Characterization of dissolved organic matter from source to sea using fluorescence and absorbance spectroscopy

Abstract

Dissolved organic matter fluorescence, absorbance and dissolved organic carbon were measured from source to sea in the River Tyne catchment, of approximately 2935 km(2) and encompassing areas of contrasting land use. The catchment has three major tributaries: the North Tyne which has good water quality, high dissolved organic carbon concentrations and visible water colour from the high proportion of peat in its upper catchment; the South Tyne which has good water quality with typical riverine dissolved organic carbon concentrations and drains from limestone uplands; and the Derwent, a more urbanized catchment which is increasingly impacted by treated sewage effluent discharges towards its mouth. Thirty sample sites, 23 along the three main tributaries and seven within the estuary, were sampled on six occasions over the period 2002-2003. High absorbance at 340 nm and dissolved organic carbon concentration identify N Tyne waters due to the peaty headwaters, but no downstream trends in these parameters are observed in any of the tributaries, in contrast to the estuary where a rapid decrease is observed in both. Fluorescence in contrast demonstrated downstream trends in both intensity and wavelength, especially in the Derwent as it is increasingly impacted by anthropogenic dissolved organic matter. Elevated protein-like fluorescence intensity also fingerprints sewage effluent within the estuary. The absorbance coefficient at 340 nm was found to have the strongest correlation to dissolved organic carbon concentration, greater than all fluorescence intensity parameters measured. However, fluorescence analysis permits the source of the dissolved organic matter to be determined, and therefore has implications for understanding its fate in estuaries and the ocean.