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. Detailed cell counts are presented for the years 1993, 1994, and 1995, of which 1994 was exceptional with a low spring peak but a much higher peak in June/July. Over tidal flats in the western Dutch Wadden Sea, I found Phaeocystis colony accumulation and cell densities one to two orders of magnitude higher than in the Marsdiep tidal inlet; colonies settled 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 processes. The small benthic gastropod Hydrobia ulvae at low tide grazed on colonies settled on the sediment. This loss of colonies from the water column by settling and grazing may help in rapidly decreasing P. globosa cell numbers after the spring peak in the Wadden Sea. Massive sedimentation and subsequent mineralization of Phaeocystis colonies might induce low oxygen events and mass mortality of macrobenthos, but up to now no direct relation has been reported between Phaeocystis sedimentation and mass mortality in zoobenthos.