Biomanipulation of lake ecosystems: successful applications and expanding complexity in the underlying science

1. To illustrate advances made in biomanipulation research during the last decade, seven main topics that emerged after the first biomanipulation conference in 1989 are discussed in relation to the papers included in this special issue and the general literature. 2. The substantially higher success rates of biomanipulations in shallow as opposed to stratified lakes can be attributed to several positive feedback mechanisms relating mainly to the recovery of submerged macrophytes. 3. The role of both nutrient loading and in-lake concentrations in predicting the success of biomanipulations is emphasised and supported by empirically defined threshold values. Nutrient recycling by aquatic organisms (such as fish) can contribute to the bottom-up effects on lake food webs, although the degree can vary greatly among lakes. 4. Ontogenetic niche shifts and size-structured interactions particularly of fish populations add to the complexity of lake food webs and make scientifically sound predictions of biomanipulation success more difficult than was previously envisaged. 5. Consideration of appropriate temporal and spatial scales in biomanipulation research is crucial to understanding food web effects induced by changes in fish communities. This topic needs to be further developed. 6. An appropriate balance between piscivorous, planktivorous and benthivorous fishes is required for long-lasting success of biomanipulations. Recommended proportions and absolute densities of piscivorous fish are currently based on data from only a few biomanipulation experiments and need to be corroborated by additional and quantitative assessments of energy flow through lake food webs. 7. Biomanipulation effects in stratified lakes can be sustained in the long term only by continued interventions. Alternate stable states of food web composition probably exist only in shallow lakes, but even here repeated interventions may be needed as long as nutrient inputs remain high. 8. Biomanipulation is increasingly used as a lake restoration technique by considering the needs of all lake users (sustainability approach). The combination of water quality management and fisheries management for piscivores with positive effects for both appears to be particularly promising. 9. Biomanipulation research has contributed substantially to progress in understanding complex lake food webs, which should in turn promote a higher success rate of future whole-lake biomanipulations.