Abstract: Acute responses of four antioxidant enzymes:superoxide dismutase(SOD,EC 1.15.1.1),catalase(CAT,EC 1.11. 1 6) peroxidase(POD,EC 1.11.1.7)and ascorbate peroxidase (APX,EC1.11.1.11),of Ceratophyllum demersum to inorganic nitrogen enrichment in water column were investigated after 5—24h incubations with potassium nitrate or ammonium carbonate,and thus to evaluate the stress of inorganic nitrogen enrichment in water column on submersed macrophytes.In this experiment,the concentrations of NO 3 N were ranged from 1mg/L to 20mg/L,and the concentrations NH 4 N were ranged from 1mg/L to 100mg/L. The results show that in the ammonium treatments,CAT activity was the highest among the four enzymes.In the 5h and 15h after the treatments,the activity of CAT showed a concentration dependent increase,but there were no significant differences in CAT activity among treatments as the incubation prolonged to 24h.In the treatments with nitrate,SOD,APX and CAT activities changed significantly only at 24 hr after the treatment.SOD and CAT activities were the highest in the four enzymes.The activities of both enzymes showed a peak at 5mg/L nitrate. APX activity was the lowest among the four enzymes both in ammonium and nitrate treatments.The trend of APX was similar to that of CAT.This experiment suggested that CAT show a quick response to ammonium enrichment in water,whereas SOD and CAT show delayed response of 15—24h to that of nitrate.CAT and SOD are possible indicators for submersed macrophytes under the stress of inorganic nitrogen enrichment in water.