Environmental impact of underwater exhausted CO2 from ships and its potential as antifouling strategy. Impact and potential of underwater emissions

Several engineering projects have studied the application of underwater exhaust systems as air lubrication to reduce the water resistance of ships. The system would increase the energy efficiency and as such reduce ships emissions on a global scale. At local scale however, the underwater released exhaust gas, and especially the CO2 in it, may significantly acidify the water with unknown impact. Most available biological impact studies reflect the expected chronic future ocean acidification, which does not represent the rapid and high dose of CO2 from an underwater exhaust system. Therefore, we assessed the impact of high CO2 concentrations (resulting in pH 7.8 to 6.3) on developing microbial biofilms and barnacles. With decreasing pH to 6.3, the development of biofilm biomass was significantly reduced and a shift in taxonomic composition occurred. The barnacles tolerated acidic conditions up to pH 6.8 for 25 days although their feeding activity and growth rate decreased, and high mortality occurred in the pH 6.3 group. Moreover, we found over 50% of the barnacles in the pH 6.3 group was removed by the water force generated by 12 km h-1 sailing for 15 min. This suggests that CO2-induced acidic conditions around the ship’s hull may increase the anti-fouling property, which could be an added benefit of underwater exhaust systems on ships.