Reservoirs override seasonal variability of phytoplankton communities in a regulated Mediterranean river.

Water hydrology, temperature and transparency, as well as nutrient retention downstream of the reservoirs alter the temporal and spatial distribution patterns of phytoplankton communities in regulated rivers. The seasonal dynamics of phytoplankton communities in the Ebro was analysed in contrasting water flow periods in sections upstream and downstream of three large reservoirs, as well as in an intermediate site. Phytoplankton communities changed in response to seasonal variations in the areas not influenced by the reservoirs, but the phytoplankton distribution downstream of the reservoirs was driven by their particular hydrodynamics. The change in environmental conditions promoted by reservoirs influenced the pattern of replacement between diatoms and green algae of the upstream section. Differences in the phytoplankton community structure, abundance and environmental variables between upstream and downstream sites were maximal during low flow periods. Chlorophytes and dinoflagellates were present during low flow periods upstream of the reservoirs and in the intermediate site. Cocconeis cf. placentula characterized the downstream section, associated to the presence of macrophytes in that section. The present study sheds light on the consequences of river regulation under potential scenarios of climate change, and results could be used to anticipate ecological problems in large regulated rivers under these circumstances.