Biogeochemical dynamics in coastal sediments and shallow aquifers
Porubsky, William P.
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Patterns of benthic metabolism and the relative importance of assimilatory and dissimilatory processes as sinks for nitrate (NO3-) in intertidal sediments were examined. Under illuminated, nitrogen (N)-replete conditions, sequential nutrient limitation of benthic microalgae (BMA) was observed, with N limitation preceding silicate limitation; and biological assimilation dominated nitrate uptake. Conversely, under dark hypoxic and anoxic conditions, water column NO3- uptake was dominated largely by three competing dissimilatory reductive processes; denitrification (DNF), dissimilatory nitrate reduction to ammonium (DNRA), and, on one occasion, anaerobic ammonium oxidation (anammox). High sulfide concentrations negatively impacted DNF and DNRA rates, while high dissolved organic carbon (DOC):NO3- ratios favored DNRA over DNF. Under baseline conditions sediments exhibited tight coupling between photosynthesis and respiration. Nitrogen addition shifted the metabolic status of the sediments from a balance between autotrophy and heterotrophy to net autotrophy, and the sediments became a source of DOC. The role of groundwater as a source of nutrients and organics to the coastal ocean was evaluated using a combination of radium isotopes and geochemical characterization. Geochemical data indicated significant spatial variations in groundwater chemical composition and radium activity ratios indicated geographically distinct hydrological regimes. Spatial variations in microbially mediated processes, DOC distribution, and/or groundwater residence time contributed to this pattern. Radium based geochemical loading rates illustrated a substantial groundwater contribution of organics, DIC, nutrients, methane and nitrous oxide to the Okatee estuary. The groundwater biogeochemical dynamics along a shallow monitoring well transect on a coastal hammock were evaluated by density-dependent reaction transport model. A switch in the redox status of the DIN pool occurred during the spring-neap tidal transition (spring high NO3- low NH4+; neap low NO3- high NH4+). The observed N redox-switch was evaluated with regard to the relative roles of nitrification, DNF, DNRA, ammonium adsorption, and variations in inflowing water geochemistry between spring and neap tides. The latter was found to most significantly affect the observed pattern in DIN dynamics. Additionally, the fate of DOC and DIN originating from a septic system was studied. Simulation results indicated that while DNF increased ~15 fold, higher N removal rates could not keep pace with the increase in DIN loading, resulting in higher export of DIN to coastal waters.