PLANT ANIMAL INTERACTIONS Elke S. Reichwaldt Æ Herwig Stibor The impact of diel vertical migration of Daphnia on phytoplankton dynamics Received: 11 February 2005 / Accepted: 26 May 2005 / Published online: 9 July 2005 Ó Springer-Verlag 2005 Abstract Diel vertical migration (DVM) of large zoo- plankton is a very common phenomenon in the pelagic zone of lakes and oceans. Although the underlying mechanisms of DVM are well understood, we lack experimental studies on the consequences of this behaviour for the zooplankton’s food resource—the phytoplankton. As large zooplankton species or indi- viduals migrate downwards into lower and darker water strata by day and upwards into surface layers by night, a huge amount of herbivorous biomass moves through the water column twice a day. This migration must have profound consequences for the phytoplankton. It is generally assumed that migration supports an enhanced phytoplankton biomass and a change in the composition of the phytoplankton community towards smaller, edi- ble algae in the epilimnion of a lake. We tested this assumption for the first time in field experiments by comparing phytoplankton biomass and community assemblage in mesocosms with and without artificially migrating natural stocks of Daphnia hyalina. We show that DVM can enhance phytoplankton biomass in the epilimnion and that it has a strong impact on the com- position of a phytoplankton community leading to an advantage for small, edible algae. Our results support the idea that DVM of Daphnia can have strong effects on phytoplankton dynamics in a lake. Keywords Behaviour Æ DVM Æ Phytoplankton community Æ Trait-mediated interaction Æ Trophic cascade Æ Zooplankton Introduction Diel vertical migration (DVM) is a well-known phe- nomenon performed by several mesozooplankton spe- cies both in freshwater and marine ecosystems. It is thus a very widespread animal behaviour. In the case of a ‘normal’ DVM, zooplankton only spend the night in the warmer upper layer of a lake (epilimnion) but migrate downwards into the darker lower layer (hypolimnion) during the day. During this migration, an enormous amount of zooplankton biomass moves daily up and down in the waters, making it one of the world’s most massive synchronized animal migrations. In a medium- sized lake of temperate regions, the zooplankton’s bio- mass can easily exceed several tons of carbon. So far, most research has been done in freshwater ecosystems where metabolic costs due to the lower temperature in the hypolimnion are thought to be the main costs for the zooplankton (Dawidowicz and Loose 1992; Loose and Dawidowicz 1994) and avoidance of predation by visually orientated predators during day in the darker hypolimnion is thought to be the main benefit for the zooplankton (Zaret 1972; Stich and Lampert 1981; Lampert 1989). The strongest DVM can be seen in the presence of chemical trigger substances of fish (kairomones) (Dod- son 1988; Loose 1993; Loose and Dawidowicz 1994). If no such cue is present, the zooplankton usually stays in the epilimnion continuously. Therefore, the strong migration pattern in the presence of fish kairomones represents the result of a non-lethal interaction between predator (fish) and prey (zooplankton). Such interac- tions have gained more and more interest in many dif- ferent ecosystems such as lakes (Turner and Mittelbach 1990), streams (McIntosh and Townsend 1996), grass- lands (Schmitz et al. 1997), and rocky intertidal zones Communicated by Ulrich Sommer E. S. Reichwaldt (&) Æ H. Stibor Department Biologie II, Aquatische O ¨ kologie, Ludwig-Maximilians Universita¨t Mu¨nchen, Großhadener Str. 2, 82152 Martinsried, Germany E-mail: reichwaldt@mpil-ploen.mpg.de Fax: +49-4522-763310 E. S. Reichwaldt Max-Planck-Institute of Limnology, Postfach 165, 24302 Plo¨n, Germany Oecologia (2005) 146: 50–56 DOI 10.1007/s00442-005-0176-3