Combined effects of warming and nutrient enrichment on water properties, growth, reproductive strategies and nutrient stoichiometry of Potamogeton crispus

Global warming and eutrophication are two important environmental issues facing freshwater ecosystems. Especially in winter and spring, freshwater ecosystems lacking large submerged plants are the most vulnerable. However, there is a paucity of research on the combined effects of global warming and eutrophication on the water quality of freshwater ecosystems and the growth characteristics of submerged plants in winter and spring. We therefore performed a mesocosm experiment examining the combined effects of nutrient (nitrogen and phosphorus) enrichment and warming (2.65 +/- 0.07 degrees C) on the submerged plant Potamogeton crispus. We found that warming affected the morphological characteristics of P. crispus, promoted plant height, accumulated more biomass, augmented the proportion of above-ground biomass and decreased the nitrogen content of the aboveground organs. Meanwhile, warming increased more resources for asexual reproduction (turions) and reduced the number of flowers. The addition of nitrogen or phosphorus alone could significantly augment the content of nitrogen or phosphorus in vegetative organs, respectively. In addition, the effects of different nutrient enrichment on the reproduction strategy of P. crispus were different. The addition of nitrogen reduced the number of flowers, while the addition of phosphorus augmented the number of flowers and turions. In contrast, the warming and co-addition of nitrogen and phosphorus treatments had a more negative effect on P. crispus, reducing biomass, maximum fluorescence and the number of ramets, flowers and turions. Although the leaf biomass allocation increased, the turion biomass allocation decreased. This may be due to the increase in algae, turbidity and total suspended solids caused by warming and the co-addition of nitrogen and phosphorus, which stressed the growth of P. crispus. Our results suggest that global warming and eutrophication can affect the structure and function of aquatic ecosystems by inhibiting the growth of submerged plants, which may lead to further fragile stability of freshwater ecosystems in winter and spring.