Submerged macrophytes are important for maintaining a clear water state in shallow lakes but are now threatened by the ongoing climate changes involving more extreme weather events. We explored the effects of extreme precipitation (higher water level, higher loading of nutrients (phosphorus and nitrogen)) on the growth of the submerged macrophyte Vallisneria denseserrulata Makino in a 10-month long mesocosm experiment. We used three macrophyte communities (one, three and six species) and four simulated scenarios of water level and nutrient loading (constant 75 cm, no nutrient loading; gradual water level increase from 75 cm to 150 cm over three months; extreme water level increase to 150 cm within one day at the same nutrient loading as in the gradual increase; constant 75 cm and the same nutrient loading as in the two former treatments). Leaf chlorophyll fluorescence, biomass, morphology of macrophytes and periphyton biomass were recorded. In the treatment with an extreme increase in water level, macrophyte biomass declined, whereas plant height and leaf number remained unchanged compared with the treatments where only nutrient but not water levels were increased. The combined effect of increased water level and enhanced nutrient loading (gradual and extreme change) did not differ significantly from the constant 75cm treatment without additional nutrient loading. No significant differences were found in epiphyton biomass among the water level treatments, while epipelon biomass was marginally higher and plant volume inhabited lower in the two treatments with a water level increase. Changes in macrophyte assemblages only had limited effects on maximum leaf width and the relative electron transport rate of V. denseserrulata. In conclusion, V. denseserrulata was overall resilient to the major
simulated changes in precipitation, except when the water level increased suddenly, and only marginally influenced (leaf chlorophyll fluorescence parameter) by the composition of the macrophyte assemblage.