Lake Koronia, Greece: Shift from Autotrophy to Heterotrophy with Cultural Eutrophication and Progressive Water-Level Reduction

Lake Koronia, a Ramsar site, is shallow, polymictic, hypertrophic and until recently was aerially the fourth largest lake in Greece. Although exceeding 5 m in the past, lake depth has declined progressively from 3.8 m in 1980 to < 1 m in 1997, reducing surface area and water volume by 50% and 80%, respectively. Specific conductivity increased from 1300 μS cm ^-1 in 1977 to >6000 μS cm ^-1 in 1991. Increased phosphate concentrations from the late 1970's (8-45 μg L ^-1 ) to the late 1990's (100-1000 μg L ^-1 ) document that the previously eutrophic system with a limited littoral zone switched to hypertrophy dominated by massive cyanobacteria blooms. Oxygen saturation of the water column increased progressively from about 80% in 1983 to full saturation about 1993, after which it decreased progressively to only 20% saturation in 1997. In spite of cyanobacteria dominance, community metabolism of the lake switched from progressively increasing autotrophy to rapidly advancing heterotrophy associated with progressive water-level reduction leading to fish extirpation in the lake.