The effect of freshwater inflow on net ecosystem metabolism in Lavaca Bay, Texas

Abstract

Estuaries and other coastal ecosystems depend on freshwater inflow to maintain the gradients in environmental characteristics that define these transitional water bodies. Freshwater inflow (FWI) rates in many estuaries are changing due to changing land use patterns, water diversions for human consumption, and climate effects, but there are no standard criteria to determine minimum inflow rates. An ecological indicator is required so models can be produced to predict how changing hydrology might affect estuarine metabolic rates, productivity, or impairment. One indicator of estuarine metabolic rates is net ecosystem metabolism (NEM). It is hypothesized that metabolic rates (as indicated by NEM) will depend on recently delivered FWI. To test this hypothesis, daily NEM was calculated from high frequency changes in dissolved oxygen measurements in a shallow water estuary, Lavaca Bay, Texas, USA and related to FWI and other environmental conditions. There was a significant relationship between NEM and cumulative ten-day FWI in upper Lavaca Bay, with more heterotrophic conditions occurring after high FWI events. No significant relationship existed between NEM and FWI in lower Lavaca Bay, where other environmental conditions, such as tidal forcing, may be more influential. An empirical model simulating NEM's response to FWI in upper Lavaca Bay was more accurate during moderate to high flows than during low base-flow conditions. Thus, quantity of FWI and distance from the inflow point source constrained the dependence of NEM on FWI. NEM can be quite variable over time and within a bay dominated by a river at one end and ocean exchange at the other end. The range of environmental gradients over time and within an estuary will determine how representative NEM at a single location is for an entire estuary. However, during high flow period pulses NEM can accurately predict shifts from balanced to heterotrophic conditions. Therefore, use of the NEM model as an indicator of FWI effects is constrained to regions in estuaries that are most effected by FWI because of proximity to the source or the size of FWI pulse events. The unique geologic, geographic, and climate signature's of individual estuaries will dictate the fit of this model in space and time. Inclusion of other environmental factors, such as temperature or irradiance, is necessary to improve the NEM model during low base-flow conditions. It is concluded that the NEM model does provide a useful indicator of FWI effects on ecosystem metabolic rates as it works well in the upper regions of Lavaca Bay and during freshwater inflow pulses where and when freshwater inflow has more influence than other environmental factors.