Diel vertical migration (DVM) is a common behaviour of many pelagic zooplankton species. While the causes (mostly predator avoidance) and ecophysiological consequences of DVM for zooplankton have been well studied, little is known about the consequences of DVM for the pelagic food web. DVM creates a temporal and spatial grazer-free niche for edible phytoplankton, and theoretical models predict that parts of the phytoplankton community should use this niche. Conceivably, DVM could also cause nutrient transport between separated water layers. We experimentally investigated the influence of DVM of the zooplankton species, Daphnia hyalina, on the nutrient and phytoplankton dynamics of an oligotrophic lake. We used 10-m deep field enclosures with a 4-m deep, well-mixed surface layer. The enclosures contained either migrating or nonmigrating Daphnia populations; temperature was kept nearly constant across the entire enclosure depth. Our results show that DVM had significant quantitative and qualitative effects on the phytoplankton community. There was no measurable net nutrient transport between hypolimnion and epilimnion. The gelatinous green alga Planktosphaeria gelatinosa, was the dominant algal species in our experiment. Its abundance decreased in DVM treatments, and thus also influenced the total biomass and diversity of phytoplankton communities.

In many lakes, zooplankton show a distinct diel vertical migration (DVM) behavior, especially during periods of stratification. Excretion products of these zooplankton could potentially cause an upward nutrient transport and consequent nutrient enrichment for phytoplankton in the epilimnion. We quantified the upward transport of phosphorus by the cladoceran Daphnia DVM experimentally by adding a radioactive tracer (33P) to the hypolimnion of large indoor mesocosms and measuring tracer accumulation in the epilimnion over time. During the daytime, when all Daphnia were found in the hypolimnion, no phosphorus transport from the hypolimnion into the epilimnion took place. As soon as the Daphnia started their upward migration, around dusk, we observed a continuous increase in phosphorus concentration in the epilimnion. The amount of phosphorus transported was in a biologically meaningful range. Our results strongly suggest that Daphnia vertical migration presents a continuous nutrient supply for the epilimnion.