تغییرات بارگذاری فسفر داخلی و کیفیت آب در یک دریاچه هیپرتروفی طی 40 سال تلاش های مختلف مدیریت

Variations of internal phosphorus (P) loading and water quality in the hypertrophic Lake Tuusulanjärvi (southern Finland) were studied over a period covering 40 years (1970–2010). The lake has hosted numerous different management efforts. Diversion of sewage waters away from the lake in 1979 resulted in a considerable reduction of external P loading. Due to diffuse loading from agricultural areas, external loading however still exceeds the critical loading of the lake, thus having an effect on the water quality. The total P concentration of surface layers has decreased but is still on a hypertrophic level (90 μg l−1). The high productivity of the lake is maintained also by intensive internal P loading, and the sediment has a potential to release P to the water for decades even if external loading would be reduced to a tolerable level. Internal P loading has not decreased over the studied decades despite numerous within-lake management efforts (aeration with different methods, food web management). In opposite, our results demonstrated that destratification applied since 1998 resulted in a persisting internal P loading. Destratification has increased the concentration of oxygen and decreased the concentration of soluble P in deep water, but at the same time it has accelerated P release from aerobic bottoms. This can be due to elevated near-bottom temperatures, enhanced liberation of organic P through accelerated mineralization, and increased sediment resuspension by aeration-induced turbulence. Additionally, increasing wind velocities may have a role in the increasing aerobic internal loading. Food web management has compensated for the amplified P-cycling, revealed by the decreasing chlorophyll:total P ratio.