Graduate Theses & Dissertations

Effects of Silver Nanoparticles on Lower Trophic Levels in Aquatic Ecosystems
Due to their effective antibacterial and antifungal properties, silver nanoparticles (AgNPs) have quickly become the most commonly used nanomaterial, with applications in industry, medicine and consumer products. This increased use of AgNPs over the past decade will inevitably result in an elevated release of nanoparticles into the environment, highlighting the importance of assessing the environmental impacts of these nanomaterials on aquatic ecosystems. Although numerous laboratory studies have already reported on the negative effects of AgNPs to freshwater organisms, only a handful of studies have investigated the impacts of environmentally relevant levels of AgNPs on whole communities under natural conditions. This thesis examines the effects of chronic AgNP exposure on natural freshwater littoral microcrustacean, benthic macroinvertebrate and pelagic zooplankton communities. To assess the responses of these communities to AgNPs, I focused on a solely field-based approach, combining a six-week mesocosm study with a three-year whole lake experiment at the IISD – Experimental Lakes Area (Ontario, Canada). Our mesocosm study tested the effects of AgNP concentration (low, medium and high dose), surface coating (citrate- and polyvinylpyrrolidone [PVP]-coated AgNPs), and type of exposure (chronic and pulsed addition) on benthic macroinvertebrates in fine and stony sediments. Relative abundances of metal-tolerant Chironomidae in fine sediments were highest in high dose PVP-AgNP treatments; however, no negative effects of AgNP exposure were seen on biodiversity metrics or overall community structure throughout the study. I observed similar results within the whole lake study that incorporated a long-term addition of low levels of AgNPs to an experimental lake. Mixed-effects models and multivariate methods revealed a decline in all species of the littoral microcrustacean family Chydoridae in the final year of the study within our experimental lake, suggesting that this taxon may be sensitive to AgNP exposure; however, these effects were fairly subtle and were not reflected in the overall composition of littoral communities. No other negative effects of AgNPs were observed on the pelagic zooplankton or benthic macroinvertebrate communities. My results demonstrate that environmentally relevant levels of AgNPs have little impact on natural freshwater microcrustacean and benthic macroinvertebrate communities. Instead, biodiversity metrics and community structure are primarily influenced by seasonal dynamics and nutrient concentrations across both lakes. This thesis highlights the importance of incorporating environmental conditions and the natural variability of communities when examining the potential risks posed by the release of AgNPs into the environment, as simplistic laboratory bioassays may not provide an adequate assessment of the long-term impacts of AgNPs on freshwater systems. Author Keywords: Benthic macroinvertebrates, IISD - Experimental Lakes Area, Littoral microcrustaceans, Silver nanoparticles, Whole lake experiment, Zooplankton
Fate and Effects of Silver Nanoparticle Addition in a Lake Ecosystem
The potential release of nanoparticles into aquatic environments is raising global concerns. As antimicrobials, silver nanoparticles (AgNPs) are among the most prominent form in use. Despite this, their fate, long-term toxicity, and ecological relevance have yet to be investigated largely under natural settings with seasonality and environmental complexity. To better understand the environmental significance, we released AgNPs into Lake 222 at the Experimental Lakes Area over two years. AgNPs remained suspended in the water column and were detected throughout the lake and in the lower food web. Total Ag concentrations ranged from below 0.07 to 18.9 μg L-1 in lake water, and were highly dynamic seasonally both in the epilimnion and hypolimnion depending on the physical, chemical and biological conditions of the lake. Approximately 60% of the measured Ag mass in October was present in the sediment in 2014 and 50% in 2015 demonstrating relatively high sedimentation and removal from the water column. During winter months, Ag was largely absent in the water column under the ice. After ice melt and before summer stratification, Ag concentrations increased in the lake suggesting AgNPs may not be tightly bound to the sediment and are able re-enter the water column during spring mixing events. Despite temporal variation, total Ag was highly synchronous across spatial locations for both years, indicating rapid dispersal upon lake entry. When investigating AgNP sizes using spICPMS, size distributions were similar across spatial locations, with the 40-60 nm size class constituting approximately 60% of all particles identified. Large aggregates (>100 nm) and dissolved Ag were infrequently detected within the lake. Ag accumulated in the lower food web ranging from 0.27-16.82 μg Ag mg C-1 in the bacterioplankton and 0.17-6.45 μg Ag mg C-1 in algae (particulate fraction). Partial least squares models revealed the highest predictors of Ag accumulation were dissolved nutrients including DOC, TDN, TDP in bacterioplankton. Major predictors for particulate Ag included temperature, dissolved oxygen, and sampling date. The diversity of predictors among biological compartments emphasizes the importance of understanding the role of environmental complexity within the lower food web. Despite Ag accumulation we did not detect strong negative effects on the lake food web. An increase in particulate and bacterioplankton chlorophyll-a occurred after addition in contrast to reference lakes, which may indicate a hormetic response to low dose AgNP concentrations. Our findings provide the first whole-lake perspective regarding Ag fate and toxicity, suggesting small scale experiments may overestimate environmental responses. Author Keywords: Ecotoxicity, Fate, Lower food web, Silver Nanoparticles, Whole-lake addition
effects of parasitism on consumer-driven nutrient recycling
Daphnia are keystone consumers in many pelagic ecosystems because of their central role in nutrient cycling. Daphnia are also frequently infected, and the parasites causing these infections may rival their hosts in their ability to regulate ecosystem processes. Therefore, parasitic exploitation of Daphnia may alter nutrient cycling in pelagic systems. This thesis integrates existing knowledge regarding the exploitation of Daphnia magna by 2 endoparasites to predict parasite-induced changes in the nutrient cycling of infected hosts and ecosystems. In chapter 1, I I contextualizing the integration of these themes by reviewing the development of the fields of elemental stoichiometry and parasitology. In chapter 2, we show how the bacterial parasite, Pasteuria ramosa, increased the nitrogen (N) and phosphorus (P) release rates of D. magna fed P-poor diets. We used a mass-balance nutrient release model to show that parasite-induced changes in host nutrient accumulation rates and diet-specific changes in host ingestion rates were responsible for the accelerated nutrient release rates that we observed. In chapter 3, we extended our examination of the nutrient mass balance of infected D. magna to include another parasite, the microsporidian H. tvaerminnensis. We found differences in the effects of these two parasites on host nutrient use as well as support for the hypothesis that parasite-induced changes in Daphnia N release are caused by the effects of infection on Daphnia fecundity. In chapter 4, we examined the relationship between P concentrations and the presence and prevalence of H. tvaerminnensis in rock pools along the Baltic Sea. We found that particulate P concentrations were negatively associated with the prevalence of this parasite, a result that is consistent with the increase in P sequestration of H. tvaerminnensis-infected Daphnia that we observed in chapter 3. I discuss the potential implications of the work presented in chapters 2-4 for other parasite-host systems and ecosystems in chapter 5. Overall, the research presented here suggests that parasite-induced changes in host nutrient use may affect the availability of nutrients in the surrounding environment, and the magnitude of this effect may be linked to parasite-induced reductions in fecundity for many invertebrate hosts. Author Keywords: consumer, ingestion rates, mass-balance, nutrient-recycling, parasitism, phosphorus

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