Mesocosm experiment reveals a strong positive effect of snail presence on macrophyte growth, resulting from control of epiphyton and nuisance filamentous algae: Implications for shallow lake management

Increased nutrient loading has adverse effects on the growth of submerged macrophytes in eutrophic shallow lakes. Where growth of phytoplankton, epiphyton and filamentous algae is excessive, all may contribute to shading that limits macrophyte growth. However, when abundant, herbivorous snails may dampen this effect by reducing the biomass of epiphyton, and perhaps also of nuisance filamentous algae, both which have the potential to become more abundant in a future warmer world. We studied the effects of herbivorous snails (Radix swinhoei) on the biomass of phytoplankton, epiphyton and filamentous algae as well as the growth of the submerged macrophyte, Vallisneria denseserrulata, under contrasting nutrient loadings (low, nitrogen (N) 113 μg L⁻¹·d⁻¹ and phosphorus (P) 10 μg L⁻¹·d⁻¹; high, N 339 μg L⁻¹·d⁻¹ and P 30 μg L⁻¹·d⁻¹) in a 30 day outdoor mesocosm experiment, conducted on the shore of subtropical Lake Taihu, China. We found significant interactive effects of nutrient loading and snail presence on biomasses of epiphyton and filamentous algae and on the biomass and relative growth rate of submerged macrophytes. When snails were absent, the biomass of epiphyton and the biomass and coverage of filamentous algae all increased markedly, while the biomass, density and relative growth rate of V. denseserrulata decreased significantly with increased nutrient loading. When snails were present, biomasses of epiphyton, phytoplankton and filamentous algae were significantly reduced and growth of V. denseserrulata significantly increased under both high and low nutrient loading scenarios, and the effect was most pronounced in the nutrient-rich treatment. The present study suggests that in shallow aquatic ecosystems, herbivorous snails reduce the negative impact of nutrient loading on submerged macrophyte growth, by controlling both epiphyton and nuisance filamentous algae. How best to protect snails from fish predation in order to realize this potential under natural conditions is a matter that warrants further studies.