1999 Volume 60 Issue 3 Pages 335-345
In an upper shallow reach of the Ishite River, the stream water is completely diverted at a weir except under flood conditions, and is regenerated below the weir by groundwater. A 13-year monthly survey was undertaken 2.4 km below the weir between 10 : 00 and 14 : 00 from 1985 to 1997. The difference between observed DO minus saturated DO concentrations (ΔDO), not DO itself, correlated positively with pH among 110 samples under baseflow conditions (r=0.585, P<0.001). A diel survey also showed the parallel change in ΔDO and pH. Four surveys at 10 : 00 and 22 : 00 near the weir showed that ΔDO and pH were little changed between day and night. They increased at 2.4 km below the weir, especially in the daytime. ΔDO and pH seemed to increase all day long by an exchange of oxygen and carbon dioxide between the stream and the air, and additionally by photosynthesis in the daytime. An additional eight surveys at 22 : 00 2.4 km below the weir showed that ΔDO in the nighttime distributed within a narrow range having a mean of -1.26 mg l-1. The surplus ΔDO above this mean in the daytime is available as an index of photosynthesis in the stream. A recent 4-year monthly observation of biofilms on the stream bed containing algae and measurements of photons in the stream water showed that ΔDO was controlled by both the number of photons and the amount of biofilms.