• principal component analysis;
  • PCA;
  • water quality;
  • lake


Lake Simpele is located in South-east Finland. The surface area of the lake, which consists of its eastern and western parts, is 97 km2. The wastewater treatment plant of the Parikkala municipality is situated between the two parts in Särkisalmi. The eastern part of the lake is more under the influence of agriculture (discharges from fields). The rather polluted bay Siikalahti is also located in the eastern part of the lake. The data for this study was obtained through obligatory monitoring measurements during the period 1974–2006. The seven water quality variables used in the classifications were the dissolved oxygen saturation degree C2-sat. %, chemical oxygen demand CODMn, total phosphorus concentration, total nitrogen concentration, pH, electrical conductivity and colour. All of the time series of the eight sampling sites were used in the first classification with PCA. The dissolved oxygen saturation degree was not used as a variable in this classification, because the depth of the sampling sites varied a great deal. The sampling sites were put in order according to the pooled t-values of the years 2000–2005. The t-values of the studied decades were also pooled in order to examine the history of water quality in the different sampling sites. The changes in water quality were studied during the past three decades. Measures to protect the environment during the past decades have improved the quality of the water in Lake Simpele. The water quality of the western part of the lake has been and still is better than that of the eastern part. The epilimnion and hypolimnion were classified in one sampling site. The low dissolved oxygen saturation degree and total phosphorus concentration had a strong negative correlation. It could be seen how the low oxygen saturation degree releases phosphorus in the hypolimnion. Copyright © 2008 John Wiley & Sons, Ltd.