Ocean liming in eutrophic vs. ultraoligotrophic environments and the response of algal calcifiers

Our commitments to limit future global warming to below 2°C of the pre-industrial level are clashing with demonstrably insufficient present-day efforts to reduce CO₂ emissions. The development and implementation of Negative Emission Technologies (NETs), enabling a massive and fast CO₂ Removal (CDR), represent the most promising strategy to support an effective mitigation of the ongoing climate change within the next decade. Marine CDR (m-CDR) encompasses those technologies exploiting the ocean CO₂ storage potential, and there is an increasing number of international initiatives aimed at assessing their possible impact on marine communities. Ocean liming consists in spreading alkaline substances, such as calcium hydroxide (slaked lime), on surface ocean waters. Slaked lime reacts with surface marine waters by triggering m-CDR from the atmosphere and ocean alkalinity enhancement, thus contrasting ocean acidification. Although ocean liming has already been assessed as chemically effective and economically sustainable, the scientific scrutiny of its potential impacts on the ocean biota has just started. Previous laboratory and mesocosm experiments showed the occurrence of transient pH peaks, which may impact the pelagic ecosystem by selecting less sensitive species, and runaway precipitation of aragonite particles after concentrated and repeated liming, which reduces the efficiency of CDR and negatively affects both plankton and benthos by mechanical clogging and choking. Nutrients exert a major control on primary producers, and higher salinity may affect the carbonate kinetics by facilitating CaCO₃ precipitation. For these reasons, two mesocosm experiments of liming, funded by the EU2020 project AQUACOSM-plus and the OACIS-initiative of the Fondation-Prince-Albert-II-de-Monaco, were conducted with a comparable experimental design at the CIM-ECIMAT (University of Vigo) and CRETACOSMOS (Hellenic Centre for Marine Research) facilities. The aim was to contrasting the response to ocean liming of the eutrophic Ría de Vigo upwelling system (eastern Atlantic) and the eastern Mediterranean ultraoligotrophic and more saline setting. The preliminary results of repeated additions of slaked lime in the two different types of marine coastal waters, and the response of calcareous nannoplankton and benthic calcareous red algae (coralline algae) to the observed chemical changes are presented here, suggesting the need to optimize and modulate the mCDR techniques, in order to meet the specific geochemical and biological characteristics of the different water bodies.