ACCUMULATION AND SEDIMENTATION OF PHAEOCYSTIS-GLOBOSA IN THE DUTCH WADDEN SEA

In the Marsdiep tidal inlet, Phaeocystis globosa blooms have occurred every year since phytoplankton monitoring started in the early 1970s. A high spring peak is usually followed by lower peaks in summer, Detai led cell counts are presented for the years 1993, 1994, and 1995, of w hich 1994 was exceptional with a low spring peak but a much higher pea k in June/July. Over tidal flats in the western Dutch Wadden Sea, I fo und Phaeocystis colony accumulation and cell densities one to two orde rs of magnitude higher than in the Marsdiep tidal inlet; colonies sett led on the tidal flats, but did not become part of larger aggregates. The accumulation process resembles similar processes earlier described for fine sediments in the Wadden Sea and may be caused by similar pro cesses. The small benthic gastropod Hydrobia ulvae at low tide grazed on colonies settled on the sediment. This loss of colonies from the wa ter column by settling and grazing may help in rapidly decreasing P. g lobosa cell numbers after the spring peak in the Wadden Sea. Massive s edimentation and subsequent mineralization of Phaeocystis colonies mig ht induce low oxygen events and mass mortality of macrobenthos, but up to now no direct relation has been reported between Phaeocystis sedim entation and mass mortality in zoobenthos.