Dissolved organic carbon in coastal basins: The roles of floodplain connections and urban systems on export, quality, and processing

Human activity in urban basins can impact both DOC quality and export, but river-wetland complexes may dampen this signature. We performed synoptic surveys upstream and downstream of wetlands in urban and rural sub-basins of the Ipswich and Parker River watersheds (Plum Island Estuary Long Term Ecological Research) in northeastern Massachusetts, US. We also instrumented wetland sites to sample during storm events. We used the optical characteristics of DOC determined by fluorescence and absorbance to quantify DOC quality.  Excitation Emission Matrices (EEMS) were used to determine the relative importance of autochthonous and allochthonous sources of DOM.  In addition, we measured metabolism and denitrification to correlate fluorescence characteristics to ecosystem processes.  We predicted that at high flow conditions, DOC export increases*(DOC:CL ratio increases 10-fold below wetland sites), while the urban signature is erased because over-land and sub-surface flow-paths facilitate leaching of more recalcitrant forms of DOC from wetland plant sources.  Under low-flow conditions, autochthonous sources of DOC from the stream channel may be more prevalent and the urban signature retained.   In contrast, wetlands in rural areas are less flashy and sources of DOC more consistent with those reflecting forest and wetland sources.