Magnesium and calcium concentrations in the surface water and bottom deposits of a river-lake system

River-lake systems comprise chains of lakes connected by rivers and streams that flow into and out of them. The contact zone between a lake and a river can act as a barrier, where inflowing matter is accumulated and transformed. Magnesium and calcium are natural components of surface water, and their concentrations can be shaped by various factors, mostly the geological structure of a catchment area, soil class and type, plant cover, weather conditions (precipitation- evaporation, seasonal variations), land relief, type and intensity of water supply (surface runoffs and groundwater inflows), etc. The aim of this study was to analyze the influence of a river-lake system on magnesium and calcium concentrations in surface water (inflows, lake, outflow) and their accumulation in bottom deposits. The study was performed between March 2011 and May 2014 in a river-lake system comprising Lake Symsar with inflows, lying in the Olsztyn Lakeland region. The study revealed that calcium and magnesium were retained in the water column and the bottom deposits of the lake at 12.75 t Mg year-1 and 1.97 t Ca year-1. On average, 12.7±1.2 g of calcium and 1.77±0.9 g of magnesium accumulated in 1 kg of bottom deposits in Lake Symsar. The river-lake system, which received pollutants from an agricultural catchment, influenced the Ca2+ and Mg2+ concentrations in the water and the bottom deposits of Lake Symsar. The Tolknicka Struga drainage canal, to which incompletely treated municipal wastewater was discharged, also affected Ca2+ and Mg2+ levels, thus indicating the significant influence of anthropogenic factors.