Spectral identification and behaviour of dissolved organic fluorescent material during estuarine mixing processes

In most estuaries, a linear relationship between fluorescence intensity and salinity has been frequently reported. Accordingly, it is commonly accepted that the dissolved organic matter in an estuary is essentially conservative and is not substantially modified by dilution with sea water. In this paper we have tried to verify whether other fluorescent parameters, namely the position of the fluorescence maximum and the apparent fluorescence efficiency (A.Fl.E.), whose variations were measured during estuarine mixing of waters in different European estuaries are consistent with this conclusion. Field observations and laboratory simulation experiments show that upon mixing with sea water, an important blue shift ( 40 nm) of the fluorescence emission spectra as well a non-conservative evolution of the A.Fl.E. are observed in estuarine waters. These phenomena have been explained by a complex combination of processes involving mainly, the mixing of fresh water and marine water fluorophores having different fluorescence properties, but also, the loss of high molecular weight fluorescent matter from fresh waters possibly removed by flocculation.