AMMONIUM EXCRETION BY BENTHIC INVERTEBRATES AND SEDIMENT-WATER NITROGEN FLUX IN THE GULF-OF-MEXICO NEAR THE MISSISSIPPI RIVER OUTFLOW

Benthic macroinvertebrate biomass and ammonium excretion rates were me asured at four stations in the Gulf of Mexico near the Mississippi Riv er mouth. Calculated areal excretion rates were then compared to sedim ent-water nitrogen fluxes measured in benthic bottom lander chambers a t similar stations to estimate the potential importance of macroinvert ebrate excretion to sediment nitrogen mineralization. Excretion rates for individual crustaceans (amphipods and decapods) was 2-21 nmoles NH 4+(mg dry weight)-1 h-1. The mean excretion rates for the polychaetes, Paraprionaspio pinnata[6-12 nmoles NH4+(mg dry weight)-1 h-1] and Mag elona sp. [27-53 nmoles NH4+ (mg dry weight)-1 h-1], were comparable o r higher than previous measurements for similar size benthic or pelagi c invertebrates incubated at the same temperature (22 +/- 1-degrees-C) . Although the relatively high rates of excretion by these selective f eeders may have been partially caused by experimental handling effects (e.g., removal from sediment substrates), they probably reflected the availability of nitrogen-rich food supplies in the Mississippi River plume. When the measured weight-specific rates were extrapolated to to tal areal biomass, areal macroinvertebrate excretion estimates ranged from 7 mumole NH4+ m-2 h-1 at a 40-m deep station near the river mouth to 18 mumole NH4+ m-2 h-1 at a shallower (28-m deep) station further from the river mouth. The net flux of ammonium and nitrate from the se diments to the water measured in bottom lander chambers in the same re gion were 15-53 mumole NH4+ M-2 h-1 and -25-21 mumole NO3- M-2 h-1. Th ese results suggest that excretion of NH4+ by macroinvertebrates could be a potentially important component of benthic nitrogen regeneration in the Mississippi River plume-Gulf shelf region.