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April
3, 2007 Life as we know it,
from the most
basic microbes to our human neighbors, is carbon based. By
investigating how
carbon cycles through ecosystems, scientists can learn valuable
information
about food chains, nutrient cycling, and productivity. Because carbon
dioxide
is a greenhouse gas, with the ability to influence temperature, an
accurate
global carbon budget is needed to address climate change. On Earth, carbon is
continually
cycling through terrestrial systems, inland waters, the ocean, and the
atmosphere. Until little over a decade ago, when calculating the
terrestrial
component of the global carbon budget, inputs were limited to the ocean
and the
land. Because inland water bodies cover less than 1% of the
Earth’s surface, it
was assumed that their contribution was inconsequential. This view was
recently challenged
in an Ecosystems paper highlighting the findings of
a While rivers were
introduced into
global carbon budget assessments in the late 1990s, Cole and colleagues
argue
that current models are limited by a narrow definition of how rivers
transport
carbon. By depicting rivers as "pipes" that passively deliver
terrestrial carbon to the sea, models fail to capture the complex
transformations that occur on the journey toward the ocean. The fact
is,
according to the authors, that half of the terrestrial carbon entering
inland
waters is destined for a fate outside of the ocean’s salty
shores. Where does the
remaining
terrestrial carbon go? Approximately 40% is returned to the atmosphere
as [CO2]
and 12% is stored in sediments. This holds true across a range of
inland
systems, from lakes and rivers to reservoirs and wetlands. Carbon
budgets that
are based on the passive pipe view are flawed because in-system
transformations
fall off the balance sheets. Even if models were adjusted to embrace a
more
dynamic view of river inputs, they would need further amending to
include the
true range of inland waters. Take, for instance,
the role
played by lakes and reservoirs. By burying carbon in their sediments,
lakes
serve as important regional carbon stores. In aggregate, lakes play a
significant role in the global carbon budget. On an annual basis, they
bury 40%
as much carbon as the ocean. Reservoirs, which are steadily increasing
in
number, bury more organic carbon than all natural lake basins combined
and
exceed oceanic organic carbon burial by more than 1.5-fold. These findings
debunk the concept
that inland waters are inconsequential when accounting for the global
carbon
budget; instead they are places of complex and active carbon
transformation.
The take home message from the authors: "Continental hydrologic
networks,
from river mouths to the smallest upstream tributaries, do not act as
neutral
pipes— they are active players in the carbon cycle despite
their modest size."
As global carbon
budget models
move from static boxes to dynamic flows, future models should take into
account
the myriad of ways that inland waters contribute to the carbon cycle.
In many
cases, these aquatic systems are biogeochemical "hot spots" within
the terrestrial landscape with contributions that are significant at
regional
to global scales.
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