中小都市河川の富栄養化

河川における無機炭素収支

抄録

Deterioration of water quality in a shallow and eutrophicated river is caused by respiration and detachment of attached algae on river bed (which is refered to as Selfpollution). A model based on longitudinal inorganic carbon balance is proposed to account for selfpollution rate in such urban polluted rivers. By applying this model to the Nogawa River (a branch of the Tama River), it is found that the alkalinity in polluted rivers is not constant because of the diurnal and longitudinal fluctuations of NH⁺₄·N, NO^-₂·N and NO^-₃·N concetration.
The material balance reveals that about 347 kg/ day (in summer) or 163 kg/ day (in winter) of inorganic carbon is fixed by benthic algae in the midreaches (10 km) of the Nogawa River. If all of the fixed carbon becomes an organic carbon represented by glucouse (C₆H₁₂O₆), this amount corresponds to 925 kg BOD per day which is equal to BOD discharge of as much as 23000 peoples.
Thus it is of importance to control benthic algal growth for the purpose of reclamation of water quality in shallow urban rivers.