<p>Floodplains are increasingly subject to cattle grazing; however, the effects on dissolved organic matter (DOM) dynamics in floodplain lakes remain poorly understood. We characterized the UV–visible and fluorescence characteristics of water-extractable organic matter (WEOM) from cattle dung in a floodplain under extensive rotational grazing, assessed its potential influence on floodplain lake DOM, and evaluated the usefulness of spectral variables to detect dung-derived inputs. We hypothesized that cattle grazing modifies DOM concentration and quality in floodplain lakes through distinctive dung-derived contributions identifiable by their spectral characteristics. Results showed that dung was rich in aromatic and biologically active WEOM, in early stages of microbial processing and humification, as indicated by UV–visible characteristics and fluorescence variables: humification index (HIX &lt; 4), fluorescence index (FI = 1.6–1.9), and biological index (BIX ≈ 1). In contrast, DOM in the floodplain lake displayed significantly higher humification and lower aromaticity and molecular weight. Cattle stocking rate was not associated with significant changes in DOM concentration or quality, except for BIX (positive relationship). Variability in other spectral characteristics and DOM concentration was mainly driven by precipitation and natural successional processes. These findings suggest that extensive rotational cattle grazing exerts negligible influence on DOM in floodplain lakes, which are inherently rich in organic matter. Dung-derived DOM is likely stored in bottom sediments due to its highly aromatic character. Nevertheless, BIX appears to be a sensitive tracer of subtle dung-derived inputs. Further studies are needed to assess the spatiotemporal variability of dung-derived WEOM quality across the study area.</p>