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.

<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>

Seaweeds are attractive as a sustainable aquaculture crop for food, feed, bioenergy and biomolecules. Further, the non-value ecosystem services of seaweed cultivation (i.e. nutrient recapture) are gaining interest as an instrument towards sustainable aquaculture and for fulfilling the aims of the EU Marine Strategy Framework Directive. Environmental factors determine the yield and quality of the cultivated seaweed biomass and, in return, the seaweed aquaculture affects the marine environment by nutrient assimilation. Consequently, site selection is critical for obtaining optimal biomass yield and quality and for successful bio-mitigation. In this study, 5 sites for cultivation of Saccharina latissima were selected within a eutrophic water body to guide site selection for future kelp cultivation activities. Results were coupled to marine monitoring data to explore the relationship between environmental conditions and cultivation success. The biomass yields fluctuated 10-fold between sites due to local variations in light and nutrient availability. Yields were generally low, i.e. up to 510 g fresh weight (FW) per meter seeded line; however, the dry matter contents of protein and high-value pigments were high (up to 17% protein and 0.1% fucoxanthin). Growth performance, biomass quality and bio-mitigation potential was restricted by low availability of light and bioavailable phosphorus, and biofouling through juvenile suspension feeders was a critical factor at all cultivation sites. At specific sites, the tissue metal contents (Pb and Hg) exceeded the limit values for feed or food. Our results emphasize the importance of careful site selection before establishing large-scale cultivation, and stress the challenges and benefits of kelp cultivation in eutrophic waters.

Eelgrass coverage in Odense Fjord (Denmark) has declined by 90% since 1983, due to eutrophication and its associated pressures, and the state of low eelgrass coverage has remained stable despite 10 to 15 yr of reduced nutrient loading and improved water quality. We hypothesize that the survival of eelgrass seedlings, and thus recolonization through reproductive dispersal, is negatively affected by physical disturbances. The 3 most likely physical mechanisms involved are uprooting or burial through drifting macroalgae, Arenicola marina sediment reworking and current-driven sediment resuspension. Our hypothesis was tested by field observations during the summer of 2009, when the mortality of seedlings was followed through time. The density of seedlings decreased dramatically by 80% during the first month of observations, and no seedlings survived past August, corresponding to an average seedling mortality of 1.5% d -1. This was > 3 times higher than the mortality for seedlings protected from physical disturbance by enclosures (0.4% d -1 ), indicating that physical disturbance contributed to high seedling mortality. A significant correlation (p = 0.02) between macroalgal drift and seedling mortality suggested that ~40% of seedlings were lost due to the physical disturbance of drifting algae. In contrast, no correlations were found between A. marina reworking or resuspension and seedling mortality, despite a mobility of up to 400 cm 3 sediment m -2 d -1 by these mechanisms. Given the observed intensity of macroalgal drift, we speculate that this mechanism severely hampers eelgrass reestablishment in certain parts of Odense Fjord.KEY WORDS: Eelgrass · Macroalgal drift · Bedload transport · Fucus sp. · Sediment · Reworking · Resuspension · Arenicola marina Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 418: [119][120][121][122][123][124][125][126][127][128][129][130] 2010 elongation (Olesen & Sand-Jensen 1994, Townsend & Fonseca 1998, Boese et al. 2009). Recolonization of larger areas, therefore, depends primarily on seed dispersal and the subsequent growth of seedlings (Olesen & Sand-Jensen 1994, Plus et al. 2003, Greve et al. 2005, Jarvis & Moore 2010. Z. marina has a high capacity for sexual reproduction, whereby seeds are released during late summer and fall, resulting in densities of from 10 2 to 10 4 seeds m -2 inside eelgrass beds and up to 30-50 seeds m -2 at a distance of 15 m from established beds (Harrison 1993, Olesen & Sand-Jensen 1994, Orth et al. 1994, Olesen 1999, Probert & Brenchley 1999, Boese & Robbins 2008. However, only 5 to 15% of seeds germinate to produce seedlings the following spring (Harrison 1993, Orth et al. 2003, resulting in low seedling density (0.2 to 26 m -2 ) outside established beds (Olesen & Sand-Jensen 1994, Greve et al. 2005, Boese & Robbins 2008. Hence, net expansion of eelgrass coverage depends primarily on the survival of a low number of seedlings to form new patches and eventually to create a continuous seagra...