ZOOPLANKTON RESPONSE TO A PHYTOPLANKTON BLOOM NEAR SOUTH GEORGIA, ANTARCTICA

A shelf site near the island of South Georgia was sampled during a spr ing bloom in January 1994. Chlorophyll a (chi a) values declined from 19 to 6 mg m(-3) during the 8 d of sampling. The bloom comprised mainl y long pennate diatoms and large colonies of centric diatoms; a 200 mu m sieve retained over two-thirds of the chi a. Mesozooplankton biomas s was high, 12.3 g dry mass m(-2) within the top 200 m, and comprised mainly copepods. A series of Longhurst Hardy Plankton Recorder profile s showed that the numerical dominants (Oithona similis, Drepanopus for cipatus and pteropods) resided mainly within the top 20 m, whereas the large, biomass-dominant copepods had secondary maxima rather deeper w ithin the thermocline. Diel vertical migration was not a feature of th is community, being limited to metridiid and euchaetiid copepods. Gut fluorescence measurements on 7 large copepod species showed that all f ed during both day and night, although guts tended to be fullest durin g afternoon and night. About 20% of chi a grazed by these copepods occ urred below the mixed layer, thus representing a potentially direct ex port of carbon from the system via sinking faecal pellets. Algal carbo n rations (% body carbon ingested per day) of mixed layer copepods ran ged from 3% (Rhincalanus gigas) to 20% (small copepods). With the exce ption of R. gigas, these values from gut fluorescence agreed with inde pendent estimates from the site following the decline of chi a during incubations in ambient seawater. Despite low clearance sates, ingestio n rates (per copepodid) were at the upper end of recorded Antarctic va lues, suggesting food saturation. Calanoides acutus and R. gigas clear ed diatoms (including the highly elongated 0.5 to 1 mm forms) at maxim al rates, Metridia spp., Calanus propinquus and small copepods, by con trast, cleared dinoflagellates and ciliates faster than diatoms of sim ilar size. The total mixed layer zooplankton probably removed < 5% of daily primary production and < 5% of protozoan standing stocks per day .