Retentive structures of carbon in the North and Baltic Seas

In marine systems, carbon is sequestered through both, biological and physical, processes. This study focused on four benthic habitats acting as retentive structures, namely seagrass meadows, subtidal sediments, biogenic reefs and macroalgal forest, in the North and Baltic Seas. In both seas, large-scale losses of seagrass meadows, mussel reefs, and algal forests occurred in the 20th century. Multiple stressors, such as eutrophication, bottom trawling, pollution, overexploitation, and plant disease, led to declines in extensions of the four retentive habitats and thus in carbon sequestration. Major part of the overall retained carbon is buried in subtidal sediments, although only few deep areas, such as the Norwegian Trough, act as effective permanent sinks. Seagrass meadows are efficient in sediment trapping and thus in carbon retention, but due to their small extensions compared to subtidal sediments, their contributions to the overall regional carbon budgets are small. Algal forests are discussed as transient carbon sinks, but contribute to permanent sinks in deep areas by the excretion of fucoidan. Mussel reefs retain organic carbon in forms of faeces and pseudofaeces. Discussion on shell calcification as potential carbon source is ongoing, where it remains unclear whether mussel reefs act as net source or sink of carbon. Knowledge gaps and pronounced variability in carbon retention rates impede the calculation of regional carbon budgets. Bridging knowledge gaps on the role of eutrophication and climate change, and the functioning of carbon retention processes requires harmonized monitoring, as it is under development in Germany.