Measurements of total CO2, alkalinity and dissolved oxygen were conducted in a tidal basin with estuarine properties during three cruises. In winter CO2 and oxygen were close to equilibrium with the atmosphere. In May the basin was a large source for CO2, where the fractional gain with respect to the freshwater CO2 was as high as 42%. Also in September high pCO2 indicated a release to the atmosphere; both in May and September there was accompanying low oxygen. Water originating from the tidal flats was shown to affect the water in the adjacent tidal channel significantly. Tidal flat water is characterized by high TCO2 and alkalinity and low oxygen as compared with the surrounding water. It develops through biological processes on the tidal flats (only in the vegetative season), as well as through the local discharge of a small freshwater source with very high alkalinity (and TCO2). Using alkalinity as a tracer it was calculated that in March 12% of the fresh water in an adjacent tidal channel may originate from this source, although it constitutes only 1.5% of the total freshwater discharge.