Abstract
Principal component analysis (PCA) was applied to evaluate some of the potential factors contributing to Secchi depth (SD) in the oligotrophic alpine Lake Oze-Numa, using seven water quality parameters obtained during 1990-1992. The data on SD, water temperature (WT), chemical oxygen demand (COD), total nitrogen (TN), suspended solids (SS), total phosphorus (TP), and chlorophyl-α (CHL) were examined. The first component had large loadings on SD, WT, SS, TP, and CHL attributable to phytoplankton population. COD and TN correlated with the second component, showing natural contributions. SD was revealed to be a reliable indicator of eutrophication in the lake, as it was predicted successfully by multiple linear regression analysis using SS, TP, and CHL as independent variables. A time-series analysis of the SD records was applied to a simple estimation of the eutrophication rate of the lake. SD has decreased for the past 25 years (1965-1990) at a rate of 0.065 m⋅yr-1 or 1.2%⋅yr-1. The SD trend was partly explained by the increase in TP concentration attributable to tourists.
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