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Keywords:

  • horizontal distribution;
  • planktonic invertebrates;
  • root-mean-square velocity;
  • small-scale turbulence;
  • vertical refuge use

Summary

  1. Small-scale turbulence is a key factor in increasing encounter rates but also dispersing prey patches of planktonic organisms and can thus contribute to their distribution. Our hypotheses were that turbulence can (i) disturb the vertical refuge use of fourth instar Chaoborus flavicans larvae and (ii) also disperse them horizontally. Both were tested experimentally, across a gradient of turbulence.
  2. The vertical experiments were conducted in 46-L cylinders, providing the larvae a vertical refuge by darkening the bottom layer. The horizontal experiments were conducted in a 200-L aquarium, providing them a horizontal refuge by darkening one end of the aquarium.
  3. With both set-ups, two light treatments were tested: uniform darkness as a control and the bottom layer of the cylinder or either end of the aquarium darkened to provide a refuge. In addition to non-turbulent treatment, five different root-mean-square (RMS) velocities were generated, ranging from 0.6 to 10.2 cm s−1 in the vertical and to 10.9 cm s−1 in the horizontal experiment.
  4. Increasing turbulence had a significant effect on the distribution of C. flavicans larvae in both vertical and horizontal directions and increased the proportion of larvae in the non-preferred illuminated environment. In the vertical direction, the proportion of larvae in the refuge decreased linearly with increasing turbulence, showing a decline from 84% under calm conditions to 61% at the highest RMS velocity (10.2 cm s−1). The effect of turbulence was more prominent in the horizontal direction; without turbulence, 86% of the larvae occupied the refuge, but the proportion declined to 32% at 5.4 cm s−1 RMS velocity. In experiments with uniform darkness, the distribution of chaoborids was not affected by turbulence either in the vertical or in the horizontal direction.
  5. Our results indicate that turbulence has substantial effects on refuge use and distribution of C. flavicans larvae. Increasing turbulence can greatly diminish the ability of prey species to use their vertical refuges. Once away from their vertical refuge, chaoborids are highly vulnerable to horizontal dispersion, thus upsetting their horizontal escape behaviour. Small-scale turbulence is therefore likely to have considerable effects on food-web dynamics in lakes.