Phytoplankton primary production,chlorophyll and species composition,organic carbon and turbidity in the Marsdiep in 1990, compared with foregoing years

Annual primary production in 1990 in the Marsdiep amounted to about 250 g C.m^–2, which is lower than during the mid 1980s, but still higher than the about 150 g C.m^–2a^–1 measured during the 1960s and early 1970s. The annual curve shows a clear spring peak and a broad but lower summer peak. Chloropyll-a showed a similar annual curve, the maximum of 35 mg.m^–3 during the spring peak, as well as the annual average of 6 mg.m^–3 were lower than during the late 1970s and the 1980s. Organic carbon values for 1990 were similar to those observed in 1978 to 1984. Turbidity (at high tide) at our sampling station did not change over the period 1973 to 1990. The high winter temperature of 1989/90 did not influence the timing of the phytoplankton spring bloom. The diatom spring peak is better related to light. With more light in spring and clearer water, the peak occurs earlier. The trend of an increase of the period of occurrence ofPhaeocystis continued.Phaeocystis colonies and single cells were present almost the year round, however, the maximum cell numbers ofPhaeocystis (80,000 ml^–1) was relatively low. Newly formed colonies on spines of diatoms (Chaetoceros sp.,Bacteriastrum hyalinum) were observed in autumn. Its life-cycle remains enigmatic.Rhizosolenia indica, a warm water species, was observed for the first time in net-plankton samples, it occurred from September to December 1990.