First assessment of seagrass carbon accumulation rates in Sweden: A field study from a fjord system at the Skagerrak coast

Seagrass meadows are globally important blue carbon sinks. In northern cold-temperate regions, eelgrass ( Zostera marina ) is the dominant seagrass species, and although their sedimentary carbon stocks have been quantified across regions, information regarding the CO 2 withdrawal capacity as carbon sinks remains scarce. Here we assessed the carbon (C org ) accumulation rates (CARs) and stocks as well as the organic matter sources in five seagrass meadows in the Gullmar Fjord area on the Swedish Skagerrak coast. We found that the mean (±SD) CAR was 14 ± 3 g C org m -2 yr -1 over the last ~120–140 years (corresponding to a yearly uptake of 52.4 ± 12.6 g CO2 m -2 ). The carbon sink capacity is in line with other Z . marina areas but relatively low compared to other seagrass species and regions globally. About half of the sedimentary carbon accumulation (7.1 ± 3.3 g C org m -2 yr -1 ) originated from macroalgae biomass, which highlights the importance of non-seagrass derived material for the carbon sink function of seagrass meadows in the area. The C org stocks were similar among sites when comparing at a standardized depth of 50 cm (4.6–5.9 kg C org m -2 ), but showed large variation when assessed for the total extent of the cores (ranging from 0.7 to 20.6 kg C org m -2 for sediment depths of 11 to at least 149 cm). The low sediment accretion rates (1.18–1.86 mm yr -1 ) and the relatively thick sediment deposits (with a maximum of >150 cm of sediment depth) suggests that the carbon stocks have likely been accumulated for an extended period of time, and that the documented loss of seagrass meadows in the Swedish Skagerrak region and associated erosion of the sediment could potentially have offset centuries of carbon sequestration.