Characterization of dissolved organic matter in Lake Superior and its watershed using ultrahigh resolution mass spectrometry

With the advent of ultrahigh resolution mass spectrometry (MS), recent studies have begun to resolve molecular relationships between terrestrial and aquatic dissolved organic matter (DOM) in rivers, estuaries, mangrove swamps and their receiving oceans and lakes. Here, we extend ultrahigh resolution MS techniques to Lake Superior, the largest freshwater lake in the world by area. Solid-phase extracted samples from the western arm of the lake and its watershed, including swamp, creek, river, lake-river confluence and offshore lake sites were compared using electrospray ionization (ESI), Fourier transform ion cyclotron resonance MS (FT-ICR-MS). Results were analyzed using cluster analysis and van Krevelen diagrams. Chemical similarity appears related to hydrological proximity, terrestrial impact and flow conditions. For example, higher and lower flow samples from the same stream differ from one another. Toivola Swamp, Lake Superior and the south shore river have diverse arrays of unique molecular formulae relative to the north shore river and stream sampled in the data set. The lake's unique elemental formulae, relative to its watershed samples, are primarily in the lignin-like and reduced hydrocarbon regions of van Krevelen diagrams. The ESI-amenable lake DOM also has a higher proportion of formulae containing nitrogen or sulfur relative to the other samples. The degree of overlap among formulae within the data set is consistent with previous ESI FT-ICR-MS characterization of terrestrial, estuarine and marine OM. There appears to be a conserved portion of formulae across natural OM samples, perhaps because these compounds are intrinsically refractory or because they are commonly generated as products of natural reworking processes.