《Table 2–Relationship between water quality indicators in Tai Lake.》

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《Three decades of changes in water environment of a large freshwater Lake and its relationship with socio-economic indicators》

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The sources of COD are almost the same as for TN and TP and its increase was also significantly correlated with growth of the human population（p<0.05）and GDP（p<0.05）in the basin（Fig.3）.However，there were some dissimilarities in relationships with BOD.First，BOD had a significantly positive relationship with population without a turning point until2010.Second，the statistical relationship between BOD and per capita GDP did not conform to the Kuznets curve as did the other environmental indicators.BOD increased dramatically with industrial sewage discharge after 2000 whereas concentrations of TN，TP and COD decreased.There may be some explanations for this.For instance，BOD is an important indicator of the degree of organic pollution in water bodies，which is indirectly represented by the amount of dissolved oxygen consumed by microbial metabolism.Before the 1980s，the deterioration of water quality，mainly evident in TN and COD，was closely linked to primary industries（agricultural development），when pesticide application in the basin was 2.4 times that of the national average（Chen et al.，2010）.Afterwards，BOD has replaced these as the foremost indicator of deteriorating water quality（Li et al.，2000；Zhu et al.，2013，2015b）owing to the urbanization and the transformation of primary industries to secondary and tertiary industries，as well as the consequent increase in living standards in the basin（Qin，2008）.This can be further illustrated by Fig.4，which shows that sewage biodegradability increased evidently with population and per capita GDP from 1995 on.On the other hand，in addition to external discharges，the life cycles of phytoplankton（growth，death，and degradation）can also contribute greatly to the increase of BOD in Tai lake（Wu et al.，2011）as indicated by Table 2，which shows that BOD and Ch1-a were significantly correlated（r=0.745，p<0.01）.