Recurrent and changing seasonal patterns in phytoplankton of the westernmost inlet of the Dutch Wadden Sea from 1969 to 1985

Data for phytoplankton composition and abundance in the Marsdiep are presented for the period from 1969 to 1985 inclusive. Only a few species dominated the phytoplankton. A recurrent pattern was observed in the seasonal succession: in winter, total cell numbers were invariably low, but freshwater algae, sluiced into the Wadden Sea from IJssel Lake, showed highest densities in winter. A diatom spring peak was observed around mid-April, followed by a Phaeocystis pouchetii peak about three weeks later. Later in summer usually two more diatom peaks followed by non-diatom peaks were present. The exact timing of the spring peak varied from year to year, with the extremes being late March and early May. A relatively late spring peak usually coincided with a relatively high turbidity in the preceding winter. An increase in total cell numbers was found over the 17-year observation period. Diatoms decreased from 1969 to 1974 but have increased since then, reaching values above those of 1969 during recent years. Flagellates showed a consistent increase over the entire observation period.