Eutrophication is a serious threat to aquatic ecosystems globally with pronounced negative effects in the Baltic and other semi-enclosed estuaries and regional seas, where algal growth associated with excess nutrients causes widespread oxygen free “dead zones” and other threats to sustainability. Decades of policy initiatives to reduce external (land-based and atmospheric) nutrient loads have so far failed to control Baltic Sea eutrophication, which is compounded by significant internal release of legacy phosphorus (P) and biological nitrogen (N) fixation. Farming and harvesting of the native mussel species (Mytilus edulis/trossulus) is a promising internal measure for eutrophication control in the brackish Baltic Sea. Mussels from the more saline outer Baltic had higher N and P content than those from either the inner or central Baltic. Despite their relatively low nutrient content, harvesting farmed mussels from the central Baltic can be a cost-effective complement to land-based measures needed to reach eutrophication status targets and is an important contributor to circularity. Cost effectiveness of nutrient removal is more dependent on farm type than mussel nutrient content, suggesting the need for additional development of farm technology. Furthermore, current regulations are not sufficiently conducive to implementation of internal measures, and may constitute a bottleneck for reaching eutrophication status targets in the Baltic Sea and elsewhere. Highlights • Mussel farming is a viable internal measure to address Baltic Sea eutrophication. • Rates of nutrient removal depend on salinity at the regional scale and food availability at the local scale. • Cost effectiveness of nutrient removal by mussel farming depends also on farm type. • Total farm area needed for achieving HELCOM nutrient reduction targets is realistic.

Marine ecosystems and oceans have a vital role in sustaining life on Earth. Additionally, the oceans have an essential role in the world's economy as the main platform for global trade as well as a provider of many raw materials and resources. Due to the degradation of these marine ecosystems, the wellbeing of the seas and oceans has risen to the forefront of many global and regional political initiatives and agendas, and consequently, new concepts such as the blue economy have evolved. However, the sector lacks a clear consensus regarding its definition. In the absence of a unified definition, there is no general perception of what the blue economy sector means for the national economy of Finland. In order to efficiently and optimally regulate and manage the use of ocean resources and space, it is essential to understand the economic contribution and the role of industries linked to it. As a part of a more extensive research project called Blue Adapt, this thesis strives to identify the industries regarded as blue economy and measure their current economic contribution in Finland. In order to estimate the economic size of the blue economy sector, first the definition of the concept is clarified. The definition as well as the identification of economic activities regarded as part of it, is made based on a literature review. In the literature review, different studies are compared and analyzed and as a result, the most common activities are selected. The economic contribution of these sectors is measured by deriving data from the national accounts system and the standard structural business statistics (SBS). Further, four macro indicators are chosen to measure the economic weight of the blue economy sectors: turnover, employment, value added and exports. In addition, these macro indicators are analyzed and compared to national figures, such as the gross domestic product (GDP), gross value added (GVA), the national employment, and exports in order to gain an overview of the sector’s relative significance within the Finnish economy. This study shows that the six sectors of the blue economy generated in total a turnover between 14.6–20.2 billion euros, constituting roughly 3.5–4.9% of the national output, and created approximately 4–5.8 billion euros in value added, which covers over 2% from the national gross value added, and 1.8–2.6% from the gross domestic product. The sector employs somewhere between 53 000–71 000 people, which is 2–2.7% of the national employment of Finland. The share of exports from the national total is somewhere between 4–6.2%, and 3.5 to 5.4 billion euros. If compared to other industries in Finland, the blue economy sector is slightly larger than forestry measured by value added and possibly even larger than the forest industry, covering roughly 2.1–2.8% of the national GVA. In terms of employment, the proportion of the blue economy sector is slightly more than that of forestry and the forest industry combined, but less than that of agriculture. To conclude, as the forest industry and forestry are commonly seen as central generators of economic wellbeing in Finland, it is important to acknowledge the significance of the blue economy sector. Future policies regulating the maritime should bear in mind the economic importance and potential of the sector, as well as its role in climate change mitigation and other environmental policy goals.