Competitive Outcomes Between Floating‐Leaved and Submerged Plants Under Eutrophication Depend on Growth Form

ABSTRACT The niches of floating‐leaved plants partially overlap with those of submerged plants, leading to widespread competition between the two in freshwater ecosystems. Therefore, one of the biggest challenges for submerged plants is coexisting with floating‐leaved plants. However, the mechanisms of coexistence between floating‐leaved plants of varying densities and different growth forms of submerged plants under eutrophic conditions remain unclear. Here, we conducted a mesocosm experiment to study the combined effects of different densities and eutrophication levels of a common floating‐leaved plant ( Nymphoides peltata ) on three growth forms of submerged plants (bottom‐dwelling Vallisneria natans , erect Myriophyllum aquaticum and canopy‐forming Myriophyllum spicatum ). We found that intermediate and high densities (9 and 18 plants) of N. peltata significantly reduced the total biomass and leaf chlorophyll content of submerged plants and had a greater impact on the morphological traits of V. natans and M. spicatum . In contrast, its effects on M. aquaticum were relatively minor, causing only slight reductions in ramet number and plant height. Elevated nutrient levels alleviated the competitive pressure of intermediate density N. peltata on submerged plants but were more favourable to M. spicatum and M. aquaticum . Submerged plants of different growth forms exhibited species‐specific responses to this. V. natans responded by increasing plant height and allocating more biomass to the roots, while M. spicatum responded by producing more ramets and having a lower root/shoot ratio. The combined effect of density and nutrient level had the greatest impact on the organ nutrient content of the erect M. aquaticum . Furthermore, increased floating‐leaved plant density and higher levels of eutrophication were most detrimental to the growth of the bottom‐dwelling V. natans . Our study suggests that under eutrophic conditions, the coexistence pattern between floating‐leaved and submerged plants will change, strengthening the dominance of floating‐leaved plants and presenting greater challenges for submerged plants.