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1. Background

Human-induced eutrophication and spread of anoxic bottom waters in Baltic Sea are of major concern in region today. Anthropogenic nutrient input to the Baltic Sea has been reduced during the last decades, to mitigate the harmful effects of eutrophication on marine ecosystems, but the efforts are not being rewarded with a healthier sea. Understanding ecosystem response and resilience to multiple stressors requires research on processes and their interactions both at long and short time scales. Lessons from the past can be used as a key to understanding the underlying mechanisms of long term changes which led to present status of the ecosystem. Besides natural factors (climate, postglacial land uplift etc.) the past land use may have considerably contributed to the eutrophication and hypoxia by influencing nutrient and carbon fluxes between terrestrial and aquatic environments in the Baltic Sea catchment. Agricultural activities have been a major cause of land-cover change within the Baltic Sea catchment for the last 6000 years. The effects of land use, such as increased nutrient run off and soil erosion, affect the marine environment and interact with other environmental stressors such as climate variability, fisheries and pollution, potentially leading to major ecosystem changes. Altered carbon storage in vegetation and soil may lead to increased quantity and mobility of Dissolved Organic Carbon (DOC), brownification and oxygen depletion in surface waters and may so contribute to changes in coastal marine ecosystems. Increased DOC concentrations have recently been observed in surface waters draining into the Baltic Sea …