DOM dynamics across the sediment-water interface in a macrotidal saltmarsh
Saltmarshes are biogeochemical hotspots for organic matter transformations, with exchange of dissolved organic matter (DOM) across the sediment-water interface. However, molecular size, a crucial property characterizing DOM, remains underexplored in the transformation processes of DOM constituents across this zone. Variations in DOM properties between porewater and creek water during tidal cycles were investigated during summer and autumn in a macrotidal saltmarsh, Hangzhou Bay, China, using flow field-flow fractionation (FlFFF) combined with fluorescence excitation-emission matrix and parallel factor analysis (EEM-PARAFAC). DOM from Andong saltmarsh exhibited a predominantly autochthonous origin, with protein-like compounds accounting for approximately 75% of the total fluorescent DOM. Dissolved organic carbon (DOC) concentration, aromaticity and humification of DOM in creek water negatively correlated with tidal levels. Principal component analysis revealed distinct differences in properties and composition of DOM between creek water and porewater. DOC concentration, fluorescence indices, and humic-like components in porewater fell between those of creek water during the flood and ebb tides. Moreover, humic-like components were primarily present in <10 kDa size-fraction, with a higher proportion (56.8%) in the medium-sized (2-20 kDa) fraction of creek water during autumn. Protein-like components were observed across <3 kDa, ∼50 kDa and >100 kDa size ranges in both porewater and ebbing water. Seasonal river runoff and pulses of organic carbon incorporated into surface sediments likely influence the origins and molecular weight distribution of DOM in the saltmarsh. This study emphasized the compositional and molecular size dynamics of DOM over tidal cycles at the tidal marsh-estuarine margins of a river-dominated estuary.