Water and Nutrient Mass Balance of the Partly Meromictic Temperate Lake Verevi

Mass balances of total nitrogen and total phosphorus were calculated for Lake Verevi (area 0.126 km², maximum depth 11 m, mean depth 3.6 m), a sharply stratified small lake located in South Estonia within the borders of the town Elva. The lake has up to 10 small inflows but only three of them are nearly permanent. Accidental overflows from near-by oxidation ponds during high floods have been the major source of the nutrient load of the lake in the past. L. Verevi receives a significant part of its inflow from groundwater, which is difficult to measure. In dry years the outflow is temporary. During summer the lake is sharply thermally and chemically stratified. The spring turnover is often incomplete even in homothermal conditions, thus giving the lake some meromictic features. The influx of nitrogen exceeded the outflux at any supposed proportion (20%, 50%, 80%) of surface runoff. The lake retained 45–90% of the nitrogen influx by sedimentation and/or by denitrification. The largest nitrogen losses with loss rates more than 10 kg N d⁻¹ occurred in May and June. The calculated phosphorus retention rate became strongly negative during mixing periods. From June to November, phosphorus release from the sediment exceeded sedimentation by 205 kg in 1991 and by 79 kg in 1993. Earlier stagnation and absence of a full spring turnover in the 2000 has slowed down the recovery of the lake because less phosphorus is flushed out. However, the stronger stratification and significantly smaller phosphorus content in the epilimnion limits biological activity and as a result improves the water quality of the surface layer.