The oxygen dynamics of the Oyster Ground, North Sea. Impact of eutrophication and environmental conditions

Abstract

Abstract The aims of the study were to determine the impact of environmental factors and anthropogenic nutrient inputs on hypoxia in the Oyster Ground area (North Sea). The relationships between stratification, primary production and the oxygen dynamics of the Oyster Ground have been investigated by field research during 1988-1990. The stability of the stratification was high in 1989 compared to 1988 and 1990 at all stations. Stable stratification coincided with low oxygen concentrations (± 4 mg 11 in 1989) in the bottom mixed layer (BML). The variation of the oxygen concentration in the surface mixed layer (SML) was small. Annual primary production rates varied between 74 and 198 gC m2 in the stratified Oyster Ground. The primary production rate in the BML depended on the light attenuation of the surface layer and the position of 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 anthropogenic nutrients had to be investigated by a set of interrelating models on water and nutrient transport, stratification, primary production and oxygen consumption. Transport of nutrients, nutrient cycling and retention have been described by a two-dimensional transport model coupled to a nutrient-phytoplankton model. Calculated time series of available nutrients have been used as boundary conditions for a more detailed one-dimensional nutrient-phytoplankton-stratification model for the Oyster Ground. The oxygen economy of the BML depends on the oxygen production, consumption and vertical transport across the pycnocline. The vertical diffusion coefficient across the pycnocline increases with tidal velocity. This explains the relatively high oxygen fluxes observed at stations in the southern part of the Oyster Ground compared to more northern stations. The oxygen consumption, calculated in the model 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 by this set of models. Meteorological conditions promoting stratification, in combination with NE-winds causing accumulation of nutrients, were identifiers important factors favouring oxygen depletion. According to the simulations, a 50 percent reduction of nutrient inputs, especially of nitrogen, into the North Sea would be sufficient to prevent that the oxygen concentrations drop below 5 mg11 in the Oyster Ground region.