Turbulence and feeding behaviour affect the vertical distributions of Oithona similis and Microsetella norwegica

The small copepods Oithona similis and Microsetella norwegica are often numerically abundant and widely distributed, but the factors controlling their vertical distributions and role in carbon cycling are yet unknown. Here we examined the vertical distributions of copepods during spring and summer in the Skagerrak and during autumn in the North Sea with respect to different physiochemical factors including turbulent dissipation rate. The ambush feeder O. similis numerically dominated the copepod community; they were located in the layers with high microzooplankton abundance. M, norwegica were dominant in the Skagerrak and were observed within or just below the pycnocline; they are assumed to feed on sinking detrital aggregates. Both copepods use remote detection of either hydromechanical (O. similis) or chemical signals (M, norwegica) generated by the prey and both species migrated to deeper depths in response to elevated surface turbulence. The potential effect of turbulence on both types of feeding is theoretically shown to be negative and we suggest a turbulent dissipation rate in the range 10(-7) to 10(-6) m(2) s(-3) as a threshold triggering the observed avoidance responses.