LAUSR

Xuan Dai is a coastal bay in Phu Yen Province, Vietnam. The bay has an area of 80 km 2 and a considerable depth from 8 to 18 m. This is an ecologically diverse area with great potential for aquaculture and fishing, which brings significant benefits to the people in the region. For years, the excessive development of aquaculture has enriched the water in the bay, resulting in mass deaths of lobsters. In this study, the ASSETS index model has been applied to Xuan Dai Bay to determine the trophic status and the natural and man-made processes related to eutrophication. The study results showed that the core zone of Xuan Dai Bay (CZ) was classified with a high influencing factor, with a high eutrophic condition, with a “no change” future outlook, and with a bad final overall ASSETS grade. Ky Lo Estuary (KLE) presents a low pressure, a low eutrophic condition, a “no change” future outlook, and a good final ASSETS index. These mean that CZ had a poor tropic status or the eutrophication happened in this system; whereas, KLZ presented a good tropic condition or no eutrophication. A basic management plan and early warning monitoring should take into consideration the condition of the water body to preserve the good tropic condition in KLZ, especially during dry seasons. For CZ, three groups of solutions have been proposed, including nutrient management, ecosystem restoration, and further researches. The results of the study also reveal the significance and benefit of applying integrated methods in water quality assessment and management in coastal zones. Deep understanding of water retention time, typology, nutrient loading, and the land/water uses of a system is the key factor for atrophic status management strategy.