Effects of macrophytes and terrestrial inputs on fluorescent dissolved organic matter in a large river system

We studied the contribution of aquatic macrophytes and allochthonous sources to the pool of fluorescent dissolved organic matter (FDOM) in a large river system composed of several distinct water masses that flow alongside one another in the same riverbed. Using three dimensional fluorescence combined with parallel factor analysis (PARAFAC), we characterized FDOM found in the St. Lawrence River (Lake Saint-Pierre, Qu,bec, Canada), and from macrophyte leaching experiments. Eight fluorescent components were identified, three of which were dominant in macrophyte experiments and were similar to protein-like, autochthonous fluorophores identified in previous studies. The remaining components corresponded to humic and fulvic acids, and a principal component analysis revealed that their distribution in Lake Saint-Pierre was different than that of protein-like fluorophores, suggesting a different origin. Concentrations of dissolved organic carbon were strongly associated with the distribution of the allochthonous components. The distribution of protein-like FDOM in Lake Saint-Pierre matched that of macrophytes in the lake and the abundance of allochthonous FDOM was explained by the connectivity with the terrestrial ecosystem. Nearshore water masses carrying large loads of newly imported organic matter from proximal tributaries showed the maximum abundances and the older water masses, from the center of the lake, carried smaller quantities of terrestrial organic matter, thus originated mainly from Lake Ontario, several hundred kilometers upstream of Lake Saint-Pierre. This study demonstrates that macrophytes are a net source of protein-like FDOM and could represent an important supply of autochthonous DOM in shallow, productive environments.