Benthi-planktivorous Fish-Induced Low Water Quality of Lake Eymir Before Biomanipulation


Abstract: In this study, Lake Eymir, Eymir inflow 1, Kışlakçı brook and Eymir outflow were sampled monthly betwen March 1997and April 1998 for physico-chemical and biological parameters. Within the catchment of the lake, there are small-scale industries, sewage effluent of TEK residence and recreational activities causing high concentrations of dissolved inorganic nitrogen (DIN) and total phosphorus (TP) in the eymir inflow 1 and in Kışlakçı brook. In Lake Eymir although diversion of sewage effluent of the Mogan municipality to İmrahor valley appeared to have led to a decrease in TP and DIN concentrations compared with those found in 1994, especially the lake TP concentration was still found to be very high (annual mean: 305 µg l -1 ) this may be due to the high inflows concentrations. The lake had a high spring increase in chlorophyll-a concentration which remained relatively low throughout the summer (annual mean: 19 mg -1 ), perhaps due to very low DIN/TP ratios implying possible N-limitation at the very high TP concentration. Moreover, the Secchi depth was also relatively low throughout summer at the low chlorophyll-a concentration possibly due to feeding of the dominant fish tench ( Tinca tinca) and carp ( Cyprinus carpio) of the lake by stirring up the sediment. The densities of large-bodied zooplankters, Daphnia and Arctodiaptomus remained low in summer following the high spring densities probably due to fish prediation exerted on them by tench and carp. In Lake Eymir, top-down or fish-induced control of the water quality appeared to be the predominant factor,following the reduction of 30% of the tench and carp (biomanipulation), has led to very high Secchi depth and densities of the large grazes.

Keywords: Benthivorus fish, biomanipulation, eutrophication, chlorophyll-a planktivorous fish, Secchi depth, total phosphorus.

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