Restoration of a shallow Mediterranean Lake by biomanipulation complicated by drought

Whether fish biomanipulation is an efficient restoration technique in eutrophic warm sub/tropical lakes has been subject to recent debate. Our investigations undertaken in warm Lake Eymir, Turkey show that fish removal increased water clarity during a five-year period. Rapid re-colonisation of submerged plants, Potamogeton pectinatus and Ceratophyllum demersum, occurred. This recovery was achieved at higher total phosphorus (TP) levels than the suggested threshold for stability, probably owing to the nitrogen-limited condition of the lake. Re-establishment of vegetation coincided with significantly reduced concentrations of TP and dissolved inorganic nitrogen (DIN). Abundance of the large-bodied Daphnia pulex was low and it later disappeared completely from the zooplankton community, probably due to increased fish predation. A severe drought, occurring two years after the fish removal, significantly lowered the water level, increased the hydraulic residence time and caused an expansion of the vegetation. The drought was also associated with a significant increase in salinity, conductivity, nutrient concentrations JP and DIN) and in the abundance of Arctodiaptomus bacillifer. The in-lake nutrient amounts became more dependent on internal processes rather than on the external loading, which was very low during the drought period. When the water level rose to normal values, the concentrations of TP and soluble reactive phosphate (SRP) decreased. However, the DIN concentration significantly increased due to slowed denitrification processes in consequence of the low availability of dissolved oxygen. This increase in ammonium concentrations may have contributed to the instability of clear-water conditions by releasing the lake from its former nitrogen-limited state, since the TP levels in the lake had already surpassed critical levels. Along with this, tench (Tinca tinca) biomass increased to pre-biomanipulation levels and the pike (Esox lucius) to planktivorous fish ratio decreased with increased chlorophyll-a concentrations, largely by inedible cyanobacteria, which led to a decline in summer water clarity but not to disappearance of submerged plants. The early growth of plants was probably controlled by the spring water clarity, which remained high, and the lake maintained its macrophyte-dominated state, despite the relatively turbid conditions prevailing in summer. The highly positive effect of the Lake Eymir restoration effort contradicts the experiences from other subtropical lakes. This could be due to the fact that warm Lake Eymir being at high latitude with a strong presence of predatory fish and limited abundance of planktivorous fish species, as opposed to low altitude warm lakes. However, drought as an inherent feature of the and region may be even more important in the future as drier conditions are predicted for the Mediterranean region in consequence of the global climate change.