The nutrient emissions coming from agriculture, rural living, the atmosphere and many other diffuse sources increase coastal primary production, which leads to increased biomass of both filamentous algae and phytoplankton. Phytoplankton is the main feed for mussels and consequently this ecosystem service provided by mussel farming has been recognized by Swedish environmental authorities as a possible measure to improve coastal water quality. Concepts and management strategies on how to increase mussel farming for environmental purposes have recently been developed. The main principle is the implementation of nutrient trading as a management tool. This imposes demands on those who emit the pollution through emission quotas which are traded and bought by the emitter . The seller is a nutrient harvesting enterprise, e.g. a mussel farmer. However, the nutrient trading scheme thus far suggested by the Swedish EPA is rather complex and will probably take long time to launch.
An option to the nutrient trading system above could be a parallel to the EC environmental aid program used for agriculture operations, which would pay the mussel farmer according to what agricultural farmers are paid for spring cultivation, catch crops, buffer zones and wet lands. Such a system is natural since roughly half of the diffuse discharge of nutrients to the coastal zone has its origin from agricultural operations. However, this would likely requires political decisions at the EC level.
The Agro-Aqua recycling of nutrients
About 4% of the live mussel is carbon (C), 1.0% nitrogen (N) and 0.1 % is phosphorous (P). When the mussels are harvested, a known amount of the nutrients are recycled back from sea to land, mainly in the form of valuable sea food or mussel biomass which can be processed into mussel meal to be used in organic feed or composted into a rich organic fertilizer (eventually after having first been used for biogas production). The Agro-Aqua recycling concept may thus recycle nutrients from the sea back to land and agriculture operations. The mussel farming and harvest can be regarded as an engine running the recycling of nutrients.
Mussel farming in the Baltic
Earlier farm trials in Åland and Kalmarsund, Sweden, have shown that long-line mussel farming has definite potential, as the biomass after one year was 60 tons per hectare of farm area and could be expected to reach 120 – 180 tons per hectare in 2 to 3 years. This means that roughly 6 tons of carbon, 1.3 tons of nitrogen and 90 kg of phosphorus will be recycled through the harvest.
On-going large scale trials in Sweden have selected nets as the substrate for the mussel larvae to settle on, using pipes as floating elements. This technique is expected to be cost-effective and long-lasting. However, recent cold winters have clearly shown that the presence of ice, and especially drifting ice, calls for special solutions. Such solutions are available, e.g. in Denmark and Canada, but are at present not cost-effective enough in the context of the local economy of environmental mussel farming. One of the aims of future development is thus to improve the methods for lowering mussel farms below the surface in a simple and cost-effective way and to improve the equipment exposed to ice and ice drift. Such a developments may in turn lead to farming techniques which can be used at exposed farming sites.
In this context, SUBMARINER regional activities in Kalmar will focus on:
- The study of mussel farming techniques which can withstand ice and ice-drift while remaining cost-effective
- The study of harvesting techniques for the small and fragile Baltic mussels
- Lobby on the adoption of nutrient trading schemes as an environmental measure in the Baltic.