Salinity mediates ecosystem impacts of an invasive macrophyte across an aquatic continuum

The ecological impacts of invasive species are increasingly recognized as context-dependent, yet the role of environmental gradients in shaping these effects remains poorly understood. This is particularly relevant in transitional aquatic ecosystems, where salinity gradients can constrain plant performance and alter ecosystem functioning. We investigated how salinity influences the functional role of the invasive macrophyte Pontederia crassipes by quantifying growth, nutrient uptake, and decomposition dynamics across a freshwater–marine continuum. Using a combination of mesocosm experiments (0–5 g L⁻¹) and in-situ litterbag assays along a natural salinity gradient (0–32 g L⁻¹), we assessed how key processes linked to ecosystem functioning vary across environmental conditions. We found that plant growth and nutrient uptake declined with increasing salinity, whereas decomposition rates increased markedly along with the gradient. These contrasting responses indicate a shift in the functional role of the species, from nutrient retention in freshwater systems to enhanced nutrient release under more saline conditions. Our results demonstrate that environmental gradients strongly modulate the ecosystem impacts of invasive macrophytes, highlighting salinity as a key driver of transitions between ecosystem service provision and ecosystem degradation. This context dependency has important implications for predicting invasion impacts and for managing transitional aquatic ecosystems.