The seafloor plays a vital role in mitigating climate change by storing carbon. Yet human activities like demersal fishing undermine this function and still receive little regulatory attention. This oversight is often due to a lack of empirical data, uncertainty in first-order estimations, and low resolution of the heterogeneous seafloor, emphasizing the need for a simple, fast, reproducible quantification approach. We address this gap by presenting a new resuspension assay based on biochemical oxygen demand (BOD). The assay compares oxygen consumption rates between undisturbed sediment cores and those subjected to resuspension. Our results show that sediment resuspension increases oxygen demand in various sediment types, indicating a loss of organic carbon from storage due to its enhanced mineralization to CO2. Applying this assay on a wide range of sediments in the Hauraki Gulf, New Zealand, we further identify environmental factors influencing resuspension effects on carbon storage, which are crucial for designating marine protected areas. Our assay is notable for its simplicity and efficiency, offering a solution for assessing sediment resuspension impacts and promoting marine sustainability.