Effect of Reservoir Function on Water Quality and Phytoplankton in Lake Taylorville, Christian County, Illinois

As is true for most reservoirs in agricultural areas, Lake Taylorville is currently impacted by excess sedimentation. A system of floodplain wetlands, holding ponds, and sediment basins is being constructed on the tributaries to the reservoir in effort to reduce sediment and nutrient loads. A comprehensive twelve-month assessment of water quality has been conducted to provide a baseline for evaluating the success of this restoration project and to allow predictions regarding future management strategies. In reservoirs, a continuum of longitudinal gradients result in the establishment of three distinct zones possessing unique physical, chemical and biological properties. The function of these zones (termed the riverine zone, transitional zone and the lacustrine zone) can be used to characterize the ecosystem of a reservoir. In Lake Taylorville, only the riverine and transitional zones were found. The elimination of the lacustrine zone in Lake Taylorville is consistent with the large watershed to surface area ratio found there. Lake Taylorville can be characterized as a high flow reservoir that is more river like than lake like in function. Light penetration in Lake Taylorville was usually limited to less than a meter at the surface by high suspended solids concentrations and although nutrient concentrations were high, phytoplankton density, primary productivity and chlorophyll a concentrations were lower than expected. Carlson Trophic State Index (TSI) calculated for total phosphorus and Secchi depth suggest hypereutrophy while those calculated from chlorophyll a data indicate a lower trophic state. Algae may be unable to maximize utilization of available nutrients as a result of low light availability. Because algal bioassays and nitrogen : phosphorus ratios indicate phosphorus limitation of primary productivity, productivity and phytoplankton standing crop may increase if light regimes are improved by sediment reduction resulting from wetland creation. Wetlands have been found to be effective at denitrification as well as sediment reduction. The combination of reduced sediments and nitrates coupled with high in-lake phosphate levels could cause a shift in algal community structure from a currently Chlorophycean dominated community to a community dominated by Cyanophyceae. Future reservoir management may need to address the possibility of frequent blue-green algal blooms and perhaps target phosphorus reduction