3D Ecosystem Models as Decision Support Tools in the Gulf of Finland — the Kotka Archipelago as an Example

Abstract

The Gulf of Finland is one of the most eutrophied areas of the BalticSea. The major part of the nutrient load enters the sea outside Finnishborders from the metropolis of St. Petersburg and the river Neva.Finnish local authorities are facing a difficult task of defining waterprotection measures, when the effects of local measures have to beweighted against transboundary influences. This complex situation can bedemonstrated visually with the help of mathematical modelling.We present here a nested 3D model application for the Gulf of Finland,where focus is set,on the coast and archipelago outside the city ofKotka in the eastern Gulf of Finland. Our model describes the load andtransport of soluble nutrients, the growth of two groups ofphytoplankton and one group of littoral filamentous algae as well as thesettling, sedimentation and regeneration of detritus nutrients. Theresults of both model calibration and validation are shown. We have usedthe model to demonstrate the effects of two scenarios, the FinnishNational Water Protection Agenda and improvements in the sewagetreatment in St. Petersburg, on the algal biomasses.The effects of Finnish national measures take place in the innerarchipelago, close to the local sources of nutrient loads. This is thearea where eutrophication problems are accentuated and where most of thehuman activities take place. The effects of the measures carried out inSt. Petersburg can be seen in the middle and outer archipelago andcoastal areas further away from local nutrient sources. The influencedareas of the two scenarios are practically complementary and, thus, theycannot be understood as mutually exclusive ways to solve the problem ofeutrophication. The national measures are the only way to improve thewater quality in the heavily loaded parts of the coastline. However,without major water protection measures in St. Petersburg, theeutrophication of the whole Gulf of Finland cannot be stopped. This willnot be possible in Russia without extensive international co-operation.