Photo-biodegradation drives organic matter-mediated carbon-nitrogen-sulfur cycling in the reservoir's surface-layer:...

The synergistic regulation of dissolved organic matter (DOM) dynamics and coupled carbon-nitrogen-sulfur (C-N-S) cycling in the surface waters of cascade reservoirs by photochemical and microbial processes remains poorly understood. This study systematically examined DOM dynamics under both sunlight and dark conditions during consecutive 30-day experiments using unfiltered and filtered water from two reservoirs (Dongfeng and Yaoqiaoyu). Analyses included three-dimensional fluorescence (excitation-emission matrix, EEM) coupled with parallel factor analysis (EEM-PARAFAC), isotope techniques (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-), and SO₄^2- measurements. Results revealed that sunlight-induced DOC decoupling arises from DOM release via phytoplankton photorespiration and its simultaneous photodegradation, evidenced by an increasing DOC trend in unfiltered samples under high-temperature conditions. This DOC release was further supported by the detection of three newly generated tyrosine-like, phenylalanine-like, and protein-like substances characterized using the EEM-PARAFAC model. Microbial respiration-induced DOC changes in darkness reflected differential microbial utilization of unstable versus recalcitrant DOM fractions and exhibited reservoir specificity. During late sunlight exposure (days 15-30) under optimal temperature conditions (air temperature: 20.0-26.9 °C), sustained phytoplankton-mediated fresh DOM production, combined with simultaneous photodegradation, resulted in minimal DOC losses. This process decouple DOC dynamics, leading to a substantial increase in DOC accompanied by extensive mineralization of its organic sulfur components to SO₄^2-. NO₃^- transformation fluctuations were modulated by the phytoplankton assimilation and nitrification coupling processes, as supported by δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-. Based on these findings, we developed an integrated C-N-S cycle conceptual model that elucidates how light-dark cycles jointly drive DOM turnover by regulating phytoplankton photosynthesis, respiration, photodegradation, and microbial metabolism. Finally, this study clarifies the composition and transformation mechanisms of DOM in reservoir surface waters, providing concrete, process-based insights into how reservoirs, acting as 'reactors,' respond to and influence C-N-S cycles under climate change.