Effects of mass removal of fish on the nutrient status of a eutrophic lake in southern Finland

摘要

The nutrient status in a lake is dependent not only on the external loading, but also on the ecosystem structure (Carpenter et al. 1992, Scheffer 1998, Jeppe-sen et al. 1998, Meijer et al. 1999). At a constant level of nutrient load, function of the different trophic levels of phytoplankton, macrophytes, zooplankton and fish can directly or indirectly change the amount, balance and circulation of nutrients (Andersson et al. 1988, Scheffer 1998). Biomanipulation by removing cyprinids can change the nutrient status of a lake in several ways. The total amount of nutrients in the water pool decreases as the nutrients bound in the fish biomass are removed and as the fish-induced nutrient excretion and the release from the sediment declines (TAtrai & Istvanovits 1986, Brabrand et al. 1990, Horppi-la & Kairesalo 1990). If the cyprinid migration is lessened between habitats due to reduced food competition and increased predation pressure, the circulation of nutrients decreases (Brabrand & Faafeng 1993, Horppila et al. 1998). If the average individual growth rate increases due to younger age structure and released resources, fish bind nutrients into the fast-growing biomass (Carpenter et al. 1992, Kraft 1992). The cyprinid reduction indirectly changes the nutrient status by increasing the proportion of large cladocerans, which retain more phosphorus (P) than nitrogen (N) compared to small cladocerans (Carpenter et al. 1992). Higher grazing rate and reduced sediment disturbation may increase the Secchi depth, enabling the invasion of vegetation, which especially reduces N-concentration (Scheffer 1998). With decreasing phytoplankton biomass, sedimentation declines, which reduces P-flux from the bottom sediment (Scheffer 1998).