The Characteristics of a Temperate Eutrophic, Dimictic Lake (Lake Mikołajskie, Northern Poland)

Gross primary production equals 3160 kcal Xm−2 ×yr−1, 38% coming from the littoral. The efficiencies of both nonpredatory and predatory zooplankton production are high. The production of phytoplankton is expended mostly for sedimentation in April, for grazing by zooplankton in June, heterotrophic respiration and cumulation of dissolved and particulate organic matter in August—September and sedimentation in late autumn. Almost total elimination of zooplankton is done by predators, mostly invertebrates. About 10% of autochthonically produced plus allochthonic matter is annually removed from cycling, mostly as permanent bottom deposits. The annual load of phosphorus is 3 times higher than the permissible VOLLENWEIDER'S level. About 70% of phosphorus load, but only two percent of its reserve in the lake is annually removed from cycling. Progressing eutrophication resulted in several times increased phytoplankton biomass, decrease of phytoplankton P/B and share of nannoplankton, increase of decomposition in epilimnion, etc, during 10 years. Inspite of this, pelagic community in spring still has rather mesotrophic than highly eutrophic character. Increased eutrophication due to human impact (deforestation, agriculture, erosion) is also seen in bottom deposits since the 15th century.