Land use influences storm hysteresis of dissolved organic matter character and concentration

The character and concentration of dissolved organic matter (DOM) can change rapidly during storm events, which affects key biogeochemical processes, carbon bioavailability, metal pollutant transport, and potential disinfectant byproduct formation during drinking water treatment. We used in situ UV-Visible spectrophotometers to concurrently measure dissolved organic carbon (DOC) concentration and spectral slope ratio, a proxy for dissolved organic matter molecular weight. Measurements were made at 15-minute intervals over three years in three streams draining primarily agricultural, urban, and forested watersheds. We describe storm event dynamics by calculating hysteresis indices for DOC concentration and spectral slope ratio for 220 storms and present a novel analytical framework that compares and interprets these metrics together in a four-quadrant plotting space. DOC concentration and spectral slope ratio differed significantly among sites, and storm DOM dynamics were remarkably variable at each site and among the three sites. Distinct patterns emerged for storm DOM dynamics depending on land use / land cover of each watershed. In agricultural and forested streams, DOC concentration increased later in the storm cycle, and spectral slope ratio dynamics indicate that this delayed flux was of relatively higher molecular weight material compared to the beginning of each storm. Patterns observed at the urban stream show that DOM character generally shifts to lower molecular weight while DOC concentration increases on the falling limb, indicating either the introduction of lower molecular weight DOM, the exhaustion of a higher molecular weight DOM sources, or a combination of these factors. We show that the combination of high-frequency DOM character and DOC quantity metrics has the potential to provide new insight into short-timescale DOM dynamics and can illuminate previously unknown effects of land use / land cover.