Fluorescent dissolved organic matter cycling reveals dynamic catchment interactions in Lake Superior

Abstract:

R.M. Cory,^1,2* M. Funke,¹ B. Peterson,² A. Amado,^{1 ζ} K. McNeill,^2Ŧ J. B. Cotner¹

Lake Superior, an ultra-oligotrophic lake holding 10% of the world’s surface freshwater, contains a large pool of organic carbon (1.5 to 1.9 x 10¹³ g), despite low dissolved organic carbon (DOC) concentrations of 1-2 mg C L^-1. While Lake Superior is already experiencing climate change impacts such as increasing temperatures and extended periods of summer stratification, there remain key uncertainties in the carbon and nitrogen cycles that center on the source and quality of dissolved organic matter (DOM) in Lake Superior. We employed absorbance and fluorescence spectroscopy to asses the sources and dynamics of the colored and fluorescent fraction of DOM in Lake Superior and one its major tributaries by collecting samples throughout the water column on eight cruises spanning May through October, 2006-2008. Parallel factor analysis (PARAFAC) of the fluorescent fraction of DOM (FDOM) revealed six components associated with terrestrial, autochthonous, and/or proteinacous material. The general pattern obtained from the colored and fluorescent DOM patterns in Lake Superior was a strong removal of terrestrially-derived humic DOM with increasing importance and variability in DOM associated with autochthonous sources along the riverine to open Lake Superior water gradient. In addition, we observed the production and removal of FDOM with a distinct autochthonous signature between May and June, 2006, which may provide evidence for a rapidly cycling DOM pool in Lake Superior driven by terrestrial inputs.

¹Department of Ecology, Evolution and Behavior, University of Minnesota, Twin Cities, Saint Paul, MN, 55455. ²Department of Chemistry, University of Minnesota, Twin Cities, Minneapolis, MN, 55455. ^*Current address: Environmental Sciences & Engineering, University of North Carolina, Chapel Hill, NC, 27599. ^ŦCurrent address: Department of Environmental Science, ETH Zurich, Universitaetstrasse 16, Zurich, Switzerland. ^ζ Current address: Departamento de Oceanografia e Limnologia; Universidade Federal do Rio Grande do Norte.