Modelling phytoplankton dynamics and its application to lake management

Abstract

Conceptual models assist the understanding of complex, multivariate problems. Management models should offer reliable predictions of the outcome of alternative approaches to problems which assist the manager to decide the optimal course of action. It is rare for one model to fulfil both purposes. Excessive phytoplankton production in lakes, reservoirs and rivers presents, at best, a potentially conspicuous detraction from water quality or, at worst, a lethal cocktail which must be excluded from recreational waters and potable supplies. Thus, the difficulties may relate as much to the type of organisms present and to the problems they may cause as they do to the biomass 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 problems; the models are shown to be unhelpful and potentially misleading in the context of the questions usually asked. Even when quite general questions are submitted to generalised models, imprecision can lead to erroneous judgements. Approaches to making much more process-based models and expert systems are advocated. The ability to identify and quantify the principal regulatory 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 selected 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 modelling lake metabolism is introduced for its potential as a guide to determining management impacts and priorities at particular sites. The objective of the paper is to encourage the development of site-specific functional models which are oriented to both conceptual and management issues. We have to overcome the widespread but naive "my lake is phosphorus-limited" syndrome if we are to learn how to better manage our standing waters.