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

Abstract

Daphnids with higher density than the surrounding water body have to push water downwards to hold their position in the water column. This swimming current 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 <it>Daphnia</it> are very variable depending on the swimming behavior. We focused on 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 than the zooplankton itself which can be important for the perceptibility by mechanically sensing carnivorous predators. In water with a density gradient of 10 kg m^-4 the wake length<it>WL</it> can be described by <it>WL</it> = 1.64 (<IMG SRC="/math/plusmn.gif">0.096) x<it>L</it>^{1.58(&plusmn;0.14)} with the length of the <it>Daphnia L</it> 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 with the value expected from theory with<it>W</it>L = 10.66 (&plusmn; 0.21) x (d&rgr;/dz)^{-0.265 (&plusmn; 0.010)}. Since the intra- and inter-specific 'communication' (e.g. mate seeking, nutrient partition between phytoplankton and bacteria, predator avoidance) can be expected to be bound to the hydrodynamic properties of the wakes, different wake forms and sizes most likely have an important impact on the information sent out.