Biomanipulation in the shallow lakes: State of the art

The current state of biomanipulation was the subject of much discussion at Shallow Lakes '95. This led to a workshop focusing on the factors influencing the establishment of macrophytes and the mechanisms responsible for their stability following biomanipulation. The purpose of the current paper is to distil current knowledge on biomanipulation in shallow lakes gleaned from discussions at the conference and recent literature. Biomanipulation should be used in the theoretical context of two alternative stable equilibria, as the extreme perturbation required to move from a phytoplankton dominated state to one dominated by macrophytes. Understanding the nature of the factors and mechanisms responsible for turbid water, is critical if biomanipulation is to be appropriate. We suggest that with sufficient information, particular components of the fish community may be targeted and precise figures for removal, designed to exceed critical threshold values, may be set. Without this knowledge, a 'play-safe' strategy should be adopted and at least 75% of the fish removed. Stocking with piscivores may be a useful additional measure to fish removal. The principal objective of biomanipulation in shallow lakes is to generate a period of clear water of sufficient length to allow macrophytes to establish. To this aim, as well as for technical reasons, biomanipulation is best undertaken in winter and early spring to generate clear water as early as possible in the season. In the cases where grazing is important, this coincides with the spring peak of Daphnia spp. Biomanipulation may have to be repeated if macrophytes do not colonise effectively within the first season. The factors responsible for the lag in response of macrophytes in some cases and the potential mechanisms contributing to the maintenance of clear water in macrophyte beds are discussed. From empirical data sets from many lakes, both a relative increase in the piscivorous fish stock and a reduction in nutrient levels are thought to be important in stabilising the system in the long-term. Whether biomanipulation may lead to alternative equilibria (i.e. high diversity macrophyte communities with piscivorous fish at high P) is unknown. Further study of exceptional cases, theoretical modelling and development and analysis of more long-term (> 10 years) case histories is recommended.