Dual challenges of eutrophication and stoichiometric imbalance: Implications for coordinated nutrient management in...

Subtropical estuaries are profoundly impacted by intensive anthropogenic activities, making the inputs of terrestrial nutrients a critical focus for estuarine ecosystem management. Here, we conducted a three-year, 12-season investigation in the Jiulong River Estuary (JRE) to characterize spatiotemporal patterns in nutrient ratios and eutrophication and assess their associated environmental effects. The eutrophication index (EI) ranged from 0.32 to 386.83 (mean, 46.62 ± 73.10). EI decreased significantly along the salinity gradient (P < 0.05) and peaked in winter (P < 0.05). Across the three-year period, the indicator of coastal eutrophication potential of phosphorus (P-ICEP) remained negative, whereas N-ICEP ranged from -3.58 to 4.60 kg C/km²/day and was generally higher in winter (P < 0.05). The DIN/DIP (N/P) ratio of terrestrial nutrient flux ranged from 42.89 to 277.01 (mean, 97.38 ± 61.30), while the N/P ratio in estuarine waters ranged from 7.76 to 1259.05 (mean, 114.86 ± 140.07), indicating persistent phosphorus (P) limitation. High riverine N/P inputs likely drove nutrient-ratio imbalances and eutrophication, increased the risk of red tide outbreaks, and accelerated P consumption. Therefore, ecosystem-based estuarine management should move toward the integrated regulation of nutrient fluxes, stoichiometric ratios, and ecological effects. We further propose the combined use of ICEP and EI as a management tool for the watershed-estuary continuum, leveraging their ability to identify eutrophication potential and ecological risk while helping prevent shifts in algal communities toward harmful bloom-forming species. We recommend coordinated land-to-sea control of both N and P loads and their ratios to enable integrated, ecosystem-based scientific management from watershed to estuary.