Plankton migration is system-dependent

Posted on June 19, 2017

Lake zooplankton commonly switch microhabitats on a regular day-night basis by migrating horizontally between stands of vegetation and open waters, or by migrating vertically between deep and surface waters. These movements have been explained in terms of the need to avoid predators (especially fish) and excessive ultraviolet radiation.  However, although zooplankton are known to seek refuge in structurally complex habitats like weed beds, such habitats may not always provide good protection because they can also attract potential zooplankton predators.  The situation is further complicated in temporary ponds, where predators tend to be mainly invertebrates and amphibians rather than fish, and where shallow water depths reduce the advantages of vertical migration. In a study of zooplankton migration in an Iberian pond with a maximum depth of 1 m, researchers collected plankton samples by deploying 500-ml funnel traps at different depths in the littoral zone, where vegetation density was low, and in the centre of the pond, where vegetation density was high.  Sample analysis revealed that zooplankton did show day-night patterns of microhabitat switching, but some of these patterns were different from those previously reported for other shallow lakes and temporary ponds.  Water fleas (cladocerans) and rotifers (Keratella sp.) carried out horizontal movements: they avoided high-density weed beds by day (presumably in response to predators) but preferred weed beds by day. In contrast, cyclopoid copepods, which were less vulnerable to predation, showed a range of microhabitat selection strategies, and calanoid copepods, the group least susceptible to predation, didn’t show diel horizontal migration at all.   And vertical migration, in the form of a move toward the surface at night and a return to the bottom by day, was observed in copepods, but not in cladocerans.  So it seems that patterns of day-night migration by zooplankton aren’t fixed, but rather vary with environmental conditions.

Reference: Compte, J. et al.  2016. Microhabitat selection and diel patterns of zooplankton in a Mediterranean temporary pond   Hydrobiologia 766,201–213.