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Stream periphyton response to phosphorus loading events is constrained by antecedent conditions
(University of Waterloo, 2025-11-06) Schneider, Natalie
Phosphorus (P) loadings to streams often occur in short duration events associated with runoff from human activities. Although it has been shown that stream periphyton can uptake and assimilate event-based P, the role of antecedent P concentrations in modulating P uptake from event-based loadings and resulting effects on periphyton structure and function is not known. To assess effects of antecedent P concentration on stream periphyton response to short-term P loading events, we completed two 26-day artificial stream experiments at the Thames River Experimental Stream Sciences (TRESS) Centre in London, Canada. Experiments consisted of exposing periphyton communities in nine artificial streams to a range of 48-hour P loading event concentrations (15 to 690 μg P/L) under low (10 μg P/L) or high (50 μg P/L) antecedent P concentrations. Periphyton was sampled one day before, one day after and 10 days after P loading events to quantify periphyton structure (ash free dry mass (AFDM), chlorophyll a (chl a), P content) and function (P uptake, benthic metabolism, cellulose decomposition, biomass growth, chl a accumulation). Under low antecedent P conditions one day after the P event, P content and P uptake had a positive linear relationship with event concentration and this was similarly seen in biomass and chl a ten days after the P event. One day after the P event in high antecedent streams, P content and P uptake showed a positive linear response with P event concentration, but this additional P in periphyton did not lead to increases in biomass and chl a. Whereas, a negative linear relationship with event concentration and P uptake was seen ten days after the P event. Measures of periphyton function (benthic metabolism and cellulose decomposition) were unaffected by P event size and regardless of the antecedent condition. These findings suggest that high antecedent P concentrations caused cellular saturation of periphyton limiting the assimilation of P from event-based P loads. Therefore, streams with high antecedent P may deliver reduced water purification benefits with regards to attenuating P transport to downstream ecosystems at risk of eutrophication. Management actions to reduce antecedent P concentrations will be needed to rehabilitate ecosystem service provision in streams chronically enriched in P.
Cross-sectional Analysis of Current Care Assessment Practices in the Retirement Home Sector in Ontario
(University of Waterloo, 2025-11-06) Nasim, Anooshah
As the population of Canada ages, some older adults often have increased multimorbidity, disabilities, and frailty. As a result, they are at an increased risk of hospitalization, accelerated functional decline, and earlier institutionalization. As they face more disability and health challenges, the lack of sufficient primary, community, and home care services to support them leads many to move into retirement homes. Once there, residents continue to experience health challenges, likely as a consequence of ongoing inadequate primary care and insufficient services geared toward their needs. Yet, addressing the unmet needs of retirement home residents at the individual and population levels is made challenging by the lack of standardized information collection. While regulatory agencies stipulate that residents undergo a health assessment, there are no specific requirements as to their nature. A better understanding of their unmet needs can potentially guide better primary care planning and help identify the level of services required to deliver better resident and system outcomes. To begin, current care assessment practices and processes surrounding these assessments must first be characterized and understood before the introduction of a new standardized instrument can be contemplated.
Manifold-Aware Regularization for Self-Supervised Representation Learning
(University of Waterloo, 2025-11-04) Sepanj, Mohammad Hadi
Self-supervised learning (SSL) has emerged as a dominant paradigm for representation learning, yet much of its recent progress has been guided by empirical heuristics rather than unifying theoretical principles. This thesis advances the understanding of SSL by framing representation learning as a problem of geometry preservation on the data manifold, where the objective is to shape embedding spaces that respect intrinsic structure while remaining discriminative for downstream tasks. We develop a suite of methods—ranging from optimal transport–regularized contrastive learning (SinSim) to kernelized variance–invariance–covariance regularization (Kernel VICReg)—that systematically move beyond the Euclidean metric paradigm toward geometry-adaptive distances and statistical dependency measures, such as maximum mean discrepancy (MMD) and Hilbert–Schmidt independence criterion (HSIC). Our contributions span both theory and practice. Theoretically, we unify contrastive and non-contrastive SSL objectives under a manifold-aware regularization framework, revealing deep connections between dependency reduction, spectral geometry, and invariance principles. We also challenge the pervasive assumption that Euclidean distance is the canonical measure for alignment, showing that embedding metrics are themselves learnable design choices whose compatibility with the manifold geometry critically affects representation quality. Practically, we validate our framework across diverse domains—including natural images and structured scientific data—demonstrating improvements in downstream generalization, robustness to distribution shift, and stability under limited augmentations. By integrating geometric priors, kernel methods, and distributional alignment into SSL, this work reframes representation learning as a principled interaction between statistical dependence control and manifold geometry. The thesis concludes by identifying open theoretical questions at the intersection of Riemannian geometry, kernel theory, and self-supervised objectives, outlining a research agenda for the next generation of geometry-aware foundation models.
Towards a Novel Optical Spectroscopy Technique Using Photon Absorption Remote Sensing
(University of Waterloo, 2025-11-04) Dhillon, Jodh
Optical spectroscopy has shown great promise in the field of biomedical research. For example, works employing traditional spectroscopy approaches have demonstrated that analyzing a sample’s optical response to incoming light can effectively differentiate between healthy and diseased tissue. However, these techniques suffer from limitations due to the fact that they typically capture signals from only a single light-matter interaction type, such as absorption, scattering or fluorescence. Therefore, many traditional methods are constrained in terms of the types of samples they can feasibly analyze, as well as, potentially, the depth of their sample characterization, as they do not focus on capturing relevant information from other interaction modalities. This work employs photon absorption remote sensing (PARS) to overcome these limitations. PARS is a novel all-optical imaging technique capable of capturing radiative and non-radiative relaxation processes following electronic photon absorption. This thesis explores the initial development of the first PARS system specifically designed and optimized for optical spectroscopy applications, aimed at studying wavelength-dependent relaxation processes to characterize a wide range of liquid samples. The first step of this work was to build a non-radiative PARS spectroscopy system capable of accurately capturing the thermal and acoustic relaxation processes that arise from different ultra-violet (UV) excitation wavelengths. These signals were processed and used to construct a non-radiative PARS absorption spectrum for each sample of interest. These spectra were benchmarked against the absorption data collected from a NanoDrop spectrophotometer, which served as the ground truth in this work. This study revealed that for certain samples, such as eumelanin, which is highly absorbent to UV light and relaxes almost all absorbed energy non-radiatively, the non-radiative PARS spectroscopy system is capable of generating highly accurate absorption spectra. However, this system did not generate as close to ground truth spectra for samples that do not have as strong UV absorbing tendencies and are not as non-radiative in nature. The second step of this work was to integrate a radiative relaxation arm into the developed non-radiative PARS spectroscopy system. This pathway was configured to collect fluorescence emission spectra, which represent radiative sample relaxation, simultaneously with the collected non-radiative data. Radiative PARS absorption spectra were generated for each sample. In this way, the developed PARS system combines absorption (monitoring both relaxation pathways) and fluorescence emission spectroscopy onto a single bench-top system. The radiative PARS absorption spectra were compared to the ground truth, which revealed that molecules that are highly fluorescent in nature are more appropriately studied through the radiative relaxation arm than the non-radiative pathway. Total absorption spectra, which combine the non-radiative and radiative absorption data, were also generated, and it was determined that the absorption profiles of certain samples, such as NADH, are best studied using this approach. The final step of this work was to use the collected total absorption and fluorescence emission data from the PARS spectroscopy system to identify the composition of different mixtures of craft red and blue ink samples. Traditional linear and generalized bilinear models were employed to perform this unmixing and the results from this study indicate that the combination of the absorption and fluorescence data collected on this system allows for a more accurate identification of a mixture’s components than either data source individually. This suggests that the PARS spectroscopy system provides an increased level of detail in sample characterization compared single-modality spectroscopy systems. Ultimately, this research lays the groundwork for the development of a PARS spectroscopy system capable of being deployed in clinical settings to study samples and help inform diagnoses. This work demonstrates the feasibility of leveraging PARS for optical spectroscopy and presents a system design and framework that can be further iterated upon to enhance performance and enable a robust characterization of relevant and complex biological samples.
Polynomial Controllers for Optimal Trajectory Matching with Stability Guarantees
(University of Waterloo, 2025-11-04) Kitaev, Alexander
We formulate a trajectory matching problem in which a set of reference trajectories for a plant is given, and a control law that causes the plant’s trajectories to be as close as possible to the reference trajectories is desired. These trajectories might be generated by an implicit controller such as a model predictive control (MPC) algorithm or manually chosen by a user. This thesis presents a nonconvex optimization approach for solving the trajectory matching problem that generates explicit polynomial controllers. The value of this approach is that the explicit control laws it generates are simpler to implement, and can be used for stability analysis. Additionally, the method presented in this thesis guarantees local stability of the generated controller by ensuring local contractivity towards the generated trajectories. This thesis presents several theoretical results that justify the method described here. Firstly, a proof that the local contractivity constraints used to ensure local stability can be expressed as a set of matrix inequalities is presented, which turns an infinite set of constraints into a finite one. Secondly, a theorem that describes how symmetries in the trajectory matching problem correspond to symmetries in its solution is presented and proven, which enables a reduction in the control design problem size and resulting solution. Finally, this thesis demonstrates the method it describes on two example problems motivated by real-world applications. The first of these is stabilization and disturbance recovery for a single-machine infinite-bus (SMIB) power system, and the second is a lane change manoeuvre for Dubin’s vehicle, a simple vehicle model. In each case, the reference trajectories are generated by MPC.