The seawater carbonate system in coastal waters is often highly dynamic, but especially in tropical regions, these dynamics and their drivers are not well resolved. Understanding these dynamics better is critical on the one hand for understanding ocean acidification rates, as coastal carbon cycling processes can strongly impact pH variability. Conversely, feedbacks in the carbonate system could also significantly impact the effectiveness of ocean alkalinity enhancement (OAE) in coastal waters for sequestering CO₂. We will present data from Southeast Asian coastal waters to identify drivers of carbonate system variability and quantify longer-term rates of ocean acidification and assess the potential for feedbacks on OAE effectiveness in this region. Our data show that terrestrial dissolved organic carbon input and remineralization is a major driver of carbonate system variability in the region. By acting as a CO₂ source, this process significantly drives pH dynamics and affects ocean acidification rates. This is illustrated by monthly time-series data collected since 2018 in the Singapore Strait, which show a strong acidification trend. This trend is driven in part by climate variability that drives land–ocean carbon fluxes. However, our data also show that calcium carbonate formation and dissolution have a significant effect on the observed carbonate system variability in the region, causing net addition or removal of TA. Moreover, our Singapore Strait time series shows not only a trend of ocean acidification but also a trend of increasing TA. We attribute this increase in TA to reductions in net calcification, caused by the fact that ocean acidification is decreasing the calcium carbonate saturation state. Our data thus indicate that tropical shelf seas are already responding to anthropogenic CO₂ and ocean acidification with reduced net calcification, representing an important but still poorly quantified carbon cycle feedback. Because large-scale OAE in coastal waters will raise the calcium carbonate saturation state, our results imply that the overall CO₂ sequestration efficiency of coastal OAE will be lower, because OAE would promote more net calcification. Our data highlight the critical importance of baseline coastal carbonate system observations to better constrain acidification rates and to understand the potential for feedbacks that would reduce OAE effectiveness.