Long-term monitoring of water quality in the monomictic Ishitegawa reservoir indicates carbon limitation

The monomictic Ishitegawa Reservoir (surface area: 0.50 km; total capacity: 1.28 × 10 m; mean retention time: 0.33 y) is located on the Ishite River (N 33 53 , E 132 50 ) in Ehime Prefecture, Japan. In this reservoir, summer chlorophyll a concentrations (Chl-a) have been observed to decrease gradually from year to year, but the cause remains unclear. To determine the cause, we first examined a monthly data set from January 1983 to January 2003 that contained measures of water temperature (WT), pH, Chl-a, total phosphorus and total nitrogen taken at 0.5 m water depth at the main station in the reservoir (Sta. R). We found two significant long-term linear regressions of pH against date: a positive relationship when pH≤ 8.10 (R = 0.099, p < 0.001, n = 123) and a negative relationship when pH > 8.10 (R = 0.164, p < 0.001, n = 118). This pattern was also found at a station near the head of the reservoir where river inflow occurs (Sta. H). Low pH ( ≤ 8.10) was observed mainly in the circulation period (October – March) and high pH (> 8.10) was found during times of stratification (April – September). Next, we examined the relationship of pH = 8.1 with other limnological parameters by using a monthly data set of dissolved inorganic carbon (DIC), dissolved oxygen, and main cation and anion concentrations from February 1993 to January 2003. Concentration of free CO2 (dissolved CO2 + H2CO3) was calculated from DIC, pH, WT and ionic strength by using the dissociation equations of carbonic acid. pH was negatively related to the log partial pressure of gaseous CO2 in equilibrium with free CO2 (R 2 = 0.968, p < 0.001, n = 120). At pH 8.1, free CO2 was in equilibrium with atmospheric CO2 (360 μatm partial pressure). Chl-a concentrations during the stratification period (pH > 8.10) decreased from year to year following the gradual decrease in the free CO2 concentration and the gradual increase in pH in the preceding circulation period. This data suggests that partial carbon limitation of phytoplankton growth may have occurred under high pH (> 8.10). In addition, plastic-coated sheet fences that were installed across the upper 5 m of the water column near Sta. H during several intervals in the study period formed a strong interflow of river water below the fences, and may have strengthened carbon limitation at Sta. R.