Ecological Risks and Byproduct Impacts of Biomass Sinking in the Anoxic Black Sea

Marine biomass sinking has emerged as a promising carbon dioxide removal (CDR) strategy within the broader portfolio of climate change mitigation approaches. While the slow remineralisation rates in anoxic basins such as the Black Sea suggest that depositing agricultural residue biomass on the seafloor could achieve meaningful organic carbon sequestration, the biogeochemical and ecological consequences of doing so remain understudied. This literature review synthesises geological precedents for large-scale organic carbon burial, small-scale in situ sinking experiments, and biogeochemical modelling studies to evaluate the feasibility of anoxic deep-ocean biomass storage. The review finds that while biomass sinking is a theoretically viable CDR strategy, shifting climate conditions and the limited baseline characterisation of benthic and pelagic communities in anoxic basins introduce substantial uncertainty into projections of net carbon sequestration. Moreover, the results suggest that biomass sinking is likely to affect both localised anoxic zones and downstream surface and pelagic ecosystems through anaerobic decomposition processes, including methanogenesis and sulphate reduction, which generate methane and other chemical byproducts, as well as through the upwelling of these byproducts and the consequent disruption of microbial food webs. These findings indicate that widespread implementation of biomass sinking in the Black Sea is premature without additional in situ experimentation to quantify remineralisation rates, byproduct dynamics, and ecosystem-level impacts under projected climate conditions.