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Upper ocean plankton assemblages are known to influence the export of carbon and biominerals from the mixed layer. However, relationships between plankton community structure and the magnitude and stoichiometry of export remain poorly characterized. We present data on biogeochemical and diatom export fluxes from the annual deployment of a sediment trap in a High Nutrient, Low Chlorophyll (HNLC) area upstream of the Kerguelen Plateau (KERFIX station). The weak and tidal-driven circulation provided favorable conditions for a quantitative analysis of export processes. Particulate organic carbon (POC) fluxes were highest in spring and summer. Biogenic silica (BSi) fluxes displayed similar seasonal patterns, although BSi:POC ratios were elevated in winter. Fragilariopsis kerguelensis dominated the annual diatom export assemblage (59.8% of the total valve flux). We identified clusters of diatom species that were positively or negatively correlated to the BSi:POC ratio. Our results indicate that the differential role of certain diatom species for carbon and silicon export, previously identified from iron-fertilized productive areas, is also valid in HNLC regimes. Although annual POC export below the mixed layer of the HNLC site is twofold lower that the one previously reported in a naturally iron-fertilized area of the Kerguelen Plateau, the fraction of seasonal net community production exported is similar at both sites (~1.5%). These findings suggest that natural iron fertilization increases the strength but not the efficiency of carbon export from the mixed layer. [ABSTRACT FROM AUTHOR]

We report on the zonal variability of mesopelagic particulate organic carbon remineralization and deep carbon transfer potential during the Kerguelen Ocean and Plateau compared Study 2 expedition (KEOPS 2; October-November 2011) in an area of the polar front supporting recurrent massive blooms from natural Fe fertilization. Mesopelagic carbon remineralization (MR) was assessed using the excess, non-lithogenic particulate barium (Baxs) inventories in mesopelagic waters and compared with bacterial production (BP), surface primary production (PP) and export production (EP). Results for this early season study are compared with the results obtained during a previous study (2005; KEOPS 1) for the same area at a later stage of the phytoplankton bloom. Our results reveal the patchiness of the seasonal advancement and of the establishment of remineralization processes between the plateau (A3) and polar front sites during KEOPS 2. For the Kerguelen plateau (A3 site) we observe a similar functioning of the mesopelagic ecosystem during both seasons (spring and summer), with low and rather stable remineralization fluxes in the mesopelagic column (150-400 m). The shallow water column (~500 m), the lateral advection, the zooplankton grazing pressure and the pulsed nature of the particulate organic carbon (POC) transfer at A3 seem to drive the extent of MR processes on the plateau. For deeper stations (>2000 m) located on the margin, inside a polar front meander, as well as in the vicinity of the polar front, east of Kerguelen, remineralization in the upper 400m in general represents a larger part of surface carbon export. However, when considering the upper 800 m, in some cases, the entire flux of exported carbon is remineralized. In the polar front meander, where successive stations form a time series, two successive events of particle transfer were evidenced by remineralization rates: a first mesopelagic and deep transfer from a past bloom before the cruise, and a second transfer expanding at mesopelagic layers during the cruise. Regarding the deep carbon transfer efficiency, it appeared that above the plateau (A3 site) the mesopelagic remineralization was not a major barrier to the transfer of organic matter to the seafloor (close to 500 m). There, the efficiency of carbon transfer to the bottom waters (>400 m) as assessed by PP, EP and MR fluxes comparisons reached up to 87% of the carbon exported from the upper 150 m. In contrast, at the deeper locations, mesopelagic remineralization clearly limited the transfer of carbon to depths of >400 m. For sites at the margin of the plateau (station E-4W) and the polar front (station F-L), mesopelagic remineralization even exceeded upper 150m export, resulting in a zero transfer efficiency to depths >800 m. In the polar front meander (time series), the capacity of the meander to transfer carbon to depth >800m was highly variable (0 to 73 %). The highest carbon transfer efficiencies in the meander are furthermore coupled to intense and complete deep (>800 m) remineralization, resulting again in a near-zero, deep (>2000 m) carbon sequestration efficiency there. [ABSTRACT FROM AUTHOR]

The KEOPS2 project aims to elucidate the role of natural Fe fertilisation on biogeochemical cycles and ecosystem functioning, including quantifying the sources and processes by which iron is delivered in the vicinity of the Kerguelen Archipelago, Southern Ocean. The KEOPS2 process study used an upstream high-nutrient, low-chlorophyll (HNLC), deep water (2500 m), reference station to compare with a shallow (500 m), strongly fertilised plateau station and continued the observations to a downstream, bathymetrically trapped recirculation of the Polar Front where eddies commonly form and persist for hundreds of kilometres into the Southern Ocean. Over the Kerguelen Plateau, mean particulate (1-53 μm) Fe and Al concentrations (pFe = 13.4 nM, pAl = 25.2 nM) were more than 20-fold higher than at an offshore (lower-productivity) reference station (pFe = 0.53 nM, pAl = 0.83 nM). In comparison, over the plateau dissolved Fe levels were only elevated by a factor of ~2. Over the Kerguelen Plateau, ratios of pMn / pAl and pFe / pAl resemble basalt, likely originating from glacial/fluvial inputs into shallow coastal waters. In downstream, offshore deep-waters, higher pFe / pAl, and pMn / pAl ratios were observed, suggesting loss of lithogenic material accompanied by retention of pFe and pMn. Biological uptake of dissolved Fe and Mn and conversion into the biogenic particulate fraction or aggregation of particulate metals onto bioaggregates also increased these ratios further in surface waters as the bloom developed within the recirculation structure. While resuspension of shelf sediments is likely to be one of the important mechanisms of Fe fertilisation over the plateau, fluvial and glacial sources appear to be important to areas downstream of the island. Vertical profiles within an offshore recirculation feature associated with the Polar Front show pFe and pMn levels that were 6-fold and 3.5-fold lower, respectively, than over the plateau in surface waters, though still 3.6-fold and 1.7-fold higher respectively than the reference station. Within the recirculation feature, strong depletions of pFe and pMn were observed in the remnant winter water (temperature-minimum) layer near 175 m, with higher values above and below this depth. The correspondence between the pFe minima and the winter water temperature minima implies a seasonal cycle is involved in the supply of pFe into the fertilised region. This observed association is indicative of reduced supply in winter, which is counterintuitive if sediment resuspension and entrainment within the mixed layer is the primary fertilising mechanism to the downstream recirculation structure. Therefore, we hypothesise that lateral transport of pFe from shallow coastal waters is strong in spring, associated with snow melt and increased runoff due to rainfall, drawdown through summer and reduced supply in winter when snowfall and freezing conditions predominate in the Kerguelen region. [ABSTRACT FROM AUTHOR]

We examined phytoplankton community responses to natural iron fertilisation at 32 sites over and downstream from the Kerguelen Plateau in the Southern Ocean during the austral spring bloom in October-November 2011. Community structure was estimated from chemical and isotopic measurements (particulate organic carbon POC, 13C-POC, particulate nitrogen PN, 15N-PN, and biogenic silica BSi) on size-fractionated samples from surface waters (300, 210, 50, 20, 5, and 1 μm fractions). Higher values of 13C-POC (vs. co-located 13C-DIC source values) were taken as indicative of faster growth rates, and higher values of 15N-PN (vs. co-located 15N-NO3 source values) as indicative of greater nitrate use. Community responses varied in relation to both regional circulation and the advance of the bloom. Iron fertilised waters over the plateau developed dominance by very large diatoms (50-210 μm) with high BSi/POC ratios, high growth rates, and significant ammonium recycling as biomass built up. In contrast, downstream Polar Frontal waters with similar or higher iron supply were dominated by smaller diatoms (20-50 μm) and exhibited greater ammonium recycling. Stations in a deep water bathymetrically trapped recirculation south of the Polar Front with lower iron levels showed the large cell dominance observed on the plateau, but much less biomass. Comparison of these communities to surface water nitrate (and silicate) depletions as a proxy for export shows that the low biomass recirculation feature exported similar amounts of nitrogen to the high biomass blooms over the plateau and north of the Polar Front. This suggests that trophodynamic and export responses differed between regions with persistent low levels vs. punctual high levels of iron fertilisation. [ABSTRACT FROM AUTHOR]

The KEOPS2 project aims to elucidate the role of natural Fe fertilisation on biogeo-chemical cycles and ecosystem functioning, including quantifying the sources and processes by which iron is delivered in the vicinity of the Kerguelen Archipelago, Southern Ocean. The KEOPS2 process study used an upstream HNLC, deep water (2500 m), reference station to compare with a shallow (500 m), strongly fertilised plateau station and continued the observations to a downstream, bathymetrically trapped recir-culation of the Polar Front where eddies commonly form and persist for hundreds of kilometres into the Southern Ocean. Over the Kerguelen Plateau, mean particulate (1-53μm) Fe and Al concentrations (pFe = 13.4nM, pAl = 25.2nM) were more than 20-fold higher than at an offshore (lower-productivity) reference station (pFe = 0.53 nM, pAl = 0.83 nM). In comparison, over the plateau dissolved Fe levels were only elevated by a factor of ~2. Over the Kerguelen Plateau, ratios of pMn/pAl and pFe/pAl resemble basalt, likely originating from glacial/fluvial inputs into shallow coastal waters. In downstream, offshore deep-waters, higher pFe/pAl, and pMn/pAl ratios were observed, suggesting loss of lithogenic material accompanied by retention of pFe and pMn. Biological uptake of dissolved Fe and Mn and conversion into the biogenic partic-ulate fraction or aggregation of particulate metals onto bioaggregates also increased these ratios further in surface waters as the bloom developed within the recirculation structure. While resuspension of shelf sediments is likely to be one of the important mechanisms of Fe fertilisation over the plateau, fluvial and glacial sources appear to be important to areas downstream of the island. Vertical profiles within an offshore re-circulation feature associated with the Polar Front show pFe and pMn levels that were 6-fold and 3.5-fold lower respectively than over the plateau in surface waters, though still 3.6-fold and 1.7-fold higher respectively than the reference station. Within the re-circulation feature, strong depletions of pFe and pMn were observed in the remnant winter water (temperature-minimum) layer near 175 m, with higher values above and below this depth. The correspondence between the pFe minima and the winter water temperature minima implies a seasonal cycle is involved in the supply of pFe into the fertilized region. This observed association is indicative of reduced supply in winter, which is counterintuitive if sediment resuspension and entrainment within the mixed layer is the primary fertilising mechanism to the downstream recirculation structure. Therefore, we hypothesise that lateral transport of pFe from shallow coastal waters is strong in spring, associated with snow melt and increased runoff due to rainfall, drawdown through summer and reduced supply in winter when snowfall and freezing conditions predominate in the Kerguelen region. [ABSTRACT FROM AUTHOR]

The combination of concentrations and σ13C signatures of Particulate Organic Carbon (POC) and sterols provides a powerful approach to study ecological and environmental changes in both the modern and ancient ocean. We applied this tool to study the biogeochemical changes in the modern ocean water column during the BONUS-GoodHope survey (February-March 2008) from Cape Basin to the northern part of the Weddell Gyre. Cholesterol and brassicasterol were chosen as ideal biomarkers of the het-erotrophic and autotrophic carbon pools, respectively, because of their ubiquitous and relatively refractory nature. We document depth distributions of concentrations (relative to bulk POC) and σ13C signatures of cholesterol and brassicasterol combined with CO2 aq. surface concentration variation. While the relationship between CO2 aq. and σ13C of bulk POC and biomarkers have been reported by others for the surface water, our data show that this persists in mesopelagic and deep waters, suggesting that σ13C signatures of certain biomarkers in the water column could be applied as proxies for surface water CO2 aq. We observed a general increase in sterol σ13C signatures with depth, which is likely related to a combination of particle size effects, selective feeding on larger cells by zooplankton, and growth rate related effects. Our data suggest a key role of zooplankton fecal aggregates in carbon export for this part of the Southern Ocean (SO). Additionally, in the southern part of the transect south of the Polar Front (PF), the release of sea-ice algae during the ice demise in the Seasonal Ice Zone (SIZ) is hypothesized to influence the isotopic signature of sterols in the open ocean. Overall, the combined use of σ13C values and concentrations measurements of both bulk organic C and specific sterols throughout the water column offers the promising potential to explore the recent history of plankton and the fate of organic matter in the SO. [ABSTRACT FROM AUTHOR]

During the Bonus-GoodHope (BGH) expedition (Jan-Mar 2008) we studied the water column distribution of total 234Th and biogenic particulate Ba (Baxs) in the Atlantic sector of the Southern Ocean. The objective was to assess the export flux of particulate organic carbon (POC) from the surface to the mesopelagic twilight zone along a section between the Cape Basin and Weddell Gyre. Export production of POC was estimated from steady state and non steady state export fluxes of 234Th which were converted into POC ?uxes, using the POC/234Th ratio of large (>53 µm) suspended particles, collected via in-situ pumps. Deficits in 234Th activities were observed at all stations from the surface to the bottom of the mixed- layer. 234Th export fluxes from the upper 100m ranged from 496±57 dpm m-2 d-1 to 1195±120 dpm m-2 d-1 for the steady state model and from 149±18 dpm m-2 d-1 to 1217±146 dpm m-2 d-1 for the non steady state model calculated for a time window of 15 to 22 days preceding the timing of the present cruise. The POC/234Thp ratio of large, potentially sinking particles (>53 µm), was observed to increase with latitude, from 1.9±0.2 µmol dpm-1 and 1.7±0.3 µmol dpm-1 in the Subtropical Zone (STZ) and Subantarctic Zone (SAZ), respectively, to 3.0±0.2 µmol dpm in the Polar Front Zone (PFZ), 4.8±1.9 µmol dpm-1 at the Southern Antarctic Circumpolar Current Front (SACCF) to 4.1±1.7 µmol dpm in the northern Weddell Gyre, in line with an increasing contribution of larger cell diatoms. Steady state and non steady state POC export from the upper 100m ranged from 0.9±0.2mmolC m-2 d to 5.1±2.1mmolC m-2 d-1 and from 0.3±0.0mmolC m-2 d-1 to 4.9±3.2mmolC m-2 d-1 , respectively. From the SAZ to the SACCF, non steady state POC export production represented only 15 to 54% of the steady state POC flux, suggesting that the intensity of export had decreased over time partly due to the fact that regenerated-production based communities of small-sized phytoplankton became predominant. In contrast, for the HNLC area south of the SACCF, we found an excellent agreement between the two modeling approaches indicating that surface POC export remained rather constant there. [ABSTRACT FROM AUTHOR]

The combination of concentrations and σ13C signatures of Particulate Organic Carbon (POC) and sterols provides a powerful approach to study ecological and environmental changes both in the modern and ancient ocean, but its application has so far been restricted to the surface area. We applied this tool to study the biogeo- chemical changes in the modern ocean water column during the BONUS-GoodHope survey (Feb-Mar 2008) from Cape Basin to the northern part of the Weddell Gyre. Cholesterol and brassicasterol were chosen as ideal biomarkers of the heterotrophic and autotrophic carbon pools, respectively, because of their ubiquitous and relatively refractory nature. We document depth distributions of concentrations (relative to bulk POC) and σ13C signatures of cholesterol and brassicasterol from the Cape Basin to the northern Weddell Gyre combined with CO2 aq. surface concentration variation. While relationships between surface water CO2 aq. and σ13C of bulk POC and biomarkers have been previously established for surface waters, our data show that these remain valid in deeper waters, suggesting that σ13C signatures of certain biomarkers could be developed as proxies for surface water CO2 aq. Our data suggest a key role of zooplankton fecal aggregates in carbon export for this part of the Southern Ocean. We observed a general increase in sterol σ13C signatures with depth, which is likely related to a combination of particle size effects, selective feeding on larger cells by zooplankton, and growth rate related effects Additionally, in the southern part of the transect south of the Polar Front (PF), the release of sea-ice algae is hypothesized to influence the isotopic signature of sterols in the open ocean. Overall, combined use of σ13C and concentrations measurements of both bulk organic C and specific sterol markers throughout the water column shows the promising potential of analyzing σ13C signatures of individual ma- rine sterols to explore the recent history of plankton and the fate of organic matter in the SO. [ABSTRACT FROM AUTHOR]

We investigated the seasonal and geographical variation in the stable carbon isotope ratios of total dissolved inorganic carbon (Delta13CDIC) and suspended matter (Delta13CPOC) in the freshwater part of the River Scheldt. Two majorsources of particulate organic matter (POM) occur in this riverine system: riverine phytoplankton and terrestrial detritus. In winter thelowest Delta13CDIC values are observed due to enhanced input of CO2 from decomposition of 13C-depleted terrestrial plant detritus (average Delta13CDIC = ?/14.3). During summer, when litter input from terrestrial flora is the lowest, water column respiration on POM of terrestrial origin is also the lowest as evidenced by less negative Delta13CDIC values (average Delta13CDIC = ?9.9). In winter the phytoplankton biomass is low, as indicated by low chlorophyll a concentrations (Chl a < 4.5 Mu gl-1), compared to summer when chlorophyll a concentrations can rise to a maximum of 54 Mu gl-1. Furthermore, in winter the very narrow range of Delta13CPOC (from ?26.5 to ?27.6) is associated with relatively high C/N ratios (C/N > 9) suggestingthat in winter a major fraction of POC is derived from allochthonousmatter. In summer Delta 13CPOC exhibits a verywide range of values, with the most negative values coinciding with high Chl a concentrations and low C/N ratios (C/N < 8). This suggestspredominance of phytoplankton carbon in the total particulate carbonpool, utilising a dissolved inorganic carbon reservoir, which is already significantly depleted in 13 C. Using a simple two source mixing approach a reconstruction of the relative importance of phytoplankton to the total POC pool and of 13 C/12C fractionation by phytoplankton is attempted. [ABSTRACT FROM AUTHOR]