The effect of ocean acidification on carbon storage and sequestration in seagrass beds; a global and UK context
Introduction
Ecosystem services can be defined as “the aspects of ecosystems utilized (actively or passively) to produce human well-being” (Fisher et al., 2009). As the importance of understanding how the natural world affects human health, wealth, and well-being is recognized, research into ecosystem services has risen exponentially (Fisher et al., 2009). An ISI Web of Knowledge search into the number of papers referring to the term ‘ecosystem services’ found 41 in 1995, whilst 2020 were published in 2013, highlighting the increase in interest in this area of science. The marine environment provides a number of ecosystem services, which are essential for human health and well-being, such as provision of food, coastal protection, water purification, and climate regulation (Liquete et al., 2013). Anthropogenic carbon emissions, which cause climate change, are a concern, as climate change may weaken the provision of some ecosystem services by the natural environment (Schröter et al., 2005).
Anthropogenic CO2 release primarily comes from two sources: (1) emissions from fossil-fuel combustion and industrial processes and (2) the CO2 flux from land-use changes (especially forest clearing) (Raupach et al., 2007). The first source is of particular concern as this releases over five times the quantity of CO2 as the latter (7.9 Gt C y−1 versus 1.5 Gt C y−1), and emissions are accelerating rapidly (Raupach and Canadell, 2007). The concentration of CO2 in the atmosphere has risen from 280 ppm in preindustrial times to levels of 390 ppm in 2010, an increase of approximately 40%. In 2013 levels of 400 ppm were measured at Mauna Loa volcano for the first time in recorder history, the highest levels found on earth in millions of years (Showstack, 2013). If CO2 emissions continue under the ‘business as usual’ scenario then levels are expected to increase to over 900 ppm by the end of this century (IPCC, 2013).
Increasing CO2 is of concern as it is a greenhouse gas and alters the energy budget of the earth by changing the net balance of incoming solar radiation and outgoing infrared radiation between the troposphere and the stratosphere, leading to changes in temperature, humidity, clouds and circulation, termed radiative forcing (Kamae and Watanabe, 2013). Greenhouse gases reduce the amount of energy leaving the atmosphere leading to a warming effect (Estrada et al., 2013). Other climatic effects from the increase of these gases include increased intensity of tropical cyclones (Krishnamurti et al., 1998) and increased frequency of El Niño like conditions (Timmermann et al., 1999), alongside conditions of greater precipitation or drought (IPCC, 2007).
The ocean contributes to the key ecosystem service of climate regulation, one aspect of which is the absorption of CO2 from the atmosphere, acting as a carbon sink. In 2010 it was thought to have absorbed approximately 155 Gt C (Khatiwala et al., 2013), almost 30% of the anthropogenic CO2 released into the atmosphere since the pre-industrial times (Doney et al., 2009, Sabine and Feely, 2007). A portion of the CO2 absorbed by the oceans is stored in living biomass or sequestered in sediments, whilst a large amount remains in its inorganic form. The increase in inorganic carbon stored in the oceans has led to a decrease in pH and associated changes to seawater chemistry, and has been termed ocean acidification (OA) (Doney et al., 2009). Since the Industrial Revolution (approximately 250 years ago), the pH of ocean surface waters has dropped by 0.1 units from 8.2 to 8.1, representative of a 30% increase in hydrogen ion concentration (Guinotte and Fabry, 2008). To try and predict future climate change and associated changes to ocean chemistry, a number of potential scenarios have been developed, and pH is expected to drop to anywhere between 8.05 and 7.6 by the end of this century (Caldeira and Wickett, 2003, IPCC, 2007, IPCC, 2013, Prinn et al., 2008, Prinn et al., 2011).
Ocean acidification is a ubiquitous stressor which is likely to lead to negative consequences for marine organisms (Kroeker et al., 2010), ecosystems (Fabry et al., 2008), and ecosystem service provision (Cooley et al., 2009) in the future. For example the Pacific oyster (Crassostrea gigas) is one of the most cultivated molluscs in the world, being extensively cultured or fished in Europe, North America, Asia and Oceana. In 2011 over 650,000 tonnes were harvested, primarily through cultivation (FAO, 2014). In the UK Pacific oyster cultivation was valued at 10.137 million in 2011/12 (Herbert et al., 2012). Research has shown that ocean acidification will have deleterious effects on this species (e.g. Barton et al., 2012, Kurihara et al., 2007), which may lead to a reduction in food provision, and employment potential in the future in relation to this species.
The majority of OA research has focussed on the effect of a decrease in pH on calcifying species (Brennand et al., 2010, Courtney et al., 2013; e.g. Gazeau et al., 2007, Iglesias-Rodriguez et al., 2008). The process of calcification is particularly susceptible to a reduction in pH (Fabry, 2008), due to the decrease in carbonate availability and subsequent decrease in calcite and aragonite saturation states. Although non-calcifying photosynthetic organisms are seen as a lesser priority, the ability of marine ecosystems and species to store and sequester carbon in marine sediments under changes in pH and carbonate chemistry is of paramount importance if we are to understand how the ability of the ocean to act as a carbon sink may change under future OA scenarios.
Seagrass beds cover less than 0.2% of the ocean surface, and yet they contribute to a disproportionate amount of marine net primary productivity (NPP) (1%), and are responsible for approximately 15% of carbon storage in the oceans (Kennedy and Bjork, 2012). Seagrasses play an important role in carbon sequestration, partially due to their ability to filter particulate carbon from the water column (Greiner et al., 2013), and to the fact that their belowground component of higher plants is nutritionally poor, unpalatable, and often found in anoxic sediments, so prevents efficient decomposition (Duarte and Cebrian, 1996). The carbon stored in living biomass or sequestered in sediments by seagrasses, alongside salt marsh plants and mangroves, has been termed ‘blue carbon’ (Nelleman et al., 2009), and the importance of conserving these habitats is increasingly recognised (Laffoley and Grimsditch, 2009, Luisetti et al., 2013, Nelleman et al., 2009).
Carbon storage and sequestration is an imperative component of climate regulation, as this carbon is locked away for either short (living biomass) or long (carbon buried within sediments or in the deep sea) periods of time (e.g. Lo Iacono et al., 2008, Mateo et al., 1997, Passow and Carlson, 2012), rendering it inert. Our definition of marine carbon sequestration has been modified from the OECD definition for biological carbon sequestration (OECD, 2001), to apply to marine environments. Marine carbon sequestration is defined as ‘a biochemical process by which carbon dioxide is fixed by living organisms, including phytoplankton, seagrasses and salt marsh plants, and involving the storage of carbon in sediments, or the deep sea, with the potential to reduce atmospheric carbon dioxide levels’. Without the ability of the oceans to sequester this carbon, atmospheric CO2 concentrations would be higher, leading to climate change occurring at a more rapid pace.
This study aims to detail both current carbon storage and sequestration in seagrass beds, and also how this may change in the face of OA from both a global and UK perspective. The UK has been used as a case study due relatively good data availability, and also to fulfil a real need of the UK’s Marine Management Organisation, to inform marine planning in line with the requirements of the UK marine policy statement. A review of the evidence of OA impacts on seagrass will be summarised, alongside a review of current and potential future carbon storage and sequestration of seagrasses. Calculations of standing stock (tC ha−1 yr−1) and carbon sequestration (tC ha−1 yr−1) have been used to assess these parameters.
Section snippets
Global carbon storage and sequestration by seagrass
Seagrass meadows are estimated to cover between 170,000 and 600,000 km2 globally (McLeod et al., 2011). They are generally net autotrophic, therefore acting as global carbon sinks (Duarte et al., 2010) This is partially due to their ability to filter sediments and particulate organic carbon from the water column (sestonic particles), through a reduction in flow, wave action and turbulence (Hendriks et al., 2008). Denser meadows trap sediments more effectively than less dense meadows or bare
The impact of ocean acidification on seagrasses
The global ocean pH is expected to fall to between 8.05 and 7.6 by the end of this century (IPCC, 2013, Prinn et al., 2011). This wide range of predicted values are built on scenarios of future development which make assumptions of the speed and shape of future political and technical changes, how human activities will change and natural resource availability (Prinn et al., 2011). As research on the effect of OA on seagrass beds is limited, it was not possible to separate out the impact of
Valuing the carbon sequestration component of climate regulation by seagrass, between present day and 2100
Understanding the extent and mechanisms of carbon storage and sequestration by seagrass, and how this will change with increasing ocean acidification, is clearly valuable from a natural resource management perspective. However, monetary valuation of this service further enables the debate, making the potential costs and benefits transparent and comparable to other impacts of ocean acidification. Carbon sequestration is an important component of climate regulation and the benefits of this are a
Discussion
This study has investigated the potential change to carbon storage and sequestration by seagrass beds in response to ocean acidification. It must be taken into consideration that the benefit of increased carbon storage and sequestration by seagrass beds in response to ocean acidification is unlikely to outweigh the negative impact on other ecosystem services provided by the ocean. Many other marine ecosystem services may be negatively impacted by a decrease in pH, most notable of which may be
Conclusions
Seagrass beds provide numerous other important ecosystem services such as protection of the coastline, bioremediation of waste, food provision, and maintenance of marine biodiversity (Cullen-Unsworth and Unsworth, 2013, Jackson et al., 2012). These services have not been investigated in this study, but it must be taken into consideration that whilst the story may be positive for carbon sequestration, other ecosystem services may suffer. Seagrass-associated taxa such as fish and invertebrates
Acknowledgements
Thanks to Caroline Hattam (PML) and an anonymous reviewer for her constructive comments on the manuscript. This work was funded through the UK Ocean Acidification research programme, co-funded by the Natural Environment Research Council (NERC), the Department for Environment, Food and Rural Affairs (Defra), and the Department of Energy and Climate Change (DECC) (Grant no. NE/H017488/1).
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2022, Estuarine, Coastal and Shelf ScienceCitation Excerpt :However, one of the main challenges in attributing a value to ecosystem services relates to the lack of sufficient data to quantify the service's scale and geographical extent (Dewsbury et al., 2016; Villa and Bernal, 2017; Nordlund et al., 2017). The majority of seagrass blue carbon assessments in the UK have been restricted to basic estimations using data from studies in different regions (Garrard and Beaumont, 2014), estimated values based on extrapolations (Green et al., 2018), or outdated standing stock assessments, making them unreliable (Green et al., 2021; Gregg et al., 2021). For example, Green et al. (2018) assessed the variability of subtidal Zostera marina seagrass sediment carbon, along the west coast of England.