GUT EVACUATION RATES AND PIGMENT DESTRUCTION IN THE ANTARCTIC KRILL EUPHAUSIA-SUPERBA

Krill grazing data collected during cruises in the region of the Antar ctic Polar Front (S.A. ''Agulhas'' Voyage 70) and the South Georgia sh elf (R,V. ''Africana'' Voyage 119) during the austral summer of 1993 w ere 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 ti mes (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 feedin g activity (estimated as initial gut pigment content, G(o)) but not to ambient chlorophyll a concentration. A significant difference was fou nd 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 hig hest recorded for zooplankton organisms, 58.1 to 98.4%, and did not co vary significantly (p > 0.05) with ambient food concentration. However , the pigment lost per individual krill was strongly correlated with t he total amount of pigment ingested(p < 0.001, r(2) = 0.99). We sugges t that both gut evacuation rates and pigment destruction efficiency ma y be realistically estimated only when krill is allowed to continue in gesting particles uninterruptedly. Charcoal particle concentration sho uld be equivalent to the in situ wet weight of total seston per unit v olume. An objective criterion for the standardization of the measureme nt and calculation of k values is also proposed.