Long‐Term Expansion, Ecosystem Services and Management Trade‐Offs of Phragmites karka in Asia's Largest Lagoon

ABSTRACT Coastal lagoons are increasingly altered by hydrological modification and catchment‐derived nutrient loading, driving shifts in macrophyte dominance and creating complex management trade‐offs. In Chilika lagoon, India, the emergent macrophyte Phragmites karka has expanded, raising concerns about impacts on native macrophytes, habitat heterogeneity and biodiversity. We integrated Landsat imagery (1991–2025) with field measurements of water quality, sediment biogeochemistry, metal accumulation and population demography to evaluate its expansion implications. P. karka exhibited alternating phases of expansion and contraction, and occupied approximately 8611.21 ha in 2025. Established stands occurred under low‐salinity, and reducing conditions, and contained sediment carbon and nitrogen stocks of 50.09 ± 18.11 Mg C ha −1 and 4.81 ± 1.60 Mg N ha −1 , corresponding to approximately 0.43 million Mg C and 41,420 Mg N across the extent. Metals were below‐detection limits in surface waters but accumulated in sediments and plant tissues, with biota‐sediment accumulation factors > 1 for Mn, Ni and Co. Population demography indicated recruitment and positive growth, suggesting persistence under prevailing conditions. These findings document habitat‐specific biogeochemical attributes, but do not imply that they outweigh biodiversity concerns, establish conservation equivalence with native habitats or alter the invasive status of P. karka . Management should remain site‐specific: restoration, nutrient‐source reduction, hydrological rehabilitation, containment and removal should be prioritized where native‐community recovery is feasible or sensitive habitats are threatened. Monitored temporary retention may be considered only where restoration is constrained, risks to sensitive habitats and native communities are low and rapid removal could mobilize sediments, nutrients or contaminants. Decisions should remain conditional, reversible and periodically reassessed.