Hydrology and water quality evaluation for potential HABs under future climate scenarios

Harmful algal blooms (HABs) are increasingly a global concern and the issue of all fifty states in the U.S as it poses a threat to human health and aquatic ecosystem. This study aimed to investigate the relationship of HABs with streamflow and water quality parameters and assess the hydrology-based potential future HABs in the Ohio River Basin at Ironton (ORBI) using the Soil and Water Assessment Tool (SWAT). SWAT was calibrated and validated against potential HABs indicators including streamflow, total suspended solids (TSS), and dissolved oxygen (DO) with acceptable accuracies. Twenty-one years (2002-2022) of model simulated data were analyzed to relate the 2015 HABs occurrence in the Ohio River with unique temperature, streamflow, TSS, and DO conditions. Additionally, a future climate model was developed to project these variables for the next two decades (2023-2043) using data from the Canadian Regional Climate Model (CRCM5) for Representative Concentration Pathway (RCP) 4.5. The 2015 HABs formation was found to be associated with a series of high flows contributing to high nutrient transport followed by an extended period of low flows balancing nutrients flushing rate. The projections of average temperature, streamflow, and TSS concentration showed increments of 5%, 15%, and 28%; whereas DO concentration showed a decrement of 8%. Flood frequency analysis was conducted to better understand the HABs probability related to peak flow conditions. For the base condition, results showed 3% probability of peak flow (4550 m³/s) associated with 2015 HABs formation at the Ironton gauge station and 20% probability of the same flow in the next two decades demonstrating an increased risk of HABs and highlighting the necessity of mitigation measure implementations.