The persistent increase in atmospheric CO₂ has stimulated interest in marine Carbon Dioxide Removal (mCDR) strategies as a means to mitigate climate change. As global warming will create extreme conditions in many parts of the world, these mCDR techniques are necessary in addition to a reduction of anthropogenic CO₂ emissions. Among these, Ocean Alkalinity Enhancement (OAE) is notable for its potential for substantial global CO₂ removal and concomitant mitigation of ocean acidification. However, the localized ecological consequences, particularly on microbial and phytoplankton communities, are still unknown. Here we present results from multi-scale assessments of OAE impacts in Halifax Harbour that involved both controlled experiments and benthic field surveys at a pilot industrial Brucite dosing plant done in the Bedford Basin, NS, Canada.
We conducted two controlled mesocosm experiments (as part of the OAE Pelagic Impact Intercomparison Project - OAEPIIP) evaluating NaOH, NaHCO₃, and CaO additions to natural seawater (targeting +500 µmol.kg^-1 alkalinity addition over 19–26 days). Comprehensive monitoring included seawater parameters (alkalinity, pH, temperature, salinity), nutrient dynamics, and particulate matter (C/N), coupled with high-resolution biological assessments via flow cytometry, microscopy imaging (FlowCam), and 16S/18S rRNA metabarcoding. Our results indicate that unequilibrated alkaline treatments (NaOH, CaO) elicited limited phytoplankton taxonomic shift but significantly increased biomass, particularly of diatom species, in OAE-treatments relative to the control. A shift in the bacterial and archaeal community was detected, with bloom-associated bacteria increasing in the unequilibrated NaOH-treatments.
In a parallel approach, we analyzed the microbial community out of sedimental grabs of the Halifax Harbour at several locations, including directly below the OAE-plant outflow; sampling occurred prior to, and few weeks, six months and one year after the initiation of Mg(OH)₂ dosing. The in situ sediment communities at the industrial outfall exhibited a notable structural shift after six months and a year of dosing, whereas reference stations remained unchanged. Notably, differential abundance analysis detected a significant increase of Lutibacter, a chemoorganotrophic bacterium metabolically able to carry out denitrification and dissimilatory NO₃ reduction to ammonium (DNRA).
Taken together, these findings establish a critical baseline for predicting OAE-induced biological perturbations across pelagic and benthic niches and highlight the need for future research to quantify how such community shifts alter specific microbial ecological functions. Ultimately, our results provide important context for the assessment of the environmental impacts of OAE, which will aid in refining environmental risk assessments and informing regulatory frameworks for scalable mCDR deployment.

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