The Lake Roś
is a postglacial flow-through lake with a well-developed coastline. It is the first lake in the Great Masurian Lakes system. The lake is supplied by waters of several rivers (Święcek, Konopka, Czarna Struga, Wilkus and Jegliński Canal). The lake waters are discharged by the Pisa River into the Narew, the Vistula and the Baltic Sea. The lake water fluctuates up to 1.5 m annually, more than in most other lakes in Poland. Strong fluctuations of water level in the Roś Lake are caused by seasonal rainfall and evaporation intensity fluctuations and, to a large extent, by the so-called water management, i.e. reduction of water inflow at the Karwik lock and the Kwik weir.
Water level fluctuations pose a challenge to sedentary organisms inhabiting coastal areas below and above the water surface.
Current water levels can be found here. The minimum lake surface level recorded in the last half century at the Majdan gauge is 5 cm, the maximum is 212 cm.
High nutrient concentration ( a typical state in most lowland lakes in Poland) and high biomass of planktonic organisms result in long-term oxygen depletion of the bottom waters (>10m) in this relatively deep lake (max 32m), a few weeks after the formation of summer thermal stratification.
This state continues until the initiation of autumn mixing of the lake waters, in mid-November.
Due to lack of oxygen, this zone (death zone) is practically inaccessible to most oxic organisms for six months of the year.
A similar phenomenon occurs, in varying scale and extent, in most lakes in Poland and increasingly in the Baltic Sea.