Xenopoulos, Marguerite A.

Although stormwater management ponds (SWMPs) are frequently used to mitigate flooding in urban areas, we still do not fully understand how these systems impact water quality in a watershed. Currently, most research focuses on the effectiveness of SWMPs to retain nutrients during high flows, even though there is potential for internal nutrient releases to occur in these systems during low flows. To investigate if SWMPs act as nutrient sources or sinks during low flow conditions, we analyzed how sewershed characteristics, pond properties, and hydrological and limnological factors influenced nutrient dynamics and stoichiometry in 10 SWMPs. Our study ponds were located in Peterborough, Whitby, and Richmond Hill, which are urbanized municipalities in southern Ontario, Canada. During October 2010 to 2011, we took monthly measurements of various carbon (C), nitrogen (N), and phosphorus (P) forms. We collected samples in the inlets, permanent pools, and outlets to determine any changes in concentrations, loads, and stoichiometric ratios into and out of the ponds. At the time of sampling, we also measured a variety of hydrological and limnological parameters. Our findings indicate that more urbanized sewersheds with higher drainage densities tend to have higher inflowing particulate and dissolved nutrient loads. In addition, we found that pond properties such as depth, length-to-width ratio, volume, and age differentially influence the retention of particulate and dissolved C, N, and P forms. Influential hydrological and limnological factors were antecedent moisture conditions, season, and thermal stratification. We found higher particulate P concentrations near the sediments when the catchments were drier and the ponds were ice-free and stratified. As well, we found higher outflowing stoichiometric ratios for DOC:TDN and DOC:TDP. This indicates an enrichment of C compared to N and P and suggests biogeochemical processes may be occurring in SWMPs. Overall, our results demonstrate that SWMPs are complex aquatic systems, and we need to consider biogeochemical processes in our design and maintenance activities, so that the effectiveness of SWMPs is not compromised during low flow conditions as a result of internal nutrient releases.

Author Keywords: Carbon, Nitrogen, Phosphorus, Urban biogeochemical cycling, Urban stormwater pond

Animals are essential to freshwater biogeochemistry and productivity. Through their excretion, aquatic consumers release bioavailable nutrients and carbon that can vary with animal taxonomic rank, trophic position, and abiotic factors such as light and nutrient supply. In fresh waters, light and nutrient supply is often modulated by dissolved organic matter (DOM), a "murky" component in the water that gives it a brown color and that may indirectly affect animal nutrient and carbon excretion. Additionally, contaminants can impact animal physiology, altering metabolism and inducing stress, further affecting nutrient and contaminant excretion. The size and structure of the ecosystem, including community composition and biomass, can also impact the contribution of aquatic animals to the elemental pool. To understand these dynamics, I examined animal‐mediated elemental cycling in freshwater ecosystems across gradients of DOM concentration and composition and under contaminant exposure. I tested fish and invertebrate nitrogen, phosphorus, and DOM excretion across trophic positions during two sampling events in Lake Erie and in naturally DOM-variable streams and lakes. I also investigated the effects of chronic exposure to silver nanoparticles (AgNP) under environmentally relevant conditions on fish nutrient and silver (Ag) release. I found that aquatic animals can be a substantial nutrient contributor to the nutrient pool, particularly when their population biomass is high and ambient nutrient concentrations are low. I also detected nonlinear relationships between animal nutrient excretion and DOM characteristics that varied with taxonomic rank and trophic position and that dampened at larger ecological scales. Importantly, I identified several fish DOM excretion signatures that differed relative to ambient DOM and reported the first fish Ag excretion rates under AgNPs exposure. My results underscore the context-dependency and variability inherent in animal-mediated elemental cycling, highlighting the critical role of animals as both modifiers and conduits of nutrients, DOM, and contaminants in aquatic ecosystems.

Author Keywords: carbon, consumer-nutrient driven dynamics, ecological stoichiometry, nitrogen, phosphorus, silver nanoparticles

Dissolved organic matter (DOM) plays a vital role in lake ecosystems, yet its temporal and spatial variations in lakes remain poorly understood. This study investigates DOM dynamics in boreal lakes during the warm season and the Laurentian Great Lakes ice during winter. Sampling 10 boreal lakes in early and late June 2022, revealed subtle changes in DOM in the epilimnion and hypolimnion related to factors such as water residence time and stratification strength. In ice from the Great Lakes, lower dissolved organic carbon concentrations and a higher proportion of protein-like DOM were found compared to water, which mostly contained terrestrial-like DOM. Ice DOM composition varied with factors such as ice thickness, water nutrients, and DOM concentration in ice and water. In addition, we found that the potential release of protein-like DOM from ice to the water during spring melt is considerable and may fuel heterotrophic microbial metabolism.

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