Date of Award

8-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Forestry and Environmental Conservation

Committee Member

Dr. John H. Rodgers, Jr., Committee Chair

Committee Member

Dr. Burton C. Suedel

Committee Member

Dr. George M. Huddleston

Committee Member

Dr. Greg K. Yarrow

Committee Member

Dr. William C. Bridges

Abstract

Cyanobacteria blooms that produce endotoxins (e.g. microcystins [MCs]) are increasing spatially and temporally in critical freshwater resources worldwide. Due to the potential for human-health and ecological risks from exposures to MCs, designated uses for freshwater resources are often inhibited by MC-producing cyanobacteria blooms. As evidence builds for the risks from MC exposures, as well as financial losses resulting from colonization of MC-producing cyanobacteria in freshwater resources, effective and efficient risk management strategies are urgently needed. The overall goal of this dissertation was to contribute information that supports progress in risk intervention for MC-producing cyanobacteria as well as MCs. To support this goal, the first study involved development of a decision support system for risk management of MC-producing cyanobacteria and MCs. The decision support system included strategic literature reviews with data acceptability criteria, followed by assembly and organization of scientific information necessary to make defensible risk management decisions. Subsequent experiments in this dissertation were conducted to contribute data related to specific risk management approaches for MC-producing cyanobacteria (copper-based algaecide exposures) and MCs (solar fixed-film photocatalysis). For pulse exposures of copper-based algaecides in aquatic systems, cell density is a site specific exposure modifying factor influencing the mass of copper sorbed by the cyanobacteria population and consequent responses. To test this hypothesis, a cyanobacterium, Microcystis aeruginosa, was exposed to a copper-based algaecide for a range of cell densities to model the density-dependence of responses in terms of microcystin-LR (MC-LR) release in a series of laboratory toxicity experiments. While copper exposure concentrations eliciting comparable extents of MC-LR release ranged an order of magnitude (24-h EC50s 0.03-0.3 mg Cu/L) among cell densities of 106 through 107 cells/mL, copper doses (mg Cu/mg algae) were similar (24-h EC50s 0.005-0.006 mg Cu/mg algae). Knowledge of exposure-response relationships for specific cell densities could refine predictions for in situ exposures and responses, and in turn, decrease the likelihood of amending unnecessary copper concentrations to aquatic systems. In another experiment, hypotheses were tested regarding the fate of MCs following exposures of MC-producing cyanobacteria to a copper-based algaecide. The overall objective was to measure the influence of declining dissolved oxygen (DO) levels (following copper-based algaecide exposures to a range of cyanobacteria cell densities) on resident bacterial assemblages and MC-LR degradation, in mesocosm experiments conducted in a pond in Anderson, SC. DO concentrations had the greatest rate of decline in the highest cell density treatment, followed by medium and low cell densities. MC-LR degradation had half-lives of 1 to 1.9-d among cell densities, with the shortest half-life occuring for the lowest cell density. The relationship between cyanobacteria densities and MC-LR half-lives demonstrated the benefits of mitigating cyanobacteria in early growth stages to minimize MC exposures. In the final experiment, questions were asked regarding an advanced oxidation process that may be useful for altering MC exposures in drinking waters. Following sand filtration of solutions containing a range of cellular: aqueous MC ratios, rates of fixed-film solar photocatalysis using TiO2 were measured for aqueous MCs. Rates and half-lives were calculated as a function of both time and cumulative UV insolation, to contribute data that would be transferable among seasons and latitudes. In terms of time, half-lives for photocatalysis were approximately 111 to 138-min, and in terms of cumulative UV insolation, half-lives ranged from 0.35 to 0.38 MJ/m2. Fixed-film solar photocatalysis achieved half-lives that were less than aerobic and anaerobic biodegradation half-lives reported in peer-reviewed literature (e.g. 1 to 14-d) and could be useful for mitigating risks from MC exposures in drinking water, especially in regions that require low-energy, low-maintenance water treatment methods. This dissertation provided the first known example of a decision support system for intervening in exposures of MC-producing cyanobacteria and MCs in critical freshwater resources. Subsequent experiments contributed data for specific risk management approaches that support early intervention efforts at the source of MC exposures (i.e. cyanobacteria) and rapid transformation of MCs in drinking water using fixed-film solar photocatalysis.

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