Ammonium regeneration by Antarctic mesozooplankton: an allometric approach

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 co pepods and small euphausiids, and estimated the role of metazoans in recycl ing ammonium in the South Georgia region. Measurements were made during sum mer on animals ranging over about two orders of magnitude in body mass. A p hytoplankton bloom extended throughout the study area, and high C and dry m asses of late-stage copepodites suggested good recent feeding conditions. E xcretion rates declined roughly exponentially during the similar to1 day in cubations in filtered sea water. The patterns observed suggested that the o nset of starvation rather than the stress of capture caused this. Allometri c relationships between body mass in and excretion rate R were derived usin g the equation R = am(b). Large compilations of literature data produce a v alue 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 re flects peculiarities of polar environments, namely, lipid storage and diapa use 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 abundanc e and excretion rates from this study, we estimate that within the upper mi xed 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.