Gut evacuation rates and pigment destruction in the Antarctic krill Euphausia superba

Krill grazing data collected during cruises in the region of the Antarctic Polar Front (S.A. ''Agulhas'' Voyage 70) and the South Georgia shelf (R,V. ''Africana'' Voyage 119) during the austral summer of 1993 were analyzed to estimate the variability of crucial parameters of the gut fluorescence technique in relation to food availability and krill feeding history. Gut evacuation rates (k) and passage or throughput times (1/k) varied in the ranges of 0.101 to 0.424 h(-1) and 2.3 to 9.9 h and were strongly correlated (p <0.001, r(2) = 0.98) to krill feeding activity (estimated as initial gut pigment content, G(o)) but not to ambient chlorophyll a concentration. A significant difference was found when k values derived from incubations in filtered seawater and low charcoal particle concentrations (0.4 to 0.8 mg l(-1)) were compared with values derived from krill fed high concentrations of charcoal (6 mg l(-1)). The efficiency of gut pigment destruction was among the highest recorded for zooplankton organisms, 58.1 to 98.4%, and did not covary significantly (p > 0.05) with ambient food concentration. However, the pigment lost per individual krill was strongly correlated with the total amount of pigment ingested(p < 0.001, r(2) = 0.99). We suggest that both gut evacuation rates and pigment destruction efficiency may be realistically estimated only when krill is allowed to continue ingesting particles uninterruptedly. Charcoal particle concentration should be equivalent to the in situ wet weight of total seston per unit volume. An objective criterion for the standardization of the measurement and calculation of k values is also proposed.