Evaluation and control of eutrophication in Helsinki sea area with the help of a nested 3D-ecohydrodynamic model

Abstract Helsinki is located on the southern coast of Finland by the Baltic Sea. The ecological state of the archipelago in front of Helsinki is affected by several factors. The effects of local point and scattered loads are mixed with the transboundary effects from neighbouring countries and atmospheric deposition. Municipal waste waters of 800 000 inhabitants of the capital area are released outside the archipelago into the open Gulf of Finland (GoF). In addition, river Vantaanjoki is transporting load originating from agriculture, industry and settlements of the drainage area to the shores of Helsinki. Toxic algal blooms and annoying filamentous algal mats have become a problem to the users of the coastal zone. A local nested 3D-hydrodynamic model with water quality and ecosystem modules was set up in the Helsinki sea area. The model gives us an opportunity to examine ecosystem processes in a simplified format. The whole Gulf of Finland is included into the calculation area with low resolution (5000 m) increasing in two steps towards the coastal area around Helsinki (1000- and 250-m resolution). The model calculates water flows, transport of nutrients and growth of two groups of phytoplankton and one group of littoral filamentous algae. For model validation, a measurement program was carried out (flows, nutrients, chlorophyll- a , littoral filamentous algal biomass). Two nutrient load reduction scenarios were analysed with the validated model. The first scenario describes planned improvements in the efficiency of Helsinki municipal wastewater treatment, and the second scenario decrease in riverine load from the drainage area. The wastewater scenario decreased both phytoplankton and filamentous algal biomass by 5–15% in the vicinity of the outlet located in the outer archipelago of Helsinki. Changes in the inner archipelago and close to the shoreline were hardly detectable. The effects of the agriculture scenario on the algal biomasses stayed mainly below the arbitrarily selected detection limit of 2%. The reason for low impact is probably the effective transport of the riverine nutrients out of the estuary. The water of river Vantaanjoki has a very high concentration of suspended solids making the growth of both phytoplankton and filamentous algae clearly light limited in front of the river mouth. Low light availability decreases the algal growth and nutrient uptake allowing an effective transport of even highly biologically available nutrients. When the nutrients are transported and diluted into a large area, also the effects of load reductions easily fade below the detection limit.