Copepod Behavior in Thin Layers of Toxic Algae

Cryptic blooms of toxic phytoplankton were modeled in a custom thin layer flume for behavioral assays with calanoid copepods. Acartia tonsa and Temora longicornis were exposed to thin layers of exudates from the toxic dinoflagellate Karenia brevis (1 - 10,000 cells/mL). Planar laser-induced fluorescence (PLIF) was used to quantify the spatial structure of the layer allowing for correlation of behavioral responses with toxin levels. Both species explicitly avoided the exudate layer and the vicinity of the layer. Measures of path kinematics (swimming speed, turn frequency) by location (in-layer vs. out-of-layer) and exposure (pre-contact vs. post-contact) revealed some similarities, but also significant differences, in trends for each species. A. tonsa significantly increased swimming speed and swimming speed variability in the exudate layer and post-contact, whereas T. longicornis slightly increased both in-layer and slightly reduced both post-contact. Both species increased turn frequency in-layer and post-contact with increasing K. brevis exudate concentration. Path fractal geometry indicated that A. tonsa trajectories became more diffuse/sinuous and T. longicornis trajectories became more linear/ballistic (trending effects). Regression analyses revealed that the rate of change of behavior with increasing exudate concentration for A. tonsa was three to fifty times that of T. longicornis depending on the parameter considered. The results suggested that toxic K. brevis can essentially eliminate top-down grazer control, another sinister means by which it gains a competitive advantage over the local phytoplankton taxa.