Hydrodynamic controls on harmful algal bloom connectivity in an estuarine system: insights for monitoring

Harmful algal blooms (HABs) pose a significant threat to aquatic ecosystems and public health. In the Chowan River-Albemarle Sound system of North Carolina, recurrent HABs pose persistent ecological and management challenges. However, physical processes governing HAB movement in this region remain insufficiently characterized, limiting our understanding of bloom connectivity and effective monitoring strategies. Using a Lagrangian particle tracking framework coupled with hydrodynamic simulations and in situ observations, we investigated how physical processes shape HAB transport under contrasting river discharge conditions. Four experiments were initialized during a low-discharge period at Arrowhead and Leary Landing sampling stations during July 2019, along with two high-discharge scenarios in June 2019. Results demonstrate that bloom connectivity is jointly controlled by winds and river discharge, producing both upstream and downstream transport pathways across the estuarine network. During low-discharge periods, wind-driven circulation promoted upstream transport and cross-channel exchange, whereas high discharge enhanced downstream exports. In all scenarios, blooms showed limited downstream transport toward western Albemarle Sound. Blooms initialized at Arrowhead failed to reach the Sound within 10 days, while those initialized at Leary Landing reached Edenton Bay by day 10. Simulations also identified previously unsampled regions in northern western Albemarle Sound that are susceptible to transported blooms under specific hydrodynamic conditions. By revealing how physical processes regulate bloom connectivity, transport pathways and potential exposure locations, our framework provides insights that can inform the spatial and temporal design of HAB monitoring. Further model evaluations and explicit assessment of alternative monitoring strategies are needed before making specific monitoring recommendations.