Assessing the response of particulate matter stoichiometry to ocean alkalinity enhancement

Ocean Alkalinity Enhancement (OAE) is one of the most promising ocean-based negative emissions technologies (NETs) currently discussed. Dissolution of alkaline minerals such as olivine or quicklime in the surface seawater elevates total alkalinity (TA), thereby increasing the oceanic CO₂ uptake capacity. Depending on the mineral used, increased TA may have different consequences for marine pelagic ecosystems and water column biogeochemistry that are still unknown, creating a need for empirical data. Our study reports on the particulate matter stoichiometry of a pelagic ecosystem in response to a gradient of elevated alkalinity levels and under different alkaline mineral applications. Ten offshore mesocosms were deployed in the mesotrophic waters of the Raunefjord off Bergen, Norway, from May – July 2022. Using NaOH, a delta alkalinity gradient was created (∆TA = 0, 150, 300, 450, and 600 µmol·L^-1), resulting in two sets of five alkalinity levels each. To simulate the application of olivine (Mg₂SiO₄) vs. calcium-based minerals for OAE, corresponding amounts of MgCl₂ and CaCl₂ were each added to the respective treatments. Each mesocosm of the silicate-based OAE treatments additionally received 70 µmol·L^-1 Si(OH)_4, simulating the concomitant release of silicate under silicate-based mineral dissolution. We found significant differences in the production of biogenic silica between the two mineral simulations, indicating beneficial conditions for diatoms when silicate-based minerals are dissolved. However, the hypothesis of calcium-based mineral dissolution being favorable for calcifying organisms was not supported in our study. Neither the concentrations of particulate inorganic carbon (PIC) nor its ratio to particulate organic carbon (POC) was significantly different between TA treatments and mineral type. Additionally, increased TA had a negative effect on particulate organic nitrogen (PON) and phosphorus (POP) concentrations resulting in increased POC:PON and POC:POP ratios with higher alkalinity in both mineral simulations, yet more evident in the silicate-based treatments. Hence, the interaction of OAE and mineral effect on the particulate matter stoichiometry is possibly not induced by a single factor, yet by a variety of drivers, e.g. phytoplankton species specific physiology or food web interactions, such as grazing pressure. These results provide useful insights for better assessing ecological risks and co-benefits of OAE, like possible CO₂ limitation of primary producers or ecosystem restructuring, thus will help to inform the practical implementation of large-scale OAE applications for carbon dioxide removal.