Eutrophication Assessment for Lyndon B. Johnson Lake, a Subtropical Reservoir in Central Texas: Impacts from Urbanization and Upstream Sources

Abstract

Preliminary observations have suggested that Lyndon B. Johnson (LBJ) reservoir has become significantly more eutrophic in recent years. The reservoir receives water from the upper Colorado River drainage and from its two main tributaries, the Llano River and Sandy Creek. There has been a significant increase in urbanization directly near the reservoir which has the potential to increase nutrients from surface runoff. The goal of this study was to determine the cause(s) of the eutrophication through analysis of a long-term data base of Lake LBJ, between the years 1982-2016. Statistical techniques were applied to data from six stations over the length of the reservoir, to evaluate spatial and temporal variations in nutrient transport and symptoms of eutrophication. Early analysis confirmed that eutrophication has increased over time, and found high concentrations of ammonium and phosphorus coming from the upstream reservoir. A cluster analysis broke the reservoir down into distinct zones based on a stations water column stability. The water column stability at the dam was protected from disruption during summer stratification, allowing for seasonal anoxia and low redox conditions nearly every year in the hypolimnion. These conditions resulted in internal loading of nitrogen and phosphorus. At the other stations, the seasonal development of water column stability was weakened or completely inhibited during periods of heavy spring or summer flooding. There was an increase in developed land and population around the reservoir, which indicated another contributing factor of nutrients from surface runoff. The results suggest that eutrophication has increased over time due to localized urbanization and increased nutrients from the upstream reservoirs as well. The eutrophication has led to an increase in severe anoxia, severity of redox conditions, and internal loading of both phosphorus and nitrogen.