Food-web manipulation in a small, eutrophic Lake Wirbel, Poland: Long-term changes in fish biomass and basic measures of water quality. A case study

Abstract

A whole-lake experiment was carried out for seven years (1988-1994) in a shallow, eutrophic lake. In the first phase ( 1989-1991), repeated introductions of young-of-the-year pike (Esox lucius) were used to control the density of juvenile stages of the dominant planktivores (roach Rutilus rutilus, white bream Blicca bjoerkna and Leucaspius delineatus). The successive introductions of juvenile pike were accompanied by selective removal of large pike, roach, bream Abramis brama and white bream. No visible improvement in water quality was registered in the first three years. Assessments made after the lake was treated with rotenone, revealed the high efficiency of juvenile pike in controlling prey of vulnerable size. Though the first three age-classes were nearly exterminated, older fish (3+ to 6+) remained abundant enough to keep their planktonic prey in check. Indeed, the non-altered density of dominant cladocerans indicated that they were still heavily preyed upon. Algal biomass remained high. In October 1991, rotenone was applied to remove all fish. Immigrating fish were gradually recolonizing the lake from spring 1992 onwards. However, the process was kept under control by continued stocking with juvenile pike and intensive fishing. The years following the rotenone treatment witnessed a considerable improvement in water quality, with water transparency 30% higher, a significant 2.2-fold decrease in seston dry weight and 2.8-fold decrease in algal biomass, averaged for the summer months. These events can largely be attributed to substantial changes in the herbivorous zooplankton, above all increased density of Lake Wirbel's largest cladoceran, Daphnia cucullata.