Abstract
In a physical model of a thin planktonic layer, the estuarine copepod Acartia tonsa strongly avoided weakly stratified layers of dissolved chemical compounds from the harmful dinoflagellate Karenia brevis . Chemical‐induced changes in swimming kinematics allowed copepods to effectively avoid the layer and surrounding volume, highlighting the relevance of harmful alga‐grazer interactions at a distance that involve dissolved chemical signals. Avoidance increased significantly with increasing chemical concentration representative of a range of ecologically relevant bloom conditions (1–10 4 cells/mL equivalent). Under mid to maximal bloom conditions, Acartia displayed visually and hydrodynamically conspicuous avoidance jumps featuring large, rapid displacements with swimming speeds near those reported for predatory escape reactions. Previous findings show K. brevis is both toxic and nutritionally inadequate to A. tonsa when ingested and that exposure to dissolved chemical compounds likely rapidly suppresses effective sampling and grazing behaviors. Thus, copepods have strong incentive to sense and avoid nearby, spatially discrete patches of toxic algae in order to improve fitness by avoiding exposure and/or ingestion and associated negative impacts. Our results suggest harmful alga not only produce deleterious physiological effects in copepod grazers, but chemical‐induced behavioral responses also likely alter grazer distributions and top‐down control via avoidance reactions (reduced harmful alga‐grazer encounter rates). Additionally, predator‐prey encounter rates at higher trophic levels are likely enhanced via significant changes in copepod swimming kinematics. These combined mechanisms could protect and sustain harmful blooms contained in subsurface thin layers until blooms reach critical mass and produce widespread impacts at the ecosystem level, the “cryptic bloom” effect.