Evidence for the respiration of ancient terrestrial organic C in northern temperate lakes and streams

• Published in: Proceedings of the National Academy of Sciences - PNAS 2012-10, Vol.109 (42), p.16963-16968
• Main Authors: McCallister, S. Leigh, del Giorgio, Paul A.
• Format: Article
• Language: English
• Subjects: Bacteria - metabolism; Biogeochemistry; Biological Sciences; Carbon; Carbon Cycle - physiology; Carbon dioxide; Carbon Dioxide - metabolism; Carbon Radioisotopes - analysis; Creeks & streams; Emissions; Lakes; Lakes - chemistry; Lentic systems; Microbiology; Oxygen Consumption - physiology; Quebec; Radiocarbon; Respiration; Rivers - chemistry; Signatures; Soil respiration; Streams; Terrestrial ecosystems; Time Factors; Watersheds
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1207305109

Northern rivers and lakes process large quantities of organic and inorganic carbon from the surrounding terrestrial ecosystems. These external carbon inputs fuel widespread CO₂ supersaturation in continental waters, and the resulting CO₂ emissions from lakes and rivers are now recognized as a globally significant loss of terrestrial production to the atmosphere. Whereas the magnitude of emissions has received much attention, the pathways of C delivery and processing that generate these emissions are still not well-understood. CO₂ outgassing in aquatic systems has been unequivocally linked to microbial degradation and respiration of terrestrial organic carbon (OC) r but the nature (i.e., age and source) of this OC respired in surface waters is largely unknown. We present direct radiocarbon measurements of OC respired by bacteria in freshwater aquatic systems, specifically temperate lakes and streams in Quebec. Terrestrial OC fuels much of the respiration in these systems, and our results show that a significant fraction of the respired terrestrial OC is old (in the range of 1,000-3,000 y B.P.). Because the bulk OC pools in these lakes is relatively young, our results also suggest selective removal of an old but highly bioreactive terrestrial OC pool and its conversion to CO₂ by bacteria. The respiration of ancient ¹⁴C-depleted terrestrial C in northern lakes and rivers provides a biological link between contemporary aquatic carbon biogeochemistry and paleo-conditions in the watershed, and it implies the aquatic-mediated return to the atmosphere of C putatively considered permanently stored, thus challenging current models of long-term C storage in terrestrial reservoirs.