Effects of Freshwater Inflows on Hydrological and Biological Parameters in the San Antonio Bay System, Texas

Between September 1971 and May 1974, semi-monthly samples of biologic, water quality, and meteorologic parameters were taken at various stations in the San Antonio Bay system of Texas. Hydrographic parameters concerning current movements were measured at selected intervals. The biological data and historical commercial fish and shellfish landings statistics were subjected to computerized analysis for possible correlations with simple and cumulative lag river inflow to the estuary and various hydrologic parameters. These analyses were performed in an effort to determine the importance of freshwater inflows, their effect on estuarine biota, and the quantity and quality of fresh water required by the system. Maps based on aerial photography were used to measure the dimensions of physical features within the system. Riverflow was the major source of fresh water and thereby exerted the most influence on the hydrologic parameters of the system. Rainfall supplied much less water than riverflow and was always exceeded by evaporation when averaged annually. Riverflow was also the main source of nutrients to the bay supplying an estimated daily average of 22 metric tons of phosphorus and 26 metric tons of nitrogen. Wind and floods were the main natural sources of turbidity. Oyster abundance increased at reefs in the lower bay area and decreased in the upper bay area when increased inflows caused reduction in salinity. When the amount of freshwater inflow for any single month prior to the month in question (simple lag inflow) was correlated with various catch data, the following relationships (significant at the 99% confidence level) were considered most germane to this study: white shrimp abundance correlated positively with simple lag river inflows of 1, 2, and 4 months; brown shrimp abundance correlated negatively with simple lag inflows of 2, 4, 6, and 8 months and positively with 12 months; bay anchovy abundance correlated positively at 1, 2, and 10 months; atlantic croaker abundance correlated positively at 1, 10, and 12 months. Juvenile blue crab abundance did not correlate at the 99% confidence level with simple lag inflows. Historical inflow and commercial landing data provided evidence that river inflows between 1.6 and 2.4 million acre feet per year, if adequate amounts are received in late spring, would optimize shellfish production in the San Antonio Bay systm. Analyses of the data were not conclusive enough to predict changes in finfish production due to changes in river inflow quantity.