Date Published: December 9, 2016
Publisher: Public Library of Science
Author(s): Martin Dahl, Diana Deyanova, Silvia Gütschow, Maria E. Asplund, Liberatus D. Lyimo, Ventzislav Karamfilov, Rui Santos, Mats Björk, Martin Gullström, Xiujun Wang.
Seagrass ecosystems are important natural carbon sinks but their efficiency varies greatly depending on species composition and environmental conditions. What causes this variation is not fully known and could have important implications for management and protection of the seagrass habitat to continue to act as a natural carbon sink. Here, we assessed sedimentary organic carbon in Zostera marina meadows (and adjacent unvegetated sediment) in four distinct areas of Europe (Gullmar Fjord on the Swedish Skagerrak coast, Askö in the Baltic Sea, Sozopol in the Black Sea and Ria Formosa in southern Portugal) down to ~35 cm depth. We also tested how sedimentary organic carbon in Z. marina meadows relates to different sediment characteristics, a range of seagrass-associated variables and water depth. The seagrass carbon storage varied greatly among areas, with an average organic carbon content ranging from 2.79 ± 0.50% in the Gullmar Fjord to 0.17 ± 0.02% in the area of Sozopol. We found that a high proportion of fine grain size, high porosity and low density of the sediment is strongly related to high carbon content in Z. marina sediment. We suggest that sediment properties should be included as an important factor when evaluating high priority areas in management of Z. marina generated carbon sinks.
Seagrass ecosystems are considered highly efficient natural carbon sinks  but there is a large variation in their capacity to store carbon, depending on species composition and habitat characteristics [2,3]. While the carbon sequestration efficiency is quite well documented for many seagrass species (e.g. [4,5]) the effects of different factors influencing intraspecific variation has only recently been investigated. To get a more accurate estimate of the global seagrass carbon sink capacity cause-effect relationships need to be better understood, and as seagrass loss is accelerating  information on habitat characteristics affecting carbon storage are of importance for an efficient protection and management strategy to increase carbon storage capacity .
In this assessment of four Z. marina areas in Europe, we found a large variation in organic carbon storage where the carbon-rich sediment of the Gullmar Fjord on the Swedish Skagerrak coast was 15 times higher compared to levels in the Baltic- and Black Seas. Along with recent studies [2,8], this study shows that the environmental conditions play an essential role in determining the carbon sink capacity. We found that sediment properties highly influence carbon storage in Z. marina meadows. The results showed that high sedimentary organic carbon coincides to high content of fine grain size, high sediment porosity and low sediment density. Seagrass meadows situated in areas characterized by these sediment properties are therefore suggested to have a high potential as natural carbon sinks.