Long-term effects of liming and fertilisation in an acidic dystrophic Lake Smolak (Mazurian Lakeland, Poland)

Abstract

A dystrophic (oligotrophic), acidic (pH < 5), humic Lake Smolak was subject in 1971-1974 to an experimental liming and fertilisation (N, P, K) which simulated the eutrophication process at known nutrient inflow to the system. Liming was performed 4 times; it limited natural as well as anthropogenic acidification of the environment, this being reflected in increasing concentration of sulphates in lake water. Lake response to fertilisation consisted of its increasing resemblance to eutrophic waters. Twenty years post the experiment the lake still showed no decreasing trend of alkalinity and Ca concentration in water, and P and N contents were still high, visibility of Secchi's disc low, and COD_Mn high, suggesting that there are little chances that it would return to its original state. Lake eutrophication may be due to nutrient recycling (bacterial and detritus food web) and internal loading with P and N, related to the lowering of lake water surface and smaller area of Sphagnum mat, but possibly also to the introduction of cyprinids. This response reflects high susceptibility of such lakes to the degradation process, suggesting the need for their special protection.