Oceans play a central role in regulating Earth’s climate by absorbing, storing, and redistributing heat and CO₂ through their physical, chemical, and biological processes. Shallow shelf seas differ from the deep ocean because they are highly dynamic environments where currents and waves strongly influence the transport and deposition of organic and inorganic material. Interest in the ocean’s role in carbon storage has led to the concept of “blue carbon,” defined by the IPCC as biologically driven carbon fluxes and sinks in marine systems that can be managed. The idea that shellfish could function as carbon sinks because of the carbon stored in their shells has attracted attention, particularly within the aquaculture sector. In the German Baltic Sea, blue mussels cover about 8% of the seabed. Mussels build their shells from calcium carbonate (CaCO₃) using dissolved inorganic carbon, mainly bicarbonate ions (HCO₃⁻), from seawater. However, shell formation releases CO₂ and reduces seawater alkalinity, lowering its capacity to absorb atmospheric CO₂. After mussels die, their shells usually dissolve, increasing alkalinity and promoting CO₂ uptake. Generally, there is a lack of experimental data in the literature, thus we conducted experiments in the indoor benthocosm facilities at GEOMAR in Kiel.