NITROGEN-EXCRETION BY SOME DEMERSAL MACROZOOPLANKTON IN HERON AND ONE-TREE REEFS, GREAT-BARRIER-REEF, AUSTRALIA

Nitrogen excretion rates of demersal macrozooplankton were measured to gether with nitrogen concentrations in the water column and sediments in lagoons of Heron Reef and One Tree Reef, Great Barrier Reef, Austra lia, during August and November 1991. Excretion rates increased with b ody weight, and weight-specific excretion rates of the demersal macroz ooplankton were comparable to those of pelagic zooplankton and meiofau na in the Great Barrier Reef. Values of demersal macrozooplankton abun dance from previous studies and excretion rates from this study were c ombined to estimate fluxes of ammonium from demersal macrozooplankton in coral reef lagoons. The estimated fluxes in the water column and se diments were 12 mu M NH4 m(-2) d(-1) and 34 mu M NH4 m(-2) d(-1), resp ectively. These fluxes were compared with reported fluxes of ammonium in coral reef lagoons in the Great Barrier Reef, Australia. The estima ted flux from the demersal macrozooplankton in the water column was 29 and 9% of those reported for microheterotroph regeneration and phytop lankton utilization, respectively. It was 10% of the reported advectiv e flux during periods of low advection and 13% of the maximum efflux f rom sediments computed from diffusion models. The estimated flux from the demersal macrozooplankton in the sediments exceeded those reported for meiofauna, and was 5 to 32% and 2 to 13% of those reported for am monification and utilization in sediments, respectively. The potential importance of demersal macrozooplankton in mediating sediment-water c olumn exchanges in the absence of diffusive effluxes and when they swa rm is discussed.