The effect on freshwater inflow on macrobenthos in the Lavaca River delta and upper Lavaca Bay, Texas

A two year study on the effects of freshwater inflow on macrobenthos at selected sites in the upper portion of the Lavaca River and Bay was conducted from November 1984 through August 1986. Lowest densities occurred in lakes, and highest densities occurred in creek deltas. Sediment grain size did not have a significant effect on spatial distributions. The first year of the study had higher inflow rates than the second year. Temporal changes of inflow had a larger effect on spatial variability, because freshwater species had extended ranges during the high-flow year. Temporal variation in the benthic community during the study was caused by population changes of low salinity species, and this was associated with high inflow rates during the first year. Freshwater inflow is apparently necessary to induce the recruitment oflow salinity species in the upper bay. Chironomid larvae and the polychaete, Hobsonia florida, increased in density approximately 4 weeks after an inflow event. In contrast, the mollusks, Mulinia lateralis and Macoma mitchelli, had increased densities during the low inflow period resulting in high benthic biomass. Streblospio benedicti, a surface deposit-filter feeder, and Mediomastus californiensis, a burrowing deposit-feeder, were also positively correlated with increasing salinity during low inflow periods. Chlorophyll-a concentration in the surface water increased with inflow, indicating primary production was stimulated by inflow, and then decreased with increased salinity and increased filter feeder biomass. Non-molluskan benthic biomass was similar during wet and dry periods, and had a positive relationship with chlorophyll-a concentration. The amount, timing, and inter-annual variation of inflow influence both the spatial and temporal abundance and biomass ofbenthic macrofauna.