Vertical swimming changes the food web

Posted on December 26, 2009

Animals in the plankton commonly undergo vertical migrations, spending the daylight hours in deep water but moving into the surface layers at night.  Many research projects have clarified the benefits of vertical migration, especially in connection with predator avoidance, but much less attention has been focussed on its food web implications.  Using 10 m deep field enclosures in LakeBrunnensee, German workers examined the grazing impact of migrating water fleas (Daphnia) on the structure of plant plankton communities.  The enclosures contained gauze cages that retained Daphnia but allowed the free exchange of plant plankton with the surrounding water.  Cages in some enclosures were moved up and down to simulate the vertical migration of Daphnia, while in other enclosures, the cages were kept near the surface.  Different species of plant plankton showed positive, negative or neutral responses to migrating Daphnia, but the diversity of plant plankton was higher in the “migration” enclosures than in the “no migration” treatment.  One of the most common species was the small green alga Planktosphaeria gelatinosa.  This species was especially abundant in the upper layers of the “no migration” enclosures, where it may have benefitted from more continuous grazing by Daphnia ( it has a gelatinous cover that protects cells from digestion while allowing it to benefit from nutrients in Daphnia guts).  This study is the first to show that herbivore behaviour can affect the species diversity of plant plankton.

Reference:  Haupti, F., Stockenreiter, M., Baumgartner, M., Boersma, M. & Stibor, H. 2009.   Daphnia diel vertical migration: implications beyond zooplankton.  Journal of Plankton Research 31, 515-524.