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 s eabirds breeding on the subantarctic island of South Georgia and the overal l low primary production measured in the waters that surround the island. H owever, average phytoplankton production values may not be completely repre sentative, and local systems may exist where primary production is consider ably higher Here. we examine the distribution of phytoplankton and nutrient s 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 app eared to have been depleted to particularly low levels (<0.8 and <2 mmol m( -3), respectively), whereas nitrate concentrations remained relatively high throughout the two surveys (similar to 14-30 mmol m(-3)). However. ammoniu m, a crucial reduced nitrogen source for South Georgia phytoplankton, was p lentiful and widespread in the upper mixed layer during both surveys (maxim um >3 mmol m(-3)). An examination of upper mixed layer nutrient dynamics sh owed an apparent shortfall in phytoplankton use of nitrate-nitrogen compare d with silicate and phosphate at the western end of the island, where ammon ium-nitrogen use appeared greatest. The western end of subantarctic island of South Georgia is noted for its la rge numbers of breeding Antarctic fur seals, Arctocephalus gazella, and mac aroni penguins, Eudyptes chrysolophus, (similar to 2.6 x 10(6) and similar to 5.4 x 10(6) individuals, respectively). As land-breeding endotherms, the se animals have high metabolic costs, and they recycle dietary nitrogen rap idly. Furthermore, because they take krill advected into their foraging ran ge and return frequently to their colonies to feed pups and chicks, they co ncentrate nutrients close to the land. We evaluated the relationship betwee n the preferential use of reduced nitrogen by phytoplankton and its product ion and concentration by the land-based predators (a minimum of 1 x 10(7) m ol 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-s helf environment where currents were relatively sluggish and encompassed re tentive, eddy-like structures, which might have facilitated more stable con ditions for phytoplankton growth.