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Abstract

Stream health refers to the ability of a stream to maintain ecological integrity and support human values, and is an important concept guiding environmental assessment. Dissolved organic matter (DOM) quantity and composition, along with ecosystem metabolism parameters, are valuable indicators of stream health given their ability to represent the interdependence of ecosystem structure and function, and their sensitivity to disturbance. Despite increasing knowledge of how wildfires impact DOM, there are inconsistencies in our current understanding due to complex interactions between wildfire and watershed characteristics. Additionally, there are limited studies which meaningfully integrate DOM and metabolism, and none that contextualize their interrelatedness with respect to wildfire disturbance and stream health. In this thesis, I evaluated DOM and metabolism to characterize stream health in the Deadman watershed in British Columbia, Canada, which was recently impacted by severe wildfire. I conducted a field-based observational study, where DOM quantity and composition from spring snowmelt to baseflow in the fall was characterized through discrete sampling. Daily stream metabolic rates were estimated using hourly measurements of dissolved oxygen, water temperature, light, and streamflow. I found that seasonality of DOM was characterized by a large shunt of terrestrially derived, highly aromatic DOM during spring snowmelt, indicating high terrestrial-aquatic ecosystem connectivity. Spatial variability in DOM was highly related to the presence or absence of mid-reach lakes. Levels of specific ultraviolet absorbance at 254 nm, representative of DOM aromaticity, were among the highest reported when compared to comprehensive studies, and are likely indicative of the influence of wildfire. The aseasonality of gross primary productivity suggests constraints on metabolism related to streamflow and light. Ecosystem respiration (ER) was relatively low across most study sites, and the fraction of autotrophic respiration was high compared to other studies, indicating that ER at most study sites was primarily supported by autochthonous organic matter production. Organic carbon mineralization rates were low when compared to other studies, indicating low uptake of terrestrial organic matter. Within the context of stream health values – water quality, aquatic habitat, and support of productive fisheries – I found that DOM and metabolism dynamics indicate potential limitations to stream food web productivity.