Shallow lakes at risk: Nutrient enrichment enhances top‐down control of macrophytes by invasive herbivorous snails

In shallow lakes, submerged macrophytes can stabilise clear-water conditions and prevent phytoplankton blooms. Nutrient enrichment can promote their abundance. Above critical thresholds in water nutrient concentrations, however, submerged macrophytes have collapsed in many lakes worldwide with negative consequences for important ecosystem functions. The mechanisms behind this decline are complex and subject to debate. We conducted mesocosm and laboratory experiments to investigate the top-down effect of invasive herbivorous snails (Pomacea canaliculata) on native submerged macrophytes (Vallisneria natans) under low and moderate nutrient treatment. We tested whether effects of nutrient addition to the water column on snail herbivory were influenced by shading by phytoplankton and epiphyton, macrophyte elemental tissue content and snail nutrient release. We found that herbivorous snails had strong negative top-down effects on macrophytes, which were amplified by nutrient enrichment in the water column leading to higher macrophyte consumption rates. In moderate nutrient treatment, macrophyte consumption was faster and snails grew better than in low nutrient treatment. Additionally, snail treatments had higher nitrogen concentrations in the water leading to a negative feedback loop. Our results suggest that snail herbivory may increase the chance for macrophyte collapse and shifts of shallow lakes to turbid states, and this effect occurs at lower snail densities when nutrient concentrations increase. Shallow lakes are thus severely at risk from projected increases in nutrient loading and spread of invasive herbivorous molluscs.