Significance of subtidal sediments to heterotrophically-mediated oxygen and nutrient dynamics in a temperate estuary

Sediment-water exchanges of ammonium (NH4+), nitrate + nitrite (NOx-), filterable reactive phosphorus (FRP, primarily ortho-phosphate), and oxygen (O-2) under aphotic (heterotrophic) conditions were determined at 2-5 stations in the Neuse River Estuary, from 1987 to 1989. Shallow (1 m), sandy stations were sampled along the salinity gradient. Fluxes from deep (>2 m) sites were compared to the shallow sites in two salinity zones. Grain size became finer and organic content increased with depth in the oligohaline zone but not in the mesohaline zone. Net release of NH4+ and FRP occurred at all sites. Fluxes varied from slight uptake to releases of 200-500 mu mol m(-2) h(-1) (NH4+) and 150-900 mu mol m(-2) h(-1) (FRP). Net NOx- exchange was near zero, but were +/- 100 mu mol m(-2) h(-1) over the year. Release of NH4+ and FRP from the shallow sandy stations decreased with distance down the estuary, but O-2 uptake did not change. The deeper oligohaline site had twofold higher rates of NH4+ and FRP release and O-2 uptake than the shallow site, but no differences occurred between depths in the mesohaline zone. Temperature and organic content were important controls for all fluxes, but water column NOx- concentration was also important in regulating NOx- exchanges. Ratios of oxygen consumption to NH4+ release were near the predicted ratio (Redfield model) at oligohaline sites but increased down estuary at mesohaline sites. This may be due to greater nitrification rates promoted by autotrophy in the sediments.