Ammonium excretion by Antarctic krill Euphausia superba at South Georgia

Abstract

Excretion by Antarctic krill Euphausia superba (hereafter ''krill'') is measured typically in small containers of filtered seawater for 12-24 h, which may cause a reduction of swimming, feeding, and metabolism. If the maximum published excretion rates are realistic, krill would be a major source of regenerated 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 and an upper value of krill excretion rate at South Georgia. Experiments were on juvenile krill during October-November 1997 and January 1998. Freshly caught animals excreted 1.6-2.8 nmol ammonium mg Ϫ1 dry mass h Ϫ1; within the fivefold range of summer literature values for equivalent-sized krill. Maximum rates were determined on acclimated krill in large containers during alternating 1-d periods with and without food. During the feeding periods in saturating food concentrations, the mean daily ration was ϳ32% of body carbon d Ϫ1 and excretion was 210% (October-November) and 280% (January) of the values for freshly caught krill. This equates to a maximum loss of ϳ2% of body nitrogen d Ϫ1 . Excretion rates decreased during the 1-d periods without food, and rates during the feeding periods were ϳ30% higher than in those without food. This suggests that the lack of feeding in traditional experiments leads to roughly 30% underestimates of excretion rate. These results help to set some limits on ammonium production rates of South Georgia krill over regional scales. Our calculations suggest that the role of krill in this varies between habitats to the west of the island (insignificant) and those in the east (significant).

Krill excretion and its effect on primary production

“…3B). As expected, the compilation of our experimental krill ammonium excretion rates with the results of Huntley & Nordhausen (1995) and those of Atkinson & Whitehouse (2000) indicated a decreasing krill excretion rate with the length of incubation (Fig. 4).…”

“…Krill excretion rates measured shortly after capture were relatively high compared with published excretion rates (Hirche 1983, Huntley & Nordhausen 1995, Hernández-León et al 1999, Atkinson & Whitehouse 2000. In the absence of food, Euphausia superba has proven its resilience to starvation for > 200 d (Ikeda & Dixon 1984) by not only adopting strategies of moulting, cannibalism and shrinking in size (Ikeda & Mitchell 1982), but also by reducing its excretion rates to extremely low levels (Hirche 1983, George & Fields 1984, Atkinson & Whitehouse 2000.…”

“…Therefore, zooplankton should control primary production through both grazing and excretion. Swarms of Antarctic krill Euphausia superba represent important sources of ammonium (Atkinson & Whitehouse 2000) since large nitrogen concentrations were found within the aggregation (Johnson et al 1984, as cited in Priddle et al 1997). Swarm-forming krill should cause a temporary depletion of both copepods and phytoplankton through grazing and predation but at the same time should induce optimal phytoplankton bloom-forming conditions through the release of ammonium (Tovar-Sánchez et al 2007).…”

Diffusive boundary layers of the colony‐forming plankton alga, Phaeocystis sp.—implications for nutrient uptake and cellular growth

“…In Atkinson and Whitehouse (2000) the last two sentences of the paragraph ending at the top of the right column of p. 57 should read:…”

The potential role of ammonia as a signal molecule for procellariiform seabirds

“…These compounds include nitrogen waste products which, like DMS, may provide birds with proximate cues for detecting krill swarms. For example, freshly caught Antarctic krill Euphausia superba Dana excrete ammonium (NH 4 + ) on the order of 1.6 to 2.8 nmol mg -1 dry mass h -1 (Atkinson & Whitehouse 2000). Calculations from laboratory, ship-based, experiments suggest that copepods and small euphausids excrete ammonium into the pelagic zone around South Georgia Island at a rate of approximately 1 mmol m -2 d -1 (Atkinson & Whitehouse 2001).…”