Diel vertical migration (DVM) of zooplankton has intrigued ecologists for over
100 years. Traditional DVM theory holds that Daphnia migrate into the hypolimnion
during the daytime to escape predation pressure from visually-hunting predators, while
nighttime ascent into the epilimnion facilitates feeding and growth. It has been
demonstrated that Daphnia behavior may be altered by algal quantity, predator cues, and
temperature or light levels. However, little was known about how algal quality (in terms
of nutrient content) affects Daphnia vertical position. This dissertation examines the
effects of algal quality on the daytime habitat selection of Daphnia through laboratory
experiments and analysis of field data.
Laboratory studies examined the movements of Daphnia in a thermally-stratified
water column. When algal quality was high (C:P ~ 150) throughout the water column,
mature Daphnia were found in the epilimnion during the daytime, despite the presence of
a predator cue. However, when algal quality was low (C:P ~1500) throughout the water
column, mature Daphnia tended to spend the daytime in the hypolimnion. These habitat
preferences were not detected when the experiment was repeated using young juveniles
(age 0-5 days), though at age 6 days, juveniles’ behavior appeared to begin to shift
toward that of adults. As expected, body mass, fecundity, and net reproductive rate were
found to be positively correlated with increased environmental temperature, and
fecundity and net reproductive rate were positively correlated with increased food
A field study of 34 lakes revealed that habitat selection was not only food-quality
dependent, but also species-specific. D. retrocurva and D. mendotae exhibited a
preference for the upper lake layer when food quality was good throughout the water
column, but not when food quality was poor in the upper layer. D. pulicaria did not
exhibit any layer preference in either lake environment.
The results from this research provide insight into Daphnia’s ability to balance
the multiple factors associated with a heterogeneous vertical gradient in order to realize
maximum productivity, and thus maximum fitness, in the presence of visually-hunting
predators. The results further indicate that food quality is an important factor in
determining habitat selection, productivity, and ultimately fitness.
University of Minnesota Ph.D. dissertation. January 2009. Major: Water Resources Science. Advisor: Dr. Raymond M. Newman. 1 computer file (PDF); xi, 206 pages, appendices A-C.
Forman, Mary Rebecca.
Habitat selection and ecological stoichiometry: the role of seston C:P in Daphnia daytime spatial location..
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