LAUSR

Abstract Planktic algae have an essential role in the food web as primary producers; the determination of the ecological niche space occupied by them is thus essential in strategies aimed at sustaining the biodiversity of surface waters. In the present study, principal component analysis combined with the outlying mean index was applied to 14 water quality time series (1993–2005) derived from three previously determined homogeneous sections of the Hungarian part of the River Tisza. As a result, the seasonal distribution of the ecological n-dimensional hypervolumes was determined for the different river sections. In the first upper section, the seasonal niches overlay each other, and no clear separation could be detected. In the middle- and lower reaches, however, a clear separation between the seasons was observed. The identification of these separate niches of the various seasons as the main indicators/drivers of certain ecological communities (e.g. phytoplankton) proved possible.