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

Abstract

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, the site with most terrestrial input, has the largest and most diverse array of unique molecular formulae, including many aromatic compounds. The lake also contains unique elemental formulae, primarily in the lignin-like and reduced hydrocarbon regions of van Krevelen diagrams. Furthermore, ESI-amenable Lake Superior DOM also has a higher proportion of heteroatom (N, S and P) containing formulae other than those of 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 reworking.