Hydrochemical parameters and trophic state of an urban lake used for recreation

Every lake located in an urban area or in the vicinity of a town is a unique element of the landscape. However, lakes situated in cities are particularly endangered as they have a role of receiving water for municipal, industrial and precipitation wastes. The load of nutrients introduced with sewage is so high that it disturbs the biological balance and disrupts biogeochemical processes in an ecosystem. The object of the study was an urban Lake Sajmino (21.6 ha, max. depth 7.8 m) used for recreation. Its catchment is smaller than the topographic catchment area because some of the precipitation water is redirected outside by a storm water drainage system. The lake is fed by 3 permanent inflows (I-III). The goal of the study was to find correlations between the hydrochemical and biological parameters of the lake water and the quality of water supplied by the inflows. Simultaneously, an attempt was made to identify the hazards, in order to undertake actions in the near future preventing deterioration of the lake’s trophic state. The amounts of pollutants flowing into the lake from various sources exceeded the acceptable levels, therefore leading to an increase in its trophy, exhibited by algal blooms and limitation in the transparency of the lake water expressed as Secchi disc visibility. A significant (p ≤ 0.05) inverse correlation between Chl a and SD (r = –0.755) was found for the lake water, and between the Chl a of the lake and the pollutants of the inflows: NNH4 (I), r = 0.786, NNO2 (I), r = 0.731, NNO2 (II), r = 0.881, suspension (III), r = –0.719. TSI indicates that the eutrophic character of the lake is still unsettled. TSI (TP) > TSI (TN) > TSI (SD) > TSI (Chl a). The SD value (2.2÷3.55 m) may be caused by dissolved organic matter, the suspension or the water’s colour. Phytoplankton (Chl a ≤ 4.840 μg dm-3) could be limited by nitrogen, zooplankton feeding or toxic compounds.