A DYNAMIC-MODEL DESCRIBING THE SEASONAL-VARIATIONS IN GROWTH AND THE DISTRIBUTION OF EELGRASS (ZOSTERA-MARINA L) .1. MODEL-THEORY

In shallow coastal areas eelgrass (Zostera marina L.) can have an important influence on the biological structure. As a reflection of this a general eutrophication model for coastal marine areas has been extended to include the seasonal variations in growth of eelgrass. The basic model describes the growth of phytoplankton and zooplankton and the nutrient dynamics. The important factors controlling eelgrass growth and distribution are included in the model: water transparency, water temperature, and water depth/topography of the water bed. Factors such as growth of epiphytes on eelgrass leaves and grazing birds and invertebrates are omitted. The eelgrass submodel describes seasonal and regional variations in production and biomass of above and below ground parts. The phytoplankton and eelgrass submodels interrelate, especially at the point of water transparency, since water transparency depends on the phytoplankton (chlorophyll-a) concentration. The model has been developed and calibrated based on data from a shallow area around the island of Masnedo in the southern part of Denmark. Data on phytoplankton, zooplankton and eelgrass production and biomass as well as nutrient status and important forcing functions were collected over a period of 1 year. The developed model was found to be able to describe the variations in the data material in a satisfying way.