Structural equation model
The structural equation model indicates that both shading and snails affect primary producer biomass (Figure4). Shading and snail herbivory both negatively affected biomass of P. crispus (standardized path coefficient, SPC = −0.61 and −0.25 respectively). Snail herbivory decreased (SPC = −0.56), and shading increased (SPC = 0.32) biomass of periphyton (r 2 = 0.29). Both shading (SPC = 0.35) and snail (SPC = 0.36) significantly increased the biomass of phytoplankton (r 2 = 0.49).