Despite the importance of coastal ecosystems for the global carbon budgets, knowledge of their carbon storage capacity and the factors driving variability in storage capacity is still limited. Here we provide an estimate on the magnitude and variability of carbon stocks within a widely distributed marine foundation species throughout its distribution area in temperate Northern Hemisphere. We sampled 54 eelgrass (Zostera marina) meadows, spread across eight ocean margins and 36° of latitude, to determine abiotic and biotic factors influencing organic carbon (Corg) stocks in Zostera marina sediments. The Corg stocks (integrated over 25‐cm depth) showed a large variability and ranged from 318 to 26,523 g C/m2 with an average of 2,721 g C/m2. The projected Corg stocks obtained by extrapolating over the top 1 m of sediment ranged between 23.1 and 351.7 Mg C/ha, which is in line with estimates for other seagrasses and other blue carbon ecosystems. Most of the variation in Corg stocks was explained by five environmental variables (sediment mud content, dry density and degree of sorting, and salinity and water depth), while plant attributes such as biomass and shoot density were less important to Corg stocks. Carbon isotopic signatures indicated that at most sites <50% of the sediment carbon is derived from seagrass, which is lower than reported previously for seagrass meadows. The high spatial carbon storage variability urges caution in extrapolating carbon storage capacity between geographical areas as well as within and between seagrass species.

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 (C org stock) and carbon accumulation rates (C org 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 C org stock integrated over the top 25 cm of the sediment averaged 627 g C m −2 in Finland, while in Denmark the average C org 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 C org produced in the Finnish meadows is exported. Our analysis further showed that > 40 % of the variation in the C org 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 C org pool explained > 10 % of the variation in the C org 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.

&lt;p&gt;&lt;strong&gt;Abstract.&lt;/strong&gt; 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 (&lt;i&gt;Zostera marina&lt;/i&gt;) 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&lt;sup&gt;&amp;#8722;2&lt;/sup&gt; in Finland, while in Denmark the average Corg stock was over six times higher (4324 g C m&lt;sup&gt;&amp;#8722;2&lt;/sup&gt;). A conservative estimate of the total carbon pool in the regions ranged between 8.6&amp;#8211;46.2 t ha&lt;sup&gt;&amp;#8722;1&lt;/sup&gt;. 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 (&lt; 0.002&amp;#8211;0.033 t C y&lt;sup&gt;&amp;#8722;1&lt;/sup&gt;) were over two orders of magnitude lower compared to Denmark (0.376&amp;#8211;3.636 Corg t y&lt;sup&gt;&amp;#8722;1&lt;/sup&gt;). Our analysis further showed that &gt; 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 &lt;i&gt;Z. marina&lt;/i&gt; explained &gt; 12 % and contribution of &lt;i&gt;Z. marina&lt;/i&gt; detritus to the sediment surface Corg pool &gt; 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 &amp;#8364; ha&lt;sup&gt;&amp;#8722;1&lt;/sup&gt;, 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.&lt;/p&gt;

Seagrass ecosystems provide an array of ecosystem services ranging from habitat provision to erosion control. From a climate change and eutrophication mitigation perspective, the ecosystem services include burial and storage of carbon and nutrients in the sediments. Eelgrass (Zostera marina) is the most abundant seagrass species along the Danish coasts, and while its function as a carbon and nutrient sink has been documented in some areas, the spatial variability of these functions, and the drivers behind them, are not well understood. Here we present the first nationwide study on eelgrass sediment stock of carbon (C stock ), nitrogen (N stock ), and phosphorus (P stock ). Stocks were measured in the top 10 cm of eelgrass meadows spanning semi-enclosed estuaries (inner and outer fjords) to open coasts. Further, we assessed environmental factors (level of exposure, sediment properties, level of eutrophication) from each area to evaluate their relative importance as drivers of the spatial pattern in the respective stocks. We found large spatial variability in sediment stocks, representing 155-4413 g C m −2 , 24-448 g N m −2 , and 7-34 g P m −2 . C stock and N stock were significantly higher in inner fjords compared to outer fjords and open coasts. C stock , N stock , and P stock showed a significantly positive relationship with the silt-clay content in the sediments. Moreover, C stock was also significantly higher in more eutrophied areas with high concentrations of nutrients and chlorophyll a (chl a) in the water column. Conversely, siltclay content was not related to nutrients or chl a, suggesting a spatial dependence of the importance of these factors in driving stock sizes and implying that local differences in sediment properties and eutrophication level should be included when evaluating the storage capacity of carbon, nitrogen, and phosphorus in Danish eelgrass meadows. These insights provide guidance to managers in selecting priority areas for carbon and nutrient storage for climate-and eutrophication mitigation initiatives.

Seagrass meadows constitute important carbon sinks, and the ongoing global loss of seagrass habitats raises concerns about the release of carbon stored in their sediments. However, the actual consequences of seagrass loss for the release of carbon and nutrients remain unclear. Here, we take advantage of well‐documented historic losses of eelgrass (Zostera marina) meadows along the Swedish NW coast to assess how the contents of organic carbon (C) and nitrogen (N) in the sediment change when a meadow is lost. We find unusually high contents of C and N (on average 3.7% and 0.39% DW, respectively) in Swedish eelgrass sediments down to >100 cm depth, suggesting that these habitats constitute global hot spots for C and N storage. However, the C and N stocks were strongly influenced by wave exposure and were almost twice as high in sheltered compared to exposed eelgrass meadows. The sediment composition and stable isotope values were distinctly different in areas that have lost eelgrass meadows, with on average >2.6 times lower contents of C and N. The results indicate an erosion of >35 cm sediment following the historical eelgrass loss, and that sheltered meadows have more vulnerable sediment stocks. The results suggest that eelgrass loss has resulted in a release of 60.2 Mg C and 6.63 Mg N per hectare, with an estimated economic cost to society of 7944 and 141,355 US$/ha, respectively. The value of N storage represents one of the highest monetary values presented for an ecosystem service provided by seagrasses and shows that Swedish eelgrass meadows are particularly important for mitigating eutrophication. Following a documented loss of approximately 10 km2 of eelgrass in the study area, it is estimated that over 60,000 Mg of nitrogen was released to the coastal environment over a 20‐yr period, which constitutes over three times the annual river load of nitrogen to the Swedish NW coast. The study exemplifies the significant role of seagrass sediments as sinks for both carbon and nutrients, and that the risk of nutrient release following vegetation loss should be taken into account in the spatial management of seagrass and other coastal habitats.