Constraining the sources and cycling of dissolved organic carbon in a large oligotrophic lake using radiocarbon analyses

Prosper K. Zigah, Elizabeth C. Minor, Ann P. McNichol, Li Xu, Josef P. Werne

Research output: Contribution to journalArticlepeer-review

12 Scopus citations

Abstract

We measured the concentrations and isotopic compositions of solid phase extracted (SPE) dissolved organic carbon (DOC) and high molecular weight (HMW) DOC and their constituent organic components in order to better constrain the sources and cycling of DOC in a large oligotrophic lacustrine system (Lake Superior, North America). SPE DOC constituted a significant proportion (41–71%) of the lake DOC relative to HMW DOC (10–13%). Substantial contribution of 14C-depleted components to both SPE DOC (Δ14C = 25–43‰) and HMW DOC (Δ14C = 22–32‰) was evident during spring mixing, and depressed their radiocarbon values relative to the lake dissolved inorganic carbon (DIC; Δ14C ∼ 59‰). There was preferential removal of 14C-depleted (older) and thermally recalcitrant components from HMW DOC and SPE DOC in the summer. Contemporary photoautotrophic addition to HMW DOC was observed during summer stratification in contrast to SPE DOC, which decreased in concentration during stratification. Serial thermal oxidation radiocarbon analysis revealed a diversity of sources (both contemporary and older) within the SPE DOC, and also showed distinct components within the HMW DOC. The thermally labile components of HMW DOC were 14C-enriched and are attributed to heteropolysaccharides (HPS), peptides/amide and amino sugars (AMS) relative to the thermally recalcitrant components reflecting the presence of older material, perhaps carboxylic-rich alicyclic molecules (CRAM). The solvent extractable lipid-like fraction of HMW DOC was very 14C-depleted (as old as 1270–2320 14C years) relative to the carbohydrate-like and protein-like substances isolated by acid hydrolysis of HMW DOC. Our data constrain relative influences of contemporary DOC and old DOC, and DOC cycling in a modern freshwater ecosystem.

Original languageEnglish (US)
Pages (from-to)102-118
Number of pages17
JournalGeochimica et Cosmochimica Acta
Volume208
DOIs
StatePublished - Jul 1 2017

Bibliographical note

Funding Information:
We thank the staff at National Ocean Sciences Accelerator Mass Spectrometry Facility for radiocarbon analyses, the captain and crew of the R/V Blue Heron for their help during sampling, and the Repeta laboratory at the Woods Hole Oceanographic Institution for assistance in the HMW protein, lipid, and carbohydrate isolations for radiocarbon analysis. Two anonymous reviewers and the associate editor Orit Sivan provided comments that considerably improved earlier versions of the manuscript. This work was funded by the United States National Science Foundation grant OCE 0825600 to E.C.M. and J.P.W., a graduate student internship fellowship to P.K.Z. by the National Ocean Sciences Accelerator Mass Spectrometry Facility (OCE 0753487), and the Postdoctoral Scholar Program at the Woods Hole Oceanographic Institution to P.K.Z, with funding provided by the National Ocean Sciences Accelerator Mass Spectrometry Facility (OCE 0753487).

Publisher Copyright:
© 2017 Elsevier Ltd

Keywords

  • Atmospheric CO
  • Carbohydrate-like substance
  • Carboxylic-rich alicyclic molecules (CRAM)
  • Contemporary DOC
  • Dissolved inorganic carbon
  • Dissolved organic carbon
  • Freshwater carbon cycling
  • Heteropolysaccharides (HPS)
  • High molecular weight DOC
  • Lake Superior
  • Lipid-like fraction
  • Old DOC
  • Oligotrophic lake
  • Protein-like substance
  • Radiocarbon
  • Serial thermal oxidation
  • Solid phase extracted DOC
  • Stable isotope
  • Unhydrolysable material

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