THE OXYGEN DYNAMICS OF THE OYSTER GROUND, NORTH-SEA - IMPACT OF EUTROPHICATION AND ENVIRONMENTAL-CONDITIONS

The aims of the study were to determine the impact of environmental fa ctors and anthropogenic nutrient inputs on hypoxia in the Oyster Groun d area (North Sea). The relationships between stratification, primary production and the oxygen dynamics of the Oyster Ground have been inve stigated by field research during 1988-1990. The stability of the stra tification was high in 1989 compared to 1988 and 1990 at all stations. Stable stratification coincided with low oxygen concentrations (+/- 4 mg 1(-1) in 1989) in the bottom mixed layer (BML). The variation of t he oxygen concentration in the surface mixed layer (SML) was small. An nual primary production rates varied between 74 and 198 gC m(-2) in th e stratified Oyster Ground. The primary production rate in the BML dep ended on the light attenuation of the surface layer and the position o f the pycnocline. The production of the BML contributed from nil to 55 % of the total column production. As no historical data on the oxygen dynamics are available, the effects of changes in the input of anthro pogenic nutrients had to be investigated by a set of interrelating mod els on water and nutrient transport, stratification, primary productio n and oxygen consumption. Transport of nutrients, nutrient cycling and retention have been described by a two-dimensional transport model co upled to a nutrient-phytoplankton model. Calculated time series of ava ilable nutrients have been used as boundary conditions for a more deta iled one-dimensional nutrientphytoplankton-stratification model for th e Oyster Ground. The oxygen economy of the BML depends on the oxygen p roduction, consumption and vertical transport across the pycnocline. T he vertical diffusion coefficient across the pycnocline increases with tidal velocity. This explains the relatively high oxygen fluxes obser ved at stations in the southern part of the Oyster Ground compared to more northern stations. The oxygen consumption, calculated in the mode l depends on the amount of detritus and therefore is mainly determined by the total annual production. Differences in oxygen levels between three years of investigation were well reproduced bythis set of models . Meteorological conditions promoting stratification, in combination w ith NE-winds causing accumulation of nutrients, were identifieclas imp ortant factors favouring oxygen depletion. According to the simulation s, a 50 percent reduction of nutrient inputs, especially of nitrogen, into the North Sea would be sufficient to prevent that the oxygen conc entrations drop below 5 mg 1(-1) in the Oyster Ground region.