The Interaction Between Cyanobacterial Bloom And Two Phytoplanktivorous Aquatic Animals

The prevention and treatment of cyanobacterial bloom by the principle of biomanipulation controlling aquatic community is considered as a safe and effective alternative method in water body that the management of nutrient would be difficult to prove effective. Therefore, more attention should be paid to the interaction between cyanobacterial bloom and phytoplanktivorous animal. This thesis, with Bellamya aeruginosa and Oreochromis niloticus as the main research object, researched the regulation of accumulation and degradation of microcystin in these two kinds of phytoplanktivorous animals. A series of laboratory tests are conducted to study the influence of toxic Microcystis aeruginosa on the six kinds of enzyme of lives and DNA damage in the hepatocellular of snail after feeding exposure. The microcosm experiment is employed to investigate the short-term ecological effect of Bellamya on the bloom water body. The main results are summarized as follows:1. In the bloom pond, MC in the body of Bellamya exhibited an obvious seasonal change, and a significant positive correlation was found between the contents of MC in snail and the intracellular microcystin (IMC) of phytoplankton, but there was no significant correlation between tissues and the extracellular Microcystin (EMC). Although the concentration of MC in water body was not high in low temperature season, the value of MC in hepatopancreas remained at 1.023-1.887μg/gDW. The contents of MC in various tissues of animals showed a significant difference (P<0.01), and the amount of MC accumulation was in sequence of hepatopancreas> digestive tract> branchia> abdominal foot. The feeding exposed experiment indicated that the temperature had a significant effect on the accumulation of MC in hepatopancreas (P<0.01): the concentration of MC in hepatopancreas at 15℃was higher than that at 25℃. It was found that, at 25℃, although MC in the mixed diet was about half of BG treatment, the value of MC in hepatopancreas of snails fed by toxic M. aeruginosa + Scenedesmus (mixed) was significantly higher than that fed by toxic M. aeruginosa (BG) (P<0.05). When the degradation experiment was carried out to probe into the dynamics of MC of the tissues in the suspension without MC after feeding exposure, it was found that MC in hepatopancreas had been cleared quickly in the first ten days after removal, but the clearance rate decreased obviously after that. At 15℃the residual amount of MC in hepatopancreas was significantly higher than it was 25℃at the later stage of the degradation experiment (P<0.05).2. According to the dynamics of MC in liver, the activities of acid phosphatases (ACP) and alkaline phosphatases (ALP) change correspondingly during the period of feeding exposure and clearance. The activities of glutathione-S-transferase (GST) was induced initially and then inhibited in the mixed treatment, the change of initial activity of GST was not obvious in the BG treatment, but increased remarkably at the late stage of the exposure. The activity of these enzymes decreased in different degrees with the concentration of MC lowering during the degradation of MC. The activities of antioxidant enzymes superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPX) in liver of snail were inhibited in the mixed treatment and induced in BG treatment at 15℃, whereas the SOD and CAT activities were all induced but the GPX activities was inhibited in both treatments at 25℃. The tests indicate that the response of activities of antioxidant enzymes to MC exposure may have a correlation with the concentration of MC, ambient temperature and the duration of exposure.3. The single cell gel electrophoresis technique (Comet assay) was employed to study the damage effects of short-term feeding toxin Microcystis (24h) on DNA of hepatocyte from snail. The results showed that the MC content in the liver of B. aeruginosa all reached the maximum after 6h exposure in both experimental treatments, and the MCs content decreased afterwards. The MCs content rose again rapidly after 12h exposure (adjusting density of alga). Meanwhile, the comet tail length (TL), tail moment (TM) and tail DNA% in tissues showed a time-dependent and a dose-dependent change in response to the MC content. In the mixed treatment, the parameters of DNA damage were significantly higher than that in BG treatment, both experimental treatments higher than that in the controlled group during the experimental period.4. The microcosm experiment was conducted to investigate the short-term (10d) ecological effect of different densities Bellamya (Higher-density: 200ind./100L, Middle-density: 100ind./100L, Lower-density: 50ind./100L) on the bloom water body. The results showed that dissolved oxygen (DO) and pH decreased with the increasing density of animal, the concentration of nitrogen had no significant difference among the different treatments, there was a significant difference in the concentration of (PO43+) among the three groups (p<0.01), and the higher the concentration of PO43+ was, the faster the density of orthophosphoric acid increased. The cyanobacterial bloom were completely controlled in the higher- and middle-density treatments, the biomass of cyanophyta and phytoplankton decreased significantly, the proportion of chlorophyta and cryptophyta increased, the index of phytoplankton diversity showed a decreasing trend with the increasing density of animal. The occurrence of Bellamya also produced a prodigious influence on the community structure of zooplankton. The quantity of rotifer in the high- and middle-density treatments was significantly lower than in the controlled and low-density treatments at the later stage of the experiment. A significant positive correlation was found between the index of zooplankton diversity and the density of animal (r=0.9834).5. The feeding rate of tilapia on M. aeruginosa increased with the increase of body weight and the water temperature under the experimental condition. At 25℃, the digestibility of tilapia varied from 58.6% to 78.1%, and the mean digestibility was 67.5±6.41%. The feeding exposure indicated that there was no significant difference between BG and mixed treatment in the content of MC in fish body (P>0.05), the MC content in both treatments reached stability after 21d exposure basically. There were significant differences in the distribution of MC in different fish tissues. Although the liver and the intestine have the similar MC, they have a significantly higher content than the muscle (P<0.01). The MC degradation experiment indicated that the clearance rate of MC was in the sequence of in intestine >in liver>in muscle.