Numerical experiments investigating the influence of drag on trajectory patterns of floating macroalgae

Abstract Ocean currents are a crucial means of dispersing natural and human-made materials on the ocean surface. Macroalgae are among the most conspicuous natural dispersers, often called the ‘tumbleweeds of the ocean.’ Despite numerous studies on the subject, the relative influence of wind and surface currents on the trajectory of macroalgal dispersal remains uncertain. Previous studies have focused on kelp rafts of varying sizes, making it challenging to determine the impact of wind versus currents. These studies have also disregarded the macroalgae’s drag characteristics and surface area, which have been shown to impact the trajectory and accumulation of floating flotsam. This numerical study aims to shed light on the relative influence of wind and currents and the role of drag in determining the course and accumulation of macroalgae. By comparing simulations of virtual kelp ‘particles’ that incorporate drag and those without, this study focused on solitary kelp plants and considered the impact of morphological characteristics, flow-field combinations, and the presence of Stokes drift. Our results show that virtual kelp particles generally followed ocean currents, but the inclusion of drag caused deviations from purely Lagrangian particles’ trajectories and sheds light on the complex interplay of factors affecting macroalgal dispersal in the ocean.