Possible accumulation of harmful flagellates caused by interaction between vertical swimming and upper ocean turbulence

In the natural oceanic environment, microalgae occasionally form blooms with rates of increase that exceed their growth rates observed under laboratory conditions. We hypothesized that these elevated rates result from the accumulation caused by the combined effects of vertical swimming behavior and physical processes in the upper ocean. To test this hypothesis, we conducted numerical experiments using a Lagrangian particle-tracking model coupled with a hydrodynamic model. In a stratified ocean forced by sea surface winds and waves, we released particles simulating three fish- and shellfish-killing flagellates with different swimming speeds, namely Chattonella marina complex, Karenia mikimotoi, and Margalefidinium polykrikoides. Due to the interplay of diel vertical migration (DVM) and upper ocean turbulence, the particles accumulated vertically, exhibiting peak concentrations in the surface layer in the afternoon. These concentrations were several times higher than those observed at night or in the morning. Strong turbulence resulting from high wind speeds reduced surface accumulation and altered the depth of the maximum particle concentration compared to the still-water case (i.e., DVM only), especially for the slow-swimming Chattonella. Under low wind speeds (U₁₀ = 1.5–2.0 m s⁻¹), particles simulating the fast-swimming M. polykrikoides horizontally accumulated in streaks where surface flow converged, increasing particle concentration by up to one order of magnitude. This horizontal accumulation was caused by the balance between the upward swimming speed of M. polykrikoides and the downwelling associated with horizontal convergence driven by Langmuir circulations. Compared to algal growth, these vertical and horizontal accumulation processes occurred over shorter timescales, highlighting the importance of considering accumulation in monitoring harmful algal blooms.