Sediment nutrient dynamics on the South Atlantic Bight continental shelf

Continental shelf sediments on the South Atlantic Eight (SAB) consist of relict sands that, at depths ranging from 14-45 m, fall within the photic zone and are sites in which significant rates of benthic primary production are observed. Thus, SAB seafloor sediments are a source of organic matter to the shelf system and are possibly a sink for nutrients regenerated within the sediments. We have investigated the nutrient dynamics in SAB shelf sediments along two transects (off the coasts of Georgia and Florida) as part of a study that addresses the significance of benthic primary production in terms of overall shelf biogeochemistry. Sandy sediments were sampled with a newly designed corer that permits retention of pore water in highly permeable sands and thus avoids commonly encountered "washout" problems. Nutrient (ammonium, silicate, nitrate + nitrite, and phosphate) distributions from sediments along both transects show substantial variation in concentration magnitudes and profile shapes over short horizontal spatial scales (meters). Laboratory experiments and numerical models of diagenetic processes in these sediments suggest that (1) pore-water advection, driven by current flows over wave ripples and bioturbational features, is likely an important transport process in promoting nutrient exchange in these porous sands (more so than irrigation), and (2) reaction rates in SAB sediments are rapid and are comparable to those in nearshore muddy habitats. In spite of this variation and the apparently high rates of advective transport, time series measurements show a gradual increase in depth-integrated nutrient concentrations at one station along the Georgia transect from the spring through the fall. This increase is probably related to elevations in temperature and metabolic rates in sediments during the warmer summer months. Measured oxygen and nutrient fluxes across the sediment-water interface in paired transparent "light" chambers and opaque "dark" chambers did not conform to trends observed in shallow water systems, where oxygen is typically evolved and nutrients are consumed in the light (versus oxygen that is consumed and nutrients that are evolved in the dark). The absence of measurable or consistent fluxes on the SAB shelf is likely the result of low concentrations of nutrients in both the water column and the upper sediment layers. However, rapid regeneration rates in sediments and generally higher nutrient concentrations in deeper sediment layers suggest that sediments are an important source of the nutrients that fuel benthic primary production. Although ammonium is rapidly produced in these oxic sands, nitrate concentrations are uniformly low, and measurable nitrate generally appears in the oxic sedimentary layers. The absence of high nitrate concentrations in the suboxic or anoxic zone suggests that denitrification may not be an important process in SAB shelf sediments.