Nutrient and Light Interactions Shape Competitive Dynamics Between Two Co‐Occurring Submerged Macrophytes

Submerged macrophytes in shallow freshwater ecosystems often encounter fluctuating nutrient and light conditions, which shape their growth strategies and interspecific interactions. However, the combined effects of these environmental factors on competition and trait plasticity remain underexplored. We investigated interspecific competition and trait responses between Vallisneria natans and Myriophyllum aquaticum using a fully factorial replacement series experiment crossing two light regimes, three nutrient levels, and five planting ratios. The two light regimes were higher‐light (206.3 ± 16.8 μmol m −2 s −1 ) and lower‐light (78.4 ± 6.4 μmol m −2 s −1 ). The three nutrient levels were N1 (ambient; TN = 0.478 mg L −1 , TP = 0.001 mg L −1 ), N2 (low enrichment; TN = 3.0 mg L −1 , TP = 0.2 mg L −1 ), and N3 (high enrichment; TN = 8.0 mg L −1 , TP = 0.8 mg L −1 ). Five planting ratios of V. natans to M. aquaticum were tested: 8:0, 6:2, 4:4, 2:6, and 0:8. We measured plant biomass, morphological traits (plant height, root length, and root‐to‐shoot ratio), and tissue nutrient concentrations (total nitrogen and total phosphorus). Competition indices including relative yield (RY), relative competition intensity (RCI), and relative crowding coefficient (RCC) were calculated to quantify interspecific interactions. M. aquaticum exhibited greater biomass and tissue nutrient accumulation (total nitrogen and total phosphorus) under nutrient‐rich, higher‐light conditions, maintaining competitive dominance. In contrast, V. natans demonstrated higher morphological plasticity and competitive persistence under nutrient‐poor and shaded conditions, increasing root‐to‐shoot ratio, plant height, and root length. These traits reflected a conservative growth strategy that enabled niche differentiation. Competition indices revealed asymmetric interactions: M. aquaticum strongly suppressed V. natans under enriched conditions (low RY, high RCI), while V. natans showed higher RCC under stress, suggesting context‐dependent shifts in competitive advantage. Our findings highlight the significance of trait‐based strategies in mediating macrophyte competition and offer insights into species selection for ecological restoration in variable freshwater environments. Moreover, the strong growth and resource acquisition abilities of M. aquaticum under nutrient‐rich and well‐lit conditions may also underpin its invasive success in eutrophic freshwater ecosystems.