太田川河川水中のリンおよび窒素濃度の長期変動―植物プランクトン種の変化を引き起こす主要因として

抄録

Long-term variations of nitrogenous and phosphorus compounds were analyzed for their concentrations in the Ohta River water along with the amount of freshwater discharge using 23 years data set during 1976-1998. Both dissolved inorganic phosphorus (DIP) and total phosphorus (TP) showed significant decreasing trend from 1980 to 1998. On the other hand, all nitrogenous compounds such as mmnonia (NH₄), nitrite (N0₂), nitrate (N0₃), dissolved inorganic nitrogen (DIN) and total nitrogen (TN) showed significant decrease since 1995. These trends were judged to be the effects of the measures for the reduction of these elemental loads conducted by the Japanese Environment Agency. The DIN: DIP and TN: TP ratios, therefore, increased from 21 to 30 and from 19 to 25 during 1980-1998. While the freshwater discharge did not show any long-term trend, phosphorus and nitrogen loads, which are estimated from multiplication of the concentration by the amount of the freshwater discharge, showed significant decrease: 66% and 55% decreases for DIP and TP in comparison between the averages during 1980-1982 and during 1996-1998, and 31% and 18% decreases for DIN and TN in comparison between the averages during the entire analytical period and during 1995-1998, respectively. On the other hand, DIP/TP and DIN/TN were in the constant levels during the whole investigated period. The red-tide forming species appear to have abilities either to store phosphorus in the cellular pool or to utilize dissolved organic phosphorus (DOP). It is concluded that phosphorus reduction measure could have changed the species composition in the bay to those can be advantageous to survive even in such a low DIP condition.