Comparative abundance and distribution of major filter-feeders in the Antarctic pelagic zone

The filter-feeding plankton, herbivorous copepods, salps and euphausiids, form the basic level of metazoans in the Antarctic pelagic trophic web. This paper sets out to determine the comparative share of these taxonomic groups in the total biomass and annual production. Their most abundant representatives, four copepod species (Calanus propinquus, Calanoides acutus, Rhincalanus gigas and Metridia gerlachei), all salps and krill Euphausia superba were studied. For the first two groups net samples from six Russian expeditions in different sectors of the Antarctic were used. In total 752 samples from 118 stations were considered. The mean fresh biomass of filter-feeding copepods in the 0-1500 m layer was 18.0 g m(-2) and in the entire Antarctic 576 10(6) t. The biomass of salps in comparatively restricted rich regions exceeded 500 g m(-2) and in the remaining area was 1.2 +/- 0.8 g m(-2), giving a total quantity of 882 10(6) t. The krill abundance estimation was based on published data, using a map of its quantitative distribution compiled from commercial trawling made by Soviet fishing and scientific ships during 17 seasons [Parfenovich, S.S., 1980. O zakonomernostyakh razmeshcheniya i regionalnoi differentsiatsii mestoskoplenii krilya v Yuzhnom Okeane. VNIRO, Moskva, in Russian.]. Three main zones based on commercial characteristics were determined by this author: (I) zone of regular occurrence of dense concentrations; (2) zone of rare occurrence of concentrations; (3) zone of low-abundance dispersed krill. All available data on E. superba biomass in the Antarctic were grouped together according to these zones and their means were calculated. The biomass of krill was found to be 60.1 +/- 11.2, 3.3 +/- 1.3 and 0.8 +/- 0.4 g m(-2) fresh mass in zones i, 2 and 3, respectively, with a total of 272 10(6) t. All estimates are compared with the literature data and their validity is discussed. For the annual production determinations the obtained biomass characteristics were multiplied by published P/B coefficients. For the copepods its value was taken as 4.4 and for krill as 0.79. The total annual production of copepods was estimated as 2534 10(6) t and that of krill as 215 10(6) t (fresh mass). Salp production could not be evaluated, for lack of a sufficient database. In terms of fresh mass, the main share of filter-feeder biomass on the entire Antarctic scale belongs to salps, due to their local maxima; the second place belongs to copepods and krill comes in the third position. But after recalculation in terms of dry mass, copepods shift to the first place and salps to the last. The annual production of copepods significantly exceeds that of krill.