Transformation and fate of dissolved organic matter originating in the Suwannee River watershed: A stable isotope approach

Anthropogenic activities in a watershed can have significant impacts on the ecology of downstream coastal environments. Surface runoff and groundwater inputs from the Suwannee River watershed contribute high concentrations of nutrients (nitrates >1 mg/L) to this blackwater river before it discharges into the Gulf of Mexico. High concentrations of nutrients have the potential to affect the sustainability, productivity, and growth of the oyster and clam aquaculture industries in the estuary. In determining detrimental effects, it is important to establish a link between dissolved carbon and nitrogen, suspended particulate organic matter (SPOM), and production of clam and oyster biomass in the Suwannee River estuary.; Organic aggregates (a possible food source for the bivalves in the Suwannee River estuary) were produced in the laboratory by adding salts to unfiltered fresh organic-enriched Suwannee River surface water. They were analyzed for stable carbon and nitrogen isotopes and the results indicate that the delta 15N signatures can be used as a tracer of organic aggregate development. More importantly, however, the results suggest that delta13C signatures of organic aggregates can be used to quantify their role as a food source for estuarine fauna.; Spatial gradients in nutrient concentrations, particularly nitrate, are characteristic of estuarine systems. These gradients occur as a result of mixing processes and uptake and assimilation by phytoplankton. Isotopic fractionation associated with this uptake and assimilation can generate strong spatial gradients in the stable nitrogen isotope composition of particulate nitrogen. In essence, stable nitrogen isotopes serve as in situ tracers of the processing of nitrogen as it moves through an estuarine system.; Clams and oysters were seeded in the Suwannee River estuary near the river mouth. Clams, oysters, and SPOM were sampled monthly for approximately one year and subsequently analyzed for stable carbon and nitrogen isotopes. The results indicate that the clams consumed the same source of organic material throughout the year (marine SPOM), while the oysters' food source changed consistently with hydrologic patterns in the river (e.g., more terrestrial origin during high flow and more marine origin during low flow). Neither clams nor oysters consumed flocculated organic aggregates as a dominant food source.