Ocean alkalinity enhancement (OAE) is an important strategy for marine carbon dioxide removal (mCDR), but understanding its impacts on lower trophic levels is essential for ecosystem sustainability. In this study, we conducted (1) high-frequency monitoring of temperature, salinity, and pH using mooring sensors, along with weekly seawater sampling for carbonate chemistry and phytoplankton and bacterial assemblages in Akkeshi Bay, Japan, and (2) a NaOH-based microcosm OAE experiment for 29 days using low-temperature coastal waters (ca. 1.1°C) during the post-blooming season, following the protocol of the Ocean Alkalinity Enhancement Pelagic Impact Intercomparison Project (OAEPIIP). The pH values obtained and corrected from the mooring sensors and seawater samples exhibited higher levels (ca. 8.3) during the diatom bloom and post-bloom phases than before the bloom (ca. 8.1). For the NaOH-based microcosm OAE experiments with a 500 µmol kg^-1 of alkalinity addition, pH values remained constant values from first 8 days across three different treatments (control, equilibrated (NaOH + NaHCO₃), and unequilibrated (NaOH only) treatments) and then gradually declined toward the end of the experiment. Total alkalinity remained constant throughout incubation, whereas DIC increased, and pH declined from 10 days, with the steepest decrease in the unequilibrated treatments. No significant differences were observed in nutrients levels, chlorophyll a concentration, or maximum quantum yield of photochemistry in photosystem II (F_v/F_m) across the three treatments. These results suggest that OAE has a limited effect on nutrients, phytoplankton biomass, and photophysiology during the post-bloom season in cold coastal waters.

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