| Differences in estuarine biogeochemical process rates in response to differences in nutrient loading were investigated over two annual cycles. Overall, nutrient loading to an estuary was found to be correlated with land-use/land-cover in the adjacent watershed. Watersheds that were predominately agricultural generally had the highest nutrient export rates while watersheds that were predominately forested had the highest organic carbon, organic nitrogen, and organic phosphorus export rates. Other descriptive variables such as watershed geomorphology and interannual climate variability were also observed to be good descriptors of watershed nutrient export rates. Once nutrient and organic carbon loads were delivered to an estuary, estuarine residence times and biogeochemical processes dictated the abundance and distribution of these constituents. Phytoplankton production, benthic denitrification, and benthic nutrient flux rates were highest in the estuary receiving the agricultural runoff. The highest percent removal of incoming nutrients to an estuary, however, was observed in a tidal river portion of the urban estuary where residence times were greatest. Net ecosystem metabolism, a measure of autotrophy versus heterotrophy, showed a distinct gradient in response to the differential loading rates with the agricultural estuarine system being net autotrophic, the forested estuarine system being net heterotrophic, and an urbanized estuary being balance being autotrophy and heterotrophy. These findings are similar to results obtained from conceptual models and comparative analyses of estuarine systems. This study, however, measured for the first time nutrient loading and biogeochemical responses synoptically across multiple systems and shows a direct empirical linkage between land-use/land-cover and net ecosystem metabolism in an estuary. |
