Tides affect the marine carbonate chemistry of a coastal polynya neighbouring the Ekström Ice Shelf by movement of seawater with different physical and biogeochemical properties. The result is that the coastal polynya in the summer can switch between being a sink or a source of CO₂ multiple times a day. We encourage consideration of tides when collecting in polar coastal regions to account for tide-driven variability and to avoid overestimations or underestimations of air–sea CO₂ exchange.