THE EFFECTS OF NUTRIENT CONCENTRATION ON PURIFICAION ABILITY AND ECO-PHYSIOLOGY OF CERATOPHYLLUM DEMERSUM

Ceratophyllum demersum is a free-floating submerged angiosperm and is an important indicator of clearwater state. C. demersum can absorb nutrients from the water column and is usually used as a pioneer macrophyte in the re- storation of eutrophic lakes. The main objectives of this study were to evaluate (1) the purification ability of C. demersum to remove phosphorus and nitrogen at high levels of nutrient concentrations (N: 1030 mg/L, P: 13 mg/L); (2) the ecophysiological response of C. demersum (e. g. net phosphorus uptake by plants, biomass changes and net photosynthetic rate) under high nutrient concentrations; and (3) the potential of C. demersumused asa pioneer and constructive species for macrophyte recovery in hyper-eutrophic Lake Caohai (China). C. demersumsamples were collected from a pond near Lake Dianchi and were pre-cultured in the experimental aquaria for 30 days before experiments. The plants were transplanted and grown in four 20 cm 20 cm 60 cm aquaria with the following conditions: 12 cm depth of sediment and 18 L of tap water (control) and the other three with increased nitrogen and phosphorus concentrations (Level 1, N: 10 mg/L, P: 1 mg/L; Level 2, N: 20 mg/L, P: 2 mg/L; and Level 3, N: 30 m/L, P: 3 mg/L). The changes of nutrient concentrations, biomass of plant and periphytic algae, chlorophyll- a content of phytoplanktonic algae, net photosynthetic rate of plant, phosphorus uptake and water content of plant were evaluated during culture period (n = 30 days). We observed that C. demersum and sediment system efficiently removed phosphorus and nitrogen from water column (removal rate 80%); however, the addition of nitrogen and phosphorus decreased the efficiency. Phosphorus contents of C. demersum were significantly higher in the three treatment groups (7.0113.09 mg/g, ave. 9.03 mg/g) compared to those in the control group (2.83.17 mg/g, ave. 3.05 mg/g). In addition, we found that plant growth was obviously inhibited by high nutrient levels. Except the control, plant biomass loss, (abscission of leaves) among the treatment groups decreased 48. 3%63.3% at the end of the experiment. We also observed that net photosynthetic rates of the plants in the treatment groups significantly increased from baseline -0.0370.058 mg/(gh)to the end 0.180.44 mg/(gh); however, the change in the control group was not significant. Results indicated that some reco- very of the macrophyte may be due to the decrease of nutrient concentration at the end of culture period. We concluded that nutrient concentration significantly influenced the growth and net photosynthetic rate of C. demersum and this plant is not suitable for being used as a pioneer macrophyte in the current restoration of Lake Caohai.