Results
Water quality parameters
Shading reduced water temperature (S0, 18.12±0.60 °C; S1, 16.38±0.50 °C) and DO concentration (S0, 11.42±0.18 mg L-1; S1, 7.85±0.24 mg L-1), but increased conductivity (S0, 371.18±3.77 µs cm-1; S1, 387.33±3.16 µs cm-1), TN (S0, 1.80±0.07 mg/L mg L-1; S1, 2.63±0.07mg L-1), and TP concentration (S0, 0.033±0.01 mg L-1; S1, 0.045±0.01 mg L-1) (Table S3 and S4). TN concentration and conductivity were found to increase with snail biomass (Table S3 and S4). Nutrient loading treatment increased conductivity. Shading, nutrient loading and snail biomass also interactively affected DO concentration and conductivity (Table S3 and S4).
Effect of treatments on submerged plant growth
Nutrient loading did not affect the biomass of both submerged plants (Table 1). Shading decreased P. crispus shoot biomass and shoot number, but increased P. crispus internode length (Figure1A and Figure2A, B). Shading and snail biomass interactively affected the shoot biomass of P. crispus (Table 1, Figure1A). In the full light treatment, shoot biomass and number of P. crispus showed a humped-shape response to snail biomass, but shoot biomass declined with snail biomass in the shading treatment (Figure1A). No changes in N content and C: N ratio of P. crispus could be detected in any of the treatments (Table 1 and Figure3B, C). The P content of P. crispus decreased and C:P ratio increased under shading (Table 1 and Figure3C, E). Plant C content decreased with snail biomass during shaded conditions (Table 1 and Figure3A). Nutrient loading and snail biomass interactively affected C:P and N:P ratios of P. crispus , and the two treatments together with shading affected the P content and C:P ratio of P. crispus(Table 1).
Effect of treatments onperiphyton and phytoplankton biomass
Nutrient loading did not affect periphyton biomass. Shading significantly increased biomass of periphyton and phytoplankton. Periphyton biomass decreased with snail biomass under both full light and shaded conditions, while phytoplankton increased with increasing snail biomass only during shading (Table 1 and Figure1C, D).