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 primar y production are observed. Thus, SAB seafloor sediments are a source o f organic matter to the shelf system and are possibly a sink for nutri ents regenerated within the sediments. We have investigated the nutrie nt dynamics in SAB shelf sediments along two transects (off the coasts of Georgia and Florida) as part of a study that addresses the signifi cance of benthic primary production in terms of overall shelf biogeoch emistry. Sandy sediments were sampled with a newly designed corer that permits retention of pore water in highly permeable sands and thus av oids commonly encountered ''washout'' problems. Nutrient (ammonium, si licate, nitrate + nitrite, and phosphate) distributions from sediments along both transects show substantial variation in concentration magn itudes 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 c urrent 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 ha bitats. In spite of this variation and the apparently high rates of ad vective transport, time series measurements show a gradual increase in depth-integrated nutrient concentrations at one station along the Geo rgia transect from the spring through the fall. This increase is proba bly related to elevations in temperature and metabolic rates in sedime nts during the warmer summer months. Measured oxygen and nutrient flux es across the sediment-water interface in paired transparent ''light'' chambers and opaque ''dark'' chambers did not conform to trends obser ved in shallow water systems, where oxygen is typically evolved and nu trients 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 concent rations of nutrients in both the water column and the upper sediment l ayers. However, rapid regeneration rates in sediments and generally hi gher nutrient concentrations in deeper sediment layers suggest that se diments are an important source of the nutrients that fuel benthic pri mary production. Although ammonium is rapidly produced in these oxic s ands, 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 de nitrification may not be an important process in SAB shelf sediments.