The trophic role of the tunicate Salpa thompsoni in the Antarctic marine ecosystem

During a repeat grid survey and drogue study carried out in austral summer 1994/95, the abundance and feeding activity of salps were estimated in the Lazarev Sea region from net tows and in situ measurements of gut fluorescen ce. Throughout the survey area, Salpa thompsoni accounted for > 95% of the total salp stock while Ihlea racovitzai was consistently represented in ver y low abundances. Maximum densities of S. thompsoni, with approximate to 40 00 ind. 1000 m(-3), were recorded in the Marginal Ice Zone (MIZ) in Decembe r when chlorophyll-a concentrations were well below 1 mg m(-3). A dramatic decrease in salp stock was observed at the beginning of January, when S. th ompsoni virtually disappeared from the most productive area of the MIZ wher e chlorophyll-a concentrations had by then reached bloom levels of 1.5-3 mg (Chl-a) m(-3). In situ grazing measurements showed that throughout the cru ise S. thompsoni exhibited the highest ingestion rates per individual of an y of the most abundant components of the grazing pelagic community, with ma xima of approximate to 160 mu g (pigm) ind.(-1) d(-1). These feeding rates are 3 to 5 times higher than those previously obtained using in vitro incub ations. The total daily consumption of the population of S. thompsoni varie d from 0.3 to 108% of daily primary production. We suggest that competitive removal of food by S. thompsoni, rather than direct predation, is responsi ble for the low krill abundances generally associated with salp swarms.