| Summary: | Ocean alkalinity enhancement (OAE) is a marine carbon dioxide removal (mCDR) with the potential to remove atmospheric CO2 and increase the ocean's buffering capacity. Thus, it may aid in mitigating global warming and ocean acidification. Depending on the source mineral type (if silicate or calcium-based) and on if implemented in a pre- or non-equilibrated way, impacts on the microbial community's metabolic rates can be expected. The mineral type may introduce compounds to the system that may benefit certain phytoplankton species over others, and if non-equilibrated, changes to the pCO2 and pH may hinder primary production. To address this, an experiment with 10 pelagic mesocosms was undertaken, starting on the 7th May 2022. Two 5-step non-equilibrated TA gradients in increments of 150 µmol L-1 were implemented. One simulated a forsterite [Mg2SiO3] addition, and the other one a hydrated lime [Ca(OH)2] addition. Gross production (GP), net community production (NCP), community respiration (CR), and community metabolic balance (GP:CR), measured in vitro through oxygen production and consumption, as well as chlorophyll a, were monitored over a 53-day long period. All the biogeochemical parameters relevant to their response were measured as well. After a nutrient addition undertaken halfway through the experiment, a delay in bloom formation associated with the TA manipulation was observed. This is likely due to the previously reported, species-specific responses to low pCO2 and high pH conditions in terms of growth rates. Further research will be necessary to evaluate the implications of this findings with regards to trophic transfer, and seasonal suitability.
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