Effects of Salinity and Eutrophication Variations on the Growth of Myriophyllum spicatum

Salinization and eutrophication are increasingly severe pollution problems in wetlands. Myriophyllum spicatum is a cosmopolitan species widely used for pollution control, but its physiological responses under combined stressors remain largely unknown. Here, we used mesocosms to investigate the ecophysiological responses of M. spicatum to three ammonia nitrogen concentrations (0, 1.5, and 3 mg L−1) and two salt concentrations (0 and 5 g L−1). Shoot and stem biomass were significantly affected by both salinity and nitrogen, whereas leaf phosphorus and stem nitrogen responded only to salinity (two-way ANOVA, p < 0.05). A significant salinity–nitrogen interaction was observed for stem biomass (p < 0.05); specifically, low nitrogen alone caused no significant reduction, but under saline conditions it markedly exacerbated biomass suppression. A significant salt–nitrogen interaction was detected for stem biomass (p < 0.05), such that low nitrogen alone did not significantly reduce stem biomass but exacerbated its suppression under saline conditions. These indicate potential synergistic environmental effects and suggest that even low nutrient inputs may aggravate stress under salt exposure. Stem biomass was significantly negatively correlated with malondialdehyde content (Pearson analysis, p < 0.05). Salt–nitrogen co-stress significantly increased malondialdehyde content (Tukey HSD test), indicating enhanced lipid peroxidation and associated oxidative damage, which may represent a physiological mechanism underlying growth inhibition in M. spicatum. Our findings demonstrate the complex adaptive responses of M. spicatum and emphasize the need to consider salt–nutrient interactions in conservation and restoration practices.