Abstract:

Based on the annual water quality monitoring data from January to December 2006 in Tianmuhu Lake，distribution of dissolved oxygen and its environmental impact were analyzed.The results showed that dissolved oxygen level in Tianmuhu Lake dropped by 1~2 grades.All in all the annual water quality of reservoir area is mainly about GradeⅡ~Ⅲ (Standard for Surface Water Environment，GB 38382002)，the water quality range from Grade IV to V，even worse than V in some parts.The distribution of dissolved oxygen has obvious seasonal changes and vertical differences.In winter，the dissolved oxygen (DO) content is the highest.In spring and autumn，the dissolved oxygen content is a little lower than that in winter，and there are some small differences between the top and bottom of DO.In summer，the dissolved oxygen content is the lowest，moreover，the vertical differences are the largest and lowDO area，with the concentration of DO was <1 mg/L，appears  at the bottom of lake.DO，which is one of the key factors affecting water ecosystem，is affected by water temperature，water depth，Chlorophyll and pH，etc.We found that the increase of water temperature caused reduction of DO concentration (and saturation) in nonsummer period.However，for the middle and bottom layers，the thermal stratification is the key factor to affect the DO concentration.Moreover，depth，chlorophyll and pH have a certain impact on dissolved oxygen.There is a negative correlation between dissolved oxygen and water depth.The relation between Chlorophyll and dissolved oxygen is most significant in summer，which represents the impact of microbial activity on DO.

Key words: dissolved oxygen,lowoxygen area,environmental factors,Tianmuhu Lake