Seagrass organic carbon stocks are not correlated with seagrass abundance at local scale, but loss of seagrasses does lead to decrease in surficial sediment organic carbon at the seascape scale in Bermuda

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Abstract
Seagrass ecosystems are recognized as important carbon sinks that accumulate organic carbon (Corg) in their biomass and underlying soils. In Bermuda, seagrass meadows have declined precipitously over the last two decades, potentially leading to loss of those Corg stocks. This study found 45% lower Corg stocks in Bermuda (ca. 32.3°N) compared to lower-latitude tropical Atlantic seagrass meadows. We also report 2–3 orders of magnitude lower long-term carbon burial rates (0.10 Mg Corgha−1y− 1 over the last 1830 years) compared to global estimates, determined via 14C dating of mangrove peats buried 1m below the modern sediment-water interface. Historical data from 2008, relatively early in the decline of Bermuda’s seagrasses, showed that neither seagrass abundance nor life history strategy (persistent vs. ephemeral) were associated with Corg content of surficial sediments or the relative contribution of seagrass-derived organic matter to the soil Corg. Following the collapse of most of Bermuda’s seagrasses, Corg content of surficial sediments declined by 15% over the period 2008–2024, although this decline was not a function of seagrass loss at the site scale. This suggests that seagrass-derived Corg is deposited across the Bermuda Platform, regardless of seagrass presence or abundance, and that the regional loss in seagrasses has led to a regional-scale decline in Corg content of the surficial soils. We conclude that seagrass loss can have landscape-scale effects on sediment Corg stocks even without site-specific relationships, and that caution should be exercised when applying regional or global literature values for average Corg stocks to estimate carbon market valuations.

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