| dc.description.abstract | Humans are impacting the environment at an unprecedented scale, with anthropogenic activities altering environmental processes and cycles planet-wide. Centuries of gold mining and coal burning has more than tripled the amount of inorganic mercury in the global atmospheric pool. As this mercury is deposited across the landscape it is washed into aquatic ecosystems. Anaerobic bacteria in aquatic sediments convert the mercury into methyl mercury which readily assimilates into food webs. Methyl mercury biomagnifies in food webs to high concentrations that pose a threat to both humans and wildlife. The burning of coal, along with other fossil fuels, has also altered the global climate, which further impacts aquatic ecosystems already stressed by mercury contamination. Anthropogenic global climate change (GCC) has warmed the planet 0.85℃ since 1880 and is poised to raise average global temperatures another 0.3–4.8℃ by the end of the century. Freshwater systems will be especially hard hit by these changes due to the limited ability of many organisms to relocate and human demands on limited freshwater resources.
North American rivers are a global biodiversity hotspot for freshwater mussels. These long-lived invertebrates are ecosystem engineers that perform critical ecosystem function680s in many rivers, and they are highly imperiled due to multiple factors including land use change, habitat fragmentation, and pollution. In the southeastern U.S., rivers are experiencing more frequent and more severe droughts as a result of GCC, which have led to declines in mussel populations. These rivers are also experiencing elevated levels of mercury contamination. These two critical, emerging stressors, GCC and mercury contamination, may interact to impact freshwater systems beyond the effects of each stressor independently. My dissertation investigates the dynamics of mercury contamination alone and mercury contamination plus GCC on freshwater mussels and the ecosystem functions they provide. I asked three questions: 1) What is the extent of mercury contamination in a globally significant river in the southern U.S.? 2) How do freshwater mussels affect the movement of mercury in the ecosystem? and 3) Does toxic stress resulting from the combination of mercury contamination and GCC thermal stress interact to affect freshwater mussels and the ecosystem processes they influence?
To address the first question, I measured the mercury content of multiple species of fish and invertebrates collected from the Kiamichi River, Oklahoma, a well-studied river with high mussel and fish biodiversity. I found that concentrations of mercury in the tissue of multiple fish species in the river exceeded both the Oklahoma and federal (EPA) consumption advisory limits and that the most abundant invertebrate taxa also had the highest mercury concentrations. To address the second question, I conducted a mesocosm experiment examining the effects of mussels on mercury concentrations of benthic consumers. I collected mussels and mercury-contaminated sediments from the Kiamichi River. I added sediments to 32 recirculating mesocosms and constructed mussel communities composed of two common species across four, natural mussel densities, with each treatment replicated 8 times. Snails were added to the mesocosms to serve as primary benthic consumers. After allowing the experiment to run for four months, I measured the biomass and mercury concentration of the snail consumers. Snails had both higher overall mercury concentrations and higher mercury burdens (concentration of mercury per g snail dry weight) in the presence of higher densities of mussels. To address the third question, I performed a combination of laboratory and mesocosm experiments to examine how thermal stress from GCC combined with toxic mercury stress affects both individual mussel physiological functions and mussel community-influenced ecosystem functions. In the laboratory, I measured the respiration rates and filtering rates of two common mussel species exposed to increased temperature, increased mercury, and these stressors combined. I observed reduced respiration rates and filtration rates in both species in response to these stressors, and increased mortality in one of the species. I conducted a mesocosm experiment where I constructed three-species mussel communities, exposed them to two temperature treatments and two mercury treatments in a factorial design, and measured their influence on nutrient cycling. Mussel mortality was higher in the combined stressor treatments. Ammonia (NH3) concentrations spiked in the double stress treatments and then declined while total phosphorus (TP) showed the opposite trend. Combined we observed a declining NH3:TP in double stress treatments.
My findings have relevance both locally and broadly. Elevated concentrations of mercury in the Kiamichi River indicate that reservoirs in Oklahoma are not the only waterbodies at risk in the state for high levels of mercury even though they currently are the only ones with fish consumption advisories. Additionally, the fact that concentrations were the same throughout the river suggest that methyl mercury is being produced within the river rather than being imported from off-channel impoundments. My results also indicate that mussel-influenced ecosystems, like the Kiamichi River and similar rivers across North America, are more sensitive to mercury contamination than previously thought. These systems are at risk for both higher concentrations of mercury in consumers as well as larger burdens of mercury present in the systems. Finally, I show that two anthropogenic stressors occurring at a global scale, toxic mercury stress and thermal stress, interact negatively to affect freshwater systems from the organismal to ecosystem level. Unfortunately, both stressors are predicted to worsen over the coming decades and, given the global scope of mercury and GCC, their negative impacts on organisms and ecosystems are an additional emerging threat to imperiled freshwater systems around the world. | en_US |
