Date of Award
Doctor of Philosophy (PhD)
Civil and Environmental Engineering
Dissolved organic matter, Harmful algal bloom, New York lakes, Photoreactivity, Rain acid depsosition, Reactive Oxygen species
Civil and Environmental Engineering | Engineering | Environmental Engineering
Dissolved organic matter (DOM) is a critical component of aquatic systems that serves many purposes. Among them is the production of reactive intermediates (RIs) when irradiated with sunlight. Examples of RIs produced include the excited triplet states of dissolved organic matter (3DOM*), singlet oxygen (1O2), and hydroxyl radicals (•OH), all of which contribute to the degradation or inactivation of chemical or microbiological contaminants. It is therefore critical to understand how the sources and transformations of DOM may affect the production of RIs in sunlit aquatic systems.
The Adirondack Mountain region of New York, a historical hotspot for atmospheric sulfur and nitrogen deposition, features abundant lakes that are experiencing browning associated with recovery from acidification. Yet, much remains unknown about the photoreactivity of Adirondack lake waters. We quantified the apparent quantum yields (Фapp,RI) of photochemically produced RIs, including 3DOM*, 1O2, and •OH, for surface water samples collected from 16 representative Adirondack lakes. "Φ" _("app,3" 〖"DOM" 〗^"*" ) and "Φ" _"app,1O2" for native Adirondack Lake waters fell within ranges reported for whole waters and DOM isolates from various sources, while "Φ" _"app,•OH" were lower than those measured for other aquatic samples. Orthogonal partial least squares and multiple linear regression analyses identified the spectral slope coefficient from 290 to 400 nm (S290-400) as the most effective predictor of Фapp,RI among measured water chemistry parameters and bulk DOM properties. Фapp,RI also exhibited divergent responses to controlled pH adjustment and aluminum or iron addition simulating hypothetical scenarios relevant to past and future water chemistry conditions of Adirondack lakes. This study highlights the need for continued research on changes in photoreactivity of acid-impacted aquatic ecosystems in response to browning and subsequent impacts on photochemical processes.
Photochemical and microbial processing are the prevailing mechanisms that shape the composition and reactivity of DOM; however, prior research has not comparatively evaluated the impacts of these processes on the photoproduction of RIs from freshly sourced terrestrial DOM. We performed controlled irradiation and incubation experiments with leaf and soil samples collected from an acid-impacted lake watershed in the Adirondack Mountain region of New York to examine the effects of DOM processing on Фapp,RI, including 3DOM*, 1O2, and •OH. Photodegradation led to net reductions in "Φ" _"app,1O2" , "Φ" _("app,3" 〖"DOM" 〗^"*" ), and "Φ" _"app,•OH" , whereas (photo-)biodegradation resulted in increases in "Φ" _"app,1O2" and "Φ" _("app,3" 〖"DOM" 〗^"*" ). Photodegradation and (photo-)biodegradation also shifted the energy distribution of 3DOM* in different directions. Multivariate statistical analyses revealed the potential relevance of photo-biodegradation in driving changes in "Φ" _"app,1O2" and "Φ" _("app,3" 〖"DOM" 〗^"*" ) and prioritized five bulk DOM optical and redox properties that best explained the variations in "Φ" _"app,1O2" and "Φ" _("app,3" 〖"DOM" 〗^"*" ) along the watershed terrestrial-aquatic continuum. Our findings highlight the contrasting impacts of photochemical and microbial processes on the photoreactivity of freshly sourced terrestrial DOM and invite further studies to develop a more holistic understanding of their implications for aquatic photochemistry.
There are increased frequency and distribution of harmful algal blooms (HABs) in aquatic environments due to a combination of factors which include climate change and the presence of nutrients. The growth of these HABs in water environments alters the composition and properties of DOM. Previous studies have evaluated the production of RIs from different sources of DOM; however, no prior study has systematically evaluated the impact of HABs on the photochemical reactivity of freshwater lakes. In this study, over 250 samples were collected from 111 lakes by volunteers participating in the Citizens Statewide Lake Assessment Program (CSLAP) in New York. In comparison, nearshore cyanobacterial bloom samples and commercial strains of cyanobacteria were cultivated in the laboratory using HAB-free lake water samples. Higher median values of "Φ" _("app,3" 〖"DOM" 〗_"TMP" ^"*" ), "Φ" _"app,1O2" , and "Φ" _"app,•OH" were observed for blue-green chlorophyll a (BG-Chla)-containing samples than BG-Chla-free samples, indicating the potential impact of HABs on lake water photoreactivity. Results from the laboratory cultivation experiments indicated increased "Φ" _("app,3" 〖"DOM" 〗_"TMP" ^"*" ), "Φ" _"app,1O2" , and "Φ" _"app,•OH" during cyanobacteria growth, which corroborated our observations from the CSLAP samples. Multivariate statistical analyses prioritized BG-Chla as a top predictor of "Φ" _("app,3" 〖"DOM" 〗_"TMP" ^"*" ), "Φ" _"app,1O2" , and "Φ" _"app,•OH" for HABs-impacted lake water samples. These findings highlight the impact of HABs on the photoreactivity of freshwater lakes and invite the need for further studies to understand the implications of such impact.
Overall, this dissertation contributes to the growing knowledge about the photochemical reactivity of DOM in aquatic systems and advances our understanding towards the roles of natural processes and anthropogenic influence on the photochemical reactivity of DOM
Wasswa, Joseph, "Exploring the Roles of Organic Matter Source and Transformation for Photochemical Characterization of New York Lake Waters" (2022). Dissertations - ALL. 1619.
Available for download on Sunday, September 15, 2024