Rebecca E. Korb

Spatial and temporal operation of the Scotia Sea ecosystem: a review of large-scale links in a krill centred food web

The Scotia Sea ecosystem is a major component of the circumpolar Southern Ocean system, where productivity and predator demand for prey are high. The eastward-flowing Antarctic Circumpolar Current (ACC) and waters from the Weddell–Scotia Confluence dominate the physics of the Scotia Sea, leading to a strong advective flow, intense eddy activity and mixing. There is also strong seasonality, manifest by the changing irradiance and sea ice cover, which leads to shorter summers in the south. Summer phytoplankton blooms, which at times can cover an area of more than 0.5 million km2, probably result from the mixing of micronutrients into surface waters through the flow of the ACC over the Scotia Arc. This production is consumed by a range of species including Antarctic krill, which are the major prey item of large seabird and marine mammal populations. The flow of the ACC is steered north by the Scotia Arc, pushing polar water to lower latitudes, carrying with it krill during spring and summer, which subsidize food webs around South Georgia and the northern Scotia Arc. There is also marked interannual variability in winter sea ice distribution and sea surface temperatures that is linked to southern hemisphere-scale climate processes such as the El Niño–Southern Oscillation. This variation affects regional primary and secondary production and influences biogeochemical cycles. It also affects krill population dynamics and dispersal, which in turn impacts higher trophic level predator foraging, breeding performance and population dynamics. The ecosystem has also been highly perturbed as a result of harvesting over the last two centuries and significant ecological changes have also occurred in response to rapid regional warming during the second half of the twentieth century. This combination of historical perturbation and rapid regional change highlights that the Scotia Sea ecosystem is likely to show significant change over the next two to three decades, which may result in major ecological shifts.

An anticyclonic circulation above the Northwest Georgia Rise, Southern Ocean

Data from a variety of sources reveal a warm-core anticyclonic circulation above the Northwest Georgia Rise (NWGR), an similar to2000-m high bathymetric feature north of South Georgia. The sense of the circulation is opposite to the general cyclonic flow in the Georgia Basin. The circulation shows the characteristics of a stratified Taylor column: dimensional analysis shows that the local bathymetry and hydrography are conducive to the formation of such. ERS2 altimeter data show that the column, whilst not fully permanent, is nonetheless a recurring feature. High concentrations of chlorophyll-a are observed at the centre of the circulation, indicating that the modulation of the physical environment has significant consequences for the local biogeochemical system via enhanced primary production. Enhanced chlorophyll-a extends in a long plume from the NWGR along pathways indicated by drifters; this passive redistribution may have consequences for the larger (basin-) scale ecosystem.

The Southern Antarctic Circumpolar Current Front: physical and biological coupling at South Georgia

The coupling of physics and biology was examined along a 160 km long transect running out from the north coast of South Georgia Island and crossing the Southern Antarctic Circumpolar Current Front (SACCF) during late December 2000. Surface and near surface potential TS properties indicated the presence of three water types: a near-shore group of stations characterised by water which became progressively warmer and fresher closer to South Georgia, an offshore grouping in which sea surface temperatures and those at the winter water level were relatively warm (1.81C and 0.51C, respectively), and a third in which surface and winter water temperatures were cooler and reflected the presence of the SACCF. The transect bisected the SACCF twice, revealing that it was flowing in opposite directions, north-westward closest to South Georgia and south-eastwards at its furthest point from the island. The innermost limb was a narrow intense feature located just off the shelf break in 2000–3500 m of water and in which rapid surface baroclinic velocities (up to 35 cm s � 1 ) were encountered. Offshore in the outermost limb, shown subsequently to be a mesoscale eddy that had meandered south from the retroflected limb of the SACCF, flow was broader and slower with peak velocities around 20 cm s � 1 . Chlorophyll a biomass was generally low (o 1m g m � 3 ) over much of the transect but increased dramatically in the region of the innermost limb of the SACCF, where a deepening of the surface mixed layer was coincident with a subsurface chlorophyll maximum (7.4 mg m � 3 ) and elevated concentrations down to 100 m. The bloom was coincident with depleted nutrient concentrations, particularly silicate, nitrate and phosphate, and although ammonium concentrations were locally depleted the bloom lay within an elevated band (up to 1.5 mmol m � 3 ) associated with the frontaljet. Increased zoopl ankton abundance, higher copepod body carbon mass and egg production rates all showed a strong spatial integrity with the front. The population structure of the copepods Calanoides acutus and Rhincalanus gigas at stations within the front suggested that rather than simply resulting from entrainment and concentration within the jet, increased copepod abundance was the result of development in situ. Estimates of bloom duration, based on silicate and carbon budget calculations, set the likely duration between 82 and 122 d, a figure supported by the development schedule of the two copepod species. Given this timescale, model outputs from FRAM and OCCAM indicated that particles that occurred on the north

Rapid changes in surface water carbonate chemistry during Antarctic sea ice melt

The effect of sea ice melt on the carbonate chemistry of surface waters in the Weddell–Scotia Confluence, Southern Ocean, was investigated during January 2008. Contrasting concentrations of dissolved inorganic carbon (DIC), total alkalinity (TA) and the fugacity of carbon dioxide (fCO2) were observed in and around the receding sea ice edge. The precipitation of carbonate minerals such as ikaite (CaCO3·6H2O) in sea ice brine has the net effect of decreasing DIC and TA and increasing the fCO2 in the brine. Deficits in DIC up to 12 ± 3 ¼mol kg-1 in the marginal ice zone (MIZ) were consistent with the release of DIC-poor brines to surface waters during sea ice melt. Biological utilization of carbon was the dominant processes and accounted for 41 ± 1 ¼mol kg-1 of the summer DIC deficit. The data suggest that the combined effects of biological carbon uptake and the precipitation of carbonates created substantial undersaturation in fCO2 of 95 ìatm in the MIZ during summer sea ice melt. Further work is required to improve the understanding of ikaite chemistry in Antarctic sea ice and its importance for the sea ice carbon pump.

A comparison of satellite and cruise chlorophyll-a measurements in the Scotia Sea, Antarctica

We compared both SeaWiFS (Sea-viewing Wide Field-of-View Sensor) and MODIS (Moderate-resolution Imaging Spectroradiometer) chlorophyll-a (chl-a) measurements with simultaneous ship based data obtained during a 2003 British Antarctic Survey (BAS) research cruise. This cruise provided in situ data from a large area of the Scotia Sea containing areas of extreme contrasts in terms of chl-a concentration. We present the results of correlation analysis between the in situ ship based chl-a measurements and the satellite chl-a products (SeaWiFS, and from MODIS the semi-analytic, SeaWiFS analog OC3M, HPLC empirical algorithms). The results confirm the good correlation between SeaWiFS and in situ chl-a measurements. The results indicate Terra MODIS chl-a measurements show reduced correlation to in situ values when compared to SeaWiFS. In addition, we compared chl-a averages from the various algorithms, over wider geographical regions of greater ecological relevance than point measurements. Over an area of 3/spl deg/ /spl times/ 3/spl deg/, SeaWiFS estimates could be as much as 2 times higher than estimates from MODIS.