Biogeochemistry in the coastal zone
Weston, Nathaniel B
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Thirty years of population growth, land use change, and nutrient loading from the subwatersheds of the Altamaha River, GA were analyzed. Population growth, mainly in the upper watershed near the cities of Atlanta and Athens, resulted in increased nitrate delivery to the rivers and transport to the coastal zone. Delivery of freshwater appears to be declining, due to climate change and water withdrawal within the watershed. The effects of upriver salinity intrusion on the biogeochemistry of sediments from the tidal freshwater portion of the Altamaha River were investigated. Methanogenesis, which dominated in the freshwater sediments, declined quickly following salinity intrusion. Sulfate reduction was the dominant pathway of microbial organic matter mineralization within two weeks of salinity intrusion, although increased iron-oxide availability during initial salinity intrusion appears to have stimulated high rates of microbial iron reduction for a short period. Salinity-driven desorption of ammonium and increased rates of silica and phosphorus mineral dissolution following salinity intrusion increased overall export of ammonium, phosphate and silicate from salinity-impacted sediments. The mineralization of complex organic matter in sediments is mediated by a diverse consortium of microbes that hydrolyze, ferment, and terminally oxidize organic compounds. Patterns of estuarine sediment biogeochemistry, focusing on the role of dissolved organic carbon (DOC) and nitrogen, were determined with a multi-site, multi-season survey of estuarine porewater profiles in Georgia and South Carolina. This survey demonstrated system-scale correlations between the inorganic products of terminal metabolism (dissolved inorganic carbon, ammonium and phosphate) and sulfate depletion. DOC, as the substrate for terminal metabolism, was not correlated with other variables indicating that production and consumption of DOC were tightly coupled, and bulk DOC is largely a recalcitrant pool. Controls on the coupling between the hydrolytic/fermentative and terminal metabolic bacterial communities in estuarine sediments were further investigated using anaerobic flow-through bioreactors. We documented the temperature-driven decoupling of the production and consumption of key DOC intermediates due to variable temperature responses of these functional microbial groups. Production of labile DOC exceeded terminal oxidation at colder temperatures, resulting in accumulation of labile DOC. At higher temperatures, potential terminal oxidation rates exceeded those of labile DOC production and labile DOC availability limited rates of terminal oxidation.