Climate Change II
Estimating the Carbon Footprint
of Lakes and Reservoirs
Lindsay Yasarer
The role of lakes in the global carbon
cycle
arbon has become a notorious
element in global society. The role of
carbon dioxide (CO2) and methane
(CH4) as greenhouse gases causing
global climate change is widely known.
Consequently, tracking the global carbon
cycle and understanding sources and sinks
of carbon has become a significant line
of scientific interest. On the stage of lake
management, nitrogen and phosphorus
tend to be the main actors, with carbon
occupying a supporting role. However,
research has demonstrated that lakes,
reservoirs, and ponds have an important
role in carbon transport, processing, and
storage (Tranvik et al. 2009).
Carbon enters lake systems from
the watershed as inflows of dissolved
inorganic carbon (DIC), as well as
dissolved and particulate organic carbon
(DOC and POC). The dominant form
of carbon depends on the hydrology
and location of the water body. Carbon
can also enter the system via uptake of
atmospheric CO2. Water bodies that are
particularly eutrophic may actually show
a net influx of atmospheric carbon into the
system during summer months due to high
levels of photosynthetic algae (Balmer
and Downing, 2011). Within a waterbody CO2 is both
generated from respiration and utilized
for photosynthesis. Depending on the
productivity of the system, a great deal of
organic carbon may be generated from the
growth of algae, bacteria, aquatic plants,
and aquatic organisms. Global internal
primary production in lakes is estimated
to be 650 Tg (1 Tg = 1012 g) carbon per
year (Tranvik et al. 2009) (Figure 1; Table
1).
Organic carbon within lake systems
then becomes either buried in sediments,
C
mineralized to calcium carbonate, CO2,
or CH4, or is transported downstream.
Extrapolated sediment carbon storage
rates are between 30 to 70 Tg carbon per
year, while estimates of CO2 emissions
globally from lakes are around 530 Tg
carbon per year (Tranvik et al. 2009).
Estimates of CH4 emissions are smaller,
8-48 Tg carbon per year, but it is
important to consider that methane has
a radiative forcing 20x higher than CO2
(Bastviken et al. 2004).
Reservoirs have the potential to
produce high CO2 and CH4 emissions if
a large amount of organic soil and plant
biomass is flooded during impoundment.
Emissions are typically highest in the first
few years after impoundment and tend to
decrease over time (St. Louis et al. 2000).
Tropical reservoirs show the highest
fluxes of carbon, particularly CH4, most
likely due to higher water temperatures
that increase rates of decomposition (St.
Louis et al. 2000). A group of researchers
compiled estimated reservoir fluxes of
CO2 and CH4, along with an estimate of
the global surface area of reservoirs, to
conclude that globally reservoirs emit
approximately 270 Tg of carbon in the
form of CO2 and 53 Tg of carbon in the
form of CH4 per year. Reservoirs also
seem to have higher areal fluxes per meter
Figure 1. Illustration demonstrates carbon pathways and pools within a typical lake or reservoir.
Reservoirs would also have off-gassing of CO2 and CH4 downstream of the reservoir outlet,
which is not shown here. Uptake of CO2 refers to uptake by all photosynthetic organisms,
including phytoplankton and aquatic plants. (Illustration was developed by author based on a diagram
in Tranvik et al. 2009.)
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Table 1. Global estimates of the quantity of carbon in various pools in lakes and
reservoirs. Citations for values are included in text.
Estimated carbon sources
Global estimates of carbon (Tg-Carbon)
Lakes
Internal primary production
650N/A
Long-term sediment storage
30-70N/A
CO2 emissions
CH4 emissions
Total carbon emissions
squared of both CO2 and CH4 compared to
natural lakes (St. Louis et al. 2000).
Many assumptions have been
incorporated into these estimates.
However, they are useful to begin to tease
apart the role of lakes and reservoirs
in the global carbon cycle. One could
consider it similar to estimating a personal
carbon footprint (the net greenhouse
gas emissions emitted as a result of
lifestyle, travel, and purchases). In order
to determine your own carbon footprint
you have to make generalizations on
personal miles driven, gas mileage of
personal vehicle, meat consumption,
food sources, consumption of goods, use
of electricity, etc. One can imagine how
many variables need to be considered.
Estimating the carbon footprint of lakes
is similarly complicated. The numbers
cited above were derived using a
compilation of literature reported values
and extrapolating to estimated global lake
or reservoir areas. However, you may be
wondering: how are emissions estimated?
Measuring carbon emissions from lakes
and reservoirs
There are a variety of methods
used to track carbon emissions in the
aquatic environment. The thin boundary
layer method is a typical approach for
estimating CO2 fluxes and involves
measurements of pH, alkalinity and/or
dissolved inorganic carbon, temperature,
an estimate of wind speed, as well as
the use of basic principles of chemistry.
Briefly, pH and alkalinity are used to
estimate total inorganic carbon and then
equilibrium constants for carbonic acid
and bicarbonate, as well as Henry’s
constant are used to estimate the partial
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Reservoirs
530270
8 - 48
53
538 - 578
323
pressure of CO2 (pCO2). Fluxes can
then be estimated considering the partial
pressure of CO2 in the atmosphere
and wind speed (there are many more
details inherent in these calculations,
see Balmer and Downing [2011] for
more information). Such an approach
has been useful for making regional and
global calculations of CO2 fluxes from
water bodies and takes advantage of
data that may already be available from
water monitoring studies. However, this
approach cannot approximate CH4 fluxes.
There are also a variety of tools and
techniques available to directly sample
and measure gas fluxes. Floating static
gas chambers are a popular method
to measure diffusive fluxes, due to
convenience and low cost. Chambers
are employed at the water surface and
the air inside the chamber is sampled
over time to measure the change in
gas concentrations inside the chamber.
Plotting these measurements and
applying linear regression can estimate
the diffusive gas flux (Goldenfum 2010).
Static gas chamber design varies, as
many are made in-house. Gas samples
are either measured in-line continuously
using portable gas analyzers, or samples
are measured non-continuously in distinct
time intervals by collecting air samples
from the chamber and then analyzing gas
concentrations using gas chromatography
in the laboratory (Goldenfum 2010). Figures 2a and b show the deployment
of such a static gas chamber developed
by Dr. Zhe Li and colleagues at the
Chongqing Institute of Green and
Intelligent Technology for collecting
measurements on the Three Gorges
Reservoir in China.
Measuring methane ebullition, or
bubbling, from the sediment interface
requires a host of different techniques.
Jacob Shiba, a master’s student, and
Dr. Michael Anderson, both at the
University of California, Riverside,
use a variety of methods to study the
mechanisms of methane release from lake
sediments in Lake Elsinore, CA (shown
in Figures 3a, b and 4a, b). As shown in
the photos, methods range widely from
the “displacement and capture” method
involving a weighted funnel fitted with
a plastic tube and stopper (3a), to a
measurement station equipped with a
BioSonics hydroacoustic transducer,
which traces the bubbles escaping the
sediment through SONAR technology
(3b). Sediment can also be sampled with a
coring apparatus and brought back to the
lab for incubation and observation (4a, b).
A useful resource for those
interested in measuring freshwater CO2
and CH4 fluxes is the report, “GHG
Measurement Guidelines for Freshwater
Reservoirs,” derived from the UNESCO/
IHA Greenhouse Gas Emissions from
Freshwater Reservoirs Research Project.
This document outlines many different
approaches and provides examples on
calculating fluxes using the methods
discussed above. It is freely available
online (see references for link) and while
the focus is on reservoirs, many of the
methods would also apply to lakes or
ponds.
The big picture
Quantifying lake and reservoir
carbon fluxes is important for enhancing
our understanding of the role of inland
waters in the global carbon cycle and in
the overall climate system. For example,
such studies have demonstrated that
unaccounted methane emissions from
lakes and reservoirs add 20 percent to
previous estimates of global methane
emissions. (Tranvik et al. 2009). Estimates still do not account for
emissions from small farm ponds, which
are likely to contribute as well. Also, it
is clear from these studies that artificial
impoundments emit a large share of
greenhouse gases. With the number of
impoundments continuing to increase
around the world, quantifying the carbon
impact will become more important as
will understanding how to mitigate carbon
emissions during reservoir development.
Figure 2A and B. Students placing static floating chamber on a stretch of the Three Gorges Reservoir in China in order to measure CO2 and CH4
fluxes. (Photos are courtesy of Dr. Zhe Li, Associate Professor at the Chongqing Institute of Green and Intelligent Technology.)
Figure 3a and b. Sediment gas volume is quantified using the displacement and capture method, involving
a weighted funnel and measuring tube (a, left). Ready to deploy on Lake Elsinore, CA – telescoping poles
attached to a base that allows for rigid mounting of a BioSonics hydroacoustic transducer to collect data from
a fixed, stationary location (b, above). These data can be processed to determine the ebullition rate, as well as
the location of bubbles released through time. (Photos are courtesy of Jacob Shiba, a master’s student at the University
of California, Riverside.)
Fall 2015 / NALMS • LAKELINE
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While it is clear that lakes and
reservoirs are important actors in
the global carbon cycle, I’m not
proposing that they are “polluters”
or serious contributors to global
climate change. Especially with
respect to lakes, they represent a
natural conduit transferring carbon
from land to sea. However, we now
know that they not only transfer
this carbon, they have a large role
in either storing or emitting carbon
back to the atmosphere.
References
Balmer, M.B. and J.A. Downing.
2011. Carbon dioxide
concentrations in eutrophic
lakes: Undersaturation implies
atmospheric uptake. Inland
Waters 1: 125-132.
Bastviken, D., J.J. Cole, M.
Pace and L. Tranvik. 2004.
Methane emissions from
Figure 4a and b. Sediment samples taken from Lake Elsinore, CA, allow researchers to observe the
lakes: Dependence of lake
formation, storage, and release of gas bubbles from the sediment. Bubble escaping the sediment interface
can be observed in b (right). (Photos are courtesy of Jacob Shiba, a master’s student at the University of California,
characteristics, two regional
Riverside.)
assessments, and a global
estimate. Glob. Biogeochem.
2000. Reservoir surfaces as sources of
Lindsey Yasarer recently
Cycles 18(4).
greenhouse gases to the atmosphere: a
graduated with her Ph.D.
Goldenfum, J.A. (Ed.) 2010. GHG
global estimate. BioScience 50(9): 766
in environmental science
Measurement Guidelines for Freshwater
-775.
from the University of
Reservoirs. International Hydropower
Tranvik, L.J., J.A. Downing, J.B.
Kansas. Currently she
Association, London, UK. 138p.
Cotner, S.A. Loiselle, R.G. Striegl,
is working as the career
Available at: http://www.hydropower.
T.J. Ballatore, P. Dillon, K. Finlay,
planning specialist in
org/ghg-measurement-guidelines-forK. Fortino and L.B. Knoll. 2009.
the dean’s office at the
freshwater-reservoirs. Accessed: July 1,
Lakes and reservoirs as regulators of
University of Mississippi
2015.
carbon cycling and climate. Limnology
School of Engineering. She served NALMS as the
St. Louis, V.L., C.A. Kelly, E. Duchemin,
Oceanography 54 (6): 2298-2314.
student director in 2013 and 2014. c
J.W.M. Rudd and D.M. Rosenberg.
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