Observations and simulations of hydrography, nutrients and plankton in the southern north sea

Much of the southern North Sea is shallow. Strong tidal stirring opposes stratification, and increases turbidity through sediment resuspension. Consequently, phytoplankton growth is often light-controlled in this region, although riverine nutrients increase production and maximum biomass in waters near the continental coast, where Phaeocystis and small copepods are common. In deeper waters to the north, seasonal stratification gives rise to a ‘text-book’ annual cycle of plankton, with the spring bloom followed by a period of severe nutrient depletion. These points are illustrated with results from repeated surveys of the southern North Sea in 1988-89, and further examined using a simple mathematical model. Observations and numerical simulations are used to provide insights into nitrogen cycling and eutrophication potential, which are greatest in regions of intermediate tidal stirring, and to support a discussion of what is needed by models if they are accurately to simulate nutrient cycling.