Effects of different initial snail densities on submersed macrophyte Vallisneria spinulosa Yan and its epiphyton

Macrophytes and their epiphyton are the main primary producers in shallow lakes. Freshwater snails are the main primary consumers in freshwater ecosystems, but it is still debatable whether snail densities are an important factor that determine the growth of macrophytes and their epiphyton. We established four snail densities (0, 40, 80, and 240 ind·m^-2) to investigate the grazing impact of snails (Radix swinhoei H. Adams) on the submerged macrophyte Vallisneria spinulosa Yan and its epiphyton, and determine the dynamics of the snail populations from the interaction. Results indicated that R. swinhoei strongly reduced the biomass of the epiphyton, but promoted the biomass of the macrophytes, and at the highest initial snail density even induced higher ramet number of macrophytes. The direct grazing effects of snails on macrophytes were observed but were not significant at the moderate eutrophic level, probably because submerged macrophytes alone could not support the high densities of snails. The size structure of the snail population could quickly respond to nutrient levels of the water-bodies and food resources in the environment.