Spatial and temporal variations of bulk and colloidal dissolved organic matter in a large anthropogenically perturbed estuary

Highlights: • DOM in the estuary was primarily from autochthonous origin although terrestrial sources also contributed. • Estuarine mixing was responsible for the decrease of humic-like and protein-like DOM with salinity. • Colloidal DOM went through multiple depletion and replenishment cycles in the dry season. • Colloidal DOM was associated with both allochthonous and autochthonous origins during the wet season. In the present study, we characterized the sources of dissolved organic matter (DOM) and the processes controlling the evolution of DOM in a large anthropogenically perturbed estuary in Southern China. Three-dimensional excitation emission matrices (3D EEMs) revealed that DOM in the estuary was primarily from autochthonous origin although terrestrial sources also contributed to the DOM production. Parallel factor analysis (PARAFAC) of the EEMs dataset indicated that DOM in the estuary was a legacy from the upstream wastewater discharging activities, and estuarine mixing was the primary mechanism responsible for the decreasing concentrations of DOM components. However, non-conservative behavior of DOM components was also observed along the center transect as a result of additional removal process and spatial heterogeneity in the distribution of DOM. Seasonal influence on the concentrations and compositions of colloidal DOM in the estuary was also found. Asymmetric flow field-flow fractionation (AF4) analysis showed distinct distribution patterns of the colloidal DOM during the dry and wet seasons. Behavior of colloidal DOM was size-independent and the distribution of colloidal DOM was controlled by multiple depletion and replenishment cycles in the dry season, in contrast to those observed in the wet season. By combining both EEMs-PARAFAC modeling and AF4 technique, this study provided important information on the sources and fate of bulk and colloidal DOM in a large estuary with intensive anthropogenic influences.