Hydrological Effects on the Biogeochemical Signatures of Dissolved Organic Matter In Estuaries

Terrestrial dissolved organic matter (DOM) undergoes significant changes during transport from river to open sea. These changes include transformations and degradation by both biological and chemical processes as well as production of fresh organic matter. Since many of these processes occur simultaneously, the properties of the DOM pool represent the net changes during the passage along a hydrological path. In this study, we examined changes in multiple DOM characteristics across three Finnish estuarine gradients during spring, summer and autumn: dissolved organic carbon (DOC) concentration, colored DOM absorbance and fluorescence, stable carbon isotope signature of DOC and the molecular size distribution. Changes in these characteristics with salinity were analyzed in relation to residence time (i.e. freshwater transit time), given that increasing residence time will enhance DOM degradation while stimulating autochthonous DOM production. Results show that the investigated DOM characteristics are highly correlated, indicating common physico-chemical transformations along the salinity continuum. Residence time did not explain variations in the DOM characteristics better than salinity (60% and 65% of variance explained, respectively). Due to the typically large variations in DOM characteristics at the river end-member, conservative mixing models have shortcomings in accurately describing the occurrence and extent of deviations in DOM properties along the salinity gradient estuaries.