Food web manipulation without nutrient control: A useful strategy in lake restoration?

The results of long-term, full-scale experiments in biomanipulation (in the closer sense of top-down control of the food web) in water bodies having extremely different phosphorus loads reveal that there is a close connection between the efficiency of food web manipulation and the nutrient situation in the particular water body. Top-down control provides a high probability of improving water quality with respect to Secchi depth and algal biomass only if it is connected with a decrease in the in-lake phosphorus. If that decrease in phosphorus is not achieved for different reasons, the high probability of an efficient top-down control is limited to zooplankton and to structural changes in the phytoplankton. A lake-specific ‘biomanipulation-efficiency threshold’ of the phosphorus load should not be exceeded. The results show that integrated water quality control, i.e. a combination of the strategy of load reduction with the strategy of ‘ectotechnology’, seems to be a very promising way. Presented at the International Conference on Lake Restoration of Zürich, 3–4 November 1986