A carbon mass balance for photosynthesis, carbonate precipitation, out‐gassing, and respiration was calculated for a stream in Yorkshire, UK, from diurnal variations in total dissolved inorganic carbon (TDIC) and its isotopic composition. Complementary measurements of the photosynthetically available radiation, water temperature, pH, and dissolved calcium were taken at the springhead (site A) and downstream (site B). The difference in carbon isotope composition between the lower station and the springhead increases from a nighttime minimum of 3.4%oPDBto a daytime maximum of 7.1%o as a result of the biological and chemical processes affecting the dissolved inorganic carbon pool. The diurnal carbon balance input consisted of 194 mol carbon that entered via the spring (accounting for 94.6% of the budget) and 5.4% that was added by aquatic plant respiration. Of the output, 10.0% was fixed by photosynthesis, 12.2% was precipitated as CaCO3, and 9.4% was outgassed over 24 h. At midday 17.8% was fixed by photosynthesis, 15.2% was precipitated as CaCO3, and 8.5% was released by outgassing; 5.4% was still added by respiration. The δ13C value of TDIC showed a pronounced diurnal cycle and differed between sites A and B. The δ13C of cyanobacterial carbonate carbon was significantly higher than that of the travertine precipitated nearby and showed a downstream increase. These features relate the two types of calcareous deposits to the geochemical characteristics of the stream water and also to the biological community and its photosynthetic activity.