Net ecosystem metabolism in Texas shallow water estuaries: an indicator of freshwater inflow effects, scales of variability, and changes due to climate change and watershed development
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Ecological processes in estuarine ecosystems are driven by a suite of environmental factors. Coastal environments, as the receiving ecosystems of freshwater inflow from watersheds, have the potential to be influenced by anthropogenic and climatic watershed modifications. Net ecosystem metabolism methods may provide a useful indicator of estuarine response to changing environmental conditions. An open-water net ecosystem metabolism (NEM) method was applied to assess its usefulness as an indicator of freshwater inflow effects in a Texas shallow water estuary. The estuary became more heterotrophic as freshwater inflow increased, but this response was spatially limited to the upper reaches of the estuary. The response to increased freshwater inflow was used to produce a simple NEM model. Simulated NEM rates were not heterotrophic enough to cause conditions of dissolved oxygen impairment. High wind speeds over the surface provided sufficient diffusion to buffer NEM. It was concluded that NEM could be used as an indicator of freshwater inflow effects, but only in the upper reaches of Texas estuaries. It was hypothesized that other environmental factors, such as temperature and irradiance, could become more influential on NEM at lower freshwater inflows. Net ecosystem metabolism spatial and temporal scales of variability were determined and compared to a suite of environmental conditions. Four separate bays along the Texas coastline’s northeast-southwest freshwater inflow gradient were sampled. Inter-bay, intra-bay, and intra water column spatial scale differences and seasonal, monthly, and daily temporal scales differences in NEM were assessed. Significant differences in NEM were found at all spatial and temporal scales. Temperature, salinity, and freshwater inflow were determined to be the most influential environmental factors on NEM. A watershed scale net ecosystem metabolism model driven by freshwater inflow was produced. The model integrated watershed characteristics with daily Nexrad precipitation data to yield freshwater inflow into the bay. Climate change and watershed development scenarios were used to determine if significant changes in NEM may result from predicted environmental conditions. The model predicts a trend of more heterotrophic estuarine conditions as the magnitudes of freshwater inflow pulses increase.