Ocean liming effects on dissolved organic matter dynamics

Ocean liming has gained attention as a potential solution to mitigate climate change by actively removing carbon dioxide (CO2) from the atmosphere. The addition of hydrated lime to oceanic surface water leads to an increase in alkalinity, which in turn promotes the uptake and sequestration of atmospheric CO2. Despite the potential of this technique, its effects on the marine ecosystem are still far from understood, and there is currently no information on the potential impacts on the concentration and quality of dissolved organic matter (DOM), which is one of the largest, most complex and yet least understood mixtures of organic molecules on Earth. The aim of this study is to provide the first experimental evidence about the potential effects of hydrated lime addition on DOM dynamics in the oceans by assessing changes in its concentration and optical properties (absorption and fluorescence). To investigate the effects of liming on DOM pools with different concentrations and quality, seawater was collected from two contrasting environments: the oligotrophic Mediterranean Sea, known for its dissolved organic carbon (DOC) concentration comparable to that observed in the oceans, and the eutrophic Baltic Sea, characterized by high DOM concentration mostly of terrestrial origin. Hydrated lime was added to both waters to reach pH values of 9 and 10. Our findings reveal that the addition of hydrated lime has a noticeable effect on DOM dynamics in both the Mediterranean Sea and Baltic Sea, determining a reduction in DOC concentration and a change in the optical properties (absorption and fluorescence) of DOM. These effects, detectable at pH 9, become significant at pH 10 and are more pronounced in the Mediterranean Sea than in the Baltic Sea. These potential short-term effects should be considered within the context of the physicochemical properties of seawater and the seasonal variability.