The blue carbon of shellfish beds: storage, water quality, management and societal benefits

Shellfish beds are one of “the most imperiled marine habitats on earth”. Consequently, European flat oysters (O. edulis), horse mussels (Modiolus modiolus) and blue mussels (Mytilus edulis) are Priority Marine Habitats in the NE Atlantic and desirable conservation features in European Marine Protected Areas. In an era of rapid changes in marine space through built structures, Marine Protected Areas and Maritime Spatial Planning, there are opportunities for the management and the re-establishment of sensitive native shellfish habitats for nature conservation, water quality management, wild shell fisheries and aquaculture.

Shellfish beds are species-rich, depositional habitats that act as nursery areas for other marine species, some with commercial value, whilst, crucially, improving water quality by filtration. Multiple policy and legislative drivers across Scotland and Europe will improve the environment and increase sustainable blue economy activities, making shellfish restoration a multi-sector ‘win-win’ through a number of potential business models. The investment potential of these business models are unlocked by an ability to predict the outcomes. Research into depositional carbon storage and water filtration by shellfish is therefore of paramount importance.

Heriot-Watt University (HWU) have shown that native oysters were once present in the Dornoch Firth protected area. Taking a lead role in the Dornoch Environmental Enhancement Project, The Glenmorangie Whisky Company have launched an Anaerobic Digestion plant that will remove 95 % of the Distillery’s organic waste. What remains of the discharge (5%) could be mitigated with the restoration of shellfish, especially native oyster habitats9. At the same time, the blue mussel (Mytilus edulis) fishery in the Dornoch has unusually low stocks that might benefit from restorative management. Close-by at Noss Head MPA, carbon-rich deposits have been described under an extensive horse mussel (Modiolus modiolus) bed, a characteristic that needs to be accounted for in a changing CO2 rich climate, as it should be in other MPAs such as the Dornoch where these habitats are also found. Overall, these management scenarios can benefit directly or indirectly from an improved understanding of water filtration and carbon capture by shellfish habitats: an understanding that would unlock investment potential more widely.

This PhD project will quantify the role native oysters, horse mussels and blue mussels have in water quality improvement (nitrogen / phosphate removal) and carbon capture in the North Sea. Overall, the project will therefore clarify the investment potential for shellfish management and restoration. The project has 5 main objectives:

• To quantify the depositional rates for all three species of shellfish (i) in the lab under controlled conditions, under realistic environmental settings of flow and food availability, using the seawater flow-through facility at St Abbs Marine Station and (ii) in the field at Dornoch and St Abbs. To interpret these results in the context of other published rates for shellfish species to maximize this understanding.
• In combination with the depositional rate studies above, use respirometry to help generate a CO2 budget, as well as a carbon storage budget, for the shellfish species such that net sequestration can be assessed after the amount of CO2 produced through respiration is accounted for.
• Analyze core samples and sub-bottom profile data and other imagery to quantify the carbon stored in shellfish beds in any of the study sites
• To extrapolate findings to other areas where shellfish beds are found, were found, and where restoration might be appropriate for nature conservation and / or water quality management
• To exchange knowledge/learn from shellfish sequestration research conducted on the American oyster species through a visit to key USA institutions