Changes in Lake Norrviken after sewage diversion

Summary

In 1969 all sewage effluents were diverted from Lake Norrviken. In the two years following this diversion slight improvements of the water quality have been registered. The total N concentration has decreased from 5–6 to 2–3 mg/1, and there is evidence that N was a limiting factor for phytoplankton growth in 1970. P concentration has decreased very slowly from 300–400 to 200–300 μg/1. Phytoplankton biomass was mum smaller in 1970 than previous years, but the primary production was larger than that measured in 1961/62. Sedimentation of P in 1970 was estimated to be 3.1 g/m² and the release of P from the sediments 2.7 g/m². This means that sedimentation and release of P approximately counterbalance eam other, and further recovery of the lake is not to be expected unless the P concentration in the main tributary of the lake decreases to mum lower values than it has today.