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

The influence of the ebb tide on the abundance and distribution of bloom-fo rming species, as well as the mechanisms for the selection of those species which remain, were investigated in the shallow, tidal-flushed Urdaibai est uary, north Spain. Phytoplankton was collected monthly from May to Septembe r 1998 during different tidal conditions (neap-spring) at five stations alo ng the salinity gradient of the estuary. During the neap tides of May, Lept ocylindrus danicus dominated in the lower estuary, together with Thalassios ira guillardii and Peridinium foliaceum in the upper segment; T.guillardii experienced the broadest displacement along the estuary. During the June an d July cruises, coinciding with mid-tidal amplitudes and high temperatures, Peridinium quinquecorne densely aggregated in the upper estuary at the sla ck high tide, whereas Chaetoceros salsugineum bloomed at the intermediate s tations. Cyclotella atomus and Protoperidinium achromaticum reached high co ncentrations in the upper zone only during the late stages of the ebb. Duri ng the spring tides of September, Prorocentrum minimum, Heterocapsa pygmaea and Heterocapsa rotundata appeared in the lower marine estuary, bring wash ed out at low tide. By contrast, the diatoms Skeletonemn costatrtm, Thalass iosira guillardii and Cyclotella atomus were the most abundant species in t he 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 s eaward losses of cells during the ebb, thus allowing the development of sta ble populations in the estuary. Only Peridinium quinquecorne seems to combi ne an endogenous tidal rhythm with a photic response to remain in the tidal area of the estuary.