Effect of freshwater flow on the succession and biomass of phytoplankton in a seasonal estuary

Physico-chemical factors affecting phytoplankton succession and dynamics are examined in the upper Swan River estuary, Western Australia. Freshwater discharge affects the residence time available for different phytoplankton taxa to grow. It also influences succession between marine, estuarine and freshwater phytoplankton taxa according to the extent that it hinders intrusion of marine water into the estuary. The three major phytoplankton groups, Bacillariophyta, Dinophyta and Chlorophyta, are strongly separated temporally by season, and spatially along the estuary according to flow and salinity. Bacillariophyta exhibit the widest range of maximum potential growth rates and occur under a wide range of discharges. Dinophyta, dominated by relatively few brackish water species, have the lowest growth rates, and occur only at very low discharges. Chlorophyta, dominated by Chlamydomonas globulosa, are intermediate in their potential growth rates, and are restricted to freshwater conditions. In the Swan River estuary, nutrients appear to be less important than flow and salinity in regulating phytoplankton succession and biomass. It is highly likely that anthropogenic effects on freshwater discharge to Australian estuaries have had a significant impact on composition and biomass of phytoplankton communities. Control of freshwater discharge thus has the potential to control species assemblages, phytoplankton bloom potential, and eutrophication.