Habitat selection and ecological stoichiometry: the role of seston C:P in Daphnia daytime spatial location.

Abstract

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 quality. 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.