Abstract
The partial pressure of carbon dioxide (pCO2 ) in river water was measured. The pCO2 showed well defined diurnal variations characterized by the low values in the daylight hours, and high values in the nighttime. The amplitude of daily pCO2 ranged from 500 to 1200 ppm. Assuming that the respiration rates of biota and the advectionof carbon dioxide in river water are nearly constant all the day, the pCO2 can bealtered by three processes; carbon dioxide exchange across air and water interface, water temperature change, and biological consumption of carbon dioxide in water through photosynthesis. The relative magnitudes of all processes were evaluated. The effects of first two processes on diurnal variations of pCO2 were about 30 to 60 ppm. These values are negligibly small compared with the real diurnal variation of pCO2 . Taking into account the relationship between dissolved carbon dioxide and oxygen, weconcluded that the photosynthesis by water plants was the most important term to determine the diurnal variation of pCO2 . The pCO2 showed a maximum in warmermonths (ca. 1000 ppm), and a minimum in colder months (ca. 350 ppm).
