A general dynamic model to predict biomass and production of phytoplankton in lakes

This work presents a dynamic model to predict phytoplankton biomass and production. The model has been developed as an integral part within the framework of a more comprehensive lake ecosystem model, LakeWeb, which also accounts for production and biomass of bacterioplankton, two types of zooplankton (herbivorous and predatory), two types of fish (prey and predatory), as well as zoobenthos, macrophytes and benthic algae. The LakeWeb-model is based on ordinary differential equations (the ecosystem perspective) and gives seasonal variations (the calculation time, dt, is 1 week and Euler’s integration method has been applied). The sub-model for phytoplankton presented in this work is meant to account for all fundamental abiotic/biotic interactions and feedbacks (including predation by herbivorous zooplankton) for lakes in general. The model has not been 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. The basic aim of this dynamic model is that it should capture typical functional and structural patterns in many lakes. It accounts for how variations in (1) lake phosphorus concentrations, (2) water clarity, (3) lake morphometry, (4) water temperature, (5) lake pH and (6) predation by herbivorous zooplankton influence production and biomass of phytoplankton. An important demand for this model is that it should be driven by variables easily accessed from standard monitoring programs and maps (the driving variables are: total phosphorus, colour, pH, lake mean depth, lake area, and epilimnetic temperatures). We have demonstrated 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 regression lines when they do not provide sufficient resolution. The model has been tested in a very wide limnological domain: TP values from 3 to 300 μg/l, which covers ultraoligotrophic to hypertrophic conditions, colour values from 3 to 300 mg Pt/l, which covers ultraoligohumic to highly dystrophic conditions, pH from 3 to 11, which covers the entire natural range, and lake areas from 0.1 to 100 km2.