Influence of sewerage construction in river biota

In Yokohama, pollution of public water bodies due to domestic and industrial wastewater has been greatly reduced by the construction of sewerage systems and by guidance for and regulation of industrial wastewater. The spread of sewerage has decreased pollution due to the influx of untreated wastewater, but has also increased the share of river water occupied by final effluent, and magnified its impact on the environment. To investigate the effects of final effluent for recovery and preservation of river water quality as well as its influence on biota, the City of Yokohama carried out a biota survey on five occasions from 1982 to 2001. A survey of this type is effective for assessing environmental status, in light of the following features: 1) unlike the results of chemical analysis, the results are holistic; 2) biota represent the cumulative results of change in environmental elements over time; 3) unlike results expressed in numerical terms, improvement is readily comprehensible and clear to all.