| Traditionally, trophic cascades (resulting from predation pressure) or bottom-up control (resulting from nutrient limitation) have been proposed as the primary structuring forces regulating food web structure. More recent research indicates these forces function together when predators excrete large quantities of nitrogen (N) and phosphorus (P) after eating. This combined effect may be especially important in controlling food web structure if the predator makes daily littoral-pelagic migrations, thus translocating nutrients. The purpose of this dissertation was to determine if fish could translocate biologically meaningful amounts of N and P as a result of diurnal changes in habitat use and diet. Field research with paired trap nets found fishes in Sandy Lake, Ohio have strong diurnal activity patterns. There was no evidence of a diurnal littoral-pelagic migration. Laboratory studies found bluegills given a choice between open water and artificial macrophytes used open water significantly more when it had a higher foraging return and significantly less when it had higher predation risk. Under all foraging and predation conditions, bluegills used open water significantly more at night than during the day, indicating the potential for diurnal littoral-pelagic migration behavior. Laboratory studies found bluegills excrete significantly different amounts of N and P after eating different prey types (odonates, chironomids, or Daphnia). A modeling experiment, which combined the results of the diurnal habitat use information with the diet-specific excretion rates, found the potential exists for bluegills to translocate large quantities of N and P between littoral and pelagic habitats, which could have a strong effect on the food web structure of each habitat. |
