Estimating methane emissions from northern lakes using ice- bubble surveys

The magnitude and variability in methane (CH4) emissions from lakes are uncertain due to limitations in methods for quantifying the patchiness of ebullition (bubbling). We present a field method to estimate an important and highly uncertain source: ebullition from northern lakes. We defined four classes of CH4 bubble clusters trapped in lake ice representing distinct types of biogenic ebullition seeps that differed in flux rate. Mean annual ebullition determined through long-term (up to 700 d) continuous flux measurements of 31 seeps in three Siberian and one Alaskan lake was (mean ± standard error, 4-10 seeps per class; g CH4 seep –1 y–1): A, 6 ± 4; B, 48 ± 11; C, 354 ± 52; Hotspot, 1167 ± 177. Discrete-seep ebullition comprised up to 87% of total emissions from Siberian lakes when diffusive flux and background and seep ebullition were considered together. Including seep ebullition increased previous estimates of lake CH4 emissions based on traditional methods 5to 8-fold for Siberian and Alaskan lakes. Linking new ebullition estimates to an established biogeochemical model, the Terrestrial Ecosystem Model, increased previous estimates of regional terrestrial CH4 emissions 3to 7-fold in Siberia. Assessment of the method revealed that ebullition seeps are an important component of the terrestrial CH4 budget. They are identifiable by seep type by independent observers; they are consistent predictors of flux rate in both Siberia and Alaska; and they allow quantification of what was previously a large source of uncertainty in upscaling CH4 emissions from lakes to regions. *Corresponding author: (formerly published as Katey M. Walter) Current address: Water and Environmental Research Center, University of Alaska Fairbanks, Fairbanks, AK 99775-7000 Acknowledgments We thank S. P. Davidov for active contributions throughout the research; D. A. Draluk and A. Strohm for assistance with long-term flux measurements in winter; P. M. Anthony, C. Arp, M. L. Geai, L. McFadden, L. Oxtoby, E. T. Ricter, A. Strohm, and C. Thompson for additional field and lab assistance; J. Chanton for use of his stable isotope laboratory; the Northeast Science Station in Cherskii, Russia and Toolik Field Station, Alaska for logistic support; and M. Engram, D. Valentine, B. Finney, and R. Ruess for editorial comments on the manuscript. Siberian research funding was provided by the National Science Foundation through the Russian-American Initiative on Shelf-Land Environments of the Arctic (RAISE) of the Arctic System Science Program (ARCSS) and Polar Programs grant #0093677, Environmental Protection Agency STAR Fellowship Program, and the NASA Headquarters under the Earth System Science Fellowship Grant NGT5. The Alaskan research was funded by NSF Polar Programs IPY grant #0732735 and DOE #DE-NT0005665. DOI 10:4319/lom.2010.8.592 Limnol. Oceanogr.: Methods 8, 2010, 592–609 © 2010, by the American Society of Limnology and Oceanography, Inc. LIMNOLOGY and OCEANOGRAPHY: METHODS