New insights into the size distribution of fluorescent dissolved organic matter in estuarine waters

Abstract

The study of the fate and composition of organic matter in aquatic systems is of interest since organic matter is believed to play a key role in biogeochemical cycles. Fluorescent dissolved organic material (FDOM) represents a major fraction of dissolved organic matter (DOM) but is under explored, particularly in estuarine environments. In this study, 3D fluorescence excitation–emission matrix (EEM) spectroscopy was combined with tangential ultrafiltration (UF) to investigate the changes in FDOM properties during the mixing of fluvial and marine waters in two estuaries on the French Atlantic coast. Water samples were sequentially filtered through three membranes of decreasing molecular weight (MW) cut off points (3 kDa, 1 kDa and 500 Da). Four ultrafiltration fractions were obtained and were analysed using EEM spectroscopy, as well as for their DOC content. Satisfactory fluorescence and DOC mass balances were obtained after the sequential UF fractionation despite the variable DOC concentration and ionic strength of the estuarine samples, showing the reliability of the UF systems. DOC distribution in the different size classes was observed to vary between the upstream and downstream parts of the estuaries and to be different in the two estuaries. The value of combining UF and EEM spectroscopy was shown, since the coupling allowed observation and isolation of some fluorophores which could not be distinguished in the spectra of bulk samples. Thus, the β fluorophore, observed mainly in marine samples until now, and which is associated with material of autochthonous biological origin, was present in the lowest size fraction (MW < 500 Da) from all water samples, including freshwater ones. The γ fluorophore, characteristic of protein-like compounds, was observed in the small size fractions of some fresh and brackish water samples, but it was not visible in the spectra of the corresponding bulk water samples. The differences in fluorescence properties of the size fractions of DOM were further evaluated by determining three fluorescence indices: the fluorescence index ( f 450/ f 500), the humification index (HIX) and the index of recent autochthonous contribution (BIX). The origin and degree of transformation of DOM can be assessed through the calculation of these indices. We showed that the most humified OM was associated with different size fractions, depending on water sample origin, and was characterised by intermediate MW (higher for fresh than marine water). We also showed that DOM distribution and characteristics differ between the Gironde and Seine estuaries.