Quantifying the carbon benefits of ending bottom trawling

Sala, et al. 1 suggest that seafloor disturbance by industrial trawlers and dredgers results in 0.58 to 1.47 Pg of aqueous CO2 emissions annually, owing to increased organic carbon mineralisation in sediments after trawling. We agree that bottom trawling disrupts natural carbon flows in seabed ecosystems due to sediment disturbance, resuspension and changes in the biological community and that it is important to estimate the magnitude of this effect. We disagree however that their assessment represents a ‘best estimate’. Firstly, they critically assume that organic carbon in undisturbed sediment is inert, and that only disturbance by trawling remineralises organic carbon, an assumption at odds with decades of geochemical research. Secondly, they greatly overestimate the volume of sediment where carbon is mineralised after trawling. Thirdly, they ignore secondary effects, such as the removal of bioturbating benthic fauna and sedimentary nutrient release, which could lead to more preservation and production of organic carbon. Together these issues result in an upward bias in the estimated CO2 emissions by one or more orders of magnitude.