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 ( 3 DOM*), singlet oxygen ( 1 O 2 ), and hydroxyl radicals ( • OH). Photodegradation led to net reductions in Φ app, 1 O 2 , Φ app, 3 DOM* , and Φ app, • OH , whereas (photo-)biodegradation resulted in increases in Φ app, 1 O 2 and Φ app, 3 DOM* . Photodegradation and (photo-)biodegradation also shifted the energy distribution of 3 DOM* in different directions. Multivariate statistical analyses revealed the potential relevance of photo-biodegradation in driving changes in Φ app, 1 O 2 and Φ app, 3 DOM* and prioritized five bulk DOM optical and redox properties that best explained the variations in Φ app, 1 O 2 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.