Competition between different groups of phytoplankton for nutrients in the southern North Sea

A simple numerical model for the simulation of nutrient competition of two pelagic phytoplankton groups is discussed by which the indirect effect of silicate availability to flagellate biomass is examined. The model consists of six state variables: two nutrients (phosphate and silicate), two functional groups of phytoplankton (diatom biomass and flagellate biomass), detritus and benthos. The model is forced with a 9 component spectral model for solar radiation, with a seasonal cycle in water temperature, variable cloudiness with a periodicity of 5 days and turbulent mixing by tidal stirring and wind. The considered processes are primary production, using the Monod-Michaelis-Menten uptake kinetics and Steele's light function, respiration, mortality, detrital and benthal remineralization. A fixed Redfield ratio of C: Si: P of 106: 16: 1 is used. The system is computed with a Lagrangian model, using dynamically passive, but interactive tracers. The behaviour of the autonomous system is discussed and the effect of eutrophication is investigated in different runs with increasing phosphate concentration, with decreasing silicate concentration and both, A comparison with the measurements of Helgoland Roads shows a qualitative and quantitative good agreement. The annual net primary production is also in good agreement with observations. The simulation with increasing phosphate concentration only affects an increase of flagellate biomass due to the silicate limitation of the spring bloom of diatoms. A combination of phosphate increase and silicate decrease, however, leads to a shift in composition of functional groups with decreasing diatom biomass and increasing flagellate biomass. These results suggest that the indirect effect of silicate reduction in coastal waters favours strong flagellate bloom because of higher availability of riverborne nutrient loads. (C) 1998 Elsevier Science B.V.