Copepod avoidance of thin chemical layers of harmful algal compounds

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 (1–104 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.

在一个薄浮游层的物理模型中，河口哲水蚤（Acartia tonsa）强烈避开由有害的短凯伦藻（Karenia brevis）产生的溶解性化合物的弱分层。化学诱导的游泳运动学变化使哲水蚤能够有效地避开该层及其周围区域，突显了涉及溶解性化学信号的远距离有害藻类 - 食草动物相互作用的相关性。在代表一系列生态相关的水华条件（1 - 10⁴个细胞/毫升当量）的化学浓度增加时，回避行为显著增加。在中等至最大水华条件下，哲水蚤表现出视觉上和水动力上明显的回避跳跃，其位移大且迅速，游泳速度接近捕食逃避反应所报道的速度。先前的研究结果表明，短凯伦藻在被哲水蚤摄入时既有毒又在营养上不适合哲水蚤，并且暴露于溶解性化合物可能会迅速抑制有效的采样和摄食行为。因此，哲水蚤有强烈的动机去感知并避开附近空间上离散的有毒藻类斑块，以通过避免接触和/或摄入以及相关的负面影响来提高适应性。我们的研究结果表明，有害藻类不仅对哲水蚤食草动物产生有害的生理影响，而且化学诱导的行为反应也可能通过回避反应改变食草动物的分布和自上而下的控制（降低有害藻类 - 食草动物的相遇率）。此外，通过哲水蚤游泳运动学的显著变化，较高营养级的捕食者 - 猎物相遇率可能会提高。这些综合机制可以保护和维持包含在次表层薄层中的有害水华，直到水华达到临界规模并在生态系统层面产生广泛影响，即“隐蔽水华”效应。