LAUSR

Abstract Problems associated with eutrophication are increasing in freshwater reservoirs worldwide due to many factors, the majority of which are attributed directly to human development. Eutrophic bodies of water are much more susceptible to algal blooms, including potentially harmful species, as well as associated taste-and-odor compounds and toxic secondary metabolites. To assess the likelihood of eutrophication and harmful blooms, two reservoirs in the Highland Lakes of central Texas, which are located within the same drainage basin and climate zone, were observed for the duration of an extended drought period lasting from 2010 to 2015. This brief investigation examined a suite of physiochemical characteristics to determine which environmental factors had the most significant impacts on the eutrophication of a reservoir coupled with the abundance of microalgae. Standard water quality parameters that have been hypothesized to influence eutrophication were measured and compared with chlorophyll-a fluorescence as a proxy for phytoplankton abundance. Samples were taken every other month and each reservoir responded differently to the drought – Lake Lyndon B. Johnson (LBJ) became eutrophic while Lake Travis remained mesotrophic, according to Carlson’s Trophic State Index. Linear models using corrected Akaike Information Criterion analyses indicated that the primary indicators for microalgae in Lake LBJ were organic and inorganic nitrogen, but the indicators for Lake Travis were rainfall, chloride, sulfate, and conductivity. Preserved phytoplankton samples were analyzed from both reservoirs across seasons in 2013 and 2014 to supplement the models. Cyanobacteria were a significant factor in both lakes – samples indicated that the diazotroph and harmful genus, Aphanizomenon, was the most abundant cyanobacterium in Lake LBJ, while Limnothrix was dominant in Lake Travis. Although diazotrophic, it was hypothesized that Aphanizomenon used free nitrogen when available rather than fixing atmospheric nitrogen, thus allowing for rapid establishment and leaving less free nitrogen for plankton downstream. This study indicated that an increase in inorganic and organic nitrogen following a period of drought could amplify the potential for harmful blooms of Aphanizomenon in the upper Highland Lakes of central Texas.