Abstract: Dissolved oxygen(DO) is one of the key factors affecting aquatic ecosystem and changes with water temperature.A study of spatial-temporal distribution of dissolved oxygen in Tianmuhu,a reservoir 12 km2 in water surface and 14 meters in depth(maximum),was conducted during 2006 and 2007.It was found that in non-summer seasons the increase in water temperature caused reduction of the concentration and saturation of DO in the surface water layer,but in summer it led to increase in DO concentration and likely to over-saturation of DO due to the activities of plankton.In the middle and bottom water layers,temperature stratification is the key factor affecting DO concentration.When there was a thermocline in the water body,the bottom water was in hypoxia,and when thermocline disappeared,DO restored in the bottom water.As a result,the bottom water followed the alternation of hypoxia-DO restoration-hypoxia in a year.The hypoxia of the bottom water layer in summer caused release of ammonia nitrogen from sediments into water,thus leading to rise of the ammonia concentration in the surface water.The investigation and analysis of ammonia nitrogen concentration in the water body before and after dredging demonstrate that dredging of bottom sediments could effectively reduce the release of endogenous nitrogen in summer,thus mitigating the impact of the variation of DO concentration in the water on the release of nitrogen from sediments.

Key words: dissolved oxygen, temperature stratification, nitrogen release, Tianmuhu