Diel fluctuations of conductivity and alkalinity were measured in a macrophyte-dominated stretch of the Piako River, a rural, lowland, softwater stream in New Zealand. Both quantities exhibited elevated levels at dawn and depressed levels in the early evening suggesting that the variations might be connected with the diel cycles of macrophyte photosynthesis and respiration. A chemical analysis was used to determine which ions induced the diel variations. For low-flow periods with minimal allochthonous inputs, the changes in conductivity were correlated with calcium, magnesium and the ionised components of total inorganic carbon (bicarbonate, HCO₃^–, and carbonate, CO₃^2–). The changes in alkalinity were correlated with fluctuations of calcium and magnesium. The latter result was not anticipated based on solubility product calculations. Diel cycles of groundwater inputs explained 60% of average conductivity variations and 30% of average alkalinity variations between dusk and dawn. Other mechanisms also contribute to the observed changes and we speculate that localised calcite production and dissolution may be occurring.