Seasonal Variation in Fluorescent Dissolved Organic Matter Composition in Poyang Lake, China

Seasonal hydrological variation can reorganize fluorescent dissolved organic matter (FDOM) in floodplain lakes; yet, its expression in Poyang Lake remains uncertain. We assessed campaign differences in FDOM composition by comparing 32 excitation–emission matrices from 16 fixed sites sampled during the dry and wet periods of 2024 using parallel factor analysis (PARAFAC), fluorescence indices, paired tests with Benjamini–Hochberg correction, and principal component analysis. Rank 4 fitted better and resolved two humic-like and two protein-like regions, although incomplete validation limited component-specific interpretation. In the wet period, C2 and combined humic-like maximum fluorescence intensity (Fmax) decreased (q = 0.00836), whereas bulk total organic carbon (TOC) and total Fmax did not differ. Mean protein-like contribution rose from 25.0% to 37.5% (q = 0.00322); the biological index increased, the humification index decreased, and the fluorescence index was unchanged. The first two principal components explained 78.11% of the variance, and an exact paired multivariate test detected an overall period difference (p = 3.05 × 10−5). No component–environment correlation survived correction or differed between periods. FDOM composition therefore underwent a campaign-specific reorganization without a corresponding change in bulk carbon, but the two-campaign design and incomplete validation preclude causal or source-specific inference.