Seasonal and storm-driven changes in chemical composition of dissolved organic matter: a case study of a reservoir and its forested tributaries

Many drinking water supplies are located in forested watersheds, which operate as an important source of dissolved organic matter (DOM). In this study, monthly sampling campaigns were conducted from a reservoir (Daecheong Reservoir, South Korea) and its forested tributaries for five consecutive months (June to October) to examine the variations of DOM composition. Excitation-emission matrix fluorescence spectroscopy combined with parallel factor analysis (EEM-PARAFAC) was applied to track the variations of different fluorescent components within bulk DOM. Selected samples were further separated into hydrophobic (Ho) and hydrophilic (Hi) fractions. Water quality and DOM composition varied greatly with the sampling locations including the upstream and the downstream tributary sites, and the reservoir. Non-metric multidimensional scaling (NMDS) provided the information on the DOM sources and the potential processes leading to the observed DOM changes. Four of the five fluorescent components, identified by EEM-PARAFAC, were well correlated with the flow rates of the tributaries, suggesting hydrological control on DOM composition. The greatest effects were found on two terrestrial humic-like components (C1 and C2). The Ho fraction of DOM was more abundant for the post-storm samples versus the non-storm samples, supporting the important roles of hydrology on the changes in chemical composition of DOM. The amounts of the DOM resin fractions, either Ho or Hi, showed strong relationships with C1 and C2, suggesting that DOM fluorescence could be successfully applied to estimate different DOM chemical constituents in forested watersheds.