CONTRIBUTION OF SALPS TO CARBON FLUX OF MARGINAL ICE-ZONE OF THE LAZAREV SEA, SOUTHERN-OCEAN

In order to estimate the in situ grazing rates of Salpa thompsoni and their implications for the development of phytoplankton blooms and for the sequestration of biogenic carbon in the high Antarctic, a repeat- grid survey and drogue study were carried out in the Lazarev Sea durin g austral summer of 1994/1995 (December/January). Exceptionally high g razing rates were measured for S. thompsoni at the onset of a phytopla nkton bloom (0.2 to 0.8 mu g chlorophyll a l(-1)) in December 1994, wi th up to similar or equal to 160 mu g of plant pigments consumed by an individual salp of 7 to 10 cm length per day. Dense salp swarms exten ded throughout the marginal ice zone, consuming up to 108% of daily ph ytoplankton production and 21% of the total chlorophyll a stock. Due t o the much faster sinking rates and higher carbon content of salp faec al pellets, the efficiency of downward carbon flux through salps is mu ch higher than through the other major grazers, krill and copepods. S. thompsoni can thus export large amounts of biogenic carbon from the e uphotic zone to the deep ocean. With the observed ingestion rates duri ng December 1994, this flux could have attained levels of up to 88 mg C m(-2) d(-1), accounting for the bulk of the vertical transport of ca rbon in the Lazarev Sea. However, in January 1995, when phytoplankton concentrations exceeded a threshold level of 1.0 to 1.5 mu g chlorophy ll a l(-1), salps experienced a drastic reduction in their feeding eff iciency, possibly as a result of clogging of their filtering apparatus . This triggered a dramatic reversal in the relationship, during which a dense phytoplankton bloom developed in conjunction with the collaps e of the salp population. Increases in the biomass and geographic rang e of the tunicate S. thompsoni have occurred in several areas of the s outhern ocean, often in parallel with a rise in sea-surface temperatur e during sub-decadal periods of warming anomalies.