Ocean Alkalinity Enhancement (OAE) has been proposed as a carbon dioxide removal (CDR) strategy that increases seawater alkalinity, thereby enhancing the uptake of atmospheric CO₂. However, the potential ecological and biogeochemical impacts of OAE remain poorly understood. As part of the Ocean Alkalinity Enhancement Pelagic Impact Intercomparison Project (OAEPIIP), we examined how a coastal phytoplankton community responds to OAE using surface seawater collected from Akkeshi Bay, Hokkaido, Japan. A 19-day microcosm experiment was conducted using nine 55 L plastic tanks. Three treatments were established: a control, an unequilibrated treatment receiving NaOH only, and an equilibrated treatment receiving both NaOH and NaHCO₃. Both OAE treatments increased total alkalinity (TA) by approximately 500 µmol kg⁻¹. The alkalinity additions resulted in clear increases in TA and dissolved inorganic carbon (DIC), confirming successful manipulation of the carbonate system. OAE had little effect on most measured biological and biogeochemical variables, including concentrations of inorganic nitrogen (nitrate, nitrite, and ammonia), phosphate,  chlorophyll a, as well as phytoplankton and bacterial communities. A notable exception was observed for silicate dynamics. Silicate levels became significantly lower in the unequilibrated treatment from Day 14 onward than in the control and equilibrated treatments. These patterns indicate that the carbonate chemistry changes associated with unequilibrated treatments, particularly elevated pH, could have altered silicon utilization and/or remineralization processes. The suppression of silicate recovery in the unequilibrated treatment suggests that OAE may influence silicon cycling in coastal ecosystems. Further investigation is necessary to understand the mechanisms behind this response.

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