, DOM. Carbonyl carbon and CO photoproduction were not correlated, implying that carbonyl moieties are not quantitatively important in CO photoproduction. CO photoproduction efficiency of aqueous solutions of monomeric aromatic compounds that are common constituents of organic matter varied with the nature of ring substituents. Specifically, electron donating groups increased, while electron withdrawing groups decreased CO photoproductivity, supporting our conclusion that carbonyl substituents are not quantitatively important in CO photoproduction. Significantly, aromatic CO photoproduction efficiency spanned 3 orders of magnitude, indicating that variations in the CO apparent quantum yields of natural DOM may be related to variations in aromatic DOM substituent group chemistry.
We report the results of an experiment in the Northeast Atlantic in which sulphur hexafluoride (SF6) was released within an eddy and the behaviour of trace gases, nutrients and productivity followed within a Lagrangian framework over a period of 24 days. Measurements were also made in the air above the eddy in order to estimate air–sea exchange rates for some components. The physical, biological and biogeochemical properties of the eddy resemble those of other eddies studied in this area, suggesting that the results we report may be applicable beyond the specific eddy studied. During a period of low wind speed at the start of the experiment, we are able to quantitatively describe and balance the nutrient and carbon budgets for the eddy. We also report concentrations of various trace gases in the region which are similar to those observed in other studies and we estimate exchange rates for several trace gases. We show that the importance of gas exchange over other loss terms varies with time and also varies for the different gases. We show that the various trace gases considered (CO2, dimethyl sulphide (DMS), N2O, CH4, non-methane-hydrocarbons, methyl bromide, methyl iodide and volatile selenium species) are all influenced by physical and biological processes, but the overall distribution and temporal variability of individual gases are different to one another. A storm disrupted the stratification in the eddy during the experiment, resulting in enhanced nutrient supply to surface waters, enhanced gas exchange rates and a change in plankton community, which we quantify, although overall productivity was little changed. Emphasis is placed on the regularity of storms in the temperate ocean and the importance of these stochastic processes in such systems.
During May 2001 and May 2002, the structure and function of the microbial community within and outside the Cyprus quasi-stationary warm-core eddy in the Levantine Basin of the eastern Mediterranean was studied down to the depth of the bathypelagic layer. We present here the detailed description of the microbial food web in one of the most oligotrophic and P-starved marine systems on earth. The isothermal layer was at the depth between 20 and 260/300 m at the core of the eddy, and between 20 and 100/110 m outside. Nitrate and phosphate were found at higher concentration between 100 and 500/800 m outside the eddy compared within the core of the eddy, but the vertical diffusive flux of nitrate and phosphate across the pycnocline was higher within the core of the eddy. There were only minor differences in microbial abundance in the euphotic layers of the two sites. It is suggested that the differences in the areal supply of nutrients to the isothermal layer, between the two sites, resulted in essentially a similar volumetric supply of nutrients to the euphotic layer. This suggests that the results of this study can be applied to describe the microbial food web within the euphotic layer over the larger area of the Levantine Basin, which exhibits ultra-oligotrophic and P-starved conditions. Primary production and abundances of the microbial community were somewhat higher in May 2001 than in May 2002, possibly because of higher nutrient fluxes in the euphotic layer, which are probably the result of deeper winter mixing in 2001, although a later onset of winter mixing or increased dust supply could not be discounted. In the euphotic layer, heterotrophs (bacteria, heterotrophic nanoflagellates (HNF), and ciliates) dominated (60–70%) the microbial carbon biomass. Heterotrophic ciliates were found to be much more abundant in the upper 50 m of the water column, while no consistent pattern was found for bacteria and HNF throughout the euphotic layer. Autotrophs showed a maximum distribution at the deep chlorophyll maximum found between 100 and 130 m. In the euphotic layer, the relationships between biomass and production for phytoplankton and bacteria suggested a higher top-down control on the phytoplankton in the upper ∼50 m and a consistently tight top-down control on the bacterial biomass throughout the euphotic layer. The phosphate addition experiment in the Cyprus Eddy suggests that the close predator–prey relationships within the microbial heterotrophic community were required for the rapid transfer of a limiting element to higher trophic levels without biomass oscillations in the P-fertilized surface mixed layers (0–20 m). The results from the unmodified system in this study suggested that the rapid P transfer mechanisms would function only in the upper ∼50 m, while the element transfer would be based on a predator–prey relationship with more conceivable biomass oscillations in the deeper waters (∼100–160 m) of the euphotic layer. In the Mediterranean Sea, nutrient concentrations, POC export, and integrated chlorophyll and primary production all tend to decrease toward the east. Our results together with a literature survey showed that abundances of the microbial components in the euphotic layer were not consistently lower in the study area than in the northwestern Mediterranean and that abundances of bacteria and HNF found in the mesopelagic and bathypelagic layers of the study area were within the reported ranges and quite similar to those found in the northwestern Mediterranean. This suggests that the oligotrophic status and the low export production are not reflected in the abundance of the microbial components down to the bathypelagic layer of the eastern Mediterranean.
Since the mid-1980s, our understanding of nutrient limitation of oceanic primary production has radically changed. Mesoscale iron addition experiments (FeAXs) have unequivocally shown that iron supply limits production in one-third of the world ocean, where surface macronutrient concentrations are perennially high. The findings of these 12 FeAXs also reveal that iron supply exerts controls on the dynamics of plankton blooms, which in turn affect the biogeochemical cycles of carbon, nitrogen, silicon, and sulfur and ultimately influence the Earth climate system. However, extrapolation of the key results of FeAXs to regional and seasonal scales in some cases is limited because of differing modes of iron supply in FeAXs and in the modern and paleo-oceans. New research directions include quantification of the coupling of oceanic iron and carbon biogeochemistry.
The Lagrangian Southern Ocean Iron Release Experiment (SOIREE) allowed study of a gradually evolving iron-mediated phytoplankton bloom in water labelled with the inert tracer sulfur hexafluoride, SF6. This article describes a pelagic carbon budget for the mixed layer in SOIREE and assesses the extent to which closure of the budget is achieved. Net community production (NCP) converted 837mmolm−2 of inorganic carbon to organic carbon in 12.0d after the first iron addition. A large fraction (41%) of NCP remained as particulate organic carbon in the mixed layer of the iron-enriched patch, while 23% was lost by horizontal dispersion and 0–29% was exported. The closure of the carbon budget is hampered by the lack of measurements of dissolved organic carbon (DOC), by a major uncertainty in carbon export, and by use of empirical conversion factors in estimates of carbon biomass and metabolic rates. Lagrangian carbon-budget studies may be improved by direct measurement of all major carbon parameters and conversion factors. Carbon cycling in the SOIREE bloom resembled that in ‘natural’ algal blooms in the open Southern Ocean in some respects, but not in all. Daily NCP in the SOIREE bloom (70mmolm−2d−1) was higher than in natural blooms, partly because other studies did not account for horizontal dispersion, were for longer periods or included less productive areas. The build-up of POC stock and carbon export as a fraction of NCP in SOIREE were in the lower range of observations elsewhere.
Surface seawater fugacity of carbon dioxide (fCO2) was measured during the Subarctic Ecosystem Response to Iron Enrichment Study (SERIES), July 9–August 5, 2002. Three ships sampled the iron-fertilized waters near Ocean Station P (50°N, 145°W): the Canadian CCGS J.P. Tully (July 9–23, 2002), the chartered Mexican M/V El Puma (July 9–28, 2002), and the Japanese fisheries research ship M/V Kaiyo Maru (July 24–August 5, 2002). Data used here are from the CCGS J.P. Tully and the M/V Kaiyo Maru. From the onset of the experiment to the peak of the iron-induced diatom bloom on day 19, sea-surface fCO2 decreased from 350 to 265 μatm and average DIC concentration in the upper 30 m decreased from 2030 to 1990 μmol kg−1. Changes in fCO2 in and near the iron patch as observed from the CCGS J.P. Tully and later from the M/V Kaiyo Maru were used to estimate CO2 drawdown and air-sea fluxes, and in generating a carbon budget during the growth phase (days 3–19) of the experiment. Without considering patch dilution, sources of dissolved inorganic carbon to the patch (1.6±0.25 mol m−2) were nearly double the sum (0.87±0.34 mol m−2) of the sinks: accumulations of dissolved organic and particulate carbon, and the flux of particulate carbon to sediment traps below the patch. However, the budget is balanced after considerations of the effects of patch expansion on property concentrations within the patch. A comparison with other iron fertilization experiments from 1995 to present was made to assess the CO2 drawdown values.
An in situ mesoscale iron-fertilisation experiment in the eastern sub-Arctic Pacific (SERIES) was undertaken to test the Iron Hypothesis, that increasing iron supply would stimulate phytoplankton production and particulate organic carbon (POC) export to deep water. Patch dispersion was monitored for 26 days, using an inert tracer (SF6) and biological tracers (chlorophyll-a and fCO2), and we examine the vertical and lateral evolution of the patch, and the influence of dilution on the biological and biogeochemical response to iron addition. Vertical dispersion of the added iron was initially restricted to the upper 12 m by near-surface stratification, although the vertical flux to the lower mixed layer at this time significantly exceeded the unperturbed rate of iron supply. Calculation of vertical diffusion rates (Kz) provided an estimate of the unperturbed Fe flux across the seasonal pycnocline of 0.3–1.5 nmol/m2/d. The iron/tracer patch partially advected around an anticyclonic Haida Eddy that originated off the west coast of Canada in 1999–2000. Lateral patch evolution was initially dominated by current strain, stretching it into a filament of ∼300 km2 by day 11 and reaching a maximum area of 1300 km2 by day 23. Sustained high winds and intrusion of external waters between days 11 and 18 altered patch geometry and advection. Two scenarios for patch evolution are presented of a single exponential dilution at 0.1/d, and a variable dilution in which dilution increased from 0.078/d to 0.16/d (days 11–18) before decreasing to 0.05/d. Dilution rates were used to constrain dissolved iron dynamics, with iron regeneration rates indirectly estimated from biological iron uptake and lateral dilution losses. Lateral entrainment supplied ∼6–7 μmol/L silicic acid and 4.6 μmol/L nitrate to the patch centre by day 20, equivalent to 37% and 45%, respectively, of total biological uptake. Indirect estimates of phytoplankton nitrate uptake from patch dilution indicated a maximum rate of 1.4 μmol/L/d, in agreement with measured rates. The cumulative entrainment of 392–500 mmol dissolved inorganic carbon (DIC)/m2 at the patch centre by day 20 was of the same order as the total biological DIC uptake and POC accumulation. The potential impacts of a mid-experiment increase in dilution were explored; these included elevated entrainment of silicic acid when concentrations in the patch were growth limiting for phytoplankton, and decreased cell aggregation. Both factors could potentially have delayed the onset of bloom termination and export, and increased the longevity of the SERIES phytoplankton bloom.
Phosphate addition to surface waters of the ultraoligotrophic, phosphorus-starved eastern Mediterranean in a Lagrangian experiment caused unexpected ecosystem responses. The system exhibited a decline in chlorophyll and an increase in bacterial production and copepod egg abundance. Although nitrogen and phosphorus colimitation hindered phytoplankton growth, phosphorous may have been transferred through the microbial food web to copepods via two, not mutually exclusive, pathways: (i) bypass of the phytoplankton compartment by phosphorus uptake in heterotrophic bacteria and (ii) tunnelling, whereby phosphate luxury consumption rapidly shifts the stoichiometric composition of copepod prey. Copepods may thus be coupled to lower trophic levels through interactions not usually considered.
Carbon monoxide (CO) atmospheric mixing ratios and surface-water concentrations were determined during Atlantic Meridional Transect cruise number 10, April–May 2000. Atmospheric CO increased from south (mean=74±9 ppbv) to north (mean=151±19 ppbv) with a steep increase around the intertropical convergence zone. Surface-water CO (0.2–2.6 nmol L−1) showed pronounced diurnal variations with afternoon maxima exceeding pre-dawn minima 5–7 fold. Modest regional variations, as indicated by maximum daily CO concentrations, were also observed. Highest CO maxima occurred at ∼11.5°N, where high solar irradiance was combined with elevated coloured dissolved organic matter (CDOM) levels and modest winds, while lowest CO maxima occurred during periods of high winds and lowest solar irradiance near the western European margin at 45°N. Atlantic Ocean CO emissions were estimated to be 1.5±1.1 Tg CO-C yr−1 based on near-instantaneous atmospheric CO, sea-surface CO and windspeeds from the cruise. However, as spatial and temporal variability in both terms was considered to be unique to the timing and path of the cruise, the mean Atlantic diel cycle of sea-surface CO concentration was estimated by pooling all cruise data into 1-h sections, yielding a mean of 0.94 nmol L−1; and diurnal variations from 0.4 to 1.6 nmol L−1. Using the mean diurnal cycle, the Atlantic and global open-ocean sources of CO to the atmosphere were estimated to be 0.9±0.6 and 3.7±2.6 Tg CO-C yr−1, respectively. Therefore it is our contention that IPCC-2001 (Prather, M., Ehhalt, D., Dentener, F., Derwent, R., Dlugokencky, E., Holland, E., Isaksen, I., Katima, J., Kirchhoff, V., Matson, P., Midgley, P., Wang, M., 2001. Chapter 4: Atmospheric chemistry and greenhouse gases. In: Houghton, J.T., Ding, Y., Griggs, D.J., Noguer, M., van der Linden, P.J., Dai, X., Maskell, K., Johnson, C.A. (Eds.), Climate Change 2001: The Scientific Basis. Contribution of working group 1 to the third assessment report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, pp. 239–287; 21 Tg CO-C yr−1) overestimates the source of atmospheric CO from the global ocean by around 5 fold.
Sunlight-initiated photolysis of chromophoric dissolved organic matter (CDOM) is the dominant source of carbon monoxide (CO) in the open-ocean. A modelling study was conducted to constrain this source. Spectral solar irradiance was obtained from two models (GCSOLAR and SMARTS2). Water-column CDOM and total light absorption were modelled using spectra collected along a Meridional transect of the Atlantic ocean using a 200-cm pathlength liquid waveguide UV-visible spectrophotometer. Apparent quantum yields for the production of CO (AQYCO) from CDOM were obtained from a parameterisation describing the relationship between CDOM light absorption coefficient and AQYCO and the CDOM spectra collected. The sensitivity of predicted rates to variations in model parameters (solar irradiance, cloud cover, surface-water reflectance, CDOM and whole water light absorbance, and AQYCO) was assessed. The model's best estimate of open-ocean CO photoproduction was 47±7 Tg CO-C yr−1, with lower and upper limits of 38 and 84 Tg CO-C yr−1, as indicated by sensitivity analysis considering variations in AQYs, CDOM absorbance, and spectral irradiance. These results represent significant constraint of open-ocean CO photoproduction at the lower limit of previous estimates. Based on these results, and their extrapolation to total photochemical organic carbon mineralisation, we recommend a downsizing of the role of photochemistry in the open-ocean carbon cycle.
We report over 600 absorption spectra (250–800 nm) of unfiltered surface waters, <250 m depth, collected at daily stations during three Atlantic Meridional Transect cruises (AMT9, 10, 11) between the UK and Uruguay. AMT cruises 9 and 11 were southbound (15 September 1999–13 October 1999 and 12 September 2000–11 October 2000, respectively), while AMT10 was northbound (12 April 2000–8 May 2000). Absorption coefficients at 300 nm, a300, ranged from 0.13 to 1.32 m−1, showed insignificant differences between filtered and unfiltered samples, and were therefore attributed to chromophoric dissolved organic matter (CDOM). A non-linear single exponential regression provided the best fit to our CDOM absorbance spectra and was used to parameterise the spectral slope, S, of the monotonic absorbance decrease with increasing wavelength over the wavelength ranges 290–350 nm (S290–350) and 250–650 nm (S250–650). We observed distinct patterns in the latitudinal and depth distribution of CDOM absorbance characteristics. Distinct subsurface a300 maxima, characterised by lowest spectral slope values (S290–350=0.010 nm−1 and S250–650=0.014 nm−1) were observed in the vicinity of the deep chlorophyll maximum (DCM) in open ocean and upwelling regions, and indicated in-situ production of CDOM. Converserly, the surface a300 minima and surface S290–350 and S250–650 maxima in these regions were attributed to CDOM photo-oxidation. In order to assess the nature of the observed CDOM variability along our AMT transects, we grouped our data into 12 individual, oceanographic provinces, and further into two seasons (spring: April–May, and Autumn: September–October) and three depth zones (surface mixed layer, pycnocline, and below pycnocline). Comparisons between individual provinces, seasons and depth zones indicated that CDOM variability was dominated by regional factors and depth distribution patterns, while seasonal variability was generally less important in our data. Based on depth distribution patterns together with analyses of inter-relations between a300, and S290–350, S250–650 we propose that our data reflect the presence of two CDOM end-members, each characterised by distinct spectral slope factors, S290–350 and S250–650. The first CDOM end-member (S290–350=0.010 and S250–650=0.014 nm−1) was situated in the vicinity of the DCM, and was attributed to CDOM production from phytoplankton-derived organic matter via planktonic foodweb interactions. The second end-member (S290–350=0.028 nm−1 and S250–650=0.029 nm−1) was attributed to microbial CDOM production. We propose that the CDOM distribution along the AMT cruise track is controlled by autochthonous production near the DCM and subsequent photo-oxidation in surface waters.
Nitrogen fixation and δ15N were measured within a warm-core eddy during an in-situ phosphate experiment (CYCLOPS) in the Eastern Mediterranean, with experimental procedures performed on unconcentrated, bulk water. Mean rates of 129 nmol-N L-1d-1 were measured at control stations in the absence of phosphate (<2 nmolL-1), inferring the bioavailability of DOP to diazotrophs. In P-enriched waters, rates increased by 48% to 197 nmol-N L-1d-1 five days after addition. δ15N of particulate material was homogenous throughout the upper mixed layer, but changed with time at both amended and control stations from +3.8‰ on day 1, to -0.6‰ on day 4-5 before returning to +3.1‰ after 9 days. This trend matched the observed response in other components of the biota and biogeochemistry in the P-enriched patch. In-vitro addition experiments indicated that diazotrophy was not limited by Fe availability.
1] The large-scale iron enrichment conducted in the NE Pacific during the Subarctic Ecosystem Response to Iron Enrichment Study (SERIES) triggered a phytoplankton bloom dominated successively by nanophytoplankton and large diatoms. During the first 14 days, surface dimethyl sulfide (DMS) levels increased both inside (up to 22 nmol L À1) and outside (up to 19 nmol L À1) the patch, with no consistent Fe effect. Later, DMS concentrations became sixfold lower inside the patch than outside. In this study, we used a DMS budget module embedded in a one-dimensional ocean turbulence model to investigate the contribution of the interacting physical, photochemical, and biological processes to this particular DMS response. Temporal variations in biological net DMS production were reconstructed using an inverse modeling approach. Our results show that short-term (days) variations in both the physical processes (i.e., turbulent mixing and ventilation) and the biological cycling of DMS are needed to explain the time evolution of DMS concentrations both outside and inside the Fe-enriched patch. The biological net DMS production was generally high (up to 0.35 nmol L À1 h À1) and comparable outside and inside the patch during the first 10 days, corresponding to the observed accumulation of DMS inside and outside the patch. Later, it became negative (net DMS biological consumption) inside the patch, suggesting a change in dimethylsulfoniopropionate bacterial metabolism. This study stresses the importance of short-term variations in biological processes and their sensitivity to the physical environment in shaping the DMS response to iron enrichment.
An improved knowledge of iron biogeochemistry is needed to better understand key controls on the functioning of high-nitrate low-chlorophyll (HNLC) oceanic regions. Iron budgets for HNLC waters have been constructed using data from disparate sources ranging from laboratory algal cultures to ocean physics. In summer 2003 we conducted FeCycle, a 10-day mesoscale tracer release in HNLC waters SE of New Zealand, and measured concurrently all sources (with the exception of aerosol deposition) to, sinks of iron from, and rates of iron recycling within, the surface mixed layer. A pelagic iron budget (timescale of days) indicated that oceanic supply terms (lateral advection and vertical diffusion) were relatively small compared to the main sink (downward particulate export). Remote sensing and terrestrial monitoring reveal 13 dust or wildfire events in Australia, prior to and during FeCycle, one of which may have deposited iron at the study location. However, iron deposition rates cannot be derived from such observations, illustrating the difficulties in closing iron budgets without quantification of episodic atmospheric supply. Despite the threefold uncertainties reported for rates of aerosol deposition (Duce et al., 1991), published atmospheric iron supply for the New Zealand region is ∼50-fold (i.e., 7-to 150-fold) greater than the oceanic iron supply measured in our budget, and thus was comparable (i.e., a third to threefold) to our estimates of downward export of particulate iron. During FeCycle, the fluxes due to short term (hours) biological iron uptake and regeneration were indicative of rapid recycling and were tenfold greater than for new iron (i.e. estimated atmospheric and measured oceanic supply), giving an "fe" ratio (uptake of new iron/ uptake of new + regenerated iron) of 0.17 (i.e., a range of 0.06 to 0.51 due to uncertainties on aerosol iron supply), and an "Fe" ratio (biogenic Fe export/uptake of new + regenerated iron) of 0.09 (i.e., 0.03 to 0.24).
The cycling of phosphorus in the Mediterranean (CYCLOPS) team investigated phosphate limitation in the Eastern Mediterranean and conducted a phosphate addition experiment in 2002 at the centre of an anticyclonic eddy south of Cyprus. The 2002 and other cruises generated a small database of chlorophyll-a (chl-a) profiles that enabled investigation of the performance of a variety of standard and regional bio-optical algorithms for remote sensing retrievals of chl-a in the region. The standard SeaWiFS OC4V4 and MODIS chlor_a2 algorithms overestimated chl-a as previously reported while a regional algorithm proposed by Bricaud et al. [2002. Algal biomass and sea surface temperature in the Mediterranean basin: intercomparison of data from various satellite sensors, and implications for primary production estimates. Remote Sensing Environment 81, 163–178] and the semi-analytical MODIS chlor_a3 gave improved retrievals. SeaWiFS mean chl-a maps are presented for the Eastern Mediterranean for each month between September 1997 and August 2004 and as multi-annual “climatological” images. The former showed that chl-a in the region decreased over the duration of the time series with reductions in the centre of the eddy, tracked using a quasi-Lagrangian approach, of approximately 33% between 1997 and 1998 and 2002 and 2003. This was not correlated with deep winter mixing represented as heat loss from the sea-surface or dust deposition represented as daily EP-TOMS aerosol index and annual aluminium deposition on the Israeli coast. It is hypothesised that the variations in chl-a are partly a function of the eddy dynamics. Daily SeaWiFS observations show that the 2002 phosphate release was conducted at a period of decreasing chl-a between the winter maximum and summer oligotrophic conditions; however, the rate of seasonal decrease was less than that observed in situ during the week following the phosphate release.
The Eastern Mediterranean is the largest oceanic ecosystem that is phosphate-limited. To determine the impact of a transient input we executed a phosphate addition experiment in the surface waters of the Cyprus Eddy (33.31N 32.31E), and compared the ecosystem response with surrounding unperturbed water. A tracer, sulphur hexafluoride (SF 6), added with the phosphate, enabled tracking of the patch when phosphate concentration declined to detection limits, and provided quantitative estimates of mixing, dilution and patch volume. The patch expanded to 4400 km 2 over 9 days with a lateral diffusion rate of 2372 m 2 /s that was consistent with previous tracer releases in eddies. Mixed layer phosphate concentration was $110 nmol/l immediately post-release, and declined to o5 nmol/l after 6 days. A phosphate budget was developed using SF 6 as a proxy to discriminate between dilution and biological pathways, with dilution resulting in loss of $75% of added phosphate from the patch centre by day 3. Non-conservative phosphate loss was largely due to biological incorporation into particulate-P, of which 50% accumulated at the patch centre whilst the remainder was removed by lateral dilution by day 3. Non-conservative phosphate loss at the patch centre was 15–15.5 nmol/l by day 4, which was equal to the cumulative biological P uptake of 15.6 (75.6) nmol/l P and concurred with two other independent estimates of P uptake. This closure of the phosphate budget infers that the transfer of added P to mesozooplankton and higher consumers was not significant within the timescale of the experiment, despite the observed biomass increase that followed phosphate addition. Although patch dilution significantly reduced phosphate concentration and particulate accumulation, and so the apparent biological response to the added phosphate, analysis suggests that lateral mixing would not prevent bacterial biomass accumulation at the growth rates observed, suggesting that another factor such as grazing was responsible.
The cycling of phosphorus in the Mediterranean (CYCLOPS) team investigated phosphate limitation in the Eastern Mediterranean and conducted a phosphate addition experiment in 2002 at the centre of an anticyclonic eddy south of Cyprus. The 2002 and other cruises generated a small database of chlorophyll-a (chl-a) profiles that enabled investigation of the performance of a variety of standard and regional bio-optical algorithms for remote sensing retrievals of chl-a in the region. The standard SeaWiFS OC4V4 and MODIS chlor_a2 algorithms overestimated chl-a as previously reported while a regional algorithm proposed by Bricaud et al. [2002. Algal biomass and sea surface temperature in the Mediterranean basin: intercomparison of data from various satellite sensors, and implications for primary production estimates. Remote Sensing Environment 81, 163–178] and the semi-analytical MODIS chlor_a3 gave improved retrievals. SeaWiFS mean chl-a maps are presented for the Eastern Mediterranean for each month between September 1997 and August 2004 and as multi-annual “climatological” images. The former showed that chl-a in the region decreased over the duration of the time series with reductions in the centre of the eddy, tracked using a quasi-Lagrangian approach, of approximately 33% between 1997 and 1998 and 2002 and 2003. This was not correlated with deep winter mixing represented as heat loss from the sea-surface or dust deposition represented as daily EP-TOMS aerosol index and annual aluminium deposition on the Israeli coast. It is hypothesised that the variations in chl-a are partly a function of the eddy dynamics. Daily SeaWiFS observations show that the 2002 phosphate release was conducted at a period of decreasing chl-a between the winter maximum and summer oligotrophic conditions; however, the rate of seasonal decrease was less than that observed in situ during the week following the phosphate release.
The south east Levantine basin of the eastern Mediterranean is a uniquely P starved system with a nitrate:phosphate ratio in the deep water of 25–28:1, a PON:POP ratio of 27–32:1 and a DON:DOPUV ratio of ∼100:1 (which probably represents a DON:DOPTOTAL of ∼50:1). The C:N:P ratio of nutrients accumulated in the deep water from decomposed organic matter was 106:8.5–10.8:0.34–0.43 similar to the measured ratios for dissolved and particulate organic matter and much higher than the Redfield ratio. It is concluded that the P limitation of the eastern Mediterranean is due to the lack of P within the system and not in the preferential removal of P relative to N.
Microbial uptake of orthophosphate was studied before and during a Lagrangian experiment where orthophosphate was added to the surface mixed layer in the Cyprus Gyre, Eastern Mediterranean, a region previously hypothesized to be characterized by P-limited growth of both phytoplankton and heterotrophic bacteria. The addition of ca. 110 nM orthophosphate to a ca. 16 km2 patch in situ led, within 1 day, to an increase in particulate-P from 8 to ca. 15 nM, a result in good agreement with a previous microcosm bioassay indicating this system to have a maximum capacity for orthophosphate consumption of between 10 and 25 nM phosphate. In samples of unperturbed water taken before the addition, outside, or below the experimental patch, orthophosphate turnover time (Tt) was <4 h, argued to be consistent with the assumption of diffusion-limited phytoplankton growth. Upon addition, Tt increased to 94 h. Estimates of maximum potential uptake rate (Vmax) for orthophosphate in unperturbed water exceeded by more than one order of magnitude the biological P-requirement (ν) as obtained from stoichiometric conversion of C-based primary and bacterial production values to estimated P-requirement. Upon addition of orthophosphate, Vmax decreased to a level comparable to ν. The observations are consistent with the assumption of P-starved cells before and P-replete cells with excess external orthophosphate after the addition. Orthophosphate uptake in unperturbed water was dominated by<1 μm organisms (mean ±SD between samples 0.56±0.03 μm). In samples with higher turnover time, orthophosphate uptake was shifted towards larger organisms, culminating after 5 days with a near doubling in mean size (1.08 μm). The size distribution of particulate-P standing stock had a mean size of 10 μm, indicating the presence of a substantial biomass of micro-organisms larger than those involved in P-uptake. Comparison of the measured particulate-P with microscope-based biomass estimates indicated a microbial food web dominated by heterotrophic organisms (70% of particulate-P), distributed with ca. 25% of total particulate-P in heterotrophic bacteria, ca. 40% in heterotrophic flagellates, and ca. 5% in ciliates. Concentration of bioavailable phosphate (Sn) estimated from the relationship Sn=νTi indicated Sn values<1 nM PO4 before the addition, increasing afterwards. Estimates of the sum Kt+Sn for the 0.6–0.2 μm size fraction were in the range 1–7 nM PO4 before and outside patch, suggesting this sum to be dominated by the half-saturation constant Kt. Kt+Sn increased to 69 nM after addition, then dropped over the following week back to background levels. As reported elsewhere in this volume, there was a decline in the observed chlorophyll concentrations, but a positive response in copepods. Less clear than the effects at the level of osmotroph physiology were the subsequent responses expected in the food web. Two possible mechanisms are discussed: (1) a positive response in bacterial production and the subsequent food chain of bacterial predators, and (2) a positive response in phytoplankton predators due to a shift in food quality rather than in food quantity.
We initiated and mapped a diatom bloom in the northeast subarctic Pacific by concurrently adding dissolved iron and the tracer sulfur hexafluoride to a mesoscale patch of high-nitrate, low-chlorophyll waters. The bloom was dominated by pennate diatoms and was monitored for 25 d, which was sufficiently long to observe the evolution and termination of the bloom and most of the decline phase. Fast repetition–rate fluorometry indicated that the diatoms were iron-replete until day 12, followed by a 4–5-d transition to iron limitation. This transition period was characterized by relatively high rates of algal growth and nutrient uptake, which pointed to diatoms using intracel-lularly stored iron. By days 16–17, the bloom was probably limited simultaneously by both iron and silicic acid Acknowledgements We thank the captains, crews, and participating scientists onboard the vessels John P Tully, El Puma, and Kaiyo Maru during this study. We also thank Bill Crawford, Sheila Tows, and Frank Whitney (Institute of Ocean Sciences, Sidney, Canada) for shore-side logistical support. We acknowledge the support of Maurice Levasseur (University of Laval, Quebec, Canada) in providing unpublished data for this manuscript. We thank NASA and Orbimage for the provision of SeaWiFS satellite images presented in Figs. 1 and 8.
Microbial uptake of orthophosphate was studied before and during a Lagrangian experiment where orthophosphate was added to the surface mixed layer in the Cyprus Gyre, Eastern Mediterranean, a region previously hypothesized to be characterized by P-limited growth of both phytoplankton and heterotrophic bacteria. The addition of ca. 110 nM orthophosphate to a ca. 16 km2 patch in situ led, within 1 day, to an increase in particulate-P from 8 to ca. 15 nM, a result in good agreement with a previous microcosm bioassay indicating this system to have a maximum capacity for orthophosphate consumption of between 10 and 25 nM phosphate. In samples of unperturbed water taken before the addition, outside, or below the experimental patch, orthophosphate turnover time (Tt) was <4 h, argued to be consistent with the assumption of diffusion-limited phytoplankton growth. Upon addition, Tt increased to 94 h. Estimates of maximum potential uptake rate (Vmax) for orthophosphate in unperturbed water exceeded by more than one order of magnitude the biological P-requirement (ν) as obtained from stoichiometric conversion of C-based primary and bacterial production values to estimated P-requirement. Upon addition of orthophosphate, Vmax decreased to a level comparable to ν. The observations are consistent with the assumption of P-starved cells before and P-replete cells with excess external orthophosphate after the addition. Orthophosphate uptake in unperturbed water was dominated by<1 μm organisms (mean ±SD between samples 0.56±0.03 μm). In samples with higher turnover time, orthophosphate uptake was shifted towards larger organisms, culminating after 5 days with a near doubling in mean size (1.08 μm). The size distribution of particulate-P standing stock had a mean size of 10 μm, indicating the presence of a substantial biomass of micro-organisms larger than those involved in P-uptake. Comparison of the measured particulate-P with microscope-based biomass estimates indicated a microbial food web dominated by heterotrophic organisms (70% of particulate-P), distributed with ca. 25% of total particulate-P in heterotrophic bacteria, ca. 40% in heterotrophic flagellates, and ca. 5% in ciliates. Concentration of bioavailable phosphate (Sn) estimated from the relationship Sn=νTi indicated Sn values<1 nM PO4 before the addition, increasing afterwards. Estimates of the sum Kt+Sn for the 0.6–0.2 μm size fraction were in the range 1–7 nM PO4 before and outside patch, suggesting this sum to be dominated by the half-saturation constant Kt. Kt+Sn increased to 69 nM after addition, then dropped over the following week back to background levels. As reported elsewhere in this volume, there was a decline in the observed chlorophyll concentrations, but a positive response in copepods. Less clear than the effects at the level of osmotroph physiology were the subsequent responses expected in the food web. Two possible mechanisms are discussed: (1) a positive response in bacterial production and the subsequent food chain of bacterial predators, and (2) a positive response in phytoplankton predators due to a shift in food quality rather than in food quantity.
CYCLOPS was a European Framework 5 program to further our understanding of phosphorus cycling in the Eastern Mediterranean. The core of CYCLOPS was a Lagrangian experiment in which buffered phosphoric acid was added to a <4×4 km patch of water together with SF6 as the inert tracer. The patch was followed for nine days in total. Results obtained prior to the experiment showed that the system was typically ultra-oligotrophic and P-starved with DON:DOP, PON:POP and DIN:DIP all having ratios greatly in excess of 16:1 in surface waters. To our surprise, we found that although the added phosphate was rapidly taken up by the microbial biota, there was a small but significant decrease in chlorophyll a and no increase in primary production, together with an increase in heterotrophic bacterial activity, ciliate numbers and in the gut fullness and egg numbers in the zooplankton community. A microcosm experiment carried out using within-patch and out-of-patch water showed that the phytoplankton community were N and P co-limited while the bacteria and micrograzers were P-limited. Thus this system tends to N and P co-limitation of phytoplankton productivity in summer possibly caused by bioavailable DIN being converted into non-bioavailable forms of DON. On the basis of the data collected within the programme it was concluded that this behavior could be explained by three non-mutually exclusive processes described as (1) trophic by-pass in which the added phosphate gets directly to the grazing part of the predatory food chain from the heterotrophic bacteria bypassing the phytoplankton compartment phosphate, (2) trophic tunnelling in which phosphate is rapidly taken up by both phytoplankton and bacteria via rapid luxury consumption. This causes an immediate change in the phosphorus content but not the abundance of the prey organisms. The added P then “reappears” as responses at the predator level much more rapidly than expected, and (3) mixotrophic by-pass in which inorganic nutrients, including the added P, are taken up by mixotrophic ciliates directly, bypassing the phytoplankton. For details of the results of this study and the processes described, the readers are referred to the relevant papers within this volume. The implications of these results for nutrient cycling in the Eastern Mediterranean are discussed. In particular it is noted that the efficient and rapid grazing observed in this study might explain why the system although impacted by anthropogenic nutrient input has shown little or no measurable change in microbial productivity since added nutrients are rapidly transferred out of the photic zone via the by-pass and tunnelling processes and are exported from the basin. It is also suggested that fish productivity is higher than has been suggested by conventional food chain models due to this grazing. Two possible reasons for the unusual P-starved nature of the basin are presented.
Phosphate addition to surface waters of the ultraoligotrophic, phosphorus-starved eastern Mediterranean in a Lagrangian experiment caused unexpected ecosystem responses. The system exhibited a decline in chlorophyll and an increase in bacterial production and copepod egg abundance. Although nitrogen and phosphorus colimitation hindered phytoplankton growth, phosphorous may have been transferred through the microbial food web to copepods via two, not mutually exclusive, pathways: (i) bypass of the phytoplankton compartment by phosphorus uptake in heterotrophic bacteria and (ii) tunnelling, whereby phosphate luxury consumption rapidly shifts the stoichiometric composition of copepod prey. Copepods may thus be coupled to lower trophic levels through interactions not usually considered.
Phosphate addition to surface waters of the ultraoligotrophic, phosphorus-starved eastern Mediterranean in a Lagrangian experiment caused unexpected ecosystem responses. The system exhibited a decline in chlorophyll and an increase in bacterial production and copepod egg abundance. Although nitrogen and phosphorus colimitation hindered phytoplankton growth, phosphorous may have been transferred through the microbial food web to copepods via two, not mutually exclusive, pathways: (i) bypass of the phytoplankton compartment by phosphorus uptake in heterotrophic bacteria and (ii) tunnelling, whereby phosphate luxury consumption rapidly shifts the stoichiometric composition of copepod prey. Copepods may thus be coupled to lower trophic levels through interactions not usually considered.
Iron supply has a key role in stimulating phytoplankton blooms in high-nitrate low-chlorophyll oceanic waters. However, the fate of the carbon fixed by these blooms, and how efficiently it is exported into the ocean's interior, remains largely unknown. Here we report on the decline and fate of an iron-stimulated diatom bloom in the Gulf of Alaska. The bloom terminated on day 18, following the depletion of iron and then silicic acid, after which mixed-layer particulate organic carbon (POC) concentrations declined over six days. Increased particulate silica export via sinking diatoms was recorded in sediment traps at depths between 50 and 125 m from day 21, yet increased POC export was not evident until day 24. Only a small proportion of the mixed-layer POC was intercepted by the traps, with more than half of the mixed-layer POC deficit attributable to bacterial remineralization and mesozooplankton grazing. The depletion of silicic acid and the inefficient transfer of iron-increased POC below the permanent thermocline have major implications both for the biogeochemical interpretation of times of greater iron supply in the geological past, and also for proposed geo-engineering schemes to increase oceanic carbon sequestration.
Axial profiles of dissolved carbon monoxide(CO) from four surveys of the Scheldt estuaryconfirmed that the estuary is a source ofatmospheric CO, with an emission range of 4–404nmol m–2 h–2. Surface water COconcentration and atmospheric emission werespatially variable, with an order of magnitudedifference between the upper and lower estuaryin spring, and seasonally variable with highestlevels in spring and lowest in winter. AnnualCO emission was estimated to be 700 (396–1032) 103 mol, equivalent to 0.02–0.05% ofdissolved organic carbon (DOC) input to theestuary. CO photoproduction rates were an orderof magnitude greater in the upper estuary inspring and correlated with DOC concentration.Total CO production from DOC photodegradationwas estimated to be 8.5–18 103 mol COd–1, equivalent to 0.21–0.44% of riverineDOC input in spring. The deficit betweenproduction and emission suggests that microbialCO oxidation accounts for 68% of photoproducedCO, with highest oxidation rates at lowsalinities. The results indicate that suspendedparticulate material indirectly influencesestuarine CO distribution and emission.Assuming that the Scheldt is representative, estuaries do notcontribute significantly to the oceanic or global CO budgets.
Nitrous oxide (N2O) profiles were obtained at stations inside and outside an area of iron-fertilised surface water at 61°S 140°E during the Southern Ocean Iron Enrichment Experiment (SOIREE). Surface N2O saturation and air–sea flux during SOIREE (98–103%; −1.18–1.75 μmol/m2/d) were consistent with that obtained between 58°S 158°E and 49°S 162°E (99–104%; −0.3–4.7 μmol/m2/d), and confirmed predicted flux estimates for this region. Turbulent eddy diffusion across the pycnocline supplied an average 38% of the air–sea N2O flux, indicating a production mechanism in the upper 80 m. There was no significant difference in N2O saturation and flux between stations inside and outside the patch, although a N2O saturation maximum in the pycnocline at most stations inside the iron-fertilised patch was not present at stations outside. The mean N2O profile for the stations outside the patch was used as a control to identify pycnocline N2O production, which increased during SOIREE and co-varied with iron-mediated increases in phytoplankton biomass. The mechanisms for iron-mediated N2O production in the pycnocline are considered. On longer timescales, the decrease in radiative forcing resulting from carbon fixation and CO2 uptake during SOIREE may be subsequently offset by 6–12% by N2O production. Furthermore, analysis of scenarios of large-scale Southern Ocean fertilisation supports previous observations that any decrease in radiative forcing due to CO2 drawdown may be partially or totally negated by an increase in N2O production.
This volume is dedicated to the Southern Ocean Iron RElease Experiment (SOIREE), the first in situ iron fertilisation experiment performed in the polar waters of the Southern Ocean. SOIREE was an interdisciplinary study involving participants from six countries, and took place in February 1999 south of the Polar Front in the Australasian-Pacific sector of the Southern Ocean. Approximately 3800 kg of acidified FeSO4.7H2O and 165 g of the tracer sulphur hexafluoride (SF6) were added to a 65-m deep surface mixed layer over an area of ∼50 km2. Initially, mean dissolved iron concentrations were ∼2.7 nM, but decreased to ambient levels within days, requiring subsequent additions of 1550–1750 kg of acidified FeSO4.7H2O on days 3, 5 and 7 of the experiment. During the 13-day site occupation there were iron-mediated increases in phytoplankton growth rates, with marked increases in chlorophyll a (up to 2 μg l−1) and production rates (up to 1.3 g C m−2 d−1). These resulted in subsequent changes in the pelagic ecosystem structure, and in the cycling of carbon, silica and sulphur, such as a 10% drawdown of surface CO2. The SOIREE bloom persisted for >40 days following our departure from the site, as observed via SeaWiFS remotely sensed observations of Ocean Colour. Papers in this volume report in detail on aspects of this study, from the oceanographic setting of the experiment to a modelling simulation of the SOIREE bloom. A CD-ROM accompanies this volume and contains the main SOIREE datasets and ancillary information including the pre-experiment ‘desktop’ database study for site-selection, and satellite images of the SOIREE bloom.
Biological and biogeochemical change in the surface mixed layer of an anticyclonic eddy at 60°N in the North Atlantic were monitored within a Lagrangian time-series study using the tracer sulphur hexafluoride (SF6). Four ARGOS buoys initially released at the patch centre remained closely associated with the SF6 patch over a 10-day period, with the near-circular eddy streamlines contributing to the stability and coherence of the patch. Progressive deepening of the surface mixed layer was temporarily interrupted by a storm, which increased mixed-layer nitrate and accelerated the transfer of SF6 to the atmosphere. Diapycnal exchange of SF6 was relatively rapid due to the shallow pycnocline gradient, and a vertical eddy diffusivity (Kz) of 1.95 cm2 s−1 at the base of the mixed layer was estimated from vertical SF6 profiles at the patch centre. Application of Kz to the nutrient gradients inferred vertical nitrate and phosphate fluxes of 1.8 and 1.25 mmol m−2 d−1, respectively, for the pre-storm period, which accounted for 33% and 20% of the reported in vivo uptake rates. Integration of the vertical nitrate flux and decline in surface layer nitrate suggest a total loss of 0.54 mmol N m−3 d−1 during the 5-day pre-storm period, of which in vivo nitrate consumption only accounted for 49%. Vertical transport of ammonium regenerated in the pycnocline accounted for up to 25% of in vivo phytoplankton uptake. The results suggest that the contribution of vertical turbulence to the mixed-layer nutrient pool was less important than that recorded in other regions of the open ocean, inferring that advective processes are more significant in an eddy. This study also emphasises the potential of SF6 for oceanic Lagrangian time series studies, particularly in dynamic regions, and in constraining estimates of new production.
The effect of iron supply on phytoplankton growth and the marine carbon cycle was tested in situ at 61°S 141°E in the Southern Ocean Iron Release Experiment (SOIREE). On 9 February 1999 iron and the tracer sulphur hexafluoride (SF6) were added to the mixed layer with additional iron infusions after 3, 5 and 7 days. A small decrease of the fugacity of carbon dioxide (fCO2) and dissolved inorganic carbon (DIC) by iron-induced algal growth was observed 4–5 days after the first infusion. From then onwards fCO2 and DIC steadily decreased, and the iron-enriched waters became a sink for atmospheric CO2. The region with surface-water fCO2 drawdown closely matched the shape of the patch, as indicated by SF6. Surface-water fCO2 and DIC drawdown were relatively constant across the patch, whereas SF6 decreased from the patch centre outwards. This pointed to uniform algal carbon uptake, not limited by iron, in the patch. After 13 days surface-water fCO2 and DIC in the patch centre had decreased by 32–38 μatm and 15–18 μmol kg−1, respectively. Surface-water fCO2 outside the patch had increased by 8 μatm, partly as a result of surface-water warming. The iron-induced fCO2 change exceeded seasonal fCO2 variability in this region by a factor of two. From the surface-water fCO2 distribution we estimate a net DIC drawdown of 1353 t of carbon (±14%) (1 t=106 g) across the patch after 12 days, assuming uniform drawdown in the upper 50 m. Correction for vertical diffusion and air–sea exchange results in a gross DIC drawdown of 1408 t of carbon. The decrease of fCO2 and DIC, integrated over the mixed layer, was remarkably similar in size after 13 days of SOIREE as changes observed after 6–9 days during IronEx II, if we consider the 4–5 days lag in algal carbon uptake at the Southern Ocean site. SOIREE has demonstrated in situ the occurrence of algal iron limitation and of iron-induced carbon uptake in these Southern Ocean waters. The subsequent fate of the fixed inorganic carbon can only be speculated upon.
Ship-mounted ADCP and buoy data are used to fit an azimuthal velocity profile to a N.E. Atlantic mesoscale eddy chosen as a site for a Lagrangian biogeochemical survey, the cruise forming part of the UK Plankton Reactivity in the Marine Environment (PRIME) initiative. Together with the buoy-derived locus of the eddy centre, the ADCP-derived velocity field allows observations of a sulphur hexafluoride (SF6) tracer release within the eddy to be put into a non-translating, non-rotating frame. Analysis of the transformed data suggests that the patch of tracer, though spreading, remained coherent throughout the nine-day survey. A linear regression on the square of the radial patch width versus time gives an estimate for the effective horizontal diffusion coefficient of . This is consistent with previous estimates of diffusion rates at the O(10 km) lengthscale of the patch. Theory predicts a corresponding along-streamline spreading time of ∼14 days. This implies that the tracer patch mixed little with the surrounding waters during the first five days of the survey, suggesting that biogeochemical processes were little affected by lateral mixing during this period. The theory is inapplicable at later times because the area was struck by a storm at the start of the sixth day, which resulted in the halving of SF6 concentrations. Using a model of a circular eddy with the calculated velocity profile, the dispersion of the initial patch of tracer is simulated for different diffusivities, the results confirming the estimate of along-streamline mixing time and demonstrating the enhancement of azimuthal diffusion by the radial shear of the flow.
During the 13 day Southern Ocean Iron RE-lease Experiment (SOIREE), dissolved iron concentrations decreased rapidly following each of three iron-enrichments, but remained high (>1 nM, up to 80% as FeII) after the fourth and final enrichment on day 8. The former trend was mainly due to dilution (spreading of iron-fertilized waters) and particle scavenging. The latter may only be explained by a joint production-maintenance mechanism; photoreduction is the only candidate process able to produce sufficiently high FeII, but as such levels persisted overnight (8 hr dark period) —ten times the half—life for this species—a maintenance mechanism (complexation of FeII) is required, and is supported by evidence of increased ligand concentrations on day 12. The source of these ligands and their affinity for FeII is not known. This retention of iron probably permitted the longevity of this bloom raising fundamental questions about iron cycling in HNLC (High Nitrate Low Chlorophyll) Polar waters.
High-resolution (1 degree x 1 degree longitude) seasonal and annual nitrous oxide (N sub(2) O) concentration fields for the Arabian Sea surface layer using a database containing more than 2400 values measured between December 1977 and July 1997 was computed. N sub(2)O concentrations are highest during the southwest (SW) monsoon along the southern Indian continental shelf. Annual emissions range from 0.33 to 0.70 Tg N sub(2)O are dominated by fluxes from coastal regions during the SW and northeast monsoons. Our revised estimate for the annual N sub(2)O flux from the Arabian Sea is much more tightly constrained than the previous consensus derived using averaged in-situ data from a smaller number of studies. However, the tendency to focus on measurements in locally restricted features in combination with insufficient seasonal data coverage leads to considerable uncertainties of the concentration fields and thus in the flux estimates, especially in the coastal zones of the northern and eastern Arabian Sea. The overall mean relative error of the annual N sub(2)O emissions from the Arabian Sea was estimated to be at least 65%
Sulphur hexafluoride (SF6) has potential as a transient tracer of recently ventilated water masses, as its atmospheric burden continues to increase. Northern Arabian Sea hydrography was examined using measurements of atmospheric and dissolved SF6, CFC-11, CFC-12 and CFC-113. Persian Gulf Water (PGW) was characterized by its SF6 signal, and the time elapsed since its formation was evaluated by two approaches. Four ventilation age estimates were derived from SF6/CFC-11, SF6/CFC-12, CFC-113/CFC-11 and CFC-113/CFC-12, and their agreement at the oceanic stations confirms the validity of SF6 as a transient tracer. A second approach, of correcting SF6 partial pressure for PGW dilution by an optimal mixing model and referencing to the atmospheric SF6 chronology, provided a relative tracer age. This indicated a PGW flow of 0.016 (+/-0.003) m/s across the northern Arabian Sea, with an associated oxygen consumption of 10.1 mumol/l p.a. that exceeds tracer-derived estimates but confirms rates derived from export flux.
The growth of populations is known to be influenced by dispersal, which has often been described as purely diffusive. In the open ocean, however, the tendrils and filaments of phytoplankton populations provide evidence for dispersal by stirring. Despite the apparent importance of horizontal stirring for plankton ecology, this process remains poorly characterized. Here we investigate the development of a discrete phytoplankton bloom, which was initiated by the iron fertilization of a patch of water (7 km in diameter) in the Southern Ocean. Satellite images show a striking, 150-km-long bloom near the experimental site, six weeks after the initial fertilization. We argue that the ribbon-like bloom was produced from the fertilized patch through stirring, growth and diffusion, and we derive an estimate of the stirring rate. In this case, stirring acts as an important control on bloom development, mixing phytoplankton and iron out of the patch, but also entraining silicate. This may have prevented the onset of silicate limitation, and so allowed the bloom to continue for as long as there was sufficient iron. Stirring in the ocean is likely to be variable, so blooms that are initially similar may develop very differently.

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Top co-authors (50)

Philip W. Boyd
  • University of Tasmania
Andrew J. Watson
  • University of Exeter
Edward Abraham
  • Dragonfly Data Science
Mike J Harvey
  • National Institute of Water and Atmospheric Research
E. Malcolm S. Woodward
  • Plymouth Marine Laboratory
Karl A. Safi
  • National Institute of Water and Atmospheric Research
Andrew Marriner
  • National Institute of Water and Atmospheric Research
Andrew Ross Bowie
  • University of Tasmania
Peter L. Croot
  • National University of Ireland, Galway

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