The effects of diel vertical migration of Daphnia on zooplankton-phytoplankton interactions

Zooplankton populations which perform diel vertical migration (DVM)
only spend the night in surface water layers but migrate downwards
into the lower water layers during the day. The intention of this
study was to investigate effects of DVM of Daphnia on phytoplankton
dynamics and Daphnia life history parameters in a lake. I conducted
field and laboratory experiments in which I compared ‘migration’
with ‘no-migration’ situations. It is generally assumed that
phytoplankton communities in the epilimnion of stratified lakes
profit from the presence of DVM. This might be caused by less
grazing due to intermittent grazing and/or less grazing due to
lower population densities of migrating Daphnia populations (as
they migrate into the colder, lower hypolimnion during the day
which leads to a lower temperature-dependent population growth). In
a first laboratory experiment I showed that an enhanced
phytoplankton biomass could develop under a migration regime solely
due to intermittent grazing. I further showed that edible
phytoplankton species with higher intrinsic growth rates benefited
more from intermittent grazing than edible species with lower
intrinsic growth rates. Field experiments also indicated that
phytoplankton biomass in the epilimnion was higher when subject to
a migrating zooplankton population and that additionally different
phytoplankton community compositions arose from different migration
regimes (‘migration’/’no-migration’). For example edible algae were
at an advantage when zooplankton migrated, whereas large, inedible
algae species had an advantage when zooplankton populations did not
migrate. In an additional laboratory experiment I also showed that
these changes in phytoplankton composition had strong feedback
effects on life history parameters of daphnids and that food
conditions experienced by migrating daphnids were advantageous. In
a further laboratory experiment I exposed two Daphnia species to
either constant or regularly changing temperature regimes to study
whether a fluctuating temperature regime – as experienced by
migrating daphnids – implies costs for daphnids. Somatic growth
rates of juvenile Daphnia in the regularly changing temperature
regime were almost as low as under constant low temperature
conditions indicating that a regular change in temperature involves
high costs. The results of my study indicate that DVM has a strong
modulating effect on zooplankton-phytoplankton interactions in a
lake.