Ammonium excretion by Antarctic krill Euphausia superba at South Georgia

Excretion by Antarctic krill Euphausia superba (hereafter "krill") is measu red typically in small containers of filtered seawater for 12-24 h, which m ay cause a reduction of swimming, feeding, and metabolism. if the maximum p ublished excretion rates are realistic, krill would be a major source of re generated nitrogen in the South Georgia area because of their high biomass there. Because literature values are variable, depending on season, feeding history and the experimental set-up, our aim was to measure both a mean an d an upper value of krill excretion rate at South Georgia. Experiments were on juvenile krill during October-November 1997 and January 1998. Freshly c aught animals excreted 1.6-2.8 nmol ammonium mg(-1) dry mass h(-1); within the fivefold range of summer literature values for equivalent-sized krill. Maximum rates were determined on acclimated krill in large containers durin g alternating 1-d periods with and without food. During the feeding periods in saturating food concentrations, the mean daily ration was similar to 32 % of body carbon d(-1) and excretion was 210% (October-November) and 280% ( January) of the values for freshly caught krill. This equates to a maximum loss of similar to 2% of body nitrogen d(-1). Excretion rates decreased dur ing the 1-d periods without food, and rates during the feeding periods were similar to 30% higher than in those without food. This suggests that the l ack of feeding in traditional experiments leads to roughly 30% underestimat es of excretion rate. These results help to set some limits on ammonium pro duction rates of South Georgia krill over regional scales. Our calculations suggest that the role of krill, in this varies between habitats to the wes t of the island (insignificant) and those in the east (significant).