carbon cycling in the arctic

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DOI: 10.1126/science.1258235 , 870 (2014); 345 Science Lars Tranvik Carbon cycling in the Arctic This copy is for your personal, non-commercial use only. clicking here. colleagues, clients, or customers by , you can order high-quality copies for your If you wish to distribute this article to others here. following the guidelines can be obtained by Permission to republish or repurpose articles or portions of articles ): August 21, 2014 www.sciencemag.org (this information is current as of The following resources related to this article are available online at http://www.sciencemag.org/content/345/6199/870.full.html version of this article at: including high-resolution figures, can be found in the online Updated information and services, http://www.sciencemag.org/content/345/6199/870.full.html#related found at: can be related to this article A list of selected additional articles on the Science Web sites http://www.sciencemag.org/content/345/6199/870.full.html#ref-list-1 , 2 of which can be accessed free: cites 7 articles This article http://www.sciencemag.org/cgi/collection/oceans Oceanography subject collections: This article appears in the following registered trademark of AAAS. is a Science 2014 by the American Association for the Advancement of Science; all rights reserved. The title Copyright American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the Science on August 21, 2014 www.sciencemag.org Downloaded from on August 21, 2014 www.sciencemag.org Downloaded from

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Page 1: Carbon cycling in the Arctic

DOI: 10.1126/science.1258235, 870 (2014);345 Science

Lars TranvikCarbon cycling in the Arctic

This copy is for your personal, non-commercial use only.

clicking here.colleagues, clients, or customers by , you can order high-quality copies for yourIf you wish to distribute this article to others

  here.following the guidelines

can be obtained byPermission to republish or repurpose articles or portions of articles

  ): August 21, 2014 www.sciencemag.org (this information is current as of

The following resources related to this article are available online at

http://www.sciencemag.org/content/345/6199/870.full.htmlversion of this article at:

including high-resolution figures, can be found in the onlineUpdated information and services,

http://www.sciencemag.org/content/345/6199/870.full.html#relatedfound at:

can berelated to this article A list of selected additional articles on the Science Web sites

http://www.sciencemag.org/content/345/6199/870.full.html#ref-list-1, 2 of which can be accessed free:cites 7 articlesThis article

http://www.sciencemag.org/cgi/collection/oceansOceanography

subject collections:This article appears in the following

registered trademark of AAAS. is aScience2014 by the American Association for the Advancement of Science; all rights reserved. The title

CopyrightAmerican Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by theScience

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Page 2: Carbon cycling in the Arctic

INSIGHTS | PERSPECTIVES

870 22 AUGUST 2014 • VOL 345 ISSUE 6199 sciencemag.org SCIENCE

The arctic landmass is covered by nu-

merous shallow ponds, lakes, and

streams (see the photo). On page 925

of this issue, Cory et al. ( 1) show that

sunlight drives the production of sub-

stantial amounts of carbon dioxide in

these waters through photochemical oxida-

tion of organic matter. The results have im-

portant implications for the carbon cycle in

the Arctic and how it responds to, and pos-

sibly even magnifies, climate change.

Inland waters—lakes, rivers, and streams,

as well as reservoirs created by humans—

cover only a few percent of Earth’s continents

and were long considered to be merely a

pipe that transports carbon from land to sea

( 2). But recently, scientists have shown that

these waters are responsible for transform-

ing and transporting substantial amounts

of carbon ( 3): Sediments in inland waters

store carbon, whereas the waters themselves

emit carbon dioxide ( 4) and methane ( 5) to

the atmosphere. Each year, in-

land waters emit an estimated

1 gigaton of carbon as carbon

dioxide to the atmosphere ( 6).

For comparison, the global net

flux of carbon dioxide into the

oceans is about 2 gigatons of

carbon per year.

Much of the organic carbon

in lakes comes from soils and

wetlands. This dissolved or-

ganic carbon (DOC) is a mix-

ture of colloidal and dissolved

substances, which carry the

brownish color often seen in

lake water. Bacteria slowly de-

grade DOC to form carbon diox-

ide, most of which is emitted to

the atmosphere. Photochemical

processes driven by ultraviolet

and visible sunlight also con-

tribute to DOC degradation,

creating carbon dioxide, as well

as substances that are easier for

bacteria to metabolize than the

original DOC ( 7). This phenom-

enon has been widely studied

in the laboratory and in lakes,

but information about its wider

importance is only beginning to emerge ( 8).

Cory et al. performed a large set of ex-

periments in ponds, lakes, streams, and riv-

ers across the 8000-km2 Kuparuk Basin at

the North Slope of Alaska. They incubated

samples under different light conditions to

tease apart how much sunlight and bacte-

ria contribute to the production of carbon

dioxide in these waters. The authors take

into account several factors, including the

importance of different wavelengths in the

sunlight, changes in light with water depth,

and the amount of incoming solar radiation.

They conclude that sunlight is a dominant

driver of carbon processing in the water

column. The results, scaled up to represent

all inland waters of the Kuparuk basin, sug-

gest that photochemical processes cause 70

to 95% of all degradation of DOC to carbon

dioxide (“mineralization”) in these waters.

Previous studies at lower latitudes have

pointed to a less dominant role of sunlight

in driving DOC processing. Koehler et al. re-

cently concluded that direct photochemical

mineralization is responsible for about 10%

of the emissions of carbon dioxide from all

lakes on Earth ( 8). Sequential photochemi-

cal-bacterial mineralization may account for

a similar amount ( 9), giving a global contri-

bution of photochemical mineralization of

about 20%. In contrast, Cory et al. ( 1) suggest

that photochemical mineralization in arctic

waters drives up to 40% of the carbon diox-

ide emissions.

The authors propose several possible rea-

sons for the larger relative share of mineral-

ization attributed to sunlight in the Arctic.

The most plausible may be the predomi-

nance of very shallow waters (see the photo).

In shallow waters, sunlight penetrates most

of the water column, leaving only a small

dark zone where sunlight-independent mi-

crobial degradation dominates the decay of

the organic matter.

It remains unclear whether the high rela-

tive importance of photochemical mineral-

ization is specific to the Arctic. If the findings

of Cory et al. also apply outside of the Arctic,

the global estimate that photochemical min-

eralization causes about 10 to 20% of lake

carbon dioxide emissions ( 8) may be too low.

Thawing of permafrost soils may con-

tribute large amounts of DOC, with a great

potential to be mineralized by the Sun. On

the other hand, the increasing DOC will also

stimulate microbial mineralization. Higher

concentrations of colored DOC may change

the light climate in lakes and ponds in fa-

vor of bacterial mineralization, resulting in

a more modest future importance of pho-

tochemical mineralization relative to other

processes than reported by Cory et al. Re-

gardless of the relative roles of microbes and

sunlight as the arctic warms up, their study

points to an important mechanism that con-

trols how much carbon is evaded to the at-

mosphere and how much is carried to the

Arctic Ocean. ■

REFERENCES

1. R. M. Cory et al., Science 345, 925 (2014). 2. Intergovernmental Panel on Climate Change (IPCC),

Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, S. Solomon et al., Eds. (Cambridge Univ. Press, Cambridge/New York, 2007).

3. J. J. Cole et al., Ecosystems (N. Y.) 10, 172 (2007). 4. P. A. Raymond et al., Nature 503, 355 (2013). 5. D. Bastviken, L. J. Tranvik, J. A. Downing, P. M. Crill, A.

Enrich-Prast, Science 331, 50 (2011). 6. IPCC, Climate Change 2013: The Physical Science Basis,

in Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, P. F. Stocker et al., Eds. (Cambridge Univ. Press, Cambridge/New York, 2013).

7. R. G. Wetzel, P. G. Hatcher, T. S. Bianchi, Limnol. Oceanogr. 40, 1369 (1995).

8. B. Koehler, T. Landelius, G. A. Weyhenmeyer, N. Machida, L. J. Tranvik, Global Biogeochem. Cycles 10.1002/2014GB004850 (2014).

9. W. L. Miller, M. A. Moran, Limnol. Oceanogr. 42, 1317 (1997).

Carbon cycling in the Arctic

The power of sunlight. Cory et al. ( 1) evaluate the role of sunlight

in carbon dioxide production in arctic inland waters in the Kuparuk

Basin, Alaska. The authors consider both direct and indirect (bacteria-

mediated) pathways and find a surprisingly strong role of sunlight. 10.1126/science.1258235

By Lars Tranvik

Sunlight drives the emission of carbon from arctic waters

BIOGEOCHEMISTRY

Limnology, Department of Ecology and Genetics, Uppsala University, Norbyvägen 18D, 75236, Uppsala, Sweden. E-mail: [email protected]

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Published by AAAS