Environmental Engineering Reference
In-Depth Information
smaller scales that showed that rivers and lakes tend to emit CO
2
to the atmosphere,
Richey et al. (2002)
calculated the loss of CO
2
from the entire Amazon River, its tributaries,
and its flooded flood plain. They estimate that about 0.5 Pg C/y is released as CO
2
from
the “wet” parts of the Amazon basin. Scaling up to all tropical forests,
Richey et al. (2002)
put the estimate at 0.9 Pg C/y. This loss of gaseous C from the aquatic parts of the terres-
trial system is 10-fold larger than the fluvial export of DOC plus DIC to the ocean.
The CO
2
evaded to the atmosphere could explain the difference between the top-down
and bottom-up estimates of the Amazon basin C balance.
Recall that the fates of NEP are burial plus export. The CO
2
evaded from the aquatic
systems arises either because terrestrially derived organic C is metabolized in the river
(e.g., this came from terrestrial export, E), or because soil CO
2
is transported into the river
directly. It is important to note that a local flux tower on land misses both of these pro-
cesses. Soil CO
2
that was transported in water is terrestrial R that never made in into the
local atmosphere of the forest. So, for the Amazon, local flux towers might measure NEP
that is 0.5 Pg C/y larger than would be seen by large-scale atmospheric modeling that
included these lateral losses. There are many other possible explanations for the discrep-
ancy. For example,
Lloyd et al. (2007)
suggest that eddy flux towers underestimate terres-
trial R in the Amazon for aerodynamic reasons. The point here is that at large spatial
scales, the lateral, aquatic fluxes in DOC, DIC, and CO
2
can affect the apparent C balance
of terrestrial ecosystems.
CONCLUDING REMARKS
The answer to the question, “What factors regulate the carbon cycle?” is complex for
several reasons. First, there are many interrelated factors, some of which are biological
(tree growth), physical (diffusion of CO
2
into the ocean), or both (the preservation of
organic matter in sediments, or the transport of DOC into a lake). Second, what we mean
by regulation depends on both the temporal and spatial boundaries of the system. Over
short timescales (minutes), the physiology of stomates affects the local atmosphere that is
in contact with a forest. Over decadal timescales, drought, fire, land clearing, and the use
of fossil fuels affect the concentration of CO
2
in the entire atmosphere. Over geologic time-
scales, the long-term preservation of organic C in sediments and rocks and the return of
CO
2
during subduction, volcanism, and reverse weathering are the major controls. The
ecosystem approach allows the scientist to set temporal and spatial boundaries that make
a particular problem tractable and offer different insights into “the answer.”
References
Alin, S.R., Johnson, T.C., 2007. Carbon cycling in large lakes of the world: A synthesis of production, burial, and
lake-atmosphere exchange estimates. Global Biogeochem. Cycles 21. doi: 10.1029/2006GB002881.
Bacastow, R.B., Keeling, C.D., Lueker, T.J., Wahlen, M., Mook, W.G., 1996. The
13
C Suess effect in the world
surface oceans and its implications for oceanic uptake of CO
2
: Analysis of observations at Bermuda. Global
Biogeochem. Cycles 10, 335
346.
Balch, W.M., Fabry, V.J., 2008. Ocean acidification: Documenting its impact on calcifying phytoplankton at basin
scales. Mar. Ecol Prog Ser 373, 239
247.
