Factors controlling silicon and nitrogen biogeochemical cycles in high nutrient,low chlorophyll systems (the Southern Ocean and the North Pacific): Comparison with a mesotrophic system (the North Atlantic)

A 1-D coupled physical-biogeochemical model is used to study the seasonal cycles of silicon and nitrogen in two High Nutrient Low Chlorophyll (HNLC) systems, the Antarctic Circumpolar Current (ACC) and the North Pacific Ocean, and a mesotrophic system, the North Atlantic Ocean. The biological model consists of nine compartments (diatoms, nano-flagellates, microzooplankton, mesozooplankton, two types of detritus, nitrate, ammonium and silicic acid) forced by irradiance, temperature, mixing and deep nitrate and silicic acid concentrations. At all sites, nanophytoplankton standing crop variations are low, in spite of variations in primary production, because of a “top–down” control by microzooplankton. Although nanophytoplankton sustain more than 60% of the annual primary production in these areas, their contribution to the export production does not exceed 1% of the total. The differences in the seasonal plankton cycle among these regions come mainly from differences in the dynamics of large phytoplankton (here diatoms). In the ACC, the chlorophyll maximum remains <1.5 mg m⁻³, as an unfavourable light/mixing regime and a likely trace-metal limitation keep diatoms from blooming. In the northeast Pacific, trace-metal limitation seems to keep diatoms from blooming throughout the year. In both these systems, light or iron limitations induce high Si/N uptake ratios. Incidentally these high Si/N uptake ratios lead to a net excess of silicic acid utilization over nitrate, and to a subsequent silicic acid limitation during the summertime. In the North Atlantic, under favourable light/mixing regime and nutrient-replete conditions at the onset of the growing period, diatoms outburst and sustain a bloom >3.5 mg Chl-a m⁻³. Thereafter, mesozooplankton grazing pressure and silicic acid limitation induce the collapse of the chlorophyll maximum and the persistence of lower chlorophyll concentrations in summer. Although the ACC and the North Pacific show HNLC features, they support a high biogenic silica production (1.9 and 1.07 mol Si m⁻² yr⁻¹) and export flux (0.79 and 0.61 mol Si m⁻² yr⁻¹), compared to the North Atlantic (production: 0.23 mol Si m⁻² yr⁻¹, export: 0.12 mol Si m⁻² yr⁻¹). The differences in Si production and export between the HNLC systems and the mesotrophic North Atlantic come from both higher Si concentrations and Si/N uptake ratios in the HNLC areas compared to the North Atlantic. Also, the low dissolution rate of biogenic silica compared to nitrogen degradation rate, and the inhibition of nitrate uptake by ammonium, reinforce the net excess of silicic acid utilization over nitrate. As a result, the model also illustrates the efficiency of the silica pump for the three sites: about 50% of the biogenic silica synthesized in the euphotic layer is exported out of the first 100 m, while only 4–11% of the particulate organic nitrogen escapes recycling in the surface layer.     

Primary productivity,differential size fraction and pigment composition responses in two Southern Ocean in situ iron enrichments

Two in situ iron-enrichment experiments were conducted in the Pacific sector of the Southern Ocean during summer 2002 (SOFeX). The “north patch,” established within the Subantarctic Zone (∼56°S), was characterized by high nitrate (∼21 mmol m⁻³) but low silicic acid (2 mmol m⁻³) concentrations. North patch iron enrichment increased chlorophyll (Chl) by 12-fold to 2.1 mg m⁻³ and primary productivity (PP_EU) by 8-fold to 188 mmol C m⁻² d⁻¹. Surprisingly, despite low silicic acid concentrations, diagnostic pigment and size-fraction composition changes indicated an assemblage shift from prymnesiophytes toward diatoms. The “south patch,” poleward of the Southern Boundary of the Antarctic Circumpolar Current (SBACC) (∼66°S), had high concentrations of nitrate (∼27 mmol m⁻³) and silicic acid (64 mmol m⁻³). South patch iron enrichment increased Chl by 9-fold to 3.8 mg m⁻³ and PP_EU 5-fold to 161 mmol C m⁻² d⁻¹ but, notably, did not alter the phytoplankton assemblage from the initial composition of ∼50% diatoms. South patch iron addition also reduced total particulate organic carbon:Chl from ∼300 to 100; enhanced the presence of novel non-photosynthetic, but fluorescent, compounds; and counteracted a decrease in photosynthetic performance as photoperiod decreased. These experiments show unambiguously that in the contemporary, high nitrate Southern Ocean increasing iron supply increases primary productivity, confirming the initial premise of the Martin Iron Hypothesis. However, despite a 5-fold increase in PP_EU under iron-replete conditions in late summer, the effect of iron on annual productivity in the Southern Ocean poleward of the SBACC is limited by seasonal ice coverage and the dark of polar winter.     

The metabolic balance between autotrophy and heterotrophy in the western Arctic Ocean

The goal of this study was to explore how net community production (NCP) is influenced by the relationship between primary production and community respiration in the western Arctic Ocean. Plankton NCP and respiration were determined by measuring changes in oxygen in light and dark bottle incubations, respectively. Rates of NCP averaged over shelf, slope and basin waters were positive in summer 2002 (57±191 mmol O₂ m⁻² d⁻¹) and spring 2004 (85±86 mmol O₂ m⁻² d⁻¹) and negative in summer 2004 (−25±176 mmol O₂ m⁻² d⁻¹). Determinations of NCP obtained from bottle incubations were similar to rates inferred from in situ changes in dissolved inorganic carbon. An examination of the spatial variability of primary production and community respiration indicated that respiration is distributed more uniformly than primary production. A spatial offset between photosynthesis and respiration from the shelf to the Arctic basin was present in spring 2004, but was not seen at other times. NCP and the potential for export appear to be dependent on an uncoupling of primary production and community respiration. NCP continued into the summer after the stock of NO₃⁻ had been depleted. Our data suggest that the uniform distribution of respiration relative to primary production is an important factor influencing NCP and the potential for export in the western Arctic.     

Dissolved inorganic carbon,alkalinity, nutrient and oxygen seasonal and interannual variations at the Antarctic Ocean JGOFS-KERFIX site

JGOFS-KERFIX (KERguelen point FIXe) time-series station, located south of the polar front in the Indian sector of the Antarctic Ocean, was occupied monthly between January 1990 and March 1995. Annual cycles of dissolved inorganic carbon (DIC), total alkalinity (TALK), oxygen (O₂) and nutrients (nitrate, silicate, phosphate and ammonia) in the upper ocean are presented for this site. From seasonal drawdown of nutrients and DIC, we estimate a spring–summer net community production of 3.2±0.5 mol m⁻² and C/N/P ratios of 100/16/1. The Si/N ratio varies between 1.8 and 3, suggesting low iron concentrations. The spring–summer biogenic silicon export derived from silicate drawdown is 1.18 mol m⁻², consistent with model estimates of silicate export at this site. Seasonal and interannual variations of oxygen, nitrate and DIC due to physical and biological processes are quantified using a simple month-to-month budget formulation. From these budgets, an annual net community production of 5.7±3.3 mol m⁻² yr⁻¹ is estimated, about twice the averaged spring–summer production, indicating that, at KERFIX, there is a positive net community production throughout the year. Air–sea CO₂ fluxes show that KERFIX is a strong CO₂ sink for the atmosphere of 2.4–5.1 mol m⁻² yr⁻¹ in 1993, depending on the gas exchange formulation used. A 2.1–3.3 mol m⁻² yr⁻¹ outgassing of O₂ is observed at KERFIX except in 1993 and 1994 where a decreasing trend of temperature induces an increase of O₂ solubility.     

Diatom biomass and productivity in oceanic and plume-influenced waters of the western tropical Atlantic ocean

Whereas diatoms (class Bacillariophyceae) often dominate phytoplankton taxa in the Amazon estuary and shelf, their contribution to phytoplankton dynamics and impacts on regional biogeochemistry are poorly understood further offshore in the western tropical Atlantic Ocean (WTAO). Thus, relative contribution of diatoms to phytoplankton biomass and primary production rates and associated environmental conditions were quantified during three month-long cruises in January–February 2001, July–August 2001, and April–May 2003. The upper water column was sampled at 6 light depths (100%, 50%, 25%, 10%, 1% and 0.1% of surface irradiance) at 64 stations between 3° and 14°N latitude and 41° and 58°W longitude. Each station was categorized as ‘oceanic’ or ‘plumewater’, based on principal component analysis of eight physical, chemical and biological variables. All stations were within the North Brazil Current, and plumewater stations were characterized by shallower mixed layers with lower surface salinities and higher dissolved silicon (dSi) concentrations than oceanic stations. The major finding was a much greater role of diatoms in phytoplankton biomass and productivity at plumewater stations relative to oceanic stations. Mean depth-integrated bSi concentrations at the plumewater and oceanic stations were 14.2 and 3.7 mmol m⁻², respectively. Mean depth-integrated SiP rates at the plumewater and oceanic stations were 0.17 and 0.02 mmol m⁻² h⁻¹, respectively. Based on ratios of SiP and PP rates, and typical Si:C ratios, diatoms contributed on average 29% of primary productivity at plumewater stations and only 3% of primary productivity at oceanic stations. In contrast, phytoplankton biomass (as chlorophyll a concentrations) and primary production (PP) rates (as ¹⁴C uptake rates) integrated over the euphotic zone were not significantly different at plumewater and oceanic stations. Chlorophyll a concentrations ranged from 8.5 to 42.4 mg m⁻² and 4.0 to 38.0 mg m⁻² and PP rates ranged from 2.2 to 11.2 mmol m⁻² h⁻² and 1.8 to 10.8 mmol m⁻² h⁻² at plumewater and oceanic stations, respectively. A conservative estimate of annual integrated SiP in offshore waters of Amazon plume between April and August is 0.59 Tmol Si, based on mean SiP rates in plumewaters and satellite-derived estimates of the area of the Amazon plume. In conclusion, river plumewaters dramatically alter the silicon dynamics of the WTAO, forming extensive diatom-dominated phytoplankton blooms that may contribute significantly to the global Si budget as well as contributing to energy and matter flow off of the continental shelf.     

Sulfate reduction in Black Sea sediments: in situ and laboratory radiotracer measurements from the shelf to 2000 m depth

Sulfate reduction rate measurements by the ³⁵SO₄²⁻ core injection method were carried out in situ with a benthic lander, LUISE, and in parallel by shipboard incubations in sediments of the Black Sea. Eight stations were studied along a transect from the Romanian shelf to the deep western anoxic basin. The highest rates measured on an areal basis for the upper 0–15 cm were 1.97 mmol m⁻² d⁻¹ on the shelf and 1.54 mmol m⁻² d⁻¹ at 181 m water depth just below the chemocline. At all stations sulfate reduction rates decreased to values <3 nmol cm⁻³ d⁻¹ below 15 cm depth in the sediment. The importance of sulfate reduction relative to the total mineralization of organic matter was very low, 6%, on the inner shelf, which was paved with mussels, and increased to 47% on the outer shelf at 100 m depth. Where the oxic–anoxic interface of the water column impinged on the sea floor at around 150 m depth, the contribution of sulfate reduction increased from >50% just above the chemocline to 100% just below. In the deep sea, mean sulfate reduction rates were 0.6 mmol m⁻² d⁻¹ corresponding to an organic carbon oxidation of 1.3 mmol m⁻² d⁻¹. This is close to the mean sedimentation rate of organic carbon over the year in the western basin. A comparison with published data on sulfate reduction in Black Sea sediments showed that the present results tend to be higher in shelf sediments and lower in the deep-sea than most other data. Based on the present water column H₂S inventory and the H₂S flux out of the sediment, the calculated turnover time of H₂S below the chemocline is 2100 years.     

Picophytoplankton in the equatorial Pacific: vertical distributions in the warm pool and in the high nutrient low chlorophyll conditions

Abundance distribution and cellular characteristics of picophytoplankton were studied in two distinct regions of the equatorial Pacific: the western warm pool (0°, 167°E), where oligotrophic conditions prevail, and the equatorial upwelling at 150°W characterized by high-nutrient low-chlorophyll (HNLC) conditions. The study was done in September–October 1994 during abnormally warm conditions. Populations of Prochlorococcus, orange fluorescing Synechococcus and picoeukaryotes were enumerated by flow cytometry. Pigment concentrations were studied by spectrofluorometry. In the warm pool, Prochlorococcus were clearly the dominant organisms in terms of cell abundance, estimated carbon biomass and measured pigment concentration. Integrated concentrations of Prochlorococcus, Synechococcus and picoeukaryotes were 1.5×10¹³, 1.3×10¹¹ and 1.5×10¹¹ cells m⁻², respectively. Integrated estimated carbon biomass of picophytoplankton was 1 g m⁻², and the respective contributions of each group to the biomass were 69, 3 and 28%. In the HNLC waters, Prochlorococcus cells were slightly less numerous than in the warm pool, whereas the other groups were several times more abundant (from 3 to 5 times). Abundance of Prochlorococcus, Synechococcus and picoeukaryotes were 1.2×10¹³, 6.2×10¹¹ and 5.1×10¹¹ cells m⁻², respectively. The integrated biomass was 1.9 g C m⁻². Prochlorococcus was again the dominant group in terms of abundance and biomass (chlorophyll, carbon); the respective contributions of each group to the carbon biomass were 58, 7 and 35%. In the warm pool the total chlorophyll biomass was 28 mg m⁻², 57% of which was divinyl chlorophyll a. In the HNLC waters, the total chlorophyll biomass was 38 mg m⁻², 44% of which was divinyl chlorophyll a. Estimates of Prochlorococcus, Synechococcus and picoeukaryotes cell size were made in both hydrological conditions.     

Basin-scale variability of phytoplankton biomass,production and growth in the Atlantic Ocean

The latitudinal distributions of phytoplankton biomass, composition and production in the Atlantic Ocean were determined along a 10,000-km transect from 50°N to 50°S in October 1995, May 1996 and October 1996. Highest levels of euphotic layer-integrated chlorophyll a (Chl a) concentration (75–125 mg Chl m⁻²) were found in North Atlantic temperate waters and in the upwelling region off NW Africa, whereas typical Chl a concentrations in oligotrophic waters ranged from 20 to 40 mg Chl m⁻². The estimated concentration of surface phytoplankton carbon (C) biomass was 5–15 mg C m⁻² in the oligotrophic regions and increased over 40 mg C m⁻² in richer areas. The deep chlorophyll maximum did not seem to constitute a biomass or productivity maximum, but resulted mainly from an increase in the Chl a to C ratio and represented a relatively small contribution to total integrated productivity. Primary production rates varied from 50 mg C m⁻² d⁻¹ at the central gyres to 500–1000 mg C m⁻² d⁻¹ in upwelling and higher latitude regions, where faster growth rates (μ) of phytoplankton (>0.5 d⁻¹) were also measured. In oligotrophic waters, microalgal growth was consistently slow [surface μ averaged 0.21±0.02 d⁻¹ (mean±SE)], representing <20% of maximum expected growth. These results argue against the view that the subtropical gyres are characterized by high phytoplankton turnover rates. The latitudinal variations in μ were inversely correlated to the changes in the depth of the nitracline and positively correlated to those of the integrated nitrate concentration, supporting the case for the role of nutrients in controlling the large-scale distribution of phytoplankton growth rates. We observed a large degree of temporal variability in the phytoplankton dynamics in the oligotrophic regions: productivity and growth rates varied in excess of 8-fold, whereas microalgal biomass remained relatively constant. The observed spatial and temporal variability in the biomass specific rate of photosynthesis is at least three times larger than currently assumed in most satellite-based models of global productivity.     

The supply of nutrients due to vertical turbulent mixing: A study at the Porcupine Abyssal Plain study site in the northeast Atlantic

As part of a multidisciplinary cruise to the Porcupine Abyssal Plain (PAP) study site (49°00′N 16°30′W), in June and July 2006, observations were made of the vertical nitrate flux due to turbulent mixing. Daily profiles of nitrate and turbulent mixing, at the central PAP site, give a mean nitrate flux into the euphotic zone of 0.09 (95% confidence intervals: 0.05–0.16) mmol N m⁻² d⁻¹. This is a factor of 50 lower than the mean observed rate of nitrate uptake within the euphotic zone (5.1±1.3 mmol N m⁻² d⁻¹). By using our direct observations to ‘validate’ a previously published parameterisation for turbulent mixing, we further quantify the variability in the vertical turbulent flux across a roughly 100×100 km region centred on the PAP site, using hydrographic data. The flux is uniformly low (0.08±0.26 mmol N m⁻² d⁻¹, the large standard deviation being due to a strongly non-Gaussian distribution) and is consistent with direct measurements at the central site. It is demonstrated that on an annual basis convective mixing supplies at least 40-fold more nitrate to the euphotic zone than turbulent mixing at this location. Other processes, such as those related with mesoscale phenomena, may also contribute significantly.     

Diapycnal nutrient fluxes on the northern boundary of Cape Ghir upwelling region

In this study we estimate diffusive nutrient fluxes in the northern region of Cape Ghir upwelling system (Northwest Africa) during autumn 2010. The contribution of two co-existing vertical mixing processes (turbulence and salt fingers) is estimated through micro- and fine-structure scale observations. The boundary between coastal upwelling and open ocean waters becomes apparent when nitrate is used as a tracer. Below the mixed layer (56.15±15.56 m), the water column is favorable to the occurrence of a salt finger regime. Vertical eddy diffusivity for salt (K_s) at the reference layer (57.86±8.51 m, CI 95%) was 3×10⁻⁵ (±1.89×10⁻⁹, CI 95%) m² s⁻¹. Average diapycnal fluxes indicate that there was a deficit in phosphate supply to the surface layer (6.61×10⁻⁴ mmol m⁻² d⁻¹), while these fluxes were 0.09 and 0.03 mmol m⁻² d⁻¹ for nitrate and silicate, respectively. There is a need to conduct more studies to obtain accurate estimations of vertical eddy diffusivity and nutrient supply in complex transitional zones, like Cape Ghir. This will provide us with information about salt and nutrients exchange in onshore–offshore zones.     

Phytoplankton dynamics in the NE subarctic Pacific

Ocean Station Papa (OSP, 50°N 145°W) in the NE subarctic Pacific is characterised as high nitrate low chlorophyll (HNLC). However, little is known about the spatial extent of these HNLC waters or the phytoplankton dynamics on the basin scale. Algal biomass, production and size-structure data are presented from winter, spring and summer between 1992 and 1997 for five stations ranging from coastal to open-ocean conditions. The inshore stations (P04–P16) are characterised by the classical seasonal cycle of spring and late summer blooms (production >3 g C m⁻² d⁻¹), diatoms are not Fe-stressed, and growth rate is probably controlled by macronutrient supply. The fate of the phytoplankton is likely sedimentation by diatom-dominated spring blooms, with a pelagic recycling system predominating at other times. The offshore stations (P20/OSP) display low seasonality in biomass and production (OSP, mean winter production 0.3 g C m⁻² d⁻¹, mean spring/summer production 0.85 g C m⁻² d⁻¹), and are dominated by small algal cells. Low Fe availability prevents the occurrence of diatom blooms observed inshore. The main fate of phytoplankton is probably recycling through the microbial food web, with relatively low sedimentation compared to inshore. However, the supply of macro- and micro-nutrients to the coastal and open ocean, respectively, may vary between years. Variability in macro-nutrient supply to the coastal ocean may result in decreased winter reserve nitrate, summer nitrate limitation, subsequent floristic shifts towards small cells, and reduced primary production. Offshore, higher diatom abundances are occasionally observed, perhaps indicating episodic Fe supply. The two distinct oceanic regimes have different phytoplankton dynamics resulting in different seasonality, community structure and fate of algal carbon. These differences will strongly influence the biogeochemical signatures of the coastal and open-oceanic NE subarctic Pacific.     

Major contribution of autotrophy to microbial carbon cycling in the deep North Atlantic’s interior

Current estimates point to a mismatch of particulate organic carbon supply derived from the surface ocean and the microbial organic carbon demand in the meso- and bathypelagic realm. Based on recent findings that chemoautotrophic Crenarchaeota are abundant in the mesopelagic zone, we quantified dissolved inorganic carbon (DIC) fixation in the meso- and bathypelagic North Atlantic and compared it with heterotrophic microbial activity. Measuring ¹⁴C-bicarbonate fixation and ³H-leucine incorporation revealed that microbial DIC fixation is substantial in the mesopelagic water masses, ranging from 0.1 to 56.7 μmol C m⁻³ d⁻¹, and is within the same order of magnitude as heterotrophic microbial activity. Integrated over the dark ocean’s water column, DIC fixation ranged from 1–2.5 mmol C m⁻² d⁻¹, indicating that chemoautotrophy in the dark ocean represents a significant source of autochthonously produced ‘new organic carbon’ in the ocean’s interior amounting to about 15–53% of the phytoplankton export production. Hence, chemoautotrophic DIC fixation in the oxygenated meso- and bathypelagic water column of the North Atlantic might substantially contribute to the organic carbon demand of the deep-water microbial food web.     

Particulate organic carbon export fluxes and size-fractionated POC/234Th ratios in the Ligurian,Tyrrhenian and Aegean Seas

Measurements of ²³⁴Th/²³⁸U disequilibria and particle size-fractionated (1, 10, 20, 53, 70, 100 μm) organic C and ²³⁴Th were made to constrain estimates of the export flux of particulate organic C (POC) from the surface waters of the Ligurian, Tyrrhenian and Aegean Seas in March–June 2004. POC exported from the surface waters (75–100 m depth) averaged 9.2 mmol m⁻² d⁻¹ in the Ligurian and Tyrrhenian Seas (2.3±0.5–14.9±3.0 mmol m⁻² d⁻¹) and 0.9 mmol m⁻² d⁻¹ in the Aegean Sea. These results are comparable to previous measurements of ²³⁴Th-derived and sediment-trap POC fluxes from the upper 200 m in the Mediterranean Sea. Depth variations in the POC/²³⁴Th ratio suggest two possible controls. First, decreasing POC/²³⁴Th ratios with depth were attributed to preferential remineralization of organic C. Second, the occurrence of maxima or minima in the POC/²³⁴Th ratio near the DCM suggests influence by phytoplankton dynamics. To assess the accuracy of these data, the empirical ²³⁴Th-method was evaluated by quantifying the extent to which the ²³⁴Th-based estimate of POC flux, P_POC, deviates from the true flux, F_POC, defined as the p-ratio (p-ratio=P_POC/F_POC=S_Th/S_POC, where S=particle sinking rate). Estimates of the p-ratio made using Stokes’ Law and the particle size distributions of organic C and ²³⁴Th yield values ranging from 0.93–1.45. The proximity of the p-ratio to unity implies that differences in the sinking rates of POC- and ²³⁴Th-carrying particles did not bias ²³⁴Th-normalized POC fluxes by more than a factor of two.     

Organic matter budget in the Southeast Atlantic continental margin close to the Congo Canyon: In situ measurements of sediment oxygen consumption

A study of organic carbon mineralization from the Congo continental shelf to the abyssal plain through the Congo submarine channel and Angola Margin was undertaken using in situ measurements of sediment oxygen demand as a tracer of benthic carbon recycling. Two measurement techniques were coupled on a single autonomous platform: in situ benthic chambers and microelectrodes, which provided total and diffusive oxygen uptake as well as oxygen microdistributions in porewaters. In addition, sediment trap fluxes, sediment composition (Org-C, Tot-N, CaCO₃, porosity) and radionuclide profiles provided measurements of, respectively input fluxes and burial rate of organic and inorganic compounds.The in situ results show that the oxygen consumption on this margin close to the Congo River is high with values of total oxygen uptake (TOU) of 4±0.6, 3.6±0.5 mmol m⁻² d⁻¹ at 1300 and 3100 m depth, respectively, and between 1.9±0.3 and 2.4±0.2 mmol m⁻² d⁻¹ at 4000 m depth. Diffusive oxygen uptakes (DOU) were 2.8±1.1, 2.3±0.8, 0.8±0.3 and 1.2±0.1 mmol m⁻² d⁻¹, respectively at the same depths. The magnitude of the oxygen demands on the slope is correlated with water depth but is not correlated with the proximity of the submarine channel–levee system, which indicates that cross-slope transport processes are active over the entire margin. Comparison of the vertical flux of organic carbon with its mineralization and burial reveal that this lateral input is very important since the sum of recycling and burial in the sediments is 5–8 times larger than the vertical flux recorded in traps.Transfer of material from the Congo River occurs through turbidity currents channelled in the Congo valley, which are subsequently deposited in the Lobe zone in the Congo fan below 4800 m. Ship board measurements of oxygen profiles indicate large mineralization rates of organic carbon in this zone, which agrees with the high organic carbon content (3%) and the large sedimentation rate (19 mm y⁻¹) found on this site. The Lobe region could receive as high as 19 mol C m⁻² y⁻¹, 1/3 being mineralized and 2/3 being buried and could constitute the largest depocenter of organic carbon in the South Atlantic.     

Biological versus physical processes as drivers of large oscillations of the air–sea CO2 flux in the Antarctic marginal ice zone during summer

The fugacity of CO₂ and abundance of chlorophyll a (Chla) were determined in two long transects from the Polar Front to the Antarctic Continent in austral summer, December 1995–January 1996. Large undersaturations of CO₂ in the surface water were observed coinciding with high Chla content. In the major hydrographic regions the mean air–sea fluxes were found to range from −3 to +7 mmol m⁻² d⁻¹ making these regions act as a sink as well as a source for CO₂. In the total 40-d period, the summation of the several strong source and sink regions revealed an overall modest net source of 0.3 mmol m⁻² d⁻¹, this based on the Wanninkhof (J. Geophys. Res. 97 (1992) 7373) quadratic relationship at in situ windspeed. A simple budget approach was used to quantify the role of phytoplankton blooms in the inorganic carbonate system of the Antarctic seas in a time frame spanning several weeks. The major controlling physical factors such as air–sea flux, Ekman pumping and upwelling are included. Net community production varies between −9 and +7 mmol m⁻² d⁻¹, because of the large oscillations in the dominance of autotrophic (CO₂ fixation) versus heterotrophic (CO₂ respiration) activity. Here the mixed layer depth is the major controlling factor. When integrated over time the gross influx and efflux of CO₂ from air to sea is large, but the net residual air/sea exchange is a modest efflux from sea to atmosphere.     

A time-series study of particulate matter export in the North Pacific Subtropical Gyre based on 234Th : 238U disequilibrium

Depth profiles of total ²³⁴Th (dissolved+particulate) were collected at Station ALOHA (22°45N, 158°00W) in the North Pacific Subtropical Gyre during 9 cruises from April 1999 to March 2000. Samples were collected and processed by a new 2 L technique that enables more detailed depth resolution then previous ²³⁴Th studies. Significant zones of particle export (²³⁴Th deficiency) and particle remineralization (²³⁴Th excess) were measured both temporally and with depth. ²³⁴Th derived particulate carbon (PC) and nitrogen (PN) fluxes were determined with steady-state and non-steady-state models and PC/²³⁴Th and PN/²³⁴Th ratios measured with both in situ pumps and free-drifting particle interceptor traps deployed at 150 m. ²³⁴Th based export estimates of 4.0±2.3 mmol C m⁻² d⁻¹ and 0.53±0.19 mmol N m⁻² d⁻¹, were approximately 60% higher than those measured in PIT style sediment traps from the same time period, 2.4±0.2 mmol C m⁻² d⁻¹ and 0.32±0.08 mmol N m⁻² d⁻¹. Most of this difference is attributable to two large export events that occurred during October and December 1999, when traps undercollected for ²³⁴Th by a factor of 2 to 4. ²³⁴Th export (ThE) ratios based on ²³⁴Th derived PC flux/¹⁴C based primary production ranged from 4% to 22% (average=8.8%). Our results confirm the recent estimates of C export by Emerson et al. (Nature 389 (1997) 951) and Sonnerup et al. (Deep-Sea Research I 46 (1999) 777) and indicate that C export from the oligotrophic ocean must be considered when discussing C sequestration in global climate change.     

Export fluxes of particulate organic carbon in the Chukchi Sea: A comparative study using 234Th/238U disequilibria and drifting sediment traps

Large-volume sampling of ²³⁴Th and drifting sediment trap deployments were conducted as part of the 2004 Western Arctic Shelf–Basin Interactions (SBI) spring (May 15–June 23) and summer (July 17–August 26) process cruises in the Chukchi Sea. Measurements of ²³⁴Th and particulate organic carbon (POC) export fluxes were obtained at five stations during the spring cruise and four stations during the summer cruise along Barrow Canyon (BC) and along a parallel shelf-to-basin transect from East Hanna Shoal (EHS) to the Canada Basin. ²³⁴Th and POC fluxes obtained with in situ pumps and drifting sediment traps agreed to within a factor of 2 for 70% of the measurements. POC export fluxes measured with in situ pumps at 50 m along BC were similar in spring and summer (average = 14.0 ± 8.0 mmol C m^− 2 day^− 1 and 16.5 ± 6.5 mmol C m^− 2 day^− 1, respectively), but increased from spring to summer at the EHS transect (average = 1.9 ± 1.1 mmol C m^− 2 day^− 1 and 19.5 ± 3.3 mmol C m^− 2 day^− 1, respectively). POC fluxes measured with sediment traps at 50 m along BC were also similar in both seasons (31.3 ± 9.3 mmol C m^− 2 day^− 1 and 29.1 ± 14.2 mmol C m^− 2 day^− 1, respectively), but were approximately twice as high as POC fluxes measured with in situ pumps. Sediment trap POC fluxes measured along the EHS transect also increased from spring to summer (3.0 ± 1.9 mmol C m^− 2 day^− 1 and 13.0 ± 6.4 mmol C m^− 2 day^− 1, respectively), and these fluxes were similar to the POC fluxes obtained with in situ pumps. Discrepancies in POC export fluxes measured using in situ pumps and sediment traps may be reasonably explained by differences in the estimated POC/²³⁴Th ratios that arise from differences between the techniques, such as time-scale of measurement and size and composition of the collected particles. Despite this variability, in situ pump and sediment trap-derived POC fluxes were only significantly different at a highly productive station in BC during the spring.     

Standing stocks,production, and respiration of phytoplankton and heterotrophic bacteria in the western Arctic Ocean

Standing stocks and production rates for phytoplankton and heterotrophic bacteria were examined during four expeditions in the western Arctic Ocean (Chukchi Sea and Canada Basin) in the spring and summer of 2002 and 2004. Rates of primary production (PP) and bacterial production (BP) were higher in the summer than in spring and in shelf waters than in the basin. Most surprisingly, PP was 3-fold higher in 2004 than in 2002; ice-corrected rates were 1581 and 458 mg C m⁻² d⁻¹, respectively, for the entire region. The difference between years was mainly due to low ice coverage in the summer of 2004. The spatial and temporal variation in PP led to comparable variation in BP. Although temperature explained as much variability in BP as did PP or phytoplankton biomass, there was no relationship between temperature and bacterial growth rates above about 0 °C. The average ratio of BP to PP was 0.06 and 0.79 when ice-corrected PP rates were greater than and less than 100 mg C m⁻² d⁻¹, respectively; the overall average was 0.34. Bacteria accounted for a highly variable fraction of total respiration, from 3% to over 60% with a mean of 25%. Likewise, the fraction of PP consumed by bacterial respiration, when calculated from growth efficiency (average of 6.9%) and BP estimates, varied greatly over time and space (7% to >500%). The apparent uncoupling between respiration and PP has several implications for carbon export and storage in the western Arctic Ocean.     

Time-series observation of POC fluxes estimated from 234Th in the northwestern North Pacific

Time-series measurements of ²³⁴Th activities and particulate organic carbon (POC) concentrations were made at time-series stations (K1, K2, K3, and KNOT) in the northwestern North Pacific from October 2002 to August 2004. Seasonal changes in POC export fluxes from the surface layer (∼100 m) were estimated using ²³⁴Th as a tracer. POC fluxes varied seasonally from approximately 0 to 180 mg C m⁻² d⁻¹ and were higher in spring–summer than in autumn–winter. The export ratio (e-ratio) ranged from 6% to 55% and was also higher in spring–summer. Annual POC fluxes were estimated to be 31 g C m⁻² y⁻¹ in the subarctic region (station K2) and 23 g C m⁻² y⁻¹ in the region between the subarctic and subtropical gyres (station K3). POC fluxes and e-ratios in the northwestern North Pacific were much higher than those in most other oceans. The annual POC flux corresponded to 69% of annual new production estimated from the seasonal difference of the nutrient in the Western Subarctic Gyre (45 g C m⁻² y⁻¹). These results indicate that much of the organic carbon assimilated in the surface layer of the northwestern North Pacific is transferred to the deep ocean in particulate form. Our conclusions support previous reports that diatoms play an important role in the biological pump.     

Carbon and biogenic silica export influenced by the Amazon River Plume: Patterns of remineralization in deep-sea sediments

The Amazon River Plume delivers freshwater and nutrients to an otherwise oligotrophic western tropical North Atlantic (WTNA) Ocean. Plume waters create conditions favorable for carbon and nitrogen fixation, and blooms of diatoms and their diazotrophic cyanobacterial symbionts have been credited with significant CO₂ uptake from the atmosphere. The fate of the carbon, however, has been measured previously by just a few moored or drifting sediment traps, allowing only speculation about the full extent of the plume's impact on carbon flux to the deep sea. Here, we used surface (0.5 m) sediment cores collected throughout the Demerara Slope and Abyssal Plain, at depths ranging from 1800 to 5000 m, to document benthic diagenetic processes indicative of carbon flux. Pore waters were extracted from sediments using both mm- and cm-scale extraction techniques. Profiles of nitrate (NO₃) and silicate (Si(OH)₄) were modeled with a diffusion-reaction equation to determine particulate organic carbon (POC) degradation and biogenic silica (bSi) remineralization rates. Model output was used to determine the spatial patterns of POC and bSi arrival at the sea floor. Our estimates of POC and Si remineralization fluxes ranged from 0.16 to 1.92 and 0.14 to 1.35 mmol m⁻² d⁻¹, respectively. A distinct axis of POC and bSi deposition on the deep sea floor aligned with the NW axis of the plume during peak springtime flood. POC flux showed a gradient along this axis with highest fluxes closest to the river mouth. bSi had a more diffuse zone of deposition and remineralization. The impact of the Amazon plume on benthic fluxes can be detected northward to 10°N and eastward to 47°W, indicating a footprint of nearly 1 million km². We estimate that 0.15 Tmol C y⁻¹ is remineralized in abyssal sediments underlying waters influenced by the Amazon River. This constitutes a relatively high fraction (~7%) of the estimated C export from the region.; the plume thus has a demonstrable impact on C_org export in the western Atlantic. Benthic fluxes under the plume were comparable to and in some cases greater than those observed in the eastern equatorial Atlantic, the southeastern Atlantic, and the Southern Ocean.