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.