FOOD-WEB MANIPULATION IN A SMALL, EUTROPHIC LAKE-WIRBEL, POLAND - THEEFFECT OF REPLACEMENT OF KEY PREDATORS ON EPIPHYTIC FAUNA

The effect of fish removal on the invertebrate fauna associated with S tratiotes aloides was studied in a shallow, eutrophic lake. The biomas s of invertebrate predators was approximately 2.5 times higher in the invertebrate-dominated year (1992) than in the fish-dominated year (19 91), while the density of non-predatory invertebrates in 1991 was ca h alf that in the invertebrate-dominated year. The decrease was due to a sharp fall in the density of epiphytic chironomids, with the density of plant-mining chironomids being far less affected. Marked declines i n the density of non-predatory invertebrates in the invertebrate-domin ated year were most probably caused by invertebrate predators. Once fr eed from suppression induced by fish, invertebrate predators were able to control the density of epiphytic prey more effectively than fish.