Assessment of Eutrophication Characteristics and Evaluation of the First-Generation Eutrophication Model in the Nearshore Waters of Shantou City

Based on the water quality monitoring results from 2002 to 2018 in the coastal waters of Shantou City, this study used eight methods, including the single-factor index method, eutrophication index method, nutrient status quality method, comprehensive index method, organic pollution index method, nitrogen–phosphorus ratio method, potential eutrophication evaluation, and dissolved oxygen saturation parameter method, to evaluate the eutrophication and organic pollution status of the area. The applicability of the first-generation eutrophication model was assessed from the perspective of red tide occurrence, and a comparison was made among the various evaluation methods. The results indicate that the coastal waters of the study area were mostly in a phosphorus-limited state, with a few periods of nutrient deficiency and a very small portion of time in a moderately nutritious state with both nitrogen and phosphorus limitations. The overall N/P ratio showed an increasing trend primarily due to the gradual increase in inorganic nitrogen content. In terms of evaluating eutrophication based on red tide symptoms, the comprehensive index method showed better performance in reflecting eutrophication conditions, while the eutrophication index method and dissolved oxygen saturation parameter method highlighted problematic areas but often exaggerated their extent. The nutrient status quality method and organic pollution index method did not provide distinctive indications and could not verify red tide symptoms. The nitrogen–phosphorus ratio method and potential eutrophication evaluation could reflect certain nutrient structures and eutrophication characteristics but also failed to verify red tide symptoms. Furthermore, this study found a certain correlation between the occurrence of red tide and the ratio of nitrate nitrogen to ammonia nitrogen, which consistently ranged from 1.15 to 1.94 during red tide events. Therefore, whether this ratio can serve as a characteristic value for red tide occurrence and be used for early warning and monitoring still requires further validation.