Variations in size and fluorescent components of dissolved organic matter in a negative estuary, the Fox River plume

Natural dissolved organic matter (DOM) is heterogeneous in composition and reactivity along the aquatic continuum. However, the PARAFAC-derived fluorescent DOM components are normally obtained from EEM spectra from a large number of samples with different DOM abundance and composition in aquatic environments. Although DOM in normal estuaries has been widely investigated, much is still unknown about the mixing behavior and environmental fate of DOM with different molecular sizes and composition in a negative estuary, where ion strength decreases from river to plume. In this study, water samples were collected from a series of stations in a negative estuary from the Fox River to open Green Bay, Lake Michigan. Using flow field-flow fractionation (FlFFF) and EEM measurements of bulk samples and size-fractionated subsamples, variations in DOM molecular size, optical properties, and PARAFAC-derived fluorescent components were evaluated for individual samples along the river plume. Changes in EEM spectra with DOM molecular size from each sample using FlFFF and EEM-PARAFAC show four fluorescent DOM components for most stations, but three fluorescent DOM components for the most open water station. Statistical inter-model comparison identified four components (C₄₇₅, C₃₂₀, C₄₁₀ and C₂₉₀) including three humic-like and one protein-like components for all stations. The F-max of each component showed that humic-like components (C₄₇₅, C₃₂₀ and C₄₁₀) behaved conservatively during river-bay mixing in the estuary, while the protein-like (C₂₉₀) increased in general with decreasing conductivity along the river plume. The mixing behavior of the three humic-like components was similar to changes in humification index value, while the protein-like component was coupled with the biological index values.