In situ grazing rates and daily ration of Antarctic krill Euphausia superba feeding on phytoplankton at the Antarctic Polar Front and the Marginal Ice Zone

Abstract

Measurements of krill gut pigment content, evacuation rates and digestive efficiency were obtained during January 1993 in the Atlantic sector of the Southern Ocean, between the Antarctic Marginal Ice Zone (MIZ) and the Polar Front Zone (PFZ). These \Yere combined with net and acoustically derived abundance and biomass data to estimate the in situ grazing of Euphausia superba on the phytoplankton assemblages. Individual ingestion rates of krill were 1.5 to 3 times higher than rates previously obtained with in vitro incubations. Gut pigment levels and evacuation rates varied in the range of 0.01 to 10 pg chlorophyll a equivalents (chl a equiv.) ind:' and 0.10 to 0.31 h-', respectively. Pigment losses to non-fluorescing products during digestion were very high, in the range of 67 to 90'): of the total pigment ingested, indicating that some of the gut pigment levels previously obtained w ~t h o u t correction fol-digestive losses may have been underestimated by up to an order of m a g n ~t u d e .