published_or_final_version / Ecology and Biodiversity / Doctoral / Doctor of Philosophy

published_or_final_version / Environmental Management / Master / Master of Science in Environmental Management

The transfer of prey resources between ecosystems can have dramatic consequences for both recipient and donor systems by altering food web stability and the likelihood of trophic effects cascading across the ecosystem boundary. Landscape-scale factors influence the importance, direction and magnitude of energy flows, but may also alter the ability of consumer organisms to respond to spatio-temporal changes in allochthonous prey availability. Here, I used flood and drying disturbance gradients to investigate interactions between these two processes on populations of a riparian fishing spider Dolomedes aquaticus (Pisauridae). The abundance of aquatic insects with a winged adult stage, a major component of the diet of D. aquaticus, was markedly higher at less flood-prone rivers and declined with increasing flood disturbance. It was expected that spider populations would be largest at these stable rivers where the aquatic prey abundance was highest. However, a habitat (loose, unembedded riverbank rocks) manipulation revealed that the lack of scouring floods at these sites led to habitat-limited populations, preventing response to the increased prey resource. In fact a peak shaped relationship of spider biomass and abundance was found, with the largest spider populations at intermediately disturbed rivers. In addition, patchy habitat availability was the most likely cause of the small scale (4 m2) aggregation of spiders seen at the most stable and disturbed rivers. These patterns were also associated with strong interactions between the spiders. Stable isotope analysis of field collected spiders and an experimental manipulation of spider densities and food availability indicated that cannibalism rates were likely to be significantly higher at stable and disturbed rivers than those intermediate on the disturbance gradient. Differences in D. aquaticus population size structure and life history traits across the flood disturbance gradient were driven by interactions between resource availability, environmental stability and cannibalism rates. To separate the effects of habitat availability and aquatic prey abundance I used drying rivers, as the amount of aquatic insect prey alters as the water recedes. Desiccation mortality and low aquatic prey biomass most likely caused the spiders' spatial distribution and size class structure to alter in drying river reaches, potentially also leading to differences in cannibalism rates. Overall, cross-ecosystem transfers of prey had large impacts on the distribution, cannibalism rates and life history traits of D. aquaticus but their effects were modified by the nature of the ecosystem boundary. Thus river flow regime controlled the magnitude of the subsidy and its use by a consumer. Hence, cross-ecosystem subsidies will not always lead to larger consumer populations and consumer responses will depend on interactions between large-scale processes.

Dynamical systems theory and bifurcation are used to analyze some simple models of nutrient limited phytoplankton growth. The models are restricted to batch culture type conditions allowing the use of a mass balance constraint. Two popular models from the literature, the Michaelis-Menton-Monod or M³ model, and the Droop internal nutrient model are analyzed and found to yield unreasonable predictions for certain ambient environmental conditions. The M³ model predicts that the population size becomes unbounded at equilibrium for certain values of the parameters. The Droop model predicts that the amount of nutrient left over during a nutrient uptake experiment would be very small, regardless of how large the initial external nutrient concentration is. Numerical comparisons of data with the predictions from both models demonstrate that the conditions for unreasonable behavior could occur both in cultures and in natural aquatic ecosystems. In the predicted nutrient concentration at uptake equilibrium is several orders of magnitude off. Two specific enzyme mechanisms for nutrient transport are proposed as alternatives to current models. The models differ in the assumptions made about how the backflow reaction with the enzymes responsible for transport proceeds. A nutrient uptake equation for each model is derived directly from the enzyme kinetics, while the equation for growth in population size is taken from the Droop model. The dynamics of both models are analyzed, treating the nutrient uptake equations with the singular perturbation assumption. The simple model predicts that the external nutrient concentration at uptake equilibrium should be a constant percentage of the internal concentration, while in the inhibition uptake model, the population size could exhibit relaxation type oscillations during the batch culture steady state. Qualitative evidence supporting both models is discussed. Applications of these models to water quality simulation and implications for theoretical ecology are discussed.

Robust Vallisneria americana was cultured for restoration purposes. Preliminary studies, with various iron treatments, were conducted to ascertain the amount of phosphorous release into the water column from sediments. There was a significant difference in the amount of phosphorous released if commercial sediment was used with a low iron amendment or without an iron amendment. The second study consisted of planting V. americana on two different sediment types while supplying half of the plants with additional CO2. Plants grown on pond sediment with additional CO2 had significantly more biomass. In the third study all plants were grown on pond sediment, and half were treated with CO2. All plants that were treated with additional CO2 had significantly more biomass than those that were aerated.

Pharmaceutical contamination is increasing in the environment and the consequences this will bring are of growing concern. The highest contamination of pharmaceuticals can be found in aquatic ecosystems and the organisms of these systems are therefore of utmost importance to research in order to understand the ecological consequences of pharmaceutical contamination. This report will reveal the effect contamination can have on an important apex predator often found in temperate aquatic systems, the Northern pike (Esox lucius) when exposed to the psychiatric pharmaceutical Oxazepam. The predatory performance of pike was studied before and after Oxazepam exposure by monitoring how fast each pike caught three prey of roach (Rutilus rutilus), as well as observing the amount of failed predation attempts when hunting the roach. The exposed pike displayed more failed predation attempts after exposure as opposed to the control group. Furthermore it took the exposed pike longer to catch all three roach after exposure while the mean for the control group decreased. Hence, Oxazepam exposure seem to have an effect on predation performance of pike but no definite conclusion could be drawn about to which extent this affects the foraging success and thereby the survival of the pike considering the complex nature of aquatic systems. More studies are therefore needed in order to determine the full effect pharmaceutical contamination can have on complex aquatic ecosystems and more specifically on an apex predator.

The purpose of the present study was to determine the relative importance of aquatic and terrestrial beetles as possible vehicles in the passive overland dispersal of viable microorganisms.

Laboratory studies conducted at 5, 15, 25, and 35 C measured changes in gilling rates and oxygen consumption of green sunfish in response to exposure to bentonite clay suspensions. The tests indicate that gilling rates are not affected by bentonite clay suspensions below 2125 FTU at 5 C, 1012 FTU at 15 C, and 898 FTU at 25 C. At turbidity levels exceeding 1012 FTU at 15 C and 898 at 25 C, gilling rates increased 50-70%. Tests were inconclusive at 35 C. Oxygen consumption rates were found to be unaffected by turbid suspensions below 3500 FTU at all four temperatures. Evidence suggests that increased gilling rates under highly turbid conditions are a means of compensating for reduced respiratory efficiency and a strategy for maintaining a constant oxygen uptake. Evidence indicates that the cost of increased gilling rates is probably met by a reduction in activity.

The tidal-fresh James River experiences recurring blooms of toxin-producing cyanobacteria, including Microcystis aeruginosa. However, cyanobacteria cell densities in the James are relatively low. Our purpose was to identify key factors suppressing cyanobacteria growth in the face of eutrophication. A mesocosm experiment was designed to test the effects of light, mixing and grazing on cyanobacteria abundance, with nutrients held constant. We predicted that toxic cyanobacteria would be most abundant under stagnant conditions, with enhanced light, with no bivalve grazers present. Abundances of indicator gene copies and phytoplankton counts supported this hypothesis. However, chlorophyll-a, phycocyanin, and the toxin microcystin were all found to be most abundant under mixed conditions with ambient light. Statistically, light and mixing were important in controlling toxic cyanobacteria abundance, with little to no effect observed for bivalve grazers. Our findings suggest that toxin production may be regulated by factors independent from those driving algal growth and cyanobacteria abundance.

Food supply has been suggested as the main determinant of reproductive success in birds. Riparian species can take advantage of seasonal pulses of both terrestrial and aquatic prey, though aquatic resources are often overlooked in studies of diet and reproductive ecology. This study investigates the flux of both aquatic (mayfly) and terrestrial (caterpillar) prey resources and nestling diet of the Prothonotary Warbler throughout the breeding season in two eastern Virginia sites. One site had significantly higher aquatic prey (mayfly) availability. Nestling diet was generally reflective of prey availability, and nestlings grew faster at the site with high aquatic prey availability. At the site with low aquatic prey availability, nestling growth rates and condition were positively correlated with the amount of aquatic prey in the diet. Our results provide evidence that aquatic subsidies are an important resource for nestlings, and are crucial to understanding the breeding ecology of riparian species.