The phytoplankton spring bloom in Dutch coaqtal waters of the North Sea

In the eastern part of the Southern Bight of the North Sea several sub areas could be distinguished, each with a characteristic spring bloom and species succession pattern. Regional differences in spring bloom timing were in accord with theoretical considerations in which (on the assumption of a vertically homogeneous distribuvion of phytoplankton) turbidity, incident radiation and water depth are key factors. The development rate of the spring bloom in Southern Bight waters can therefore be expected to change from year to year due to shifts in the balance between the effect of turbidity on the phytoplankton and the increasing solar radiation in spring. The growing season does not start anywhere in the southern North Sea before the mean light intensivy of the water column has passed 0.03 gcal cm-2min-1. After this the spring bloom development rate is most rapid in either clear or shallow waters (phytoplankton concentrations over 1 mm3/l from February to early March in offshore waters and in the French Flemish Banks area), slowest in turbid Dutch coastal waters (more than 1 mm3/l not before the end of March). Conversion of solar energy into organic matter is least efficient in the most turbid zone closest to the Rhine outflow (more than 1 mm3/l of phytoplankton not before the latter half of April); this, plus the lateness of vernal biomass increase, causes a decreased nutrient consumption in the coastal zone most under the influence of eutrophication. Consequently the effect of nutrient enrichment by the Rhine on primary production in the southern North Sea will be perceptible over an increasingly large area with an increase in turbidity and vice versa. During the first half of the growing season of 1974 (February to July) primary production (of particulate plus dissolved organic matter) in the Dutch coastal zone (salinity less than 32‰ S) was 80 to 90 g C/m2; in offshore waters (32 to 35‰ S) 130 to 150 g C/m2. The diatom spring bloom is controlled by silicate, while the maximum size of the succeeding Phaeocystis crop (April to early May) may still be set by phosphate, in spite of the greatly increased P loading of the eastern part of the Southern Bight in the last decades. Chlorophyll a concentrations at the surface usually correspond with surface primary production for a given incident radiation; but chlorophyll cannot be used as an index of phytoplankton concentration (measured as cell volume). Phytoplankton fluctuations in any one region do not only depend upon rate of primary production or of grazing, but also upon currents that may transport phytoplankton patches through that region