A model for the plankton system of the Broa reservoir, Sao Carlos, Brazil

In this paper a model, called ELLOBO running in STELLA II, was set to descr ibe the plankton system of the Broa reservoir (SP). The three state variabl es of the model are: phytoplankton, zooplankton, and the fish Astyanax fasc iatus. The forcing variables are: temperature, nitrate, phosphorus and sola r radiation. The model did not consider the cycling of nutrients inside the reservoir. The results show that: temperature is the principal forcing var iable in the phytoplankton dynamic and in the subsequent evolution of the w hole system. The zooplankton predation was described by Odum's equation, an d there is a strong random component in zooplankton grazing, which was esse ntial for the model, because zooplankton estimates have high variance. One must collect data in a short space of time (maybe daily) to better explain the zooplankton and phytoplankton variation. Validation was performed using simple statistics (arithmetic mean, standard deviation) and the results sh ow concordance between observed and simulated values. Overhead was used to calibrate some parameters and to validate the model. The highest overhead v alue (5%) imply in the better accordance between estimated and;observed sta te variables values. We believe this approach in Broa reservoir will provid e an useful tool for future research and it could be used comparatively in other continental aquatic ecosystems. (C) 2000 Elsevier Science B.V. All ri ghts reserved.