MICROBIAL DYNAMICS IN COASTAL WATERS OF EAST ANTARCTICA - HERBIVORY BY HETEROTROPHIC DINOFLAGELLATES

Heterotrophic dinoflagellates and their herbivory were quantified at a coastal site in East Antarctica in the vicinity of the Australian Ant arctic station of Davis (68 degrees 35' S, 77 degrees 58' E). The stud y period, 14 January to 11 February 1994, coincided with the growth an d decline of a diatom-dominated phytoplankton bloom. Nine taxa of hete rotrophic dinoflagellates, including 2 naked and 7 armoured forms, wer e identified and selected for the determination of standing stock and grazing rates. All 9 taxa selected for grazing rate measurements showe d an increase in abundance and biomass during the phytoplankton bloom. Total abundance and biomass increased exponentially from 14 January t o reach a maximum abundance, when the 9 taxa were combined, of 46 400 cells l(-1) on the 31 January, equivalent to a standing stock of 114.5 mu g C l(-1). Taxon-specific grazing rates were determined at in situ predator and prey concentrations by tracing C-14 through a 3 compartm ent (water, phytoplankton, heterotrophic dinoflagellate) model. Mean t axon-specific clearance rates varied more than 10-fold from 0.028 mu l cell(-1) h(-1) in Diplopeltopsis spp, to 0.318 mu l cell(-1) h(-1) in a Protoperidinium sp. In contrast, mean taxon-specific rates of inges tion varied only 3-fold from 0.72 pg chl a cell(-1) h(-1) in Diplopelt opsis spp. to 2.38 pg chl a cell(-1) h(-1) in the same Protoperidinium sp. The total ingestion rate of the 9 taxa was 29.7 ng chl a l(-1) h( -1) on 31 January, of which 92 % was consumed by the 3 most abundant t axa, Gyrodinium sp.1, Gyrodinium sp.2 and Diplopeltopsis spp. This act ivity represented 6.7 % of the water column cleared, 4.8 % of autotrop hic biomass and 25 % of daily primary production ingested per day. The estimated rates of specific ingestion and growth of heterotrophic din oflagellates were lower in the coastal waters of East Antarctica than in laboratory studies carried out at higher temperatures. However, whe n the environmental parameters, predator size and prey type and concen tration are taken into account, values measured in the present study a re tenable, comparing well with other in situ measurements. Furthermor e, the estimates of grazing impact on phytoplankton biomass and produc tion illustrate that heterotrophic dinoflagellates play an important p art in the biotic control of phytoplankton production and therefore of carbon flux through the food web of coastal waters of East Antarctica .