Sediment characteristics as an important factor for revealing carbon storage in Zostera marina meadows: a comparison of four European areas

Abstract. The seagrass ecosystem is an important natural carbon sink but the 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 areas of Europe (Gullmar Fjord on the Swedish west coast, Askö in the Baltic Sea, Sozopol in 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 carbon storage varied both among and within areas, where the Gullmar Fjord had a 15 times higher carbon storage compared to Askö and Sozopol. We found that high carbon content in Z. marina sediment is strongly related to a high proportion of fine grain size, high porosity and low density of the sediment. We suggest that sediment characteristics should be highlighted as an important factor when evaluating high priority areas in management of Z. marina generated carbon sinks.