Date of Award

May 2020

Degree Type

Thesis

Degree Name

Master of Science

Department

Freshwater Sciences and Technology

First Advisor

Harvey A Bootsma

Committee Members

Joseph H Aldstadt, Rebecca D Klaper

Keywords

biomagnification, food web, Lake Michigan, PCBs, stable isotopes, trophic magnification

Abstract

Polychlorinated biphenyls (PCBs) are a group of persistent organic pollutants known to contribute to several adverse health conditions in humans including cancers and a suite of liver diseases. While there exist 209 unique PCB congeners, the World Health Organization has identified 12 that pose the greatest health risk to humans due to these congeners’ functional similarity to dioxins, another notoriously toxic class of contaminants. Along with methylmercury, PCBs are the primary drivers behind fish consumption advisories in the Great Lakes. These guidelines are informed primarily by surveys of contaminants in freshwater biota. However, the proliferation of invasive species, such as dreissenid mussels and round gobies, has dramatically restructured the food web and potentially the flow of contaminants in Lake Michigan.

This research examined the relationship between food web structure and PCB concentrations in Lake Michigan biota. A second objective was to determine how invasive species have shifted trophic structure and what implications these alterations might have on contaminant transfer. Aquatic biota were sampled throughout the southern basin of Lake Michigan and analyzed for lipid content, WHO PCB congeners, and stable isotopes of nitrogen and carbon. Nitrogen isotopes were used to calculate trophic level, while carbon isotopes indicated the primary energy source of an organism. PCBs were extracted from lyophilized tissue using a microwave-assisted extraction technique and a hexane:acetone extraction solution. Crude extracts underwent silica and Florisil clean-up protocols to remove interferences prior to gas chromatography ¬– mass spectrometric determination.

Stable isotope analysis revealed substantial shifts in trophic structure between 2002-2003 and 2019. Lake trout have significantly dropped in mean trophic level while slimy sculpin are relying more heavily on nearshore carbon relative to the 2002-2003 results. The PCB congeners observed in the highest concentrations in Lake Michigan biota in 2002-2003 closely paralleled those manufactured most commonly. Lipid content and PCB concentration showed a strong significant relationship. Following lipid correction, WHO congeners were shown to increase by a factor of 2.8 per trophic level. In addition, specific congeners appeared to magnify at different rates. Multiple regression analyses revealed that trophic level and lipid content were the best predictors for concentration of total WHO congeners as well as for congeners 105 and 118 specifically. Lake trout, deepwater sculpin, slimy sculpin, bloater chub, and lake whitefish all displayed elevated concentrations of WHO congeners relative to other species. Lake trout and yellow perch displayed an exponential relationship between total length and concentration of WHO congeners.

Lake Michigan’s PCB trophic magnification effect in 2002-2003 was within range of that observed in other ecological systems. Possible explanations for the disparities between lakes are seasonal sampling biases, differences in primary productivity, and differential rates of PCB metabolism by fauna. For lake trout and yellow perch, changes in feeding behavior as they grow in size results in an increase of trophic level, thereby increasing PCB loads for larger members of these species. The trophic magnification factor and species-specific PCB results reported in this study enhance the scientific understanding of contaminant transfer in freshwater systems.

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