Abstract: The enrichment of organic matter is a striking feature of lake eutrophication1However,distribution of organic matter associated with extracellular enzymes (especially oxidoreductases) in sediments is inadequately studied1In this paper,we described the distributions of oxidoreductase activities in shallow lakes to reveal their roles in the degradation of organic matters in sediments and the effects on eutrophication1The spatial distribution of polyphenol oxidase and peroxidase in sediments of nine lakes (Lake Luhu,Lake Liangzi,Lake Houguan,Lake Niushan,Lake Sanjiao,Lake Longyang,Lake Moshui and Lake Yuehu in Hubei Province and Lake Taihu in Jiangsu Province) in the middle and lower reaches of the Yangtze River were studied and their correlations with microorganisms were also analyzed1The activities of the two enzymes studied among and within the lakes were unevenly distributed,the sediments with the significantly higher contents of organic matter showed the higher activities,which were observed in the inlet of waste water discharge,the zone with plant residues and the undredged sites1Vertically polyphenol oxidase activities were high at surface sediment with higher organic matter contents1Hence,the organic matters derived from the different sources could induce enzymes1The peroxidase activities varied slightly with increasing sediment depth,a result similar in the dredged and undredged areas,which was probably due to the formation of humic-enzyme complex by the peroxidase and humus after the enzyme induction1There was a significantly positive correlation between the activities of polyphenol oxidase and peroxidase,therefore,the two enzymes coupled in the process of decomposition of organic matters and humification.Bacteria and Actinomyces (not fungi) were dominant species and may be the main producers of the enzymes. In short,the sediment of eutrophic Lakes would respond to the organic matters enrichment by shaping numbers and compositions of microorganisms (Bacteria and Actinomyces) and inducing polyphenol oxidase and peroxidase that oxidize organic matters to produce the humus in the end1At the same time,the enzyme could be stabilized by the formation of humicenzyme complex. The multiple roles of oxidoreductases in the process of lake eutrophication were discussed in the paper1It may accelerate the process of eutrophication by promoting anaerobic inorganic phosphorus release because of oxygen consumption by polyphenol oxidase,forming loose detritus layer to restrict submerged macrophyte restoration,and affecting division and growth of cell to cause algae bloom1On the other hand,it may combat eutrophication process through immobilizing nutrient substances such as amino acids and polysaccharides by humus and inhibiting activity of alkaline phosphatase and nitrification, thereby, it delaying phosphorus and nitrogen cycling1Meanwhile, the formation of humicenzyme complex may sustain the activities mentioned above1In conclusion,the process of enzymatic decomposition of organic matter in lake sediment is complicated,and the underlying mechanisms should be studied further more.