With the purpose of finding out whether seasonal water level fluctuations could affect water quality in a reservoir subjected
to those changes, trends in environmental variables and in phytoplankton and zooplankton assemblages were analysed. The
reservoir’s hydrological cycle was characterized by three regimes. The maximum level phase lasted from January to the
beginning of June, the emptying phase existed between mid-June to the beginning of September and the minimum level
phase lasted from mid-September to the beginning of the first autumn/winter rain events. The highest values of total
phosphorus, soluble reactive phosphorus, nitrate, water colour and chlorophyll a were found during the minimum level
phase. The phytoplankton assemblage was dominated by taxa typical of meso-eutrophic environments during the emptying
and minimum level phases. However, during the maximum level phase, taxa generally found in more oligotrophic systems
were observed here also. Similar to other disturbed systems, the zooplankton assemblage was dominated by Rotifera,
except in summer and autumn when the cladoceran Ceriodaphnia quadrangula and/or the copepod Tropocyclops prasinus
became dominant. Although those shifts seem to be related to water level variations, further research is needed to evaluate
to what extent they might also be induced by other seasonal factors acting independently of water fluctuations. Based upon
the obtained data, suggestions for reservoir management are proposed.
