In the scope of the Coastbusters FINESSE project, various environmental and economic co-benefits of mussel reefs as a nature-based solution for coastal protection will be assessed. One of them, is their potential to alter carbon sequestration rates of the soft sediments in the Belgian North Sea. The presence of mussel reefs on soft sediments affects carbon cycling through a variety of counteracting processes, which has made the assessment of their potential as a carbon sink or source heavily debated. Respiration and shell formation release CO2 into the water column, whereas shell dissolution decreases pCO2 and biodeposition can enable carbon burial in the sediment. By combining data from in situ-measurements with mesocosm experiments in a biogeochemical modelling approach, we aim to quantify the impact of these processes on the carbon cycle and determine whether mussel reefs act as a carbon source or sink.