中文摘要：

本文设计了双因子（氮负荷升高、有无螺类）受控实验,研究了氮负荷升高（太湖正常氮负荷的3倍）对螺-附着藻-苦草关系的影响.结果表明,氮负荷升高处理明显提高了水体总氮、总溶解氮、叶绿素a等指标的含量,显著增加了附着生物干重,进而抑制了苦草生长;与正常氮负荷处理组相比,3倍氮负荷处理组中苦草的相对生长率、株高和叶片数分别降低了33%、25%和13%.环棱螺降低了附着生物干重和水体叶绿素a含量,明显促进了苦草生长.氮负荷升高和环棱螺存在对水体各项理化指标、附着生物干重和苦草的各项生长指标均无交互作用.此外,氮负荷升高还降低了环棱螺的生长速率.分析认为,氮负荷升高对沉水植物生长的抑制机理主要体现在浮游藻类与附着生物生物量增加的抑制效应;环棱螺的存在虽然在一定程度上减弱了这种效应,但由于氮负荷升高还同时使环棱螺生长率降低,削弱了其对附着生物的牧食压力,从而使得附着生物对苦草生长的抑制作用加强.因此,氮负荷升高使螺-附着藻-沉水植物之间的生态关系失衡,也可能是氮浓度较高的富营养湖泊中沉水植被稀少的重要原因.

英文摘要：

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 33%,25% and 13%,respectively. The snails reduced the periphyton biomass and the concentration of chlorophyll 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 thegrowth 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.