Tidally driven distribution of phytoplankton blooms in a shallow, macrotidal estuary

The influence of the ebb tide on the abundance and distribution of bloom-forming species, as well as the mechanisms for the selection of those species which remain, were investigated in the shallow, tidal-flushed Urdaibai estuary, north Spain. Phytoplankton was collected monthly from May to September 1998 during different tidal conditions (neap-spring) at five stations along the salinity gradient of the estuary. During the neap tides of May, Leptocylindrus danicus dominated in the lower estuary, together with Thalassiosira guillardii and Peridinium foliaceum in the upper segment; T.guillardii experienced the broadest displacement along the estuary. During the June and July cruises, coinciding with mid-tidal amplitudes and high temperatures, Peridinium quinquecorne densely aggregated in the upper estuary at the slack high tide, whereas Chaetoceros salsugineum bloomed at the intermediate stations. Cyclotella atomus and Protoperidinium achromaticum reached high concentrations in the upper zone only during the late stages of the ebb. During the spring tides of September, Prorocentrum minimum, Heterocapsa pygmaea and Heterocapsa rotundata appeared in the lower marine estuary, bring washed out at low tide. By contrast, the diatoms Skeletonemn costatrtm, Thalassiosira guillardii and Cyclotella atomus were the most abundant species in the upper reaches, peaking during the ebb. Physical trapping and high water residence times served to retain blooming species in the upper estuary. The intense growth of the estuarine diatoms may compensate for the advective seaward losses of cells during the ebb, thus allowing the development of stable populations in the estuary. Only Peridinium quinquecorne seems to combine an endogenous tidal rhythm with a photic response to remain in the tidal area of the estuary.