Development of the diatom-Phaeocystis spring bloom in the Dutch coastal zone of the North Sea:The silicon depletion versus the daily irradiance threshold hypothesis

The Dutch coastal zone of the North Sea is characterized by high nutrient inputs and low water column irradiance due to high concentrations of suspended matter. The vernal phytoplankton blooms are dominated by diatoms and the flagellate Phaeocystis (Haptophyta). Two hypotheses that predict the timing of the Phaeocystis bloom were tested with field data collected in 1992. The first one, stating that the Phaeocystis bloom starts when silicon has been depleted by a preceding diatom bloom, could not be bracketed with observations. The second hypothesis, predicting that Phaeocystis blooms under nitrogen-and phosphorus-replete conditions after a daily irradiance threshold has been passed could be supported by field observations. Furthermore, it is shown that the Dutch coastal zone is not continuously homogeneously mixed. It is argued that this is an important factor in the life cycle of Phaeocystis and in foam formation on nearby shores. Grazing by the ciliate Strombidium sulcatum and heterotrophic gymnodinoid flagellates accounted for 92% of Phaeocystis cell loss after the bloom.