SUMMARY 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.

The vertical distribution of various phosphorus (P)-binding forms, associated potential P-binding partners and the composition of dry material were investigated in the bottom sediments of the dimictic oligotrophic Lake Stechlin and the dimictic eutrophic Lake Feldberger Haussee. Reductant soluble P (Fe- and Mn-bound) at the sediment surface (0−1 cm) was considerably higher in the oligotrophic Lake Stechlin (1.29 g kg−1) than in the eutrophic Lake Haussee (0.32 g kg−1). The amounts of dissolved, loosely adsorbed, metal oxide- and calcium carbonate bound P were higher in the eutrophic lake. The depth profiles of the investigated P species indicated that the mobilization of Fe- and Mn-bound P is the most important mechanism of P-release in oligotrophic lakes, whereas the mobilization of recently sedimented labile organic bound P seems to be the driving force of P-release in eutrophic lakes. In both lakes autochthonous calcite precipitations occurs during the summer months. The coprecipitation of P with calcite is an important self-cleaning mechanism in eutrophic hardwater lakes and contributes to the permanent burial of P in the sediments. Although, the precipitation of calcite is inhibited by the presence of high concentrations of soluble reactive P, the coprecipitation of P with calcite seems to be enhanced.

Dedicated to Prof. Ju¨rgen Benndorf on the occasion of his 65th birthday. AbstractChlorophyll a (chla) concentration was evaluated as a predictor of phytoplankton biomass across a broad trophic gradient of lakes (oligotrophic -highly eutrophic). First, a literature survey was conducted to collect information on the proportion of chla in phytoplankton biomass. As a result of this study (n ¼ 21) a mean value of 0.505%70.197 S.D. chla per unit wet weight of phytoplankton was calculated. Second, analyses were performed on 756 paired measurements from an unpublished database where the specific chla content of phytoplankton biomass was regressed against phytoplankton standing stocks and chla concentration. Within an interval of 0.1-50 g m À3 of phytoplankton wet weight, a substantial decrease in chla proportion from approximately 2.5% to 0.18% was found. Likewise, the proportion in phytoplankton wet weight decreased from 0.7% to 0.15% across a chla concentration interval of 0.001-0.150 g m À3 . These results had a significant impact both on chla-based biomass calculations and the subsequent comparison with phytoplankton biomasses resulting from microscopic counts. Assuming the microscopic method was a measure of the ''true'' phytoplankton standing stocks, then the precision by which phytoplankton biomass might be predicted based on chla measurements is clearly better when using variable proportions as compared to a constant conversion factor. The same holds for temporal coherence between annual records of phytoplankton biomass. The temporal fit was apparently better when relating the results of microscopic counts and biomass estimation based on variable proportions of chla in phytoplankton biomass. Nevertheless, this effect diminished as the tropic status of the lakes increased. Because of their variable specific chla content, separate taxonomic groups of phytoplankton differently affected the proportion of chla in total ARTICLE IN PRESS www.elsevier.de/limno 0075-9511/$ -see front matter (P. Kasprzak). phytoplankton wet weight. Chlorophyceae, Cryptophyceae and cyanobacteria had a high impact, while Bacillariophyceae, Dinophyceae and Chrysophyceae were of lesser importance.

Biomanipulation, the reduction of planktivorous fish to enhance filter-feeding zooplankton, has been used to rehabilitate eutrophied lakes. However, efficacy and long-term success were dependent on nutrient load, lake morphometry and biomanipulation measures. The ongoing focus on sustainable use of aquatic resources offers the chance to perform lake rehabilitation using a combined strategy of nutrient load reduction and traditional inland fisheries management techniques. Particularly in Central and Western Europe where piscivorous fish are the target species of most commercial and recreational fisheries, an enhancement of the piscivores by stocking and harvest regulations may act successfully in the co-management of ecosystem and fisheries. Guidelines are presented on how biomanipulation can be used as in lake rehabilitation by considering the objectives and constraints of traditional fisheries management. Alternatives in the decision tree are elucidated by examples from

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