ØSTENSJØVANNET – A SHALLOW HYPERTROPHIC LAKE IN NORWAY WITH SIGNIFICANT INTERNAL PHOSPHORUS LOADING DURING SUMMER
Lake Østensjøvannet is a shallow hypertrophic lake situated about 30 km south of Oslo. The water quality in the lake during the last hundred years has deteriorated greatly. The main reason for this is supply of wastewater and drainage from agricultural activities. Investigations in the summers of 1977 and 1978 indicated very high internal phosphorus loading and rather insignificant values in 1987. In the two former years, the primary productivity was measured as high as 6 g C m–2day–1, a value which is roughly considered to be the highest obtainable in lakes. Cyanobacteria were the dominating phytoplankton. The expected phosphorus concentration in the lake water during summer was calculated and the difference between the measured and the expected values
is assumed to represent internal loading. High concentrations of phytoplankton in the lake in summer and associated increases in pH through photosynthesis, stimulate phosphorus release and additional pH increase. Laboratory experiments with sediment cores suggest that the lake sediments could be a significant contributor to the internal phosphorus loading in the lake.