Seasonal Variability in Phosphorus Concentrations in Strongly Eutrophic Lake Chełmżyńskie

The results of the monthly examinations of Lake Chełmżyńskie carried out during the vegetation periods in the years 2007–2009 made it possible to define seasonal variability in the concentrations of phosphorus compounds. The examinations showed that the bottom deposits constitute a considerable source of phosphates. The release of phosphates from the deposits began as early as in mid May and with the occurrence of anoxic conditions. A particularly high concentration of phosphates in the near-bottom layer was recorded in the southeast pool of the lake (max. 0.658 mg P l⁻¹). The central part of the lake showed a lower concentration of phosphates near the bottom, yet due to the partial thermal stratification of its waters, it was the main place for the transport of the bio-element to the euphotic zone. The intense transport of phosphates occurred mainly during the short periods of strong winds. The analysis of the meteorological data obtained from the Integrated Natural Environment Monitoring—Station Koniczynka prove that winds, whose mean daily velocity values exceeded 8.0 m s⁻¹, brought about particularly intense and deep water mixing. A very significant increase in the concentration of phosphorus compounds in the epilimnion was recorded in the years 2007 and 2008, and it resulted in the violent development of phytoplankton. The results of the examinations conducted in the years 2007–2009 showed that Lake Chełmżyńskie was classified as a strongly eutrophic lake. The ecological condition of the lake was defined as poor according to the Water Framework Directive. Reclamation works including the inactivation of phosphates in the bottom deposits are indispensable in order to improve the ecological and trophic state of the lake.