Modelling phytoplankton dynamics and its application to lake management

Conceptual models assist the understanding of complex, multivariate problem s. Management models should offer reliable predictions of the outcome of al ternative approaches to problems which assist the manager to decide the opt imal course of action. It is rare for one model to fulfil both purposes. Ex cessive phytoplankton production in lakes, reservoirs and rivers presents, at best, a potentially conspicuous detraction from water quality or, at wor st, a lethal cocktail which must be excluded from recreational waters and p otable supplies. Thus, the difficulties may relate as much to the type of o rganisms present and to the problems they may cause as they do to the bioma ss that may be achieved. Examples will be cited of some particular problems of lake and reservoir management that have been confronted in recent years . The range of model solutions available to plankton biologists is reviewed . The philosophies of these are unsympathetic to specific management proble ms; the models are shown to be unhelpful and potentially misleading in the context of the questions usually asked. Even when quite general questions a re submitted to generalised models, imprecision can lead to erroneous judge ments. Approaches to making much more process-based models and expert syste ms are advocated. The ability to identify and quantify the principal regula tory processes in operation, including the effects of light, turbidity and physical mixing and those relating to the trophic structure, is highlighted . Dynamic simulations, based upon the population ecology of several selecte d species simultaneously can give reasonable fits to observable phenomena. Applying altered model components to simulate viable options can be tested for their likely comparative impacts. A yet more recent approach to modelli ng lake metabolism is introduced for its potential as a guide to determinin g management impacts and priorities at particular sites. The objective of t he paper is to encourage the development of site-specific functional models which are oriented to both conceptual and management issues. We have to ov ercome the widespread but naive "my lake is phosphorus-limited" syndrome if we are to learn how to better manage our standing waters.