Ammonium Excretion by Benthic Invertebrates and Sediment-Water Nitrogen Flux in the Gulf of Mexico near the Mississippi River Outflow

Abstract

Benthic macroinvertebrate biomass and ammonium excretion rates were measured at four stations in the Gulf of Mexico near the Mississippi River mouth. Calculated areal excretion rates were then compared to sediment-water nitrogen fluxes measured in benthic bottom lander chambers at similar stations to estimate the potential importance of macroinvertebrate excretion to sediment nitrogen mineralization. Excretion rates for individual crustaceans (amphipods and decapods) was 2–21 nmoles NH4+ (mg dry weight)−1 h−1. The mean excretion rates for the polychaetes, Paraprionaspio pinnata [6–12 nmoles NH4+ (mg dry weight)−1h−1] and Magelona sp. [27–53 nmoles NH4+ (mg dry weight)−1h−1], were comparable or higher than previous measurements for similar size benthic or pelagic invertebrates incubated at the same temperature (22±1°C). Although the relatively high rates of excretion by these selective feeders 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 total areal biomass, areal macroinvertebrate excretion estimates ranged from 7 μmole NH4+ m−2h−1 at a 40-m deep station near the river mouth to 18 μmole NH4+ m−2h−1 at a shallower (28-m deep) station further from the river mouth. The net flux of ammonium and nitrate from the sediments to the water measured in bottom lander chambers in the same region were 15–53 μmole NH4+ m−2h−1 and −25–21 μmole NO3− m−2h−1. These 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.