Photochemical Production of Singlet Oxygen in Adirondack Long-Term Monitoring Lakes of Varying Browning Status.

Widespread browning of surface waters in boreal and temperate regions of the Northern Hemisphere has been documented through long-term monitoring of color or dissolved organic matter (DOM) over recent decades. While the ecological implications of browning have received considerable attention, its impacts on the photochemical production of reactive intermediates from DOM remain understudied despite their importance for biogeochemical processes and contaminant fate in sunlit surface waters. To address this gap, we investigated singlet oxygen (1O2) production in 37 lakes within the Adirondack Long-Term Monitoring (ALTM) program. Wavelet coherence tests confirmed the synchrony between dissolved organic carbon (DOC) and color as well as the time-scale-dependent influence of regional atmospheric and hydroclimatic factors on DOC dynamics in these lakes. Hydrogeological conditions of lake watersheds (e.g., hydrologic connectivity and surficial geology) and seasonal variations in DOM quality jointly shaped the spatiotemporal patterns of the apparent quantum yields of 1O2. Within the euphotic zone, depth-averaged steady-state concentrations of 1O2 were higher in lakes experiencing more intense browning, as operationally defined through trend analyses of long-term water chemistry data; however, the relevance of 1O2-mediated reactions depends on the time scales of photochemical transformation relative to lake flushing. Overall, our study provides an initial assessment of 1O2 production in relation to lake browning and highlights the need for long-term photochemical measurements for an improved assessment.