Zooplankton response to a phytoplankton bloom near South Georgia, Antarctica

Abstract

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 mv3 during the 8 d of sampling. The bloom comprised mainly long pennate diatoms and large colonies of centric diatoms; a 200 pm sieve retained over two-thirds of the chl a. Mesozooplankton biomass was high, 12.3 g dry mass me2 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, blomass-dominant copepods had secondary maxima rather deeper within the thermocline. D ~e l vertical migration was not a feature of this community, being limited to inetridlid and euchaet~id copepods. Gut fluorescence measurements on 7 large copepod species showed that all fed during both day and nlght, 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 d e c l ~n e 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 gjgas cleared diatoms (including the hlghly elongated 0.5 to 1 mm forms) at maximal rates. Metridla spp., Calanuspropinquus 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.