Size and structure of 'footprints' produced by Daphnia: impact of animal size and density gradients

Daphnids with higher density than the surrounding water body have to push w ater downwards to hold their position in the water column. This swimming cu rrent is mainly the result of the movement of the second antennae and only to a small extent due to the filter current. The water structures under the Daphnia are very variable depending on the swimming behavior. We focused a n the hop and sink behavior, where the downward-pushed water masses form a wake which can be described as a mushroom-shaped vortex. The volume of this structure increases linearly with time. The wake volume is much larger tha n the zooplankton itself which can be important for the perceptibility by m echanically sensing carnivorous predators. In water with a density gradient of 10 kg m(-4) the wake length W-L can be described by W-L = 1.64 (+/- 0.0 96) x L-1.58 ((+/- 0.14)) With the length of the Daphnia L in millimeters. The wake length, measured after 5 s, remained constant for gradients up to 1 kg m(-4). Above this value, the wake length declines in good agreement wi th the value expected from theory with W-L = 10.66 (+/- 0.21) x (d rho/dz)( -0.265 (+/- 0.010)). Since the intra- and inter-specific 'communication' (e .g. mate seeking, nutrient partition between phytoplankton and bacteria, pr edator avoidance) can be expected to be bound to the hydrodynamic propertie s of the wakes, different wake forms and sizes most likely have an importan t impact on the information sent out.