Biomanipulation in shallow lakes: state of the art

The current state of biomanipulation was the subject of muchdiscussion at Shallow Lakes '95. This led to a workshop focusing onthe factors influencing the establishment of macrophytes and themechanisms responsible for their stability followingbiomanipulation. The purpose of the current paper is to distilcurrent knowledge on biomanipulation in shallow lakes gleaned fromdiscussions at the conference and recent literature.Biomanipulation should be used in the theoretical context of twoaltemative stable equilibria, as the extreme perturbation requiredto move from a phytoplankton dominated state to one dominated bymacrophytes. Understanding the nature of the factors and mechanismsresponsible for turbid water, is critical if biomanipulation is tobe appropriate. We suggest that with sufficient information,particular components of the fish community may be targeted andprecise figures for removal, designed to exceed critical thresholdvalues, may be set. Without this knowledge, a 'play-safe' strategyshould be adopted and at least 75% of the fish removed. Stockingwith piscivores may be a useful additional measure to fish removal.The principal objective of biomanipulation in shallow lakes is togenerate a period of clear water of sufficient length to allowmacrophytes to establish. To this aim, as well as for technicalreasons, biomanipulation is best undertaken in winter and earlyspring to generate clear water as early as possible in the season.In the cases where grazing is important, this coincides with thespring peak of Daphnia spp. Biomanipulation may have to berepeated if macrophytes do not colonise effectively within thefirst season. The factors responsible for the lag in response ofmacrophytes in some cases and the potential mechanisms contributingto the maintenance of clear water in macrophyte beds are discussed.From empirical data sets from many lakes, both a relative increasein the piscivorous fish stock and a reduction in nutrient levelsare thought to be important in stabilising the system in thelong-term. Whether biomanipulation may lead to alternativeequilibria (i.e. high diversity macrophyte communities withpiscivorous fish at high P) is unknown. Further study ofexceptional cases, theoretical modelling and development andanalysis of more long-term (>10 years) case histories isrecommended.[PUBLICATION ABSTRACT]