A comparison of chlorophyll/nutrient dynamics at two survey sites near South Georgia, and the potential role of planktonic nitrogen recycled by land-based predators

There is an apparent mismatch between the high carbon demand of seals and seabirds breeding on the subantarctic island of South Georgia and the overall low primary production measured in the waters that surround the island. However, average phytoplankton production values may not be completely representative, and local systems may exist where primary production is considerably higher Here. we examine the distribution of phytoplankton and nutrients along with physical oceanographic variables measured during two mesoscale surveys of two sites adjacent to South Georgia (January 1996 and December 1996- January 1997). Chlorophyll a concentrations were consistently higher (by up to an order of magnitude during one cruise) at the western end of the island (maximum >30 mg m(-3)). Surface phosphate and silicate at times appeared to have been depleted to particularly low levels (3 mmol m(-3)). An examination of upper mixed layer nutrient dynamics showed an apparent shortfall in phytoplankton use of nitrate-nitrogen compared with silicate and phosphate at the western end of the island, where ammonium-nitrogen use appeared greatest. The western end of subantarctic island of South Georgia is noted for its large numbers of breeding Antarctic fur seals, Arctocephalus gazella, and macaroni penguins, Eudyptes chrysolophus, (similar to 2.6 x 10(6) and similar to 5.4 x 10(6) individuals, respectively). As land- breeding endotherms, these animals have high metabolic costs, and they recycle dietary nitrogen rapidly. Furthermore, because they take krill advected into their foraging range and return frequently to their colonies to feed pups and chicks, they concentrate nutrients close to the land. We evaluated the relationship between the preferential use of reduced nitrogen by phytoplankton and its production and concentration by the land-based predators (a minimum of 1 x 10(7) mol N d(-1) in January for the colonies in the study area). We examined the predators' potential for the redistribution of plankton nitrogen in an on-shelf environment where currents were relatively sluggish and encompassed retentive, eddy-like structures, which might have facilitated more stable conditions for phytoplankton growth