Fertile sediment and ammonium enrichment decrease the growth and biomechanical strength of submersed macrophyte Myriophyllum spicatum in an experiment

Decline of submersed macrophytes has occurred in eutrophic lakes worldwide. Little is known about effects of nutrient enrichment on biomechanical properties of submersed macrophytes. In a 30-day experiment, Myriophyllum spicatum was cultured in aquaria containing two types of sediment (mesotrophic clay vs. fertile loam) with contrasting water NH₄ ⁺ concentrations (0 vs. 3.0 mg L^?1 NH₄–N). The plant growth, shoot and root morphology, stem biomechanical properties, and stem total nonstructure carbohydrates content (TNC) were examined. The NH₄ ⁺-enriched water, particularly combined with the fertile sediment, caused adverse effects on M. spicatum as indicated by reductions in the growth, stem biomechanical properties (tensile force, bending force and structural stiffness), and TNC content. These results indicate that increased sediment fertility and water NH₄ ⁺-enrichment made the plant more fragile and vulnerable to hydraulic damage, particularly for the upper stem, implying that M. spicatum was prone to uprooting and fracture by hydraulic force, and the broken fragment from parent shoot of M. spicatum might have low-survival potential due to its low-TNC content. This may be a mechanical aspect for the decline of submersed macrophytes and makes it more difficult to restore submersed vegetation in the eutrophic lakes.