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
Limnol. Oceanogr., 44(6), 1999, 1498-1508 | DOI: 10.4319/lo.19188.8.131.528
ABSTRACT: 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 (<0.8 and <2 mmol m-3, respectively), whereas nitrate concentrations remained relatively high throughout the two surveys (~14-30 mmol m-3). However, ammonium, a crucial reduced nitrogen source for South Georgia phytoplankton, was plentiful and widespread in the upper mixed layer during both surveys (maximum >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.