Modelling production and biomasses of herbivorous and predatory zooplankton in lakes

Abstract

This work presents a dynamic model to predict two fundamental functional categories of zooplankton in lakes, herbivorous and predatory zooplankton. The model has been developed as an integral part within the framework of a more comprehensive lake ecosystem model, LakeWeb, which also accounts for phytoplankton, bacterioplankton, two types of fish (prey and predatory), as well as zoobenthos, macrophytes and benthic algae. The new model is based on ordinary differential equations, the ecosystem scale and gives seasonal variations (the calculation time is 1 week). It is meant to account for all important factors regulating the production and biomass of zooplankton and the predation on zooplankton in lakes. The model has not been calibrated and tested in the traditional way using data from a few well investigated lakes. Instead, it has been tested using empirical regressions based on data from many lakes. We have also presented new data sets and regressions based on those data for zooplankton. The basic aim of the dynamic model is that it should capture general functional and structural patterns in lakes. We have demonstrated by several model tests along limnological gradients (total phosphorus concentrations, pH, lake colour, latitude and lake size) that the new model gives predictions that agree well with the values given by the empirical regressions, and also expected and requested divergences from these regressions when they do not provide sufficient resolution. It would have been very difficult indeed to carry out such tests regarding ecosystem responses using traditional methods with extensive field studies in a few lakes. We have given algorithms for (1) production of herbivorous and predatory zooplankton, (2) elimination (related to the turnover time of zooplankton), (3) zooplankton consumption by prey fish, and the factors influencing these processes/rates. The model is driven by data easily accessed from standard monitoring programs or maps and meant to be of practical use in lake management.