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 were 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 within vitro incubations. Gut pigment levels and evacuation rates varied in the range of 0.01 to 10 μg chlorophyll a equivalents (chl a equiv.) ind.-1 and 0.10 to 0.31 h-1, 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 without correction for digestive losses may have been underestimated by up to an order of magnitude. Krill population impact on the phytoplankton stock exhibited a large variability, in the range of 0.0014 to 2.68% of total integrated chlorophyll a and 0.023 to 50.8% of primary production consumed per day. The largest variations in impact levels were associated with the method used to estimate krill abundance and biomass, with net derived estimates being much lower (by as much as 2 orders of magnitude) than those obtained from acoustic data. Daily carbon rations obtained from our measurements of pigment ingestion rates are among the lowest recorded for E. superba during the summer season and, with 1 exception, ranged between 0.15 and 1.68% of body carbon per day. A daily ration of ~13% body carbon was recorded only at 1 station in the MIZ which exhibited a dense phytoplankton bloom of ~3.5 μg chl l-1. On the basis of the energetic requirements of the summer krill population, it is suggested that throughout the PFZ and the MIZ E. superba must consume a much larger proportion of heterotrophic carbon than previously supposed. Gut content analysis suggests that this is achieved by predation on meso- and microzooplankton.