Zooplankton response to a phytoplankton bloom near South Georgia, Antarctica

A shelf site near the island of South Georgia was sampled during a spring bloom in January 1994. Chlorophyll a (chl a) values declined from 19 to 6 mg m-3 during the 8 d of sampling. The bloom comprised mainly long pennate diatoms and large colonies of centric diatoms; a 200 µm sieve retained over two-thirds of the chl a. Mesozooplankton biomass 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 profiles showed that the numerical dominants (Oithona similis, Drepanopus forcipatus and pteropods) resided mainly within the top 20 m, whereas the large, biomass-dominant copepods had secondary maxima rather deeper within the thermocline. Diel vertical migration was not a feature of this community, being limited to metridiid and euchaetiid copepods. Gut fluorescence measurements on 7 large copepod species showed that all fed during both day and night, although guts tended to be fullest during afternoon and night. About 20% of chl a grazed by these copepods occurred below the mixed layer, thus representing a potentially direct export of carbon from the system via sinking faecal pellets. Algal carbon rations (% body carbon ingested per day) of mixed layer copepods ranged from 3% (Rhincalanus gigas) to 20% (small copepods). With the exception of R. gigas, these values from gut fluorescence agreed with independent estimates from the site following the decline of chl a during incubations in ambient seawater. Despite low clearance rates, ingestion rates (per copepodid) were at the upper end of recorded Antarctic values, suggesting food saturation. Calanoides acutus and R. gigas cleared diatoms (including the highly elongated 0.5 to 1 mm forms) at maximal rates. Metridia spp., Calanus propinquus and small copepods, by contrast, cleared dinoflagellates and ciliates faster than diatoms of similar size. The total mixed layer zooplankton probably removed <5% of daily primary production and <5% of protozoan standing stocks per day.