Tackling eutrophication in Europe's most complex Archipelago

The Archipelago Sea Basin in Southwestern Finland, one of GREENHOOD’s demo-regions, is a landscape of more than 40,000 islands, shallow waters with slow renewal, and an exceptionally intricate coastline. This unique geography makes the region highly vulnerable to eutrophication. Although much of the sea is classified as having moderate ecological status, the rivers of the basin are in poorer condition, and the agriculture in the catchment remains Finland’s last HELCOM Hot Spot, a reminder of the complex pressures that continue to affect water quality.

Current status

Nearly one-third of the Archipelago Sea catchment is agricultural land, hosting 10% of Finland’s farms. Agriculture drives roughly 89% of the diffuse phosphorus load and 67% of all phosphorus load, largely due to two long-established issues: 

• Legacy phosphorus
  Around 80% of the basin’s fields are easily erodible clay soils, many of which contain large stocks of phosphorus accumulated over decades of heavy fertilisation. Although fertiliser use has decreased, the basin’s average easily soluble soil phosphorus content (14 mg/L) remains well above the national average (11 mg/L), and this stored phosphorus continues to leach into waterways.
• Partial manure surplus
  Pig and poultry production is highly concentrated, accounting for 28% of Finland’s pigs and 65% of its laying hens. This creates localised manure surpluses that exceed crop nutrient requirements, highlighting the need for manure redistribution to optimise nutrient use and minimise losses. 

Also, aquaculture, dominated by rainbow trout farming, and forestry runoff create additional pressures on the fragile Baltic Sea ecosystem, while climate change, through warmer winters, heavier rainfall, and shifting hydrology, further increases nutrient mobility.

Additionally, attention to the recycling of urban wastewater nutrients is still needed. Though the largest treatment plants in the catchment exhibit high nutrient removal efficiencies, smaller plants and sewage sludge reuse can still be enhanced.

Current responses

Finland has set a clear goal: removing the Archipelago Sea from HELCOM’s Hot Spot list by 2027. The Archipelago Sea Programme, launched in 2021, works towards fulfilling the 6 remaining criteria of the 23 criteria required to remove the Hot Spot status. The measures include improvement of manure management, tightened nutrient practices, and enhanced water protection measures.  

In practice, farmers and other stakeholders across the Archipelago Sea Basin are already implementing a rich mix of measures.  

Agricultural practices:

• Over 90% of farms participate in Finland’s agri-environmental schemes.
• Over-fertilisation is avoided through detailed soil assessments.
• Winter-time plant cover protects soils outside the growing season.
• Soil improvers such as gypsum and structural lime stabilise soils and reduce phosphorus runoff. 

Water, landscape, and urban management:

• Buffer strips, riparian areas, controlled drainage, and constructed wetlands help retain nutrients before they reach waterways.
• Forestry measures, including targeted ditch management and restoring natural water flows, reduce sediment and nutrient transport.
• Urban wastewater treatment removes the majority of phosphorus and nitrogen, reducing discharges into the basin.

These efforts are backed by a strong policy and funding framework. Yet challenges persist: economic constraints, policy uncertainty, and competition from cheaper mineral fertilisers slow the adoption of circular nutrient solutions. Access to soil and farm data is partially limited, making it harder to target measures where they are most needed. And for many farmers, complex regulation, low profitability, and the slow pace of environmental improvement create frustration. 

Where GREENHOOD steps in

GREENHOOD tackles nutrient pollution in the Archipelago Sea Basin by combining scientific research, governance innovation, and practical technology solutions. In its first year, the project has mapped nutrient challenges, soil conditions, existing technologies, and regulatory barriers, creating a shared knowledge base to guide targeted interventions.

On the ground, GREENHOOD is piloting concrete solutions for agricultural nutrient management. This includes improving the nitrogen and phosphorus ratio in pelletised poultry manure to enhance fertiliser efficiency, reduce environmental impact, and facilitate redistribution of phosphorus from surplus to deficit areas. The project also conducts field trials and risk assessments to validate these solutions, ensuring effectiveness while minimising environmental risks.