Biological consequences of tidal stirring gradients in the North Sea

Tidal stirring gradients, interacting with seasonal variation in photosynthetically active radiation, sea-surface heating, and wind stirring, are proposed as the most important controls on plankton in the southern North Sea. The hypothesis, in the form of a numerical model, is tested against observations during 1988/89 of seasonal cycles and spatial variation in phyto- and zoo-plankton. The importance of the tidal mixing front, and the effects of residual circulation and nutrient-rich river discharges, are discussed, and estimates given of microplankton community production and its fate.