Effects of salinity structure on swimming behavior and harmful algal bloom formation in Heterosigma akashiwo, a toxic raphidophyte

The formation of toxic surface blooms of the motile raphidophyte Heterosigma akashiwo often occurs too quickly to be attributed to cell reproduction. Rapid appearance of surface blooms is more consistent with the hypothesis that a dispersed cell population aggregates at the surface due to a combination of physical factors and swimming behavior. Because of the frequent association of Heterosigma bloom formation with a decrease in surface salinity, we hypothesize that a layer of lowsalinity water over a high-salinity layer will suppress nearsurface vertical mixing and this halocline will enable up-swimming cells to rapidly aggregate at the surface. For this hypothesis to be viable, Heterosigma cells must be able to swim across salinity jumps of a sufficient magnitude to temporarily suppress vertical mixing. We tested whether this requirement is satisfied by using computerized video analysis to quantify swimming behavior and vertical distribution of Heterosigma within a vertical salinity structure. Swimming behavior is affected by the presence of a salinity jump and depends on the strength of the jump: cells stopped swimming upwards and aggregated below a fresh water interface; cells reduced upward swimming speed with a salinity jump from 28 to 8‰, and upward swimming speed was unchanged in cells encountering a salinity jump from 28 to 16‰. We used observed swimming behaviors to parameterize a model of a 2-layer stratified water column in which vertical mixing is suppressed at the halocline and modeled by eddy diffusivity within each layer. The model predicts rapid aggregation of cells to the surface layer.