Ammonium uptake plasticity and allocation trade-offs may shape invasion potential in Caribbean seagrasses

Nutrient uptake traits can shape plant competition, yet nutrient acquisition and internal allocation strategies remain poorly resolved for invasive seagrasses, limiting predictions of species responses to coastal nutrient enrichment. Over the past two decades, the invasive seagrass Halophila stipulacea has rapidly colonized large areas of the Mediterranean and Caribbean, occasionally displacing native species. Competition for inorganic nitrogen, particularly ammonium, may play a key role role in determining future meadow composition and functioning. Using a novel split-chamber incubation system, we compared leaf and root ammonium ( 15 NH 4 + ) uptake kinetics and internal nitrogen transfer among three native Caribbean seagrasses ( Thalassia testudinum, Halodule wrightii, Syringodium filiforme ) and the invasive H. stipulacea . We further synthesized available data on seagrass ammonium uptake and transfer parameters to place our findings in a broader context. Across all species, leaf uptake dominated and was nearly an order of magnitude higher than root uptake. Native species exhibited high root uptake affinity and balanced allocation, consistent with nutrient-conservative strategies in oligotrophic enivronments. In contrast, H. stipulacea showed lower root uptake but exceptionally high maximum leaf uptake rates and strong root-to-leaf transfer at elevated ammonium, indicating opportunistic aboveground allocation under nutrient pulses. Some species displayed non-saturating uptake kinetics, suggesting uptake capacity may be underestimated under eutrophic conditions. Our synthesis revealed high inter- and intraspecific variability and showed that 80% of seagrass species lack kinetic parameters, highlighting major mechanistic knowledge gaps in this foundational plant group. Native Caribbean seagrasses are likely competitively favoured under low to moderate ammonium concentrations, whereas H. stipulacea may gain a context-dependent advantage under nutrient enrichment. These findings illustrate how trade-offs between nutrient-use efficiency and flexibility shape invasion potential. Closing global data gaps, monitoring nutrient levels and thereby identifying mechanistic thresholds is essential for predicting future seagrass species dominance and ecosystem functioning in increasingly eutrophic seagrass meadows.