Linking the growth of filamentous algae to the 3D-ecohydrodynamic model of the Gulf of Finland

One of the most visible symptoms of eutrophication in the Gulf of Finland is blooms of unattached filamentous algae. This decomposing algal biomass causes serious nuisance problems for recreational uses of the coastal zone, particularly when cast ashore. The nutrient availability for these macroscopic filamentous algae is regulated by a superior competitor, the phytoplankton. Nutrients are left for the weaker competitor only when the conditions are not suitable for the growth of phytoplankton. This happens during vertical mixing of the water column. The lack of stratification prevents the formation of phytoplankton blooms, but does not limit the growth of filamentous algae when still growing attached to the bottom. A simple growth model was developed to describe the growth and biomass of filamentous algae. In the absence of suitable nutrient measurements, the model was linked to a 3D-ecohydrodynamic model which generated the nutrient input data. The model was calibrated with three-year monitoring data of filamentous algal biomass at one location. Validation was carried out with one-year monitoring data from an adjacent location. The model was able to describe the number of biomass peaks and their timing with good accuracy. After further development, the filamentous algal model will be used as one of the management tools for the evaluation of the sustainable nutrient load to the Gulf of Finland.