Evaluation of the long term monitoring of phytoplankton assemblages in a canyon-shape reservoir using multivariate statistical methods

Long term monitoring of the Rimov Reservoir provided a data set for the analysis of phytoplankton composition and biomass in coincidence with physical, chemical, hydrological and biotic factors. Data from 15 years of monitoring (1984 and 1998) were statistically processed using the CANOCO software. The distribution of dominant species and whole assemblages among three ordination axes were analyzed indirectly by detrended correspondence analysis (DCA). The relationships between the species and the environmental factors were studied by canonical correspondence analysis (CCA) with special attention to the seasons. Statistical analysis specified only two types of phytoplankton assemblages interchanging each year: a cold assemblage in winter and spring and a warm assemblage in summer and fall. The transition period was a 'clear-water' period. Species composition and phytoplankton biomass of the warm assemblage are the most important variables for drinking water reservoirs, especially for the Rimov reservoir. The appearance of cyanobacteria of the potentially toxic genus Microcystis was related the highest water temperature, higher concentrations of Na+ and an undisturbed epilimnion. The appearance of the green desmid alga Staurastrum planctonicum and several species of the cyanobacterial genera Aphanizomenon and Anabaena were connected with lower summer temperature and an undisturbed epilimnion but higher water level. The third type of summer dominance, Fragilaria and/or dinoflagellates was typical for years with summer disturbances (high discharge from the epilimnion) irrespective of the temperature.