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

Harmful algal blooms (HABs), exacerbated by global warming and anthropogenic nutrient loading, pose a significant threat to aquatic ecosystems and public health. In the Chowan River-Albemarle Sound estuarine system of northeastern North Carolina, recurrent HABs pose persistent ecological and management challenges. However, the physical processes governing HAB movement in this region remain insufficiently characterized, hindering timely, targeted, and effective management. Using an active Lagrangian particle tracking framework coupled with hydrodynamic model simulations and in situ observations, we investigated how physicals 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 wind forcing and river discharge, producing both upstream and downstream transport pathways that govern the redistribution of HABs across the estuarine network. During low-discharge periods, wind-driven circulation promoted upstream transport and cross-channel exchange, whereas high discharge enhanced downstream export. In all scenarios, blooms required more than ten days to reach western Albemarle Sound, highlighting the temporal scales of ecological connectivity relevant to bloom development and exposure risk. 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 and spatial redistribution, this modeling framework provides a scientific basis for improving HAB surveillance, optimizing monitoring networks, and supporting adaptive management of estuarine ecosystems under changing environmental conditions.