Degree Name

Master of Science (MS)

Semester of Degree Completion


Thesis Director

Charles L. Pederson


Reservoir zonation occurs longitudinally as water enters from a stream into an impoundment. Inflow areas (or riverine zones) are typified by high flow rate and shallow depth whereas areas near the dam (lacustrine zones) characteristically have decreased flow and greater depth. In a typical reservoir, abiotic and biotic variables change somewhat predictably along a continuum from the riverine, through a transitional and into a lacustrine zone which can have a significant affect on the biota. Daphnia lumholtzi, a cladoceran which exhibits extreme cyclomorphosis, is an exotic species introduced to North America within the past twenty-five years. I examined the morphology of D. lumholtzi in three locations within Lake Taylorville anticipating to reflect unique riverine, transitional, and lacustrine features. Head, body, and tail lengths were determined from field collections made during 1993, 1994, 1999, and 2000.

As a result of a large watershed to surface area and high flow rates, the sites sampled did not vary in a manner predicted by reservoir limnological theory. Only riverine and transitional zones were found to exist in Lake Taylorville. Nonetheless, environmental variation that occurred among sample sites was found to significantly affect morphology of D. lumholtzi. Dissolved solids, Secchi depth, temperature, dissolved oxygen, chlorophyll a, conductivity, dissolved phosphate, and suspended phosphate either collectively or individually appear to influence spine formation. It is possible that phosphate acts as a limiting nutrient increasing the algal standing crop when available in later summer months when temperatures increase. Oxygen levels would subsequently increase as a result of elevated primary productivity while dissolved solids would decrease due to uptake by plankton. Reservoir clarity as indexed by Secchi depth increases throughout the summer with decreased precipitation and sediment loading from the watershed. Increased water clarity may increase predator search efficiency causing lesser spined morphs to be consumed. It is likely that D. lumholtzi could be responding to a proximal cue such as temperature as well as light to reduce the effects of predation. Food availability may also be playing a role in spine formation by providing a resource to produce spines.