carbon cycling in the arctic
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DOI: 10.1126/science.1258235, 870 (2014);345 Science
Lars TranvikCarbon cycling in the Arctic
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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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