Statistical analysis of 14 yr (1988-2001) of intensive phytoplankton monitoring at Station 330 in the central Belgian Coastal Zone (BCZ, Southern Bight of the North Sea) indicates that the long-term diatom biomass trend and the spring dominance of Phaeocystis colonies over diatoms are determined by the combined effect of the North Atlantic Oscillation (NAO) and freshwater and continental nitrate carried by the Scheldt. The strong correlation between diatoms and the NAO index is largely explained by the modulating effect of the latter on the water budget at the monitoring station. The relationship between Phaeocystis spring blooms and winter NAO (NAOw) is indirect, better expressed by springtime Phaeocystis dominance over diatoms because of the higher response of the latter to the NAO. The spring Phaeocystis : diatom bloom ratio is negatively (or positively) linked to positive (or negative) NAOw values. A complex cascade of events links large-scale NAO index variations with those local meteorological conditions (wind strength and direction, rainfall) that drive the hydrography and water budget of the BCZ. Local meteorological conditions in turn modulate the geographical spread of Scheldt nutrient loads in the coastal zone and ultimately regulate the magnitude of Phaeocystis spring blooms by determining winter nitrate enrichment. Hence, the absence of a linear relationship between Phaeocystis spring blooms and NAOw is explained by the nonlinear response of river-based nitrate pulses to NAO due to local wind-driven hydrodynamical forcing.
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