Dynamic water quality criteria for estuaries: Exploration and implementation in the Yangtze Estuary

Estuaries, as transitional zones between freshwater and seawater, possess unique attributes rendering the application of the Sea Water Quality Standard for assessment inappropriate and inaccurate. To enhance estuarine environmental protection and management, this study developed a dynamic salinity-based water quality criteria system for the Yangtze River Estuary (YRE), integrating existing marine and surface water standards. The estuary was partitioned into three salinity zones: Tidal-controlled Freshwater Zone (TCFZ), Salinity Transition Mixing Zone (STMZ), and Plume-influenced Marine Zone (PIMZ). Applying this system using survey data from six 2021–2022 cruises revealed a significantly improved comprehensive water quality classification compared to previous marine standards, indicating that the new criteria better characterize the true ecological conditions in each distinct zone. Class III dynamic criteria established nutrient thresholds: DIN (TCFZ: 1.080, STMZ: 0.561, PIMZ: 0.119 mg/L) and SRP (TCFZ: 0.033, STMZ: 0.027, PIMZ: 0.017 mg/L). Combined with hydrodynamic analysis, nitrogen and phosphorus were identified as primary controlling factors, driven by runoff-tide dynamics. This study proposes establishing a real-time, salinity-zone-based monitoring network, implementing seasonal nutrient flux thresholds, and strengthening controls on spring inputs and summer plankton monitoring in the YRE.