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 Antarctic station of Davis (68° 35' S, 77° 58' E). The study period, 14 January to 11 February 1994, coincided with the growth and decline of a diatom-dominated phytoplankton bloom. Nine taxa of heterotrophic dinoflagellates, including 2 naked and 7 armoured forms, were identified and selected for the determination of standing stock and grazing rates. All 9 taxa selected for grazing rate measurements showed an increase in abundance and biomass during the phytoplankton bloom. Total abundance and biomass increased exponentially from 14 January to reach a maximum abundance, when the 9 taxa were combined, of 46400 cells l-1 on the 31 January, equivalent to a standing stock of 114.5 µg C l-1. Taxon-specific grazing rates were determined at in situ predator and prey concentrations by tracing 14C through a 3 compartment (water, phytoplankton, heterotrophic dinoflagellate) model. Mean taxon-specific clearance rates varied more than 10-fold from 0.028 µl cell-1 h-1 in Diplopeltopsis spp. to 0.318 µl cell-1 h-1 in a Protoperidinium sp. In contrast, mean taxon-specific rates of ingestion varied only 3-fold from 0.72 pg chl a cell-1 h-1 in Diplopeltopsis 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 taxa, Gyrodinium sp.1, Gyrodinium sp.2 and Diplopeltopsis spp. This activity represented 6.7% of the water column cleared, 4.8% of autotrophic biomass and 25% of daily primary production ingested per day. The estimated rates of specific ingestion and growth of heterotrophic dinoflagellates were lower in the coastal waters of East Antarctica than in laboratory studies carried out at higher temperatures. However, when the environmental parameters, predator size and prey type and concentration are taken into account, values measured in the present study are tenable, comparing well with other in situ measurements. Furthermore, the estimates of grazing impact on phytoplankton biomass and production illustrate that heterotrophic dinoflagellates play an important part in the biotic control of phytoplankton production and therefore of carbon flux through the food web of coastal waters of East Antarctica.

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