単循環湖の石手川ダム湖で，夏季のクロロフィルa濃度(Chl-a)が年々減少する原因を検討した。1983年1月から20年間，毎月1度，湖内定点の水深0.5 mで，水温，pHと，Chl-a，栄養塩類の濃度を調査した。その結果，試料がpHで2群に分かれること，pH ≤ 8.10の試料群でpHが経年的に上昇する一方，pH ＞ 8.10の試料群で下降することが認められた。pH ≤ 8.10は主として循環期(10-3月)に，pH ＞ 8.10は主として成層期(4-9月)に見られた。次に，1993年2月からの10年間に得た溶存無機態炭素濃度とPh,水温，主要イオン濃度とから，炭酸の解離平衡式を用いて，遊離CO_2濃度を計算した。遊離CO_2と平衡になるCO_2ガスの分圧の対数とPhが高い負の相関を示すこと，Ph 8.1で遊離CO_2が大気中のCO_2と平衡になることが認められた。全体を通して見ると，成層期の試料群では，先行する循環期の遊離CO_2が年々減少することを反映して，Chl-aが経年的減少傾向を示した。高Ph (＞ 8.10)で部分的に炭素制限が生じたと推定された。なお，湖内上流に流入河川水を制御するフェンスが設置された時期には，フェンス下流で炭素制限が強化されたと見られた。%The monomictic Ishitegawa Reservoir (surface area: 0.50 km~2; total capacity: 1.28 × 10~7 m~3; 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~2 = 0.099, p ＜ 0.001，n =123) and a negative relationship when pH ＞ 8.10 (R~2 = 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 CO_2 (dissolved CO_2 + H_2CO_3) 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 CO_2 in equilibrium with free CO_2 = 0.968， p ＜ 0.001，n =120). At pH 8.1, free CO_2 was in equilibrium with atmospheric CO_2 (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 CO_2 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.
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