Real-time carbon budgets and the native oyster: carbon sink or source?

Authors: Hannah Z. L. Lee & William G. Sanderson

Oyster restoration is not just about restoring a species or restoring an ecosystem. In the long run, the recovery of oyster habitat has the potential to bolster lost ecosystem services, such as fish production, improved water quality and possibly carbon sequestration (Beck et al., 2011; Kellog et al., 2014; Kent et al., 2016; 2017a; 2017b; zu Ermgassen et al., In Review). Being able to quantify and predict the beneficial outcomes of restoring oyster reefs is, however, difficult and likely varies from place to place. Yet quantifying the provision of ecosystem services is necessary for us to be able to responsibly communicate the potential value of restoration, and to incorporate the potential benefits into restoration planning.

Globally, the replanting and protection of terrestrial habitats is increasingly looked to by governments seeking to meet pledged carbon emissions targets. Restoration of forests and peatland has been ongoing for more than three decades with the aim of fixing carbon dioxide (CO2),protecting carbon deposits and enhancing carbon sequestration. When disturbed, carbon stores can off-gas carbon in the form of CO2. In May 2019, for example, over 5000 hectares of Scottish peatland was lost to wildfires, whichled to the estimated off-gassing of 174,000 tonnes of carbon ≈ 6 days of Scottish carbon emissions. Conversely, carbon stores can be protected and managed to promote the accretion of carbon.

Marine habitats are important carbon stores, termed “blue carbon”. Vegetated coastal habitats such as saltmarshes, seagrasses and mangroves make up only around 2% of ocean area, but store an estimated 50% of ocean organic carbon. In recent years, blue carbon research has begun to focus on the role of non-photosynthetic and calcifying ecosystems in capturing carbon.  For example, work from St Andrews University has shown that sea lochs are sediment sinks where carbon rich material is buried and stored. 

Figure 1. Conceptual carbon budget of European flat oysters. Directionality of arrows indicates carbon deposition (downward) or carbon release (upward), arrow size gives qualitative indication of relative size of carbon flow. Oyster image by SGW Illustrations.(Lee et al., In Press: Oyster image by SGW Illustrations)

The shellfish blue carbon story is, however, complex (Figure 1). Oysters are, like all living animals, sources of carbon dioxide. The animals respire and the process of shell formation (calcification) leads to both the release and storage of carbon. But oyster reefs may also be carbon sinks. As filter feeders, oysters take in particles from the water, ingest them and then deposit them as faeces and psuedofaeces onto the seabed (Figure 1) (Kent et al., 2017a; Lee et al., In Press). As the reef grows these particles and the associated carbon can become trapped. For example, up to 2m of sediment build-up has been recorded associated with a 275 ha horse mussel reef off the Pen Llŷn (Wales) (Lindenbaum et al., 2009). Similarly, the shell material produced by the oyster contains carbon and may become buried and stored in the sediment, in some cases for thousands of years (Emerson and Archer, 1990; Fariñas-Franco et al., 2018). However, the sink or source status of oyster reefs will likely vary due to a number of variables such as sediment type,  source of sediment input, age and density of the reef (Fodrie et al., 2017; Widdows et al., 2002).

The carbon budget of the native oyster can be described as a jigsaw of carbon loss and carbon deposition (Figure 1). In real-time, biodeposition and passive sedimentation (the physical presence of bivalve shells can slow the water nearby allowing particles to fall to the seafloor) facilitates carbon deposition. Oyster respiration and growth involves the release of CO2 and the short-term storage of organic carbon in the form of tissue. However, once the animal dies this is degraded. At the same time, shell production (calcification) both releases and captures carbon. In summary the real-time carbon budget of an oyster reef can be presented as follows (Lee et al., In Press):

Net carbon deposition = (Biodeposition + sedimentation + Calcification) – (Respiration CO2 + Calcification CO2)

Whether or not oyster reefs sequester carbon by building up carbon stores is a function not only of deposition, but also of what happens to the deposited carbon in the long term (Lee et al., In Press; Ullman, Bilbao-Bastida & Grimsditch, 2013). Rates of erosion, resuspension and bioturbation of the sediments as well as the effects of human disturbance must therefore also be considered (Ullman et al., 2013). The net carbon store budget is therefore (Lee et al., In Press):

Net carbon store = Net carbon deposition – (Rate of loss e.g. erosion + resuspension + remineralisation) 

To understand the oyster carbon story, all pieces of the jigsaw need to be considered. Whether sediment stores accreted by a healthy reef, will remain for a century or millennia is yet to be confirmed. Evidence suggests that existing bivalve reefs can protect considerable stores of carbon that should be protected to avoid further releases of carbon into the atmosphere (Fodrie et al., 2017; Lindenbaum et al., 2009). As for whether restored or existing reefs act as a sink of carbon – through biodeposition, sedimentation (Lee et al., In Press; Kent et al., 2017a) and the accumulation of shell material – or a source of carbon, through respiration and calcification pools, will likely depends on a number of factors such as location, sediment type, reef density, community processes and rates of loss (Lee et al., In Press). Understanding and being able to predict the outcome of these factors in a given location is an important research question to address as oyster restoration increases, and projects seek to derive the greatest possible benefits from their efforts.


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Project affiliation statement

Hannah started her PhD in the March of 2018. The focus of Hannah’s research is the relationship between blue carbon store formation, real-time carbon budgets and native European bivalve species, particularly the European flat oyster (Ostrea edulis) and the blue mussel (Mytilus edulis). As a member of the DEEP research and dive team Hannah is involved in restoring the European flat oyster to the Dornoch Firth.

This work is part-funded by the Dornoch Environmental Enhancement Project (a partnership between The Glenmorangie Company, Heriot-Watt University and the Marine Conservation Society). This work also receives funding and support from the Scottish Blue Carbon Forum, Edinburgh Napier University, Marine Scotland Science, NatureScot and St Abbs Marine Station (Scottish Charity Number SC041328).