Contrasting Impacts of Photochemical and Microbial Processing on the Photoreactivity of Dissolved Organic Matter in an Adirondack Lake Watershed

Photochemical and microbial processing are the prevailing mechanisms that shape the composition and reactivity of dissolved organic matter (DOM); however, prior research has not comparatively evaluated the impacts of these processes on the photoproduction of reactive intermediates (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 the apparent quantum yields of RIs (Φ_app,RI), including excited triplet states of DOM (³DOM*), singlet oxygen (¹O₂), and hydroxyl radicals (^•OH). Photodegradation led to net reductions in Φ_app,¹O2, Φ_app,³DOM*, and Φ_app,^•OH, whereas (photo-)biodegradation resulted in increases in Φ_app,¹O2 and Φ_app,³DOM*. 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,³DOM* and prioritized five bulk DOM optical and redox properties that best explained the variations in Φ_app,¹O2 and Φ_app,³DOM* 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.