Blue carbon stocks in Baltic Sea eelgrass (&amp;lt;i&amp;gt;Zostera marina&amp;lt;/i&amp;gt;) meadows

Maria Emilia Röhr,Christoffer Boström,Paula Canal-Vergés,Marianne Holmer

doi:10.5194/bg-13-6139-2016

Abstract

Abstract. Although seagrasses cover only a minor fraction of the ocean seafloor, their carbon sink capacity accounts for nearly one-fifth of the total oceanic carbon burial and thus play a critical structural and functional role in many coastal ecosystems. We sampled 10 eelgrass (Zostera marina) meadows in Finland and 10 in Denmark to explore seagrass carbon stocks (Corg stock) and carbon accumulation rates (Corg accumulation) in the Baltic Sea area. The study sites represent a gradient from sheltered to exposed locations in both regions to reflect expected minimum and maximum stocks and accumulation. The Corg stock integrated over the top 25 cm of the sediment averaged 627 g C m−2 in Finland, while in Denmark the average Corg stock was over 6 times higher (4324 g C m−2). A conservative estimate of the total organic carbon pool in the regions ranged between 6.98 and 44.9 t C ha−1. Our results suggest that the Finnish eelgrass meadows are minor carbon sinks compared to the Danish meadows, and that majority of the Corg produced in the Finnish meadows is exported. Our analysis further showed that > 40 % of the variation in the Corg stocks was explained by sediment characteristics, i.e. dry density, porosity and silt content. In addition, our analysis show that the root : shoot ratio of Z. marina explained > 12 % and the contribution of Z. marina detritus to the sediment surface Corg pool explained > 10 % of the variation in the Corg stocks. The mean monetary value for the present carbon storage and carbon sink capacity of eelgrass meadows in Finland and Denmark, were 281 and 1809 EUR ha−1, respectively. For a more comprehensive picture of seagrass carbon storage capacity, we conclude that future blue carbon studies should, in a more integrative way, investigate the interactions between sediment biogeochemistry, seascape structure, plant species architecture and the hydrodynamic regime.

Highlights

Atmospheric carbon dioxide (CO2) enters the ocean via gasexchange processes at the ocean–atmosphere interface
Seagrasses cover only a minor fraction of the ocean seafloor, their carbon sink capacity accounts for nearly one-fifth of the total oceanic carbon burial and play a critical structural and functional role in many coastal ecosystems
Our analysis further showed that > 40 % of the variation in the Corg stocks was explained by sediment characteristics, i.e. dry density, porosity and silt content

Summary

Introduction

Atmospheric carbon dioxide (CO2) enters the ocean via gasexchange processes at the ocean–atmosphere interface. A large percentage of this fixed carbon is stored and sequestered in the sediments of vegetated coastal ecosystems, of which the three globally most significant are salt marshes, mangrove forests and seagrass meadows (Herr et al, 2012). Seagrass meadows only cover globally about 300 000–600 000 km of the ocean sea floor, corresponding to 0.1 to 0.2 % of the total area, their carbon sink capacity (the capacity of seagrasses to absorb and store carbon in living and dead biomass and in the sediments) may account for up to 18 % of the total oceanic carbon burial (Gattuso et al, 1998; Duarte et al, 2005; Kennedy et al, 2010; Fourqurean et al, 2012).

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Conclusion