Sources, biogeochemical cycling, and fate of organic matter in Lake Superior: an investigation using natural abundance radiocarbon and stable isotopes

Abstract

The natural abundance radiocarbon and stable isotopic distributions of bulk dissolved organic carbon (DOC), particulate organic carbon (POC), dissolved inorganic carbon (DIC), zooplankton, size-fractionated organic matter, and biochemical compound classes were used to investigate the sources, biogeochemical cycling, and fate of organic matter in the water column of Lake Superior. DIC pool appears to reset rapidly, showing radiocarbon values similar to atmospheric values from approximately 3 years previous to sampling. DIC concentrations and isotopic compositions were mostly homogeneous across the entire lake. POC was generally more depleted in stable carbon isotopic values than concurrent DOC. POC was also consistently depleted in radiocarbon (thus, older) relative to DOC and DIC. Radiocarbon ages of POC was spatially heterogeneous (range, modern to 2,840 year BP), and appear to be related to total water depth, exhibiting more older and more variable ages in the deepest basins of the lake. The ages and reactivity of bulk DOC did not change radically across the lake. DOC pool appears to be semi-reactive, recycling over up to 60 years in the entire water column. The radiocarbon signatures of the various DOC size fractions show that they recycle on similar time scales, with consistently modern (post 1950) radiocarbon values. Radiocarbon and Nuclear Magnetic Resonance (NMR) data show most of the high molecular weight dissolved organic matter (HMW DOM) originates from contemporary origin and was dominated by carbohydrates, aliphatic compounds, and acetate, with little aromatic compounds. Total hydrolyzable free carbohydrates and amino acids within HMW DOM exhibited modern radiocarbon signatures and recycled rapidly in the lake. In contrast, extractable lipid was pre-aged (20 to 2,320 years BP) due to older sources and/or general long term persistence in the lake. Coupled radiocarbon and stable carbon isotopic values indicate multiple sources, and variable formation pathways for the acid insoluble organic fraction within HMW DOM in the lake. Radiocarbon and stable isotopic values show zooplankton in Lake Superior selectively feed on within-lake produced organic matter even though other organic carbon sources represented a considerable portion of the available food resource.