Long-term effect of tourism on the recent upward trend of phosphorus in a shallow alpine lake, Japan

The trend of total phosphorus (TP) concentrations in May-October of the tourist season was analyzed to examine the effect of more than 30 years of tourism on a shallow oligo-mesotrophic lake, Lake Oze-Numa. Annual mean TP concentrations increased rapidly at the rate of 11.6%/yr, from 7.5mg/m³ to 14.4 mg/m³ in 1982-90, and a mesotrophic condition of 23.1 mg/m³ is predicted for the year 2000. TP concentrations at the end of the tourist season were higher than at the beginning due to autumn circulation of the bottom sediment, and correlated with the current year TP loadings from tourists. TP concentrations in May were the lowest for the season, and showed good correlation with previous year concentrations. The recent upward trend of phosphorus concentration was mainly brought about by long-term tourism because 10% of the TP loading by tourists was estimated to accumulate annually in the water column. Phosphorus reduction measures should be taken urgently in such a shallow lake which now shows an accelerated rate of eutrophication.