Reservoir Eutrophication Reshapes Photochemical Reactivity of Dissolved Organic Matter in River–Reservoir Systems

Abstract

Reservoirs alter riverine dissolved organic matter (DOM) by exporting reservoir-processed DOM downstream, yet how reservoir eutrophication shapes the photochemical reactivity of reservoir-released DOM remains unclear. We combined photochemical experiments with two-year bimonthly sampling across two river–reservoir systems in Japan representing eutrophic (Sagami–Shiroyama) and mesotrophic (Miyagase) conditions. The eutrophic reservoir showed increased downstream dissolved organic carbon (DOC) and chlorophyll-a (Chl-a), with DOC rising logarithmically with Chl-a (R2 = 0.65). Apparent quantum yields of reactive intermediates (ΦRIs), including hydroxyl radical (Φ•OH), triplet excited-state DOM (fTMP), and singlet oxygen (Φ1O2) exhibited contrasting downstream–upstream responses: Φ•OH was higher in the released waters of the eutrophic system, whereas fTMP and Φ1O2 increased only downstream of the mesotrophic reservoir. Exploratory structural equation modeling analysis suggested distinct statistical associations among ΔChl-a, ΔDOM indicators, and ΔΦRIs (R2: 0.20–0.82), with Φ•OH more closely associated with DOM abundance and humic-like/autochthonous signals, whereas fTMP and Φ1O2 were linked to lower-molecular-weight, optically active DOM. These results demonstrate that reservoir eutrophication can reshape the photochemical reactivity of reservoir-released DOM in reactive-intermediate-specific ways, with implications for DOM transformation, carbon cycling, and contaminant fate in regulated rivers.