Abstract: Investigation and monitoring on nitrogen pollution and eutrophication status of the large and medium reservoirs for water supply in Guangdong Province were carried out in flood season and dry season of 2000.Water temperature,DO,pH,chlorophyll a,COD_Mn,turbidity,TN,TP,NO₃-N,NH₃-N,NO₂-N and PO₄-P were determined synchronized with phytoplankton.The study revealed that the water quality of the reservoirs was still generally good.Among the 20 reservoirs,70% of them were situated in oligotrophic to medium trophic status,and 30% of them showed eutrophic features.The concentration of TN was generally higher,and nitrogen pollution was one of the important factors involved in eutrophication.TN and DIN fluctuated with the area as a result of the difference in character and environmental status.Seasonal dissimilarity existed too.Statistical analysis revealed that the concentrations of TN and DIN were positively related to chlorophyll a in dry season.However the relationship was not obvious in flooding season.As the primary composition of DINin water body,NO₃-N accounted for 72.6% of DINon the average.The concentration of NH₃-N was obviously lower than that of NO₃-N which constituted 25.5% of DIN on the average.The concentration of NO₂-N was relatively low.The concentration of TN,DINand organic nitrogen were important targets used to distinguish the trophic status of the reservoirs.And the percentage of NO₃-N and NH₃-N among DIN was important basis used to estimate the source of the pollution.

Key words: Long term fertilization trial, Crop yield, Water nutrient interaction