Ocean liming effects on dissolved organic matter dynamics

Abstract. 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 (Ca(OH)2) into 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 to be understood, and there is currently no information on the potential impacts on the concentration and quality of Dissolved Organic Matter (DOM), that is the largest, the most complex and yet the least understood mixture of organic molecules on Earth. The aim of this study is to provide the first experimental evidence about the potential effects of pH peaks, that might be generated by the Ca(OH)2 dissolution in seawater, on DOM dynamics 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 (MedSea), known for its Dissolved Organic Carbon (DOC) concentration comparable to that observed in the oceans, and the eutrophic Baltic Sea (BalSea), characterized by high DOM concentration mostly of terrestrial origin. Ca(OH)2 was added in both waters, to reach a pH of 9 and 10. Our findings reveal that the addition of hydrated lime has a noticeable effect on DOM dynamics in both the MedSea and BalSea, determining a reduction in DOC concentration and a change in the optical properties (absorption and fluorescence) of DOM. Thes effects, detectable at pH 9, become significant at pH 10 and are more pronounced in the MedSea than in the BalSea. These potential short-term effects should be considered within the context of the physico-chemical properties of seawater and the seasonal variability.