The South Georgia region is characterised by high biomass and productivity of phytoplankton, zooplankton and vertebrate predators. Important commercial fisheries have been based at the island since the late 1700s, initially exploiting seals and whales, and currently taking krill Euphausia superba and finfish. Despite studies dating from the beginning of the last century, the causes of the high productivity remain unclear. The island lies within the Antarctic Zone of the Antarctic Circumpolar Current, to the south of the Polar Front. The offshore waters to its north and east are affected by a northwards deflection of the Southern Antarctic Circumpolar Current Front, together with waters from the Weddell-Scotia Confluence. Despite a retentive circulation over the shelf, the high productivity of phytoplankton and copepods is widespread, occurring far downstream and possibly extending to the Polar Front. High phytoplankton concentrations (>20 mg chlorophyll a m , equally dominated by krill and copepods. This greatly exceeds typical values for Antarctica, and is high compared to productive northern shelves. Zooplankton, and in particular krill, appear to have a pivotal role in regulating energy flow in this food web, through selective grazing and possibly also through nutrient regeneration. Abundances of krill and copepods are negatively related across a wide range of scales, suggesting direct interaction through competition or predation. Evidence suggests that when phytoplankton stocks are low, energy flow through krill is maintained by their feeding on the large populations of small copepods. Metazoans and higher predators at South Georgia can feed across several trophic levels according to prey abundance, and they regenerate substantial quantities of reduced nitrogen. Therefore we suggest that these groups have a controlling influence on lower trophic levels, both stabilising population sizes and maintaining high rates of energy flow. Hydrography, nutrient concentrations, phytoplankton, copepod, and krill biomasses fluctuate between years. Periodically (once or twice a decade), shortages of krill in summer result in breeding failures among many of their predators. This appears to be a downstream effect from wider scale, Scotia Sea phenomena, although the processes involved are unclear. The elevated biomass and energy flows at South Georgia appear to be caused by locally enhanced growth rates; there is no evidence so far for any physical concentration mechanism. Even for krill, which do not breed there, local growth rates are probably of a similar order to the biomass removed by their main land-based predators in summer. Thus the transfer of energy to higher predators depends on local enhancement of fluxes through the food web as well as the supply of plankton to the island by the Antarctic Circumpolar Current.

No abstract

Interannual variability is a characteristic feature of the Southern Ocean ecosystem, yet the relative roles of biological and physical processes in generating these fluctuations are unknown. There is now extensive evidence that there are years when there is a very low abundance of Antarctic krill (Euphausia superba) in the South Georgia area, and that this variation affects much of the ecosystem, with the most obvious impacts on survival and breeding success of some of the major predators on krill. The open nature of the South Georgia ecosystem means this variability has large‐scale relevance, but even though there are unique time series of data available, information on some key processes is limited. Fluctuations in year‐class success in parts, or all, of the krill population across the Scotia Sea can generate large changes in the available biomass. The ocean transport pathways maintain the large‐scale ecosystem structure by moving krill over large distances to areas where they are available to predator colonies. This large‐scale physical system shows strong spatial and temporal coherence in the patterns of the interannual and subdecadal variability. This physical variability affects both the population dynamics of krill and the transport pathways, emphasizing that both the causes and the consequences of events at South Georgia are part of much larger‐scale processes.

No abstract

Nitrate utilization and ammonium utilization were studied by using three algal isolates, six bacterial isolates, and a range of temperatures in chemostat and batch cultures. We quantified affinities for both substrates by determining specific affinities (specific affinity = maximum growth rate/half-saturation constant) based on estimates of kinetic parameters obtained from chemostat experiments. At suboptimal temperatures, the residual concentrations of nitrate in batch cultures and the steady-state concentrations of nitrate in chemostat cultures both increased. The specific affinity for nitrate was strongly dependent on temperature (Q10 ≈ 3, where Q10 is the proportional change with a 10°C temperature increase) and consistently decreased at temperatures below the optimum temperature. In contrast, the steady-state concentrations of ammonium remained relatively constant over the same temperature range, and the specific affinity for ammonium exhibited no clear temperature dependence. This is the first time that a consistent effect of low temperature on affinity for nitrate has been identified for psychrophilic, mesophilic, and thermophilic bacteria and algae. The different responses of nitrate uptake and ammonium uptake to temperature imply that there is increasing dependence on ammonium as an inorganic nitrogen source at low temperatures.

No abstract

No abstract

Summary 1. The Southern Ocean is a large‐scale, relatively homogeneous upwelling ecosystem whose phytoplankton apparently grows suboptimally over much of its area. By contrast there is a wide variety of freshwater habitats in the Antarctic and in some of these phytoplankton growth efficiency is very high. The two habitats share similar temperature and irradiance regimes, but differ markedly in availability of inorganic nutrients, in grazing pressure and in the time‐ and space‐scales on which various physical processes act. 2. Concentrations of inorganic nutrients in the marine ecosystem have been represented as being in excess of phytoplankton requirements, but the ionic composition of some nutrient pools may not conform to phytoplankton preferences. 3. Nutrient‐limitation determines phytoplankton production in Antarctic lakes and gives rise to gross differences between lakes. 4. Irradiance in the water column varies greatly over the year in both marine and freshwater ecosystems. Most algae are shade‐adapted, with the ability to utilize low irradiance but with sub‐optimal response to high irradiance. However, local phytoplankton maxima may attain very high carbon fixation and growth rates. 5. Consistently low temperatures characterize both systems. Their effects on photo‐synthetic carbon uptake mirror shade‐adaptation. Division rates of marine phytoplankton may however be very much higher than predicted for ambient temperatures. 6. Vertical mixing is important in both ecosystems and influences the environment experienced by phytoplankton cells. This appears to have little effect on the average performance of phytoplankton in the strongly mixed surface water column of the Southern Ocean, where the mixed depth may exceed 100 m. This can be related partly to the shade‐adapted photosynthetic response. Euphotic depths range from 20 to 100 m. 7. Strong vertical mixing under ice‐free conditions in lakes may maximize photosynthetic efficiency, whilst distinct vertical stratification in permanently ice‐covered lakes gives rise to segregation of nutrient uptake and regeneration. 8. Physical removal of phytoplankton biomass by grazing is locally important in the Southern Ocean, in contrast to the estimated mean mesoscale impact of grazing. Vertical sedimentation losses appear important in the context of mixing depth and generation time, and may be modified by vertical circulation of water. 9. Loss of phytoplankton biomass from lakes during the ice‐free period is dominated by physical removal via the lake outflow. Grazing is generally unimportant, except where larvae of otherwise nektobenthic zooplankton hatch in synchrony with a phytoplankton maximum. Sedimentation is important under ice‐cover.