The Świerszcz stream is the longest and main watercourse in the Roztocze National Park. It drains the largest part of the Park’s area and is the most diverse one among the water ecosystems in the Park. The differentiation of water reaction along the whole of the stream course (between light acid and evident alkaline values) may confirm it. The differentiation of the stream results as well from the man-made sources of its fertilization located in the immediate drainage area (the reservation of Polish ponies, outflows from the fishponds and swimming-pond called Echo Ponds, outflows and sewage from the Brewery and from the Forest technical high school in Zwierzyniec City), as from the presence and influences of humic acids in water of the stream. The humic substances are washed out from the acid peat bogs situated in the area of outflow of stream. Polyhumic or dystrophic characteristics of water are especially important and unique to the Park as well as to the whole region of Central Roztocze, because the alkaline, hard water-bodies are predominant there. It is caused by chalk in the subsoil layer, and influences the biocenoses.