Response of Benthic Organisms to External Conditions in Matagorda Bay
Montagna, Paul A.
Evans, Anne D.
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The Lavaca-Colorado Estuary is a major estuarine system along the Texas coast that provides major economic benefit to the region by supporting a variety of agricultural, residential, industrial, and recreational functions. The Matagorda Bay Health Evaluation (MBHE) component of the LCRA/SAWS Water Project (LSWP) Study Plan was created to assess the environmental effects that could result from further changes to inflow patterns in the Matagorda Bay system. To support this assessment, a bio-energetic model, calibrated using a long-term data set of benthic biomass, was run to relate macrobenthic biomass to salinity within the estuary. This model was applied to the current study to assess the role of freshwater inflow in controlling benthic productivity. Benthic productivity was calculated for two groups of macrobenthos, suspension feeders and deposit feeders; in two bays, Lavaca Bay and Matagorda Bay. Simulations of the Lavaca-Colorado Estuary, based on a calibration of data from 1991 - 1996, fit the observed data relatively well. However, following the year 2000, simulations predicted a much higher benthic biomass than in observed data. The increase in benthic biomass is likely explained by the decrease in predator populations, particularly blue crabs, which reduced loss to predation. Simulations on deposit and suspension feeder biomass exhibited responses due to natural and simulated salinity changes in both bay systems. As salinity increased, deposit feeding biomass increased while suspension feeding biomass decreased. Total biomass in Lavaca Bay was found to increase as salinity increased, which indicates that reduced inflow rates in this bay would not harm the benthic community. Total biomass concentration in Matagorda Bay decreased initially with increasing salinity but then gradually increased. Thus, reducing the freshwater inflow may cause the upper river communities to take on downstream community appearance. This effect is probably due to the benthic community acclimating to the elevated salinity or more salt tolerant species populating the area. It is concluded that freshwater inflow plays an important role in maintaining the observed character of estuarine productivity through the combined effects of the frequency, duration, timing, and magnitude of inflow, particularly during droughts or low-flow periods.