This study developed a statistical approach to predicting the probability of the occurrence of phytoplankton species from the general properties of a water body and the specific conditions at the time of sampling. Here, we report the results for the two Dinophytes Gymnodinium uberrimum and Peridinium willei which frequently cause problems in drinking-water treatment. Our statistical approach addresses the problem of uneven distribution of samples across parameters. For this purpose, we used probability calculation to assess under which conditions the species' occurrence is a likely or an unlikely event. The ecological niches of both species turned out to be similar, as both occur in oligotrophic to mesotrophic, deep, stratified reservoirs at low phosphorus concentrations. However, their pH and temperature range differed: G. uberrimum occurred predominantly between pH 6 and 7 and at a temperature range up to 168C, whereas P. willei showed a wider pH range and a temperature range below 128C. With these results, the classification of P. willei within the functional group L o proposed by Reynolds et al. (Reynolds et al., 2002) is now underpinned with a substantial database. The results suggest G. uberrimum to fit into this association as well.

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