Study On The Growth Characteristics Of Lemna Minor And The Grazing Capability Of Ctenopharyngodon Idella And Megalobrama Amblycephala On Lemna Minor

Xiaojiang is an important tributary of Three Gorges Reservoir. Its eutrophic status has become worse since construction of Three Gorges Reservoir was completed, thereby leading to more frequent occurrence of duckweed overgrowth in this river basin. Using herbivorous fishes like grass carp（Ctenopharyngodon idella） and blunt snout bream（Megalobrama amblycephala） to control duckweed overgrowth, as a biological method,is safe and effective. The aim of this study was to figure out the feeding capability of grass carp and blunt snout bream on duckweed（Lemna minor） and the main impact factors on their feeding capabilities, to reveal growth characteristic of Lemna minor,thereby providing scientific basis for reasonable utilization of grass carp and blunt snout bream in order to control duckweed outbreak in backwater area of Xiaojiang. The effects of body weight and water temperature on _maximum food consumption(C_max) and _maximum rate of food consumption(FR_max) of grass carp and blunt snout bream fed duckweed under laboratorial conditions were studied. The effects of temperature and nutrients on growth of Lemna minor were also studied. The Lemna minor collected from Xiaojiang backwater area in Three Gorges Reservoir was cultured in indoor concrete ponds in winter and idle ponds in summer to provide enough food for the fish in the study.The main results are as follows:1. Lemna minor was cultured on an artificial medium formulated by simulating the actual levels of nitrogen and phosphorus and p H in Xiaojiang. During the culture period that lasted for 4 weeks, the growth curve of Lemna minor population was presented as J-shape and its biomass increased exponentially. The fronds number had risen to 22.667 times of that at the beginning and its fresh weight had risen by 9.756 times when compared with initial fresh weight. Exponential growth model was applied to fit a curve showing the growth pattern of Lemna minor population. The population growth equations obtained by using fronds number or fresh weight as index are N_t=31.7140·e^{0.1138 t} and N_t=42.6930·e^{0.0885 t}, respectively.2. The relationship between the growth of Lemna minor and temperature was studied by culturing Lemna minor on the same artificial medium that was designed to simulate trophic levels of nitrogen and phosphorus and p H of Xiaojiang. The experimental temperature ranges from 10℃ to 30℃ with five treatments（10℃, 15℃, 20℃, 25℃,30℃）. The growth of Lemna minor had been inhibited at 10℃, and its relative growthiv rate（RGR） had risen gradually and doubling time（DT） had descended gradually as the temperature went up; when the temperature climbed to 30℃, its growth rate reached to the _maximum, which was 0.127d^-1（fronds number） and 0.119d^-1（fresh weight）respectively; while the doubling time dropped down to the minimum, which was 5.49d（fronds number） and 5.83d（fresh weight） respectively. Thus the conclusion that could be drawn from these results was Lemna minor had a rather strong tolerance towards high temperature, not towards low temperature and it is a species that favors high temperature.3. When nitrate nitrogen that designed range of concentration is 0.5¹5.0mg/L（0.5mg/L, 1.5 mg/L, 3.0 mg/L, 5.0 mg/L, 15.0 mg/L） in experiment was used as nitrogen source, its concentration also had significant effect on Lemna minor growth. At the beginning, the growth of Lemna minor had been inhibited to a certain degree when concentration of nitrate nitrogen was relatively low as 0.5mg/L; later its RGR had risen gradually and DT had descended gradually as the concentration of nitrate nitrogen went up, but the change rate of RGR and DT slowed down; when the concentration of nitrate nitrogen climbed to 15.0mg/L, its RGR to the _maximum, which was 0.149d^-1（fronds number） and 0.157d^-1（fresh weight） respectively; while the DT dropped down to the minimum, which was 4.67d（fronds number） and 4.41d（fresh weight） respectively. The RGR and DT of Lemna minor tended to be stable as the concentration of nitrate nitrogen ranged from 5.0mg/L to 15.0mg/L（p>0.05）. It could be concluded that it is enough to satisfy the growth requirement of Lemna minor when the concentration of nitrate nitrogen reached 0.10mg/L.4. When ammonia nitrogen was used as nitrogen source, its concentration also had significant effect on Lemna minor growth. At the beginning, the growth rate of Lemna minor was founded to increase obviously as the concentration of ammonia nitrogen was increased. It reached the _maximum level when concentration of ammonia nitrogen was3.0 mg/L. And RGR had increased to 0.088 d^-1（fronds number） and 0.084 d^-1（fresh weight）, DT was only 7.91 d（fronds number） and 8.28 d（fresh weight）. Later its growth had been inhibited by toxic effect of NH3, which caused rapid decrease of growth rate. It had almost stopped growing when concentration of ammonia nitrogen was 15.0mg/L. At this point, its RGR has dropped to the minimum which was 0.009d^-1（fronds number） and0.011d^-1（fresh weight）. While the DT had risen to 76.68d（fronds number） and 67.12d（fresh weight）, which were the _maximum values.5. The impact of phosphorus on the growth of Lemna minor is relatively weaker. The growth of Lemna minor had been inhibited to a certain degree when concentration of Pwas as low as 0.02mg/L; However, with the increasing of P concentration, RGR of Lemna minor had increased apparently and DT had decreased significantly. When P concentration was increased to 0.10mg/L, both of the RGR presented using fronds number or fresh weight as its index were 0.115d^-1 and the DT was 6.02d（fronds number）and 6.03d（fresh weight） respectively. Later on, the growth rate of Lemna minor tended to be stable as P concentration was increased. It could be concluded that it is enough to satisfy the growth requirement of Lemna minor when P concentration is 0.10mg/L.6. The _maximum food consumption of grass carp increased while the _maximum rate of food consumption decreased as its body weight increases from 40.36 to 478.17 g at the same temperature（25.82±0.57 ℃, Mean±SD）. The correlation between the _maximum food consumption（g/（d·fish）） and body weight（g） was C_max（g/（d·fish））= 2.460W0.779, and the correlation between the _maximum rate of food consumption（%/d） and body weight was FR_max（%/d）= 246.0W-0.221. Water temperature（16.55³0.50℃） influenced C_max and FR_max significantly（P<0.01）. The regression equation between C_max and temperature（℃）was C_max（g/（d·fish））=0.074T2.045, and the regression equation between FR_max and temperature was FR_max（%/d）=0.164T^1.940. The model established to predict _maximum rate of food consumption of grass carp of different body weights on duckweed at different temperature was: FR_max（% /d）=0.410T^1.940W-0.221.7. The _maximum food consumption of blunt snout bream increased while the _maximum rate of food consumption decreased along with the increase of body weight from 62.93 to 411.83 g at the same temperature（30.69±0.87 ℃, Mean±SD）. The correlation between the _maximum food consumption（g/（d·fish）） and body weight（g） was C_max（g/（d·fish））= 2.528W^0.717, and the correlation between the _maximum rate of food consumption（%/d） and body weight was FR_max（%/d）= 252.8W^-0.283. Water temperature（17.95³2.53 ℃） influenced C_max and FR_max significantly（P<0.01）. The regression equation between C_max and temperature（ ℃） was C_max（g/（d·fish））=0.271T^1.618, and the regression equation between FR_max and temperature was FR_max（%/d）=0.325T^1.533. The model for prediction of the _maximum rate of food consumption of blunt snout bream imposed on different body weights and temperatures was developed as follow: FR_max（%/d）=1.228T^1.533W^-0.283.