Reconstruction of the Eutrophication in the Gulf of Finland using a Dynamic Process-Based Mass-Balance Model pp. 301-332
Authors: (Lars Håkanson, Dept. of Earth Sciences, Uppsala University, Uppsala, Sweden)
Abstract: The Gulf of Finland is a large bay in the Baltic Sea where major changes have taken place during the last 100 years. The Secchi depth has, for example, decreased from more than 7 m to about 5 m. The basic aim of this work has been to try to reconstruct the development that has taken place in this bay during the last 100 years. Since the conditions in the Gulf of Finland depend very much on both the river input of nutrients directly to the bay and the exchange of nutrients and water between the bay and the Baltic Proper, this work has focused on such interactions. The work describes how a general process-based mass-balance model (CoastMab) has been applied for the Baltic Proper and the Gulf of Finland. The model has previously been extensively tested and validated for phosphorus, suspended particulate matter, radionuclides and metals in several lakes and coastal areas. The transport processes quantified in this model are general and apply for all substances in all aquatic systems, but there are also substance-specific parts (mainly related to the particulate fraction and the criteria for diffusion). This is not a model where the user should make any tuning or change model constants. The idea is to have a model based on general and mechanistically correct algorithms describing the monthly transport processes (sedimentation, resuspension, diffusion, missing, etc.) at the ecosystem scale and to calculate the role of the different transport processes and how a given system would react to changes in inflow related to natural changes and anthropogenic reductions of water pollutants. The results presented in this work indicate that it is possible to remediate the Gulf of Finland and the Baltic Proper to the conditions that characterized the system 100 years ago. About 7000 tons of phosphorus (including 1800 tons from the tributaries to the Gulf of Finland) must then be removed on an annual basis from the present annual tributary load of about 30000 tons to the Baltic Proper. The trophic conditions in the Baltic Proper have varied relatively little during the last 25-30 years. The most marked changes in Secchi depth in the Gulf of Finland took place between 1920 and 1980.
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