Journal of Environmental Conservation Engineering

Lakes, due to the long residual period of influent, in addition to the sedimentation of contaminants, frequently have lower environmental quality standards than rivers and oceans, and rehabilitation can often be challenging. Moreover, lakes are sensitive to changes in watershed development and population growth, all causing environmental changes which can increase pollution load. The watershed of any given lake is crucial to the mitigation of sewage and contaminants and maintenance of clean water. This study focused on the sediment quality of five lakes differing in levels of water pollution control countermeasures such as sewerage system in their lake basins, and considered the relationship between the sediment characteristics and the watershed pollution loads. Nutrient salt concentration in surface sediments pore water was an effective indicator to estimate the release rate of the nutrients from the sediment. Relationship was observed between phosphorus load per lake surface area and phosphorus release rate, and it was thought that progress of water pollution control countermeasures would lower the phosphorus release rate from the sediment.

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The nutrient effluent load to Tokyo Bay has been decreasing in recent years due to the enactment of the Water Regulations. However, it is difficult to establish whether there is any direct effect of the reduction in nutrient loading on red tide frequency. In the present study, I constructed the model using parameters of the red tide criteria (red tide index) and assessed the suitability of this model to predict future red tide occurrences in the Tokyo Bay based on changes in the coefficient of determination (R²) of the model. Using the stepwise method, I developed a stable multiple regression model with a high R² for data from 1986 to 1998 from a computerized database. Particulate organic nitrogen (PON) , an index of red tide, was used as the target variable. The adjusted R² (R²_adj) of the model based on data from 1986 to 1998 was 0.772. Application of this model to new annual data from 1999 to 2003 could be well explained by the model based on previous data. However, from 2004 when the regulations were implemented, there was a substantial decrease in R², suggesting that environmental changes arising from the introduction of total loading control may affect the model developed in this study.

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