Diurnal fluctuations of electrical conductivity in a pre-alpine river: Effects of photosynthesis and groundwater exchange

SummaryDiurnal fluctuations of dissolved oxygen (DO) concentration and pH due to photosynthesis and respiration are commonly observed in rivers that support periphyton growth. Diurnal fluctuations of electrical conductivity (EC) in connection with photosynthesis have also been reported, but mostly in small, first-order streams or in streams fed by karst springs. The objective of this study is to examine the diurnal EC fluctuations in a large river and understand biological, chemical, and hydrological processes controlling the fluctuations, using long-term archived data, focused field monitoring, and laboratory experiments. The study was conducted in the Thur River draining a 1700 km2 catchment in northeastern Switzerland. The river showed distinct diurnal fluctuations of DO and pH caused by photosynthesis and respiration except during December and part of January. Fluctuations were frequently disrupted by spates with peak discharge exceeding 150 m3 s−1, which removed biofilm and periphyton. During a period of low flow (12 m3 s−1) and clear sky, photosynthesis released O2 and consumed CO2 in water during the daytime, thereby increasing pH and the saturation index of calcite. This caused calcite to precipitate and removed Ca and alkalinity from water, and reduced EC. Laboratory experiments showed that the increase in pH and the saturation index alone cannot cause calcite precipitation without the presence of periphyton. It is likely that the precipitation occurs in the microenvironment in the close vicinity of photosynthesizing cells, where the pH and the calcite saturation index are much higher than in the bulk river water. Calcite precipitation stopped during the nighttime despite supersaturated conditions, and EC gradually increased presumably due to the input of Ca and alkalinity by groundwater exchange. The study clearly showed that photosynthesis and calcite precipitation have a strong influence on the chemistry of the large river, and pointed out the need for future research examining the biogeochemical processes in the microenvironment surrounding periphyton, and the roles of river-groundwater exchange processes.

► Periphyton photosynthesis consumes CO2 in river water and raises pH.
► High pH causes calcite precipitation, which decreases electrical conductivity (EC).
► Daytime decrease and nighttime increase in EC results in distinct fluctuations.
► These complex processes have significant influence on river water quality.