Multi-scenario modeling of the Ulva growth supports risk management for oyster farming on a coastal flat in western central Taiwan

An outbreak of macroalgae Ulva caused massive oyster production losses in the coastal wetlands of western central Taiwan. This event highlighted a critical gap in real-time indicators and response strategies to immediately or preparedly mitigate such ecological disturbances. Therefore, we aim to integrate risk assessment with mathematical modeling to quantify the impacts of nutrients discharges, changes in environmental factors, and potential interventions on Ulva coverage-related mortalities of cultured oysters. We monitored water quality in the wetland and nearby drainages to provide a basis for simulating the macroalgae biomass under varying environmental conditions. Effect analysis was conducted to support assessing the risk quotients for the mortality of cultured oyster covered by Ulva. Higher concentrations of ammonium nitrogen, nitrate nitrogen, and phosphates in the water of oyster farms and drainages were detected during spring and summer. Simulation results showed that if the proportion of Ulva trapped around oyster racks increased from 0.7 to 0.9, there would be significant risks when Ulva biomass in the external system attains 8 and 2 g dry weight (gDW) L⁻¹, respectively. We also found that Ulva transported from external systems and the likelihood of Ulva trapped around oyster racks were more sensitive to the risks than ammonium nitrogen, nitrate nitrogen, and changes in surface seawater temperature. Overall, our risk-based approach not only could identify crucial factors on the mortality risk of cultured oysters due to Ulva coverage, but also could inform marine environmental management to initiate early warning strategies and adopt effective interventions to sustain marine aquaculture.