The sediment carbon stocks of intertidal seagrass meadows in Scotland

Highlights

• The sediment carbon stocks of intertidal seagrass meadows were assessed in Scotland.

• Sediment carbon density was highly variable across depth and among sites.

• The sediment carbon stocks in the top 50 cm ranged from 14.94 to 105.72 Mg C ha⁻¹.

• Seagrass plots retained 20% more organic carbon (% DW) than unvegetated plots.

Abstract

Seagrasses are highly productive ecosystems and hotspots for biodiversity, providing a plethora of benefits to the environment and to people. Their value in sequestering and storing carbon is increasingly being recognised, as the world searches for ways to mitigate the effects and slow the pace of climate change. However, many uncertainties remain, with basic information such as average carbon stocks, variability and species-specific differences missing for many regions. This study evaluates, for the first time, the carbon storage capacity of Zostera noltii and Zostera marina from intertidal seagrass meadows in Scotland. Sediment carbon stocks in the top 50 cm from vegetated and reference unvegetated plots were quantified at 10 estuaries distributed along the Scottish east and west coasts. The organic carbon stocks in the top 50 cm of the seagrass sediment ranged from a minimum of 14.94 Mg C ha⁻¹ at the Moray Firth to a maximum of 105.72 Mg C ha⁻¹ at the Firth of Forth, with a mean (±SD) of 54.79 ± 35.02 Mg C ha⁻¹ across the 10 estuaries sampled. Moreover, seagrass areas showed enhanced carbon storage compared to reference unvegetated ones, however this was highly variable across depth, and among sites and estuaries. This paper addresses key gaps in knowledge concerning the role of intertidal Scottish seagrass meadows as carbon sinks and discusses the implication of this emerging information for their effective management and conservation.

Introduction

Seagrass meadows, along with mangrove forests and tidal marshes - collectively termed coastal blue carbon habitats - are considered to be among the most productive and valuable ecosystems on the planet (Barbier et al., 2011). These habitats provide a wide range of ecosystem services. For example, they act as nursery sites, foraging grounds and predator refuges; they filter the water by recycling nutrients and removing pathogens; and they improve coastal safety by stabilising the sediment bed level (Costanza et al., 1997; Green and Short, 2003; Nordlund et al., 2016; Potouroglou et al., 2017).

Despite their importance, these vegetated coastal habitats have suffered rapid and extensive loss and degradation worldwide, with 29% of seagrass meadows, 50% of tidal marshes and >35% of mangrove forests being lost over the last 20–50 years (Barbier et al., 2011; Mcleod et al., 2011; Waycott et al., 2009). Of the known distribution of seagrasses, only one quarter (26%) occurs within Marine Protected Areas (MPAs). In contrast, 40% of warm-water coral reefs, 43% of mangroves, 42% of saltmarshes and 32% of cold-water corals are found in MPAs, making seagrasses the least protected major marine ecosystem (United Nations Environment Programme, 2020). Most seagrass losses have been driven by poor coastal zone management creating increases in nutrient concentrations and decreases in water clarity (Short and Wyllie-Echeverria, 1996). In the British Isles, there is strong evidence that most seagrass meadows have been detrimentally affected as a result of excess nutrients and turbid conditions, along with other anthropogenic impacts, such as moorings and anchoring (Green et al., 2021; Jones and Unsworth, 2016).

International climate and conservation discussions have recently focused on blue carbon habitats due to the growing recognition of their role as sites of significant carbon sequestration and storage (Himes-Cornell et al., 2018). Despite early evidence indicating that marine macrophytes can act as global carbon sinks (Smith, 1981), little policy attention was paid to carbon storage in these environments before Nellemann et al. (2009) defined ‘blue carbon’ as ‘the carbon stored and sequestered in coastal and marine ecosystems, including tidal and estuarine salt marshes, seagrass meadows, and mangrove forests’. Although estimates of the organic carbon stocks of tidal salt marshes and mangroves have been readily available, there are still large uncertainties in the figures for seagrass meadows. The large variation among datasets demonstrated by a range of studies reveals the challenge of using global estimates, or those derived from other areas, as proxies for assessing local carbon budgets (Dahl et al., 2016; Fourqurean et al., 2012; Lavery et al., 2013; Miyajima et al., 2017; Röhr et al., 2018). In addition, unvegetated areas adjacent to seagrass meadows are usually not included in such analyses. Including unvegetated areas in sampling design is important, since large stocks of sedimentary organic carbon may occur in coastal sediments free of vegetation. In assessing the current and potential contribution of seagrass to carbon storage, their ‘net impact’ - the difference in storage between vegetated and unvegetated sediments - is of most relevance.

The World Atlas of Seagrasses indicates that Scotland has more records of seagrass meadows than much of the Western European coastline (Green and Short, 2003). These records typically include only ‘presence’ data although two noteworthy exceptions provide additional information on coverage (Davison and Hughes, 1998): firstly, the 1200 ha of intertidal meadows of Zostera marina and Zostera noltii in the Moray Firth Special Area of Conservation (SAC) (east coast) (RSPB, 1995), within which Cromarty Firth is considered to have the largest seagrass meadow in the UK; secondly, the Solway Firth SAC (west coast) with a coverage of 200 ha (Hawker, 1993). To date there are no complete estimates of the total areal extent in Scotland, with the most conservative figure being 1600 ha (Burrows et al., 2014). In addition, a recent study reported a seagrass area of 1316 ha (with moderate to high confidence) for the whole of the UK; however, the authors acknowledge that inconsistences and inaccuracies occur within the datasets, with as much as a 30000-fold difference between documented and actual (ground-truthed) areas (e.g. in Hawaii, USA) (McKenzie et al., 2020). The growing interest in developing a blue carbon strategy in Scotland has led to an audit of the potential blue carbon resources in the coastal waters around Orkney (Porter et al., 2020), which includes subtidal seagrass meadows, whereas other published reports include seagrass values derived from the literature (e.g. average global sequestration rates or standing stocks) (Burrows et al., 2014, 2017). The carbon stocks of intertidal Zostera meadows for the whole of Scotland have yet to be quantified, and published carbon stocks estimates for Zostera noltii globally are very limited. To fill a major gap in available knowledge, the carbon storage capacity of the intertidal seagrasses Zostera noltii and Zostera marina was evaluated in Scotland, to the best of our knowledge for the first time. Our study aimed a) to quantify the sedimentary carbon stocks of intertidal seagrass meadows and of appropriate reference unvegetated areas, in order to infer the impact of seagrass on sediment carbon storage in Scotland, and b) to describe the variability between a range of different estuaries.