Possibilities and Limits for Controlling Eutrophication by Biomanipulation

Ecotechnology aims at the implementation of such a complex physical, chemical and/or biological ecosystem structure, which leads to a maximum capacity of the ecosystem to improve the water quality by ecosystem-internal mechanisms. Numerous ecological control variables can be used in ecotechnology, however only mortality caused by predation, feeding or grazing on all trophic levels provides the possibility of a top- down control of the community structure. There are two contrasting concepts regarding the efficiency of top-down control: (1) the “cascading trophic interactions” hypothesis (Carpenter et al., 1985), which postulates that changes at the top of the food web are transmitted down to primary producers, and (2) the “bottom-up/top-down” hypothesis (McQueen et al., 1986), which predicts that top-down effects are strong at the top but weaken near the bottom of the food web. This paper describes an examination of these concepts by summarizing the effects of strong (Bautzen reservoir) and extremely strong (Lake Grafenhain) enhancement of the stock of piscivorous fish. Both investigations refer to long-term whole-lake experiments. The results seem to confirm the “bottom-up/top-down” hypothesis. Effects at the top are strong, although sustained effects regarding dominance of Daphnia are achieved only at an “optimum” (relatively low but not zero) density of zooplanktivorous fish. Far below the optimum density an efficient invertebrate predator was able to compensate almost completely the fish predation pressure on large Daphnia. On the other hand, the optimum density can easily be exceeded during periods when high numbers of 0+ fish occur. Especially the effective control of 0+ fish of the intentionally enhanced piscivorous species is a key problem. At the bottom of the “classical” food web, the effects are restricted to a change of the phytoplankton compositions (mean biomass is not reduced). The contradiction between the latter results and other findings from lakes having lower phosphorus loadings (e. g. Stenson, 1988) can be solved only if the “bottom-up/top-down” hypothesis is combined with the hypothesis of a “biomanipulation efficiency threshold of the P-loading” (Benndorf, 1987). The latter hypothesis predicts that the desired reduction of mean phytoplankton biomass as a strong top-down effect at the bottom of the food web can be expected only below a threshold of the P-loading. Below this threshold a set of biomanipulation-induced indirect effects on phosphorus metabolism causes a reduction of the in-lake P-concentration. Consequently, sustained reduction of the phytoplankton biomass by biomanipulation can be achieved only by P-limitation, which is an indirect bottom-up effect induced by top-down control of the food web. Taking into consideration the combination of the “bottom-up/top-down” and “P-threshold” hypothesis, an attempt is made to predict the effects of biomanipulation regarding phytoplankton and water transparency under different conditions of lake morphology, P-loading and light climate.