Sediment-Water Column Oxygen and Nutrient Fluxes in Nearshore Environments of the Lower Delmarva Peninsula, USA
Sediment-water column exchanges of oxygen, dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) were measured in situ over an annual cycle for sandy and silt-clay sediment types in a shallow Chesapeake Bay (USA) inlet. Benthic oxygen and inorganic nutrient fluxes differed between sediment types. Based on metabolic rate estimates and photosynthetic pigment concentrations, nearshore sandy sediments were more productive than silt-clay sediments. Overall benthic community respiration rates were 872 mu mol m(-2) h(-1) for sandy sediments and 2220 mu mol m(-2) h(-1) for silt-clay sediments. Elevated ammonium and DIP sediment fluxes were associated with silt-clay sediments. Sandy and silt-clay sediment ammonium fluxes ranged from -44 to 358 and -30 to 615 mu mol m(-2) h(-1) respectively, with DIP fluxes ranging from -5.3 to 42.0 and -3.3 to 35.7 mu mol m(-2) h(-1). Negative nutrient flux values denote sediment uptake. Sediment ammonium and DIP fluxes were dependent on benthic aerobic respiration rates for silt-clay sediments. In contrast, sandy sediment ammonium fluxes were less dependent and DIP fluxes showed no relationship to benthic aerobic respiration rates. Ammonium and DIP flux rates were significantly reduced in transparent chambers as compared to opaque chambers indicating the importance of the benthic microalgal community. On an annual basis, sandy sediments could supply 11% of the phosphorus and 6% of the phytoplankton nitrogen requirements based on gross productivity estimates, whereas silt-clay sediments could supply 11 and 14%, respectively. Positive correlations between sandy and silt-clay sediment DIN fluxes and phytoplankton DIN assimilatory demands emphasize the importance and interdependence of sediment heterotrophic and water column autotrophic processes. Short water column DIN and DIP turnover times, on the order of hours, were characteristic of summer conditions when water column nutrient concentrations were low and silt-clay sediment nutrient fluxes high. Conversely, nutrient turnover times on the order of days were characteristic of winter conditions when water column nutrient concentrations were high and sediment nutrient fluxes low.