DIURNAL FLUCTUATIONS OF STREAM WATER CHEMISTRY CONTROLLED BY PHOTOSYNTHESIS AND GROUNDWATER EXCHANGE

Diurnal fluctuations of dissolved oxygen (DO) concentration and pH due to photosynthesis are commonly observed in rivers that support periphyton growth. Diurnal fluctuations of electrical conductivity (EC) in connection with photosynthesis have also been reported 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 1,700 km² catchment in northeastern Switzerland. The river showed distinct diurnal fluctuations of DO and pH caused by photosynthesis except during December and part of January. Fluctuations were frequently disrupted by spates with peak discharge exceeding 150 m³/s, which removed biofilm and periphytons. During a period of low flow (12 m³/s) and clear sky condition, photosynthesis released oxygen and consumed carbon dioxide 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 the supersaturated condition, 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 periphytons, and the roles of river-groundwater exchange processes.