AMMONIUM CYCLING BY ANTARCTIC ZOOPLANKTON IN WINTER

Elemental composition and excretion rates of ammonium-nitrogen of zoop lankton, ranging over more than five orders of magnitude in body size, were measured in mid-winter in coastal waters west of the Antarctic P eninsula. Excretion rates were constant for the initial 12 h of incuba tion in the four species tested, and experimental stocking densities o f up to 126 mg dry wt l(-1) did not cause variability in the rate of a mmonium production. Weight-specific excretion rates of freshly caught Euchaeta antarctica, Conchoecia sp., Thysnnoessa macrura, Euphausin su perba, and early stage copepodites of Metridia gerlachei were not sign ificantly different from those reported in summer. However, adult cope pods of M. gerlachei and Calanoides acutus appear to have reduced thei r nitrogen metabolism during winter. Turnover rates of body nitrogen i ncreased with diminishing size, ranging from < 0.5% body N d(-1) for l arge E. superba to > 7% body N d(-1) for CII and CIII copepodites of M . gerlachei. Only the nitrogen turnover rates of C. acutus were suffic iently low as to suggest that it could survive the entire austral wint er without feeding. Phytoplankton and bacterioplankton were virtually absent in both the water column and the sea-ice. We conclude that carn ivory is the dominant trophic mode of the pelagic zooplankton communit y in Antarctica during winter. Production of ammonium-nitrogen by the zooplankton community probably accounts for < 10% of the total ammoniu m regenerated prior to the annual spring bloom.