Blue carbon stocks in Baltic Sea eelgrass (<i>Zostera marina</i>) meadows

Abstract

<p><strong>Abstract.</strong> Although seagrasses cover only a minor fraction of the ocean seafloor, their carbon sink capacity account for nearly one-fifth of the oceanic carbon burial and thus play a critical structural and functional role in many coastal ecosystems. We sampled 10 eelgrass (<i>Zostera marina</i>) meadows in Finland and 10 in Denmark to explore the seagrass carbon stocks (Corg stock) and the carbon accumulation (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 627g C m<sup>−2</sup> in Finland, while in Denmark the average Corg stock was over six times higher (4324 g C m<sup>−2</sup>). A conservative estimate of the total carbon pool in the regions ranged between 8.6–46.2 t ha<sup>−1</sup>. 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. Similarly, the estimates for Corg accumulation in eelgrass meadows in Finland (< 0.002–0.033 t C y<sup>−1</sup>) were over two orders of magnitude lower compared to Denmark (0.376–3.636 Corg t y<sup>−1</sup>). Our analysis further showed that > 40 % of the variation in the Corg stocks was explained by sediment characteristics (density, porosity and silt content). In addition, the DistLm analysis showed, that root: shoot- ratio of <i>Z. marina</i> explained > 12 % and contribution of <i>Z. marina</i> detritus to the sediment surface Corg pool > 10 % of the variation in the Corg stocks, whereas annual eelgrass production explained additional 2.3 %. The mean monetary value for the present carbon storage and sequestration capacity of eelgrass meadows at Finland and Denmark, were 346 and 1862 € ha<sup>−1</sup>, respectively. We conclude that in order to produce reliable estimates on the magnitude of eelgrass Corg stocks, Corg accumulation and the monetary value of these services, more Blue Carbon studies investigating the role of sediment biogeochemistry, seascape structure, plant species architecture and hydrodynamic regime for seagrass carbon storage capacity are in urgent need.</p>

Sediment Properties as Important Predictors of Carbon Storage in Zostera marina Meadows: A Comparison of Four European Areas

“…marina meadows at Askö, where the most sheltered bays with finer grain sizes are dominated by brackish water plants, such as Potamogeton pectinatus and Zannichellia palustris [55]. Meadows situated in more exposed areas could result in a high export of the produced organic matter, as suggested by [18] instead of the carbon being accumulated in the sediment, leading to a low carbon storage potential of the area. This could also be true for the meadows in Ria Formosa, the only area in this study with a pronounced tide, where the higher hydrodynamic forces could also lead to increased sediment erosion.…”

“…For seagrass meadows, the finer grain-sized particles have shown to influence sedimentary carbon content in some seagrass areas [18], while in others it seems less important [8]. The relations between carbon storage and various sediment characteristics are more pronounced in meadows with low seagrass biomass and high proportion of finer particle sizes, while in meadows with larger seagrass species, e.g.…”

“…By reducing water velocity and facilitating sedimentation processes a seagrass meadow could increase the amount of fine particles, which thus promote high carbon storage. Grain size has recently been shown to correlate with sedimentary carbon content in some seagrass areas [18,19], especially in meadows with a low contribution of autochthonous derived carbon, although the influence of grain size on carbon storage is not universal for all seagrass species and habitats [19]. Grain size is also strongly related to sediment porosity and density, which are important factors influencing the oxygen conditions in the sediment.…”

Blue Carbon Storage Capacity of Temperate Eelgrass (Zostera marina) Meadows

“…In addition to particulate organic carbon (hereafter POC) and seagrass biomass, the seagrass sediment C org can be augmented by other carbon sources including phytoplankton, terrestrial plant detritus, macroalgae, epiphytes, and benthic microalgae (Bouillon & Boschker, ; Fry et al, ; Fry & Sherr, ; Holmer et al, ; Kennedy et al, , ; Moncreiff & Sullivan, ; Ricart et al, ; Röhr et al, ). These additional sources vary considerably in input and decomposition rates over time, thus influencing the lability and magnitude of C org stocks in seagrass sediments (Kennedy et al, , ).…”

Increased current flow enhances the risk of organic carbon loss from Zostera marina sediments: Insights from a flume experiment

“…The reduction of flow velocities by seagrass canopies and associated increased particle trapping thereby contribute to the accumulation of allochthonously derived sedimentary carbon in seagrass meadows (Agawin and Duarte ; Hendriks et al ; Kennedy et al ). In concordance, seagrass meadows in sheltered areas with less wave action have a higher sedimentary carbon content than do exposed bays (Samper‐Villarreal et al ), as environments with low hydrodynamic exposure usually promote the accumulation of sediment containing fine‐grain‐sized particles (i.e., high in silt and clay contents; Mazarrasa et al ) and low bulk density (Winterwerp and van Kesteren ), which are factors linked to high seagrass carbon storage (Dahl et al ; Röhr et al ; Gullström et al ).…”