Ammonium regeneration by Antarctic mesozooplankton: an allometric approach

Abstract

Given the importance of copepods in the Southern Ocean food web, there are few assessments of their N budgets or their role in regenerating N. In this study we measured elemental composition and ammonium-excretion rates of copepods and small euphausiids, and estimated the role of metazoans in recycling ammonium in the South Georgia region. Measurements were made during summer on animals ranging over about two orders of magnitude in body mass. A phytoplankton bloom extended throughout the study area, and high C and dry masses of late-stage copepodites suggested good recent feeding conditions. Excretion rates declined roughly exponentially during the ~1 day incubations in filtered sea water. The patterns observed suggested that the onset of starvation rather than the stress of capture caused this. Allometric relationships between body mass m and excretion rate R were derived using the equation R=amb. Large compilations of literature data produce a value of b (the body-mass scaling coefficient) of 0.7–0.8. However, in this study, b ranged from 0.57 (for C as the unit of body mass) to 0.71 for N as the unit of body mass. Such low values are also common to previous studies of feeding and excretion among Antarctic copepods. We suggest that this reflects peculiarities of polar environments; namely, lipid storage and diapause in the largest copepods. Previous studies have suggested that ammonium is a preferred N source for algae at South Georgia. Based on the monitoring of a region to the north-east of South Georgia and on zooplankton abundance and excretion rates from this study, we estimate that within the upper mixed layer the copepods and small euphausiids excrete at least one third of the ammonium potentially required by phytoplankton. Krill excretion in this area was measured in a previous study, and it appears that mesozooplankton and krill are together significant regenerators of N in parts of the South Georgia pelagic system.