Restoration of Eutrophic Lakes with Fluctuating Water Levels: A 20-Year Monitoring Study of Two Inter-Connected Lakes

Eutrophication continues to be the most important problem preventing a favorable environmental state and detrimentally impacting the ecosystem services of lakes. The current study describes the results of analyses of 20 year monitoring data from two interconnected Anatolian lakes, Lakes Mogan and Eymir, receiving sewage effluents and undergoing restoration. The first step of restoration in both lakes was sewage effluent diversion. Additionally, in hypertrophic Lake Eymir, biomanipulationwas conducted, involving removal of benthi- planktivorousfish and prohibition of pikefishing. The monitoring period included high (H) and low (L) water levels(WL) enabling elucidation of the effects of hydrological changes on lake restoration. In shallower Lake Mogan, macrophyteabundance increased after the sewage effluent diversion in periods with low water levelseven at turbid water. In comparatively deeper Lake Eymir, the first biomanipulationled to a clear water state with abundant macrophytecoverage. However, shortly after biomanipulation, the water clarity declined, coinciding with low water level(LWL) periods during which nutrient concentrations increased. A second biomanipulationwas conducted, mostly during high water level(HWL) period, resulting in a major decrease in nutrient concentrations and clearer water, but without an expansion of macrophytes. We conclude that repetitive fish removal may induce recovery but its success may be confounded by high availabilityof nutrients and adverse hydrological conditions.