Discussion
Light intensity, nutrient loading, and snail herbivory interactively affected the growth of submerged macrophytes, phytoplankton and periphyton during the start of the growing season. Our experiment indicated that shading substantially reduced the total biomass and shoot number of P. crispus compared to full light conditions. No effect of nutrient loading on growth of primary producers could be detected.P. crispus shoot biomass showed a hump-shaped response to increased snail herbivory under full light condition, but linearly decreased with increasing snail herbivory under shaded conditions. Additionally, shading promoted growth of periphyton and phytoplankton, and periphyton biomass decreased with increased snail biomass. These interactions were furthermore confirmed by a structural equation model.