Interannual and regional variability of biological variables in a coupled 3-D model of the western Baltic

The physical processes in the western Baltic are characterized by a large amount of mesoscale variability which is also reflected in the distribution patterns of the biological variables. To gain a better understanding of the effects of circulation and variable forcing on the ecosystem the non-linear interactions of physical and chemical-biological processes were incorporated in a model. The coupled 3-D model consists of a highly resolved circulation model of the southwestern Baltic (GFDL-model MOM1) and a chemical-biological model. A high spatial resolution of 1 nautical mile in the horizontal direction and a vertical spacing of 2 m is used in order to resolve the mesoscale dynamics. The chemical-biological model is a simple Nutrient-Phytoplankton-Zooplankton-Detritus (NPZD)-model which describes the lower trophic levels. Experience has shown that it is essential to include the temperature dependence of the growth rate of phytoplankton, and the remineralization rate of detritus, as well as the variable sinking velocity of phytoplankton. Model simulations from spring to autumn of the years 1994 and 1995 were performed to investigate the effect of external forcing to the chemical-biological dynamics. A simulation with a riverine nutrient source was done to study the transport of riverine material.