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River Biogeochemistry

R. M. Holmes1, J. W. McClelland2, B. J. Peterson3,
P. A. Raymond4, S. E. Tank3, A. V. Zhulidov5

1Woods Hole Research Center, Falmouth, MA
2University of Texas at Austin, Marine Science Institute, Port Aransas, TX
3Marine Biological Laboratory, Woods Hole, MA
4Yale University, New Haven, CT
5South Russia Center for Preparation and Implementation of International Projects,
Rostov-on-Don, Russia

November 15, 2011

Highlights

  • In 2010, dissolved organic carbon (DOC) flux from five of the Arctic's six largest rivers (Yenisey, Lena, Kolyma, Yukon, Mackenzie, but not the Ob') was less than the 2003-2009 average.
  • Combined DOC flux from the six largest rivers peaked in 2007 but has been lower each year since then, closely tracking the trend in their combined river discharge.

In addition to transporting freshwater to the Arctic Ocean (see the essay on River Discharge), arctic rivers transport and process a large range of natural and anthropogenic compounds. For the 2011 Arctic Report Card, we focus on dissolved organic carbon (DOC) export from the watersheds of the six largest arctic rivers, which combined cover 11.3 x 106 km2 or 67% of the pan-Arctic watershed (Fig. HTC38). DOC is the dominant form of organic carbon transported by arctic rivers, its flux is expected to change as the Arctic continues to warm, and it can be an important energy source to the microbial food web and tracer in the Arctic Ocean. In future years we will consider fluxes of additional compounds as well as biogeochemical processing within river systems.

Fig. 38 -- Map of the pan-Arctic watershed and the watersheds of the six largest arctic rivers

Fig. HTC38. Map of the 16.8 x 106 km2 pan-Arctic watershed and the watersheds of the six largest arctic rivers. Together, these six watersheds cover 11.3 x 106 km2 or 67% of the pan-Arctic watershed. The red dots show the water sampling locations of the Arctic Great Rivers Observatory (www.arcticgreatrivers.org) and the red line shows the boundary of the pan-Arctic watershed.

Several recent papers have investigated DOC flux in arctic rivers (Cooper et al. 2005, Raymond et al. 2007, Cooper et al. 2008, Holmes et al. 2011). However, none of these papers have addressed inter-annual trends. Therefore, using DOC concentration data from the Arctic Great Rivers Observatory (www.arcticgreatrivers.org) and daily discharge from Arctic-RIMS (http://rims.unh.edu) and other sources as outlined in Holmes et al. (2011), we calculated annual DOC fluxes for the six largest arctic rivers from 2003 to 2010. We used the USGS LOADEST program (Runkel et al. 2004) to calculate DOC fluxes based on discharge-concentration relationships for each river. Inter-annual variability in DOC flux is assumed to be a function of differing annual quantity and seasonal distribution of discharge, not differing discharge-concentration relationships over the 8-year period we consider. This assumption is supported by the absence of clear trends in discharge-concentration relationships among years within any single river (Fig. HTC39).

Fig. 39 -- Discharge-DOC concentration relationships for the six largest arctic rivers

Fig. HTC39. Discharge-DOC concentration relationships for the six largest arctic rivers. Each point represents a single day when a DOC sample was collected. Though the relationships differ substantially among rivers, there are no clear trends across years within individual rivers.

In 2010, calculated DOC flux from the six largest arctic rivers ranged from 0.6 Tg in the Kolyma River to 5.3 Tg in the Lena River (Fig. HTC40). DOC flux in 2010 was less than the 2003-2009 average for the Yenisey, Lena, Kolyma, Yukon, and Mackenzie rivers, but not the Ob'.

Fig. 40 -- Annual fluxes of DOC

Fig. HTC40. Annual fluxes of DOC for the 2003-2010 period for the Ob', Yenisey, Lena, Kolyma, Yukon and Mackenzie rivers.

Combined DOC flux from the six largest arctic rivers declined for the fourth straight year in 2010, closely following the pattern of their combined discharge (Fig. HTC41). Though there are clear indications of increasing arctic river discharge during the past several decades (Peterson et al. 2002; McClelland et al. 2006; Déry et al. 2009; and see the essay on River Discharge), over sub-decadal periods long-term trends are obscured by high inter-annual variability and multiyear oscillations. Indeed, the combined discharge of the six largest arctic rivers increased each year between 2003 and 2007 but has decreased each year since 2007 (Fig. HTC41). Because DOC export is strongly influenced by variations in river discharge, detection of long-term trends in DOC flux requires observations over multiple years. Lack of DOC data for this set of rivers prior to ~10 years ago hinders detection of long-term trends in DOC flux, such as might be associated with the multi-decadal increase in discharge or changing discharge-concentration relationships linked to permafrost thaw, changing vegetation, altered fire regimes, or direct human impacts on hydrology such as dams. However, ongoing data collection efforts will facilitate detection and attribution of future changes.

Fig. 41 -- Combined annual DOC flux (red and discharge

Fig. HTC41. Combined annual DOC flux (red dots) and discharge (black dots) for the six largest arctic rivers.

References

Cooper, L., R. Benner, J. McClelland, B. Peterson, R. Holmes, R. Raymond, D. Hansell, J. Grebmeier, and L. Codispoti. 2005. Linkages among runoff, dissolved organic carbon, and the stable oxygen isotope composition of seawater and other water mass indicators in the Arctic Ocean. Journal of Geophysical Research 110:G02013, doi:1029/2005JG000031.

Cooper, L. W., J. W. McClelland, R. M. Holmes, P. A. Raymond, J. J. Gibson, C. K. Guay, B.J. Peterson. 2008. Flow-weighted values of runoff tracers (δ18O, DOC, Ba, alkalinity) from the six largest arctic rivers. Geophysical Research Letters, L18606, doi:10.1029/2008GL035007.

Déry, S. J., M. A. Hernandez-Henriquez, J. E. Burford, and E. F. Wood. 2009. Observational evidence of an intensifying hydrological cycle in northern Canada. Geophysical Research Letters 36:L13402, doi:13410.11029/12009GL038852.

Holmes, R. M., J. W. McClelland, B. J. Peterson, S. E. Tank, E. Bulygina, T. I. Eglinton, V. V. Gordeev, T. Y. Gurtovaya, P. A. Raymond, D. J. Repeta, R. Staples, R. G. Striegl, A. V. Zhulidov, and S. A. Zimov. 2011. Seasonal and annual fluxes of nutrients and organic matter from large rivers to the Arctic Ocean and surrounding seas. Estuaries and Coasts, DOI 10.10007/s12237-011-9386-6.

McClelland, J. W., S. J. Dery, B. J. Peterson, R. M. Holmes, and E. F. Wood. 2006. A pan-arctic evaluation of changes in river discharge during the latter half of the 20th century. Geophysical Research Letters 33:L06715, doi:06710.01029/02006GL025753.

Peterson, B. J., R. M. Holmes, J. W. McClelland, C. J. Vorosmarty, R. B. Lammers, A. I. Shiklomanov, I. A. Shiklomanov, and S. Rahmstorf. 2002. Increasing river discharge to the Arctic Ocean. Science 298:2171-2173.

Raymond, P. A., J. W. McClelland, R. M. Holmes, A. V. Zhulidov, K. Mull, B. J. Peterson, R. G. Striegl, G. R. Aiken, and T. Y. Gurtovaya. 2007. Flux and age of dissolved organic carbon exported to the Arctic Ocean: A carbon isotopic study of the five largest arctic rivers. Global Biogeochemical Cycles 21:GB4011, doi:4010.1029/2007GB002934.

Runkel, R. L., C. G. Crawford, and T. A. Cohn. 2004. Load Estimator (LOADEST): A FORTRAN Program for Estimating Constituent Loads in Streams and Rivers. Page 69 p. U.S. Geological Survey Techniques and Methods Book 4.