A detailed hydrographic and biological survey was carried out in the region of the South-West Indian Ridge during April 2004. Altimetry and hydrographic data have identified this region as an area of high flow variability. Hydrographic data revealed that here the Subantarctic Polar Front (SAF) and Antarctic Polar Front (APF) converge to form a highly intense frontal system. Water masses identified during the survey showed a distinct separation in properties between the northwestern and southeastern corners. In the north-west, water masses were distinctly Subantarctic (>8.5°C, salinity >34.2), suggesting that the SAF lay extremely far to the south.
In the southeast corner water masses were typical of the Antarctic zone, showing a distinct subsurface temperature minimum of <2.5°C. Total integrated chl-a concentration during the survey ranged from 4.15 to 22.81 mg chl-a m–2, with the highest concentrations recorded at stations occupied in the frontal region. These data suggest that the region of the South-West Indian Ridge represents not only an area of elevated biological activity but also acts as a strong biogeographic barrier to the spatial distribution of zooplankton.
Altimetry data showing sea-surface height anomalies (in cm) for the period 6–24 April 2004.
This study was undertaken to characterise the seasonal cycle of air–sea fluxes of carbon dioxide (CO2) in the southern Benguela upwelling system off the South African west coast. Samples were collected from six monthly cross-shelf cruises in the St. Helena Bay region during 2010. CO2 fluxes were calculated from pCO2 derived from total alkalinity and dissolved inorganic carbon and scatterometer-based winds. Notwithstanding that it is one of the most biologically productive eastern boundary upwelling systems in the global ocean, the southern Benguela was found to be a very small net annual CO2 sink of -1.4 ± 0.6 mol C/m2 per year (1.7 Mt C/year). Regional primary productivity was offset by nearly equal rates of sediment and sub-thermocline remineralisation flux of CO2, which is recirculated to surface waters by upwelling. The juxtaposition of the strong, narrow near-shore out-gassing region and the larger, weaker offshore sink resulted in the shelf area being a weak CO2 sink in all seasons but autumn (-5.8, 1.4 and -3.4 mmol C/m2 per day for summer, autumn and winter, respectively).
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Southern Benguela air-sea CO2 flux (mmol/m2 per day) along the St. Helena Bay Monitoring line for daily winds (right). Red is outgassing and blue is ingassing. The dot markers show the sample location and date. The average daily flux is plotted on the left.
In this study we use the southern Benguela upwelling system to investigate the role of nutrient and carbon stoichiometry on carbonate dynamics in eastern boundary upwelling systems. Six months in 2010 were sampled along a cross-shelf transect. Data were classified into summer, autumn, and winter. Nitrate, phosphate, dissolved inorganic carbon, and total alkalinity ratios were used in a stoichiometric reconstruction model to determine the contribution of biogeochemical processes on a parcel of water as it upwelled. Deviations from the Redfield ratio were dominated by denitrification and sulfate reduction in the subsurface waters. The N:P ratio was lowest (7.2) during autumn once anoxic waters had formed. Total alkalinity (TA) generation by anaerobic remineralization decreased pCO2 by 227 μatm. Ventilation during summer and winter resulted in elevated N:P ratios (12.3). We propose that anaerobic production of TA has an important regional effect in mitigating naturally high CO2 and making upwelled waters less corrosive.
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Marine Carbonate fluxes in the southern Benguela: A schematic showing the magnitude of processes contributing to DIC and TA fluxes. Black numbers represent DIC and gray TA. The solid/dashed/dotted lines represent the thermocline and its intensity. Increases in DIC throughout all seasons were largely due to aerobic remineralization (RM). Large TA gains in autumn were due to benthic processes: denitrification (DN), sulfate reduction (SR), and calcite dissolution (CD). Strong primary production (PP) in summer reduced the surface DIC, while calcification (CL) in autumn resulted in decreased TA.
Annual landings of chokka squid (Loligo reynaudii), an important fishing resource for South Africa, fluctuate greatly, and are believed to be related to recruitment success. The ‘Westward Transport Hypothesis’ (WTH) attributes recruitment strength to variability in transport of newly hatched paralarvae from spawning grounds to the ‘cold ridge’ nursery region some 100–200 km to the west, where oceanographic conditions sustain high productivity. We used an individual-based model (IBM) coupled with a 3-D hydrodynamic model (ROMS) to test the WTH and assessed four factors that might influence successful transport – Release Area, Month, Specific Gravity (body density) and Diel Vertical Migration (DVM) – in numerical experiments that estimated successful transport of squid paralarvae to the cold ridge. A multifactor ANOVA was used to identify the primary determinants of transport success in the various experimental simulations. Among these, release area was found to be the most important, implying that adult spawning behaviour (i.e., birth site fidelity) may be more important than paralarval behaviour in determining paralarval transport variability. However, specific gravity and DVM were found to play a role by retaining paralarvae on the shelf and optimizing early transport, respectively. Upwelling events seem to facilitate transport by moving paralarvae higher in the water column and thus exposing them to faster surface currents.
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The two Cape hake species of the southern Benguela ecosystem, the shallow-water and deep-water hakes Merluccius capensis and M. paradoxus, are economically the most important marine resources in South Africa. Recruitment is a key process in the dynamics of marine organisms, yet very little is known about the early life history of Cape hakes, especially the location of spawning grounds and transport of eggs and larvae. For each species, ichthyoplankton dispersal off South Africa is simulated by coupling oceanographic simulations to an individual-based model in order to track virtual individuals. Results indicate that the most favorable spawning areas for transport to nursery areas are located off the south-western coast and the eastern Agulhas Bank, and highlight partly different drift routes followed by the two ichthyoplankton species off Cape Columbine. Transport from spawning to nursery areas is the highest in austral winter for a spawning depth ranging between 0 and 100 m. These modeling results are in broad agreement with available knowledge on the ecology of Cape hakes. The present work on Cape hakes complements previous modeling studies on anchovy and sardine in the same area. Taken together, these studies underline the correspondence between cross-shore (for hakes) or alongshore (for anchovy and sardine) transport mechanisms and the spawning strategies used by these key species of the southern Benguela ecosystem.
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As part of the Bonus-GoodHope (BGH) campaign, 15N-labelled nitrate, ammonium and urea uptake measurements were made along the BGH transect from Cape Town to ~60° S in late austral summer, 2008. Our results are categorised according to distinct hydrographic regions defined by oceanic fronts and open ocean zones. High regenerated nitrate uptake rate in the oligotrophic Subtropical Zone (STZ) resulted in low f-ratios (f = 0.2) with nitrogen uptake being dominated by ρurea, which contributed up to 70 % of total nitrogen uptake. Size fractionated chlorophyll data showed that the greatest contribution (>50 %) of picophytoplankton (<2 μm) were found in the STZ, consistent with a community based on regenerated production. The Subantarctic Zone (SAZ) showed the greatest total integrated nitrogen uptake (10.3 mmol m−2 d−1), mainly due to enhanced nutrient supply within an anticyclonic eddy observed in this region. A decrease in the contribution of smaller size classes to the phytoplankton community was observed with increasing latitude, concurrent with a decrease in the contribution of regenerated production. Higher f-ratios observed in the SAZ (f = 0.49), Polar Frontal Zone (f= 0.41) and Antarctic Zone (f = 0.45) relative to the STZ (f = 0.24), indicate a higher contribution of NO3−-uptake relative to total nitrogen and potentially higher export production. High ambient regenerated nutrient concentrations are indicative of active regeneration processes throughout the transect and ascribed to late summer season sampling. Higher depth integrated uptake rates also correspond with higher surface iron concentrations. No clear correlation was observed between carbon export estimates derived from new production and 234Th flux. In addition, export derived from 15N estimates were 2–20 times greater than those based on 234Th flux. Variability in the magnitude of export is likely due to intrinsically different methods, compounded by differences in integration time scales for the two proxies of carbon export.
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Cruise track during the Bonus Goodhope 2008 campaign in the Atlantic Southern Ocean. Red dots indicate the sampling positions for 15N uptake experiments. The hydrographic fronts, Subtropical Front (STF), Sub-Antarctic Front (SAF), Polar Front (PF), South Antarctic Circumpolar Current Front (SAccF) and Southern Boundary (Sbdy) are indicated by dotted white lines.
During the 1999 Marion Island Oceanographic Survey (MIOS 4) in late austral summer, a northbound and reciprocal southbound transect were taken along the Southwest Indian and Madagascar Ridge, between the Prince Edward Islands and 31° S. The sections crossed a number of major fronts and smaller mesoscale features and covered a wide productivity spectrum from subtropical to subantarctic waters. Associated with the physical survey were measurements of size fractionated chlorophyll, nutrients and nitrogen (NO3, NH4 and urea) uptake rates. Subtropical waters were characterised by low chlorophyll concentrations (max = 0.27.3 mg m−3 dominated by pico-phytoplankton cells (> 81%) and very low f-ratios (< 0.1), indicative of productivity based almost entirely on recycled ammonium and urea. Micro-phytoplankton growth was limited by the availability of NO3 (< 0.5 mmol m−3 and Si(OH)4 (< 1.5 mmol m−3 through strong vertical stratification preventing the upward flux of nutrients into the euphotic zone. Biomass accumulation of small cells was likely controlled by micro-zooplankton grazing. In subantarctic waters, total chlorophyll concentrations increased (max = 0.74 mg m−3 relative to the subtropical waters and larger cells became more prevalent, however smaller phytoplankton cells and low f-ratios (< 0.14) still dominated, despite sufficient NO3 availability. The results from this study favour Si(OH)4 limitation, light-limited deep mixing and likely Fe deficiency as the dominant mechanisms controlling significant new production by micro-phytoplankton. The percentage of micro-phytoplankton cells and rates of new production did however increase at oceanic frontal regions (58.6% and 11.22%, respectively), and in the region of the Prince Edward archipelago (61.4% and 14.16%, respectively). Here, water column stabilization and local Fe-enrichment are thought to stimulate phytoplankton growth rates. Open ocean regions such as these provide important areas for local but significant particulate organic carbon export and biological CO2draw-down in an overall high nutrient low chlorophyll Southern Ocean.
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Cruise tracks overlaid on bathymetry of the Northbound transect, showing XBT station positions as black dots. Together with the reciprocal Southbound transect, showing CTD station positions as white circles. The cruise tracks followed the Southwest Indian and Madagascar Ridge, between the Prince Edward Islands (PEI) and 31_ S. Productivity stations are shown as pink circles and labelled (NP1–NP6). Black arrows mark the position of the Agulhas current (AC), the Agulhas Return Current (ARC) and the Antarctic Circumpolar current (ACC). Grey lines indicate the mean
frontal positions of the Sub Tropical Front (STF), the Sub Antarctic Front (SAF) and the Polar Front (PF) according to Orsi et al. (1995).
In the Ocean, the seasonal cycle is the mode that couples climate forcing to ecosystem response in production, diversity and carbon export. A better characterisation of the ecosystem’s seasonal cycle therefore addresses an important gap in our ability to estimate the sensitivity of the biological pump to climate change. In this study, the regional characteristics of the seasonal cycle of phytoplankton biomass in the Southern Ocean are examined in terms of the timing of the bloom initiation, its amplitude, regional scale variability and the importance of the climatological seasonal cycle in explaining the overall variance. The seasonal cycle was consequently defined into four broad zonal regions; the subtropical zone (STZ), the transition zone (TZ), the Antarctic circumpolar zone (ACZ) and the marginal ice zone (MIZ). Defining the Southern Ocean according to the characteristics of its seasonal cycle provides a more dynamic understanding of ocean productivity based on underlying physical drivers rather than climatological biomass. The response of the biology to the underlying physics of the different seasonal zones resulted in an additional classification of four regions based on the extent of inter-annual seasonal phase locking and the magnitude of the integrated seasonal biomass. This regionalisation contributes towards an improved understanding of the regional differences in the sensitivity of the Southern Oceans ecosystem to climate forcing, potentially allowing more robust predictions of the effects of long term climate trends.
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A schematic summarising the response of phytoplankton biomass to the underlying physics of the different seasonal regimes. Regions in blue represent regions of low (< 0.25 mgm−3) chlorophyll concentration with either high seasonal cycle reproducibility (R2 > 0.4) (Region A, light blue) or low seasonal cycle reproducibility (R2 < 0.4) (Region B, dark blue). Regions in green represent regions of high chlorophyll concentration ( > 0.25 mgm−3) with either high seasonal cycle reproducibility (Region C, dark green) or low seasonal cycle reproducibility (Region D, light green). Mean (1998–2007) frontal positions are shown for the STF (red), the SAF (black), the PF (orange) and the SACCF (blue).
Specific gravity is an important parameter in the dispersal of marine zooplankton, because the velocity of currents, and therefore the speed of transport, is usually greatest near the surface. For the South African chokka squid (Loligo reynaudii), recruitment is thought to be influenced by the successful transport of paralarvae from the spawning grounds to a food-rich feature known as the cold ridge some 100–200 km away. The role of paralarval specific gravity on such transport is investigated. Specific gravity ranged from 1.0373 to 1.0734 g cm−3 during the yolk-utilization phase, implying that paralarvae are always negatively buoyant, regardless of yolk content. The data were incorporated into a coupled individual-based model (IBM)—Regional Ocean Modelling System model. The output showed that dispersal was dominantly westward towards the cold ridge. Also, modelled paralarval vertical distribution suggested that hydrodynamic turbulence was an important factor in dispersal. The negative buoyancy of early chokka squid paralarvae may reduce the risk of paralarvae being advected off the eastern Agulhas Bank and into the open ocean, where food is less abundant, so specific gravity may be important in enhancing the survival and recruitment of chokka squid.
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The oceanic biological carbon pump (BCP), a large (10 GT C yr−1) component of the global carbon cycle, is dominated by the sinking (export) of particulate organic carbon (POC) from surface waters. In the deep ocean, strong correlations between downward fluxes of biominerals and POC (the so-called ‘ballast effect’) suggest a potential causal relationship, the nature of which remains uncertain. We show that similar correlations occur in the upper ocean with high rates of export only occurring when biominerals are also exported. Exported particles are generally biomineral rich relative to the upper ocean standing stock, due either to: (1) exported material being formed from the aggregation of a biomineral rich subset of upper ocean particles; or (2) the unfractionated aggregation of the upper ocean particulate pool with respiration then selectively removing POC relative to biominerals until particles are dense enough to sink.
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Figure 4 caption: POC (a) calcite, (b) opal ratios in exported and upper ocean particulate pools at 18 sites in the subpolar, subtropical and tropical Atlantic Ocean. Full symbols are from the AMT study [Thomalla
et al., 2008]. Empty symbols are new observations reported here from the Iceland Basin in 2007 (auxiliary
material). Note the broken axis required to include all data points.
This study analyzes the oceanic pathway connecting the Agulhas Bank to the southern Benguela upwelling system by means of a quantitative Lagrangian interpretation of the velocity field calculated by a high-resolution numerical simulation of the ocean around the southwestern tip of Africa. The regional ocean model is forced with National Centers for Environmental Prediction surface winds over 1993–2006 and offers a relevant numerical platform for the investigation of the variability of the water transferred between both regions, both on seasonal and intraseasonal time scales. We show that the intensity of the connection fluctuates in response to seasonal wind variability in the west coast upwelling system, whereas intraseasonal anomalies are mostly related to the organization of the eddy field along the southwestern edge of the Agulhas Bank. Though the study only considers passive advection processes, it may provide useful clues about the strategy adopted by anchovies in their selection of successful spawning location and period. The pathway under investigation is of major interest for the ecology of the southern Benguela upwelling system because it connects the spawning grounds on the Agulhas Bank with the nursery grounds located on the productive upwelling off the west coast.
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The addition of iron to high-nutrient, low-chlorophyll regions induces phytoplankton blooms that take up carbon1, 2, 3. Carbon export from the surface layer and, in particular, the ability of the ocean and sediments to sequester carbon for many years remains, however, poorly quantified3. Here we report data from the CROZEX experiment4 in the Southern Ocean, which was conducted to test the hypothesis that the observed north–south gradient in phytoplankton concentrations in the vicinity of the Crozet Islands is induced by natural iron fertilization that results in enhanced organic carbon flux to the deep ocean. We report annual particulate carbon fluxes out of the surface layer, at three kilometres below the ocean surface and to the ocean floor. We find that carbon fluxes from a highly productive, naturally iron-fertilized region of the sub-Antarctic Southern Ocean are two to three times larger than the carbon fluxes from an adjacent high-nutrient, low-chlorophyll area not fertilized by iron. Our findings support the hypothesis that increased iron supply to the glacial sub-Antarctic may have directly enhanced carbon export to the deep ocean5. The CROZEX sequestration efficiency6 (the amount of carbon sequestered below the depth of winter mixing for a given iron supply) of 8,600 mol mol-1 was 18 times greater than that of a phytoplankton bloom induced artificially by adding iron7, but 77 times smaller than that of another bloom8 initiated, like CROZEX, by a natural supply of iron. Large losses of purposefully added iron can explain the lower efficiency of the induced bloom6. The discrepancy between the blooms naturally supplied with iron may result in part from an underestimate of horizontal iron supply.
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