| In eutrophic lakes, periphyton on the surface of the submerged plant could inhibit the growth of submerged plant, grazing periphyton by snail could mitigate the impact, so as to promote the growth of submerged plant, therefore, the interactions among snail, periphyton and submerged plant were an important mechanism of maintaining the development of submerged plant in eutrophic lakes. Submerged plant often is rare in higher nitrogen loading eutrophic lakes, which mechanism is unclear. Increasing nitrogen concentration may change periphyton elements composition(increasing N:P ratio), the result showed that the growth reproduction rate of the snail feeding mainly on periphyton is reduced, which may result in periphyton biomass increased, and then inhibit the growth of submerged plants. To verify this hypothesis, the project carried out by controlled experiments, analysing the response of periphyton N:P ratio on increasing nitrogen, clarifying the effects and mechanism of periphyton N:P ratio on the growth of snail; Analysing of the effects of periphyton N:P ratio on the interactions among snail, periphyton and submerged plant by lakes site experiments, and then discusses the influence mechanism of increasing nitrogen concentration on the submerged plant in the eutrophic lakes. Research results can not only rich ecological stoichiometry theory, but also provide basis for revealing the submerged plant disappeared in the shallow lakes and steady state conversion mechanism, in addition revealing to restore ecologically the eutrophication lakes. In this paper, the conclusions were expressed as follow:1. The outdoor experiment was conducted to investigate the effects of snail Parafossarulus striatulus(Benson) on the growth of Vallisneria spiralis, with three treatment groups(0, 150, 450 ind·m-2). The results showed that the snails were beneficial for V. spiralis growth. Both of the relative growth rate and plant height in the low density snail group(150 ind·m-2) increased by 20% in comparison with the control group with no snails. The high density group improved further, compared to the control group, with increases in relative growth rate, number of leaves, and plantheight of 28%, 15% and 27%, respectively. It is suggested that the snails may reduce the periphyton biomass on the leaves of the plant by grazing, contributing to the increased plant growth. This study is helpful for us to further understand the mutualistic relationship between freshwater snails and aquatic macrophytes, as well as the ecological relationships between organisms in aquatic ecosystems.2. The outdoor experiment was conducted to investigate the effects of increasing nitrogen loading on the growth of periphyton, with five treatment groups(2 mg·L-1、4 mg·L-1、6 mg·L-1、8 mg·L-1 and 10 mg·L-1). The results showed that attached algae growth had obvious four stages: slow period(0~7 days), logarithmic phase(7~21 days), stabilization(21~28 days) and decline phase(28~35 days).With the increasing of nitrogen concentration in water(2~6 mg·L-1), the biomass and growth rate of the adherent algae were increasing, biomass and growth rate reached maximum at 6 mg·L-1, when the concentration of nitrogen in water is greater than 6 mg·L-1, periphyton biomass and growth rate all decreased with the increase of nitrogen concentration in water. Content of attached algae nitrogen and nitrogen concentration in water has significantly relationship(the regression equation was y =-0.0044x4+0.0929x3-0.6529x2+1.9137 x, R2=0.94). With the increase of nitrogen concentration in water, adherent algae nitrogen content increased significantly until 8 mg·L-1, then gradually reduced. Periphyton N:P ratio increased with the increasing of nitrogen concentration in water till 6 mg·L-1 and then gradually reduced. It is suggested that adherent algae growth rate and the chemical elemental composition increased with the rising of nutrient concentration in water in a certain range, when the nutrient concentration over the certain range, the lower instead. This study is helpful for us to reveal the ecological relationship between elevating nitrogen concentration nutrient in water and periphyton.3. The controlled experiment was conducted to investigate the effects of different periphyton N:P ratios that under the culture conditions(nitrogen concentrations were1 mg·L-1、4 mg·L-1 and 7 mg·L-1) on the growth of snail. The results showed that periphyton nitrogen and carbon content in 4 mg·L-1 water nitrogen treatment group were higher than 1 mg·L-1 and 7 mg·L-1 treatment groups, that in 1 mg·L-1 waternitrogen treatment group was lowest. Periphyton C:N ratios in 4 mg·L-1 water nitrogen concentration is lower than 1 mg·L-1 and 7 mg·L-1 treatment groups, 1 mg·L-1 treatment group reached a maximum. There was a significant positive correlation between change of periphyton nitrogen content and nitrogen content in snail(the regression equation was y = 2.0003x+2.8174,R2=0.769). The nitrogen and carbon content in snail in 4 mg·L-1 treatment group were higher than 1 mg·L-1 and 7 mg·L-1 treatment group, the nitrogen and carbon content in 1 mg·L-1 treatment group were the lowest. C:N ratio in snail in 4 mg·L-1 treatment group is lower than 1 mg·L-1and 7 mg·L-1 treatment group, C:N ratio in snail in 1 mg·L-1 treatment group reached a maximum. Meanwhile the growth rate and length in snail in 1 mg·L-1 treatment group were higher than 4 mg·L-1 and 7 mg·L-1 treatment group, 4 mg·L-1 treatment group were the lowest. It is suggested that adherent algae chemical elemental composition increased with the rising of nutrient concentration in water in a certain range, periphyton nitrogen content kept the same space with the content of nitrogen in snail, with the increase of periphyton nitrogen content, the nitrogen content in snail increased. The change of the chemical elemental composition lead to the food quality of periphyton for snail Bellamya aeruginosa, which was the important reason for restraining the snail growth. This study is helpful for us to understand the effect of food quality on the growth of the consumer and reveal the ecological relationship between periphyton in elevating nitrogen concentration in water and snail.4. Double two-factorial controlled experiment(high nitrogen loading; with and without snails) was conducted to investigate the effects of increasing nitrogen loading(three times as much as the normal range in Lake Taihu) on the interactions among snail Bellamya aeruginosa, periphytic algae and Vallisneria spiralis. Results indicate that increased nitrogen loading elevated the concentrations of total nitrogen, total dissolved nitrogen and chlorophyll a in water, as well as the periphyton dry mass. In contrast, increased nitrogen loading significantly decreased the growth rate of V. spiralis. The high nitrogen treatments, compared to the control, led to the decrease in relative growth rate, plant height and number of leaves by 28%, 15% and 27%, respectively. The snails reduced the periphyton biomass and the concentration ofchlorophyll a by grazing, which in turn promoted the growth of V. spiralis. However, the interaction of both increasing nitrogen loading and snail density on the interactions among the physical and chemical variables in water, periphyton dry mass and Vallisneria spiralis growth were not significant. In addition, increased nitrogen loading suppressed the growth rate of B. aeruginosa. These results suggest that increasing nitrogen loading promoted the growth of planktonic algae and periphyton biomass, which inhibited submerged macrophytes; the existence of B. aeruginosa weakened this effect to some extent. However, increasing nitrogen loading also decreased the growth rate of B. aeruginosa, and reduced grazing pressure, leading to strengthening the inhibitory effects of the periphytic algae on V. spiralis. Therefore, increasing nitrogen loading interfered with the interactions among snail Bellamya aeruginosa, periphytic algae and Vallisneria spiralis, which may lead to the disappearance of submerged vegetation in the high nitrogen eutrophic lakes. |
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