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 waterflow 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 lowflow periods. Chlorophytes and dinoflagellates were present during lowflow
periods upstream of the reservoirs and in the intermediate site.Cocconeiscf.placentulacharacterized 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.