Development of salinity-zone-specific water quality criteria for total nitrogen and total phosphorus in Guangdong coastal waters, China.

Human activities and climate change have degraded coastal ecosystems by elevating nutrient concentrations, thereby exacerbating eutrophication in the coastal waters in the past decades. Increased nutrient levels lead to hypoxia, eutrophication, and frequent harmful algal blooms. However, current estuarine and marine assessments rely on Dissolved Inorganic Nitrogen (DIN) and Dissolved Inorganic Phosphorus (DIP), disconnecting them from land-based total nitrogen (TN) and total phosphorus (TP) indicator controls. To address this, we established site-specific TN and TP criteria based on salinity zonation to support integrated land-sea management. Using 9697 monitoring datasets (2012-2022), the study area was classified into four zones: tidal freshwater zone (salinity ≤2), low brackish mixed water area (2 < salinity ≤15), moderately brackish mixed water area (15 < salinity ≤25), and seawater area (salinity >25). Numeric nutrient criteria were derived using frequency distribution statistics and Classification and Regression Tree (CART) analyses to identify ecological response thresholds. The recommended TN criteria for the four respective zones were 1.80, 1.50, 0.50, and 0.40 mg/L, while TP criteria were 0.10, 0.06, 0.05, and 0.04 mg/L. Moreover, the derived reference values were compared and analyzed with those from other coastal waters, both domestically and internationally, to verify their rationality and applicability. Validation using independent data (2021-2024) indicated that TN-based assessment increased the compliance rate of "good quality" water compared to the traditional DIN criteria, effectively mitigating false negatives in water quality evaluation. These results demonstrate that the proposed criteria effectively bridge the gap between riverine discharge limits and marine quality objectives. This study provides a robust scientific foundation for the development of zoning-based nutrient management strategies, offering solutions for different ecological zones. It presents a systematic framework that integrates land-sea nutrient control, which can be applied to manage and mitigate eutrophication in complex estuarine and coastal environments.