Characterization of bulk and chromophoric dissolved organic matter in the Laurentian Great Lakes during summer 2013

The Laurentian Great Lakes contain ~ 21% of the Earth's surface freshwater and are experiencing dramatic decadal-scale changes to their water quality, biogeochemistry and ecosystem functions.We report here the first data set of dissolved organic carbon (DOC) and chromophoric dissolved organic matter (DOM), including UV–visible absorbance and derived optical properties and fluorescence excitation–emission matrices (EEMs), for open-lakesurface waters from each of the Great Lakes. Concentrations of DOC ranged from 86 to 240 μmol/L and the absorption coefficient at 254 nm (a₂₅₄) varied from 2.67 to 10.47 m^{− 1} in Great Lakes waters.UV–visible and fluorescence characterization indicated that the DOM in open-lake waters in all the Great Lakes was primarily autochthonous. Both DOC and a₂₅₄ increased from Lake Superior to Lakes Erie and Ontario where aromaticity, molecular weight and humification degree of DOM were also higher.These variations are consistent with both the water transport pathway and the trend of increasing human population and relative watershed drainage area along the Great Lakes. Parallel factor analysis on fluorescence-EEM data revealed three terrestrial humic-likeDOM components (C1, C2 and C4) and one protein-likeDOM component (C3). Open Lake Huron had the highest protein-like/humic-like DOM ratio (C3/C1), while sites in Lakes Erie and Ontario had lower C3/C1 ratios, indicating a greater proportional influence of terrestrial DOM. Changes in DOM composition along the water transport pathway in the Great Lakes indicate varying impacts from terrestrial inputs and human activities in the different lake systems.