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RETRACTED: Globally, Freshwater
Ecosystems Emit More CO
2
Than the
Burning of Fossil Fuels
Peter C. Pollard *
School of Engineering and the Built Environment, Griffith University, Southport, QLD, Australia
Freshwater emits substantial volumes of CO
2
to the atmosphere. This has largely gone
unnoticed in global carbon budgets. My aim was to quantify the CO
2
emanating from
freshwater from 66°Nto47
°S latitudes via in situ bacterial respiration (BR). I determined
BR (n= 326) as a function of water temperature. Freshwater is emitting CO
2
at a rate of
58.5 Pg C y
−1
(six times that of fossil fuel burning). Most is emitted from the Northern
Hemisphere. This is because the high northern summer temperatures coincide with most
of the world’s freshwater. Diffuse DOC sources, for example dust, may be driving high
freshwater BR. However, many sources remain elusive and not individually quantified in the
literature. We must include freshwater CO
2
emissions in climate models. Identifying,
quantifying and managing freshwater’s diffuse sources of Dissolved Organic Carbon
(DOC) will hopefully provide us with another opportunity to change our current climate
trajectory.
Keywords: freshwater DOC flux, bacterial respiration, BR, climate change, global carbon budget, freshwater global
CO
2
emissions
INTRODUCTION
Of all the water on the planet only 0.009% is in our freshwater lakes (0.0086%) and rivers (0.0002%)
(Shiklomanov, 1993). This is a miniscule fraction of the Earth’s surface water (Figure 1). Hence, the
global carbon budget focuses on the oceans with the land taking up most of the carbon. But is this
justified? Surface freshwaters mediate large transfers of organic carbon to the atmosphere and must
be considered if we want to change our current climate change track (Battin et al., 2009). The latest
IPCC, (2021) Sixth Report shows freshwater outgassing of CO
2
is a 0.3 Pg C y-1 of the total global
respiration and fire of 142 Pg C y-1.
We are now seeing the crucial global role of freshwater transitioning carbon from terrestrial to
atmospheric biomes. This perspective has come with the advent of high-resolution satellite mapping
of freshwater (Pekal et al., 2016) and limnologists collaborating as part of global freshwater research
networks (http://www.laketemperature.org/index.html) (Hamilton et al., 2015)—freely sharing
long-term data (especially that of water temperatures). Lakes and rivers are quantitatively being seen
as connecting the lithosphere to the atmosphere (Ward et al., 2017). Freshwater’s critical role in the
global carbon balance is being unraveled. With freshwater warming faster than the atmosphere at an
alarming rate (O’Reilly et al., 2015) understanding these connections has become a matter of
urgency.
Freshwater connects the soil with the oceans and the atmosphere to complete the global cycle (del
Giorgio and Williams, 2005). Aerobic freshwater bacteria respiration contributes profoundly to the
global atmospheric carbon budget (Richey et al., 2002;Cole et al., 2007;Aufdenkampe, et al., 2011;
Edited by:
Thomas Hein,
University of Natural Resources and
Life Sciences Vienna, Austria
Reviewed by:
Ni Maofei,
Guizhou Minzu University, China
Jianqiu Zheng,
Pacific Northwest National Laboratory
(DOE), United States
*Correspondence:
Peter C. Pollard
p.pollard@griffith.edu.au
Specialty section:
This article was submitted to
Freshwater Science,
a section of the journal
Frontiers in Environmental Science
Received: 26 March 2022
Accepted: 09 May 2022
Published: 06 June 2022
Citation:
Pollard PC (2022) Globally, Freshwater
Ecosystems Emit More CO
2
Than the
Burning of Fossil Fuels.
Front. Environ. Sci. 10:904955.
doi: 10.3389/fenvs.2022.904955
Frontiers in Environmental Science | www.frontiersin.org June 2022 | Volume 10 | Article 9049551
ORIGINAL RESEARCH
published: 06 June 2022
doi: 10.3389/fenvs.2022.904955
RETRACTED
Ward et al., 2017). This carbon return pathway has gone
unnoticed in the predictive mathematical modelling of our
climate. But does appear in the latest IPCC, (2021)report
showing a 0.3 Pg C y
−1
being emitted from freshwater globally.
Since 2007, estimates of the carbon emissions from
freshwater lakes and rivers have doubled every 3 years to the
current estimate of 3.8 Pg C y
−1
(Cole et al., 2007;Tranvik et al.,
2009;Raymond, et al., 2013;Ward et al., 2017). Recent reviews
aretryingtomakequantitativesenseofthecomplex
interactions of global carbon cycling across the Atmosphere,
Biosphere, Hydrosphere and Lithosphere (Borges et al., 2015;
Sawakuchi et al., 2017). They make the valid point that we are
still underestimating freshwater carbon outgassing. Some, and
myself included, have seen the tropical and sub-tropical
freshwater CO
2
emissions as disproportionally larger than
the temperate environments of the Northern Hemisphere
(Pollard and Ducklow, 2011;Ward et al., 2017). This all goes
to highlight our uncertainty of the global rate of carbon passing
from terrestrial organic carbon through the lakes and rivers to
the atmosphere.
Freshwater CO
2
outgassing measures have been biased
towards temperate Northern latitudes (Sobek et al., 2005;
2007) resulting in global estimates of around 1 Pg C y
−1
outgassing (Cole et al., 2007). While tropical freshwater
FIGURE 1 | How water is distributed on Earth. The tiny pixel over
Georgia represents all surface freshwater lakes and rivers. Illustration after
USGS depiction by Perlman, Cook and Nieman who used the data of
Shiklomanov (1993). https://water.usgs.gov/edu/gallery/global-water-
volume.html.
FIGURE 2 | Sampling sites used in this study.
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Pollard Global Freshwater CO
2
Emissions
RETRACTED
CO
2
outgassing measures range from 0.9 Pg C y
−1
from
African inland waters to 2.9 Pg C y
−1
for the Amazon (Ward
et al., 2017).
Few consider the global quantitative consequences of lakes and
rivers as major sources of CO
2
to the atmosphere (Richey et al.,
2002;Cole et al., 2007;Battin et al., 2009;Travik et al., 2009;
Sawakuchi et al., 2017). This is made even more difficult with
traditional methods that are complicated with terrestrial
dissolved inorganic carbon (DIC) inputs to freshwater
(Johnson et al., 2008;Weyhenmeyer et al., 2015). Determining
the relationship of temperature with bacterial respiration (BR)
across the globe is an important part of this study and is
independent of these terrestrial DIC inputs.
Proportionally, more terrestrial organic carbon is processed
within freshwater lakes and rivers through BR than primary
production; a major part of whole community respiration
(Mayorga et al., 2005;Pace and Prairie 2005;Cole et al., 2007;
McCallister and del Giorgio, 2008;Pollard and Ducklow, 2011;
Cardoso et al., 2013;Cole, 2013;Soares et al., 2019). Others have
shown that pCO
2
might be controlled by external groundwater
inputs of dissolved inorganic carbon rather than by internal
metabolism. Feijoó et al., 2022, Arroita, M. Messetta, M. L.
et al. (2022) showed all streams in their study were net
emitters of CO
2
, supersaturated with CO
2
, to the atmosphere,
even those that were not net heterotrophic.
Bacterial metabolic activity (respiration; mineralisation of
organic carbon) and pCO
2
supersaturating freshwater are
positively correlated with temperature (Marotta et al., 2009;
Cardoso et al., 2013). Climate change is causing global
freshwater temperatures to rise rapidly (Acuna et al., 2008;
O’Reilly et al., 2015). Yet, the impact this increase will have on
freshwater BR and subsequent ecosystem health and global
carbon balance is a big gap in our knowledge (Acuna et al., 2008).
The aims of this study of global freshwater were to: 1) quantify
BR emissions of the greenhouse gas carbon dioxide from
freshwater across latitudes; 2) compare and contrast CO
2
emissions from the Northern and Southern Hemispheres
seasonally; 3) deliver an informed discussion on the role of
freshwater in the global carbon budget; 4) Predict future CO
2
emission as freshwater temperatures rise.
MATERIALS AND METHODS
Bacterial Respiration (BR) dominates community respiration in
most freshwater lakes and rivers (Pollard and Ducklow, 2011;
Berggren et al., 2012;Cole, 2013). Hence, I simply refer to “BR”
throughout this manuscript.
Freshwater Sampling
The rate bacterial respiration converted DOC to CO
2
was
determined in situ for 337 freshwater incubations between
2008 and 2017. Lakes (n= 253), rivers (n= 55; mostly
Amazon and Mississippi) and streams (n= 18) were sampled
at sites from latitudes 66°Nto47
°S. In 2018 another 85
measurements were made in the Arctic 66°N (Great Bear
Lake). Details of each sample site are shown on a global map
(Figure 2) and in a Supplementary Table S1 in the
Supplementary Material linked to this manuscript. The
Supplementary Table S1 shows the incubation sites with
place names, country, sampling date, their corresponding
longitudes and latitudes with a brief description of the site
that includes water temperature and depth of incubation.
BR rates (mol C.m
−3
.d
−1
) were determined using the
mathematical relationship between dissolved oxygen (DO) and
time in the incubation chamber (in situ). The relationship and
correlation co-efficient (average r
2
= 0.8) for each sample is
shown in the Supplementary Table S1.
Global freshwater surface area in each 15°latitude bandwidth
is shown in Table 1. The map of global freshwater published in
Pekel et al., 2016 was used to determine these surface areas within
each 15°latitude bandwidths across the globe. Northern cf
Southern Hemisphere freshwater surface area is 2,317,000 cf
414,300 km
2
respectively. There is more than five times more
TABLE 1 | Global freshwater surface area in each 15°latitude bandwidth is shown
in this table.
Northern hemisphere ×10
3
Km
2
75°–82.5°1.4
60°–75°724.9
45°–60°932.1
30°–45°437.9
15°–30°114.6
0°–15°106.1
Southern hemisphere
0°–15°285.5
15°–30°55.5
30°–45°50.6
45°–60°25.6
The map of global freshwater published in Pekel et al., 2016 was used to determine these
surface areas within each 15°latitude bandwidth. Northern cf Southern Hemisphere
freshwater surface area is 2,317,000 cf 414,300 km
2
respectively. There is more than five
times more freshwater surface area in the Northern cf the Southern hemisphere.
FIGURE 3 | Global freshwater bacterial respiration as a function of each
whole degree of water temperature (
xμ± SE; n = number on each data point)
The average (n= 273). Percentile error for data was 20% (shown as the dotted
lines). Additional Arctic bacterial respiration data (Great Bear Lake) (n=
85) is shown as the shaded circle, was overlaid on the rest of the global
respiration data. The additional data fit well with the relationship between
bacterial respiration and water temperature.
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Pollard Global Freshwater CO
2
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RETRACTED
freshwater surface area in the Northern cf the Southern
hemisphere.
In Situ Measurement of Aquatic Bacterial
Respiration
A YSI Sonde 6920 (Yellow Springs United States) series equipped with
a YSI 6150 Rox Optical dissolved oxygen probe was used to measure
dark chamber BR as the loss of dissolved oxygen in situ.TheSonde
was set to continuously log the depth, dissolved oxygen, temperature,
salinity and pH every minute. The probe was then sealed in a dark
chamber (2 L) made of black Perspex with one-way scuba diving
regulator valves, top and bottom’of the chamber (Pollard, 2013).
The Sonde and chamber were lowered to the sampling depth,
flushed in situ and allowed to stabilise until there was no change
in the water temperature. To start the incubation the chamber
was again flushed in situ and left for 10 min. GPS co-ordinates,
time (start and finish of incubation), date and depth were noted
and later matched with the retrieved Sonde logged data. I describe
the chamber design and validate the technique elsewhere
(Pollard, 2013), a free open access publication.
The rate bacteria used oxygen in the chamber was used to
calculate the rate bacteria mineralised DOC to emit CO
2
using a
respiration quotient of 1.2 : 1 (RQ; mole of CO
2
produced per
mole of O
2
consumed). This respiratory quotient was determined
for bacterioplankton across a range of freshwater environmental
gradients (Berggren et al., 2012).
The mathematical relationship between water temperature
and the in situ BR rates (n= 273) was determined a depth of
10 m of lakes from latitudes 66 N°to 47°S. Mean monthly global
lake freshwater surface temperatures from 1991 to 2011 were used
from Layden et al. (2015). Their data set was missing
temperatures in the 15°–30°latitude band. This was
supplemented with monthly water surface temperatures for
2007 and from 2010 to 2014 for three reservoirs between
latitudes 15°S and 30°S in South East Queensland (SEQ
Water, Australia, kindly provided water temperatures from
their in situ monitoring stations).
TABLE 2 | A few Great Bear Lake water sample calculations of bacterial respiration.
The relationship between dissolved oxygen and time in the incubation chamber was determined in situ at the depths shown. This relationship in the incubation chamber was then used to
determine the rate oxygen was being consumed at time zero. This rate of oxygen consumed was then used to determine bacteria respiration rates in situ. (mol C m
−3
.d
−1
). A full copy of the
data collected can be found in the Supplementary Material attached to this manuscript.
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Pollard Global Freshwater CO
2
Emissions
RETRACTED
Some lake profile BR measurements were in deep waters
(>20 m). Two representative profiles are presented in the main
section of the paper. The remaining deep-water data can be found
in a Supplementary Table S1. I did not have enough information
to establish the global relationship of BR as a function of deep
water-temperatures. Hence only the top 10 m data is presented in
the main manuscript.
I divided the globe into 15°band widths from latitudes 75°Nto
60°S. The freshwater surface area in these bins is given in Table 1.
Using the monthly average of freshwater surface temperatures
over 20 years of Layden et al. (2015). I determined the average
monthly rates of BR using the relationship in Figure 3. With
high-resolution maps of global freshwater surface areas of Pekel
et al. (2016), global BR rates were determined for each 15°band
shown in Table 1. Cumulative rates of CO
2
freshwater emission
in the Northern and Southern Hemispheres could finally be
directly compared as Pg C y
−1
for the top 10 m depth globally
for freshwater lakes.
This top 10 m depth was chosen to calculate the global
freshwater emissions of CO
2
for three reasons. A depth of
10 m is the: average depth Lichens (1973) estimated for global
freshwater lakes. This is the typical thermal stratification depth
separating the top 10 m from the hypolimnion. Hence surface
water temperatures could be applied to this depth (Wetzel 2001;
p75). This is also the average depth of the Amazon, the world’s
largest river (Ward et al., 2017).
Data Handling
Table 2 shows a few Great Bear Lake water sample calculations of
bacterial respiration (mol C m
−3
d
−1
). The relationship between
dissolved oxygen and time in the incubation chamber was
determined in situ at the depths shown. This relationship of
dissolved oxygen with time in the incubation chamber was then
used to determine the rate oxygen was being consumed at time
zero. This rate of oxygen consumed was then used to determine
bacteria respiration rates (mol C m
−3
d
−1
)in situ.
The observational respiration data and respiration quotient
were used to determine respiration rates. The temperature-
respiration relationship was fitted using an exponential
equation in EXCEL (Microsoft Office) accompanied by an
estimate of error (dashed lines).
I have calculated the percentile error for the bacterial
respiration data points and included the error associated with
BR measurements for each whole degree of water temperature.
The number of samples (n) at each temperature is shown in
Figure 3. The mean percentile error, determined using all the
data in Figure 3, was 20%. This value was used to determine the
uncertainty of BR as a function of water temperature Figure 3
FIGURE 4 | Global freshwater bacterial respiration in each 15°latitude bandwidth. The underlying maps of global freshwater (Modified from Pekel et al., 2016) with
the 15°latitude bandwidths of bacterial respiration (BR) overlayed. Notice BR in the Northern Hemisphere is similar to BR around the equator that also aligns with the
largest surface freshwater area (also see Table 1). This combination is the reason the North Hemisphere outgases the South Hemisphere.
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Pollard Global Freshwater CO
2
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RETRACTED
(dashed lines). The same percentile error has been applied to the
predictions of increased CO
2
emissions using projected global
water temperature increases of O’Reilly et al. (2015).
All data generated or analysed duringthisstudyareincludedin
this published article and its supplementary information files. Any
additional data are available from the corresponding author on request.
RESULTS
Figure 3 is a plot of BR (mole C.m
−3
.d
−1
) as a function of water
temperature (C°) for the top 10 m between latitudes 66°Nto47
°S.
BR was best described (r
2
= 0.5) with the following exponential
function:
Bacterial respirationmol C.m−3.d−1)0.135e0.0795×temperature(+C)
(1)
Figure 4 shows bacterial respiration in each 15°latitude
bandwidth. The Northern and Southern Hemispheres can be
compared. The underlying map is of global freshwater surface
area (based on maps of Pekel et al., 2016). Notice the BR in the
Northern summer is similar to those of the equatorial regions,
coinciding with largest freshwater surface areas of the world
Table 1. BR in the North is in harmony with that of the
warmer tropics and subtropics.
Globally, the lower latitudes (0°–15°) containing the equatorial
lakes and rivers (e.g., Amazon River, South America; Lake
Victoria, East Africa; Lake Tanganyika, East Africa) showed
high rates of respiration and emission of CO
2
. However,
globally the highest rates of BR were seen in the 15°to 30°
Northern latitude band during the Northern summer (June,
July, August, and September).
For the higher Northern latitudes, summer months showed
high rates of BR, similar to equatorial latitudes. The North’s
FIGURE 5 | From 1991 to 2011 the annual rates of carbon respired from each 15°latitude bandwidth is shown for the freshwater top 10 m. The Northern
Hemisphere (not shaded) is emitting CO
2
at twice the rate of the Southern Hemispheres (shaded area); 2.25 : 1, respectively.
FIGURE 6 | Bacterial respiration is plotted as a function of water depth
for two major reservoirs. In each case highest mineralisation rates or organic
carbon were at the bottom of each reservoir. The blocked are represents the
sediment.
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Pollard Global Freshwater CO
2
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RETRACTED
combined emissions of CO
2
dominated. This was because most
of the Earth’s freshwater lies North of 30°N(Pekel et al., 2016)
Table 1. The Northern Hemisphere BR remained high all year
round in the latitude of 0°to15 °Nbin.(Figure 4). The 0°–15°S
latitude bandwidth contained the highest rates of bacterial
respiration globally; within these latitudes you find the
Amazon River and the sub-Sahara lakes and rivers of Africa
that were responsible for most of this BR. Further south BR was
limited by the lack of freshwater in southern latitudes greater
than 15°S(Pekel et al., 2016)(Table 1).
Figure 5 lets you compare the amount of CO
2
emitted from
the Northern and Southern Hemispheres. CO
2
emissions from
the Northern Hemisphere were twice those of Sothern
Hemisphere —40.5 Pg C y
−1
cf 18.0 Pg C y
−1
respectively. In
the Northern Hemisphere the higher latitudes (above 45°N)
were responsible for the bulk of global BR—CO
2
emissions. This
was due to areas of freshwater above 45°North that coincided the
Northern summer BR that are similar to those around the
equatorial regions Table 1.
Figure 6 is a plot of bacterial respiration as a function of
depth for two lakes. Quabbin Reservoir (depth 28 m),
Massachusetts, United States (Boston’sdrinkingwater
supply) in the Northern Hemisphere. The other, Lake
Wivenhoe (depth 22 m) South East Queensland, Australia
(Brisbane’s drinking water supply) is in the Southern
Hemisphere. These are temperate and sub-tropical
freshwaters, respectively. Both water bodies showed a
positive correlation (r
2
= 0.54 and 0.92, respectively) of the
rate of BR with increasing depth. This relationship was also
seen in other deep Lakes (data shown in Supplementary
Table S1).
The Arctic (Latitude 66°North): Great Bear
Lake, Canada
When you first venture onto the lake, what strikes you most is the
horizon—the lake is indistinguishable from the sky (Figure 7).
With an average depth of 72 m, the lake’s visibility goes down for
what seems forever; a remarkable 30 m. Great Bear Lake is truly
pristine.
Of all the sites sampled in this study Great Bear Lake was the
most pristine sampled and by area, it is the eighth largest
freshwater lake in the world. With a surface area of 31,080 km
2
,
it is only 420 km
2
shy of Russia’s massive Lake Baikal.
Surprising, the results fit within the trends seen for every
other lake sampled in this study irrespective of the degree
of human impact (Figure 3).
Bacterial respiration rates were measured across the lake
between a latitude of N66°41.001′and N66°53.840′. Bacterial
respiration rates averaged 6 gC.m
−3
.d
−1
(SE = 0.1, n= 85) to a
max depth of 10 m. Unlike the depth profiles of bacterial
respiration rates in man-made reservoirs seen in Figure 6,
bacterial respiration rates were much lower at depths of
between 20 and 40 m (1.2 gC.m
−3
.d
−1
(SE = 0.3, n= 5). The
sediments of Great bear Lake were not the major source of
organic carbon driving bacterial respiration in the upper water
column.
DISCUSSION
Northern Hemisphere Outgases the
Southern Hemisphere
Globally freshwater CO
2
emissions were 58.5 Pg C y
−1
. This is
6 times the current annual burning of fossil fuels of 9.97 Pg C y
−1
FIGURE 7 | Great Bear Lake. This is an Arctic lake at a Latitude of 66°N in the North Western Territories of Canada. It is the largest lake totally in Canada. By area, it
is the eighth largest freshwater lake in the world with a surface area of 31,080 km
2
.
FIGURE 8 | Global map of annual averaged Gross Primary Production
(GPP) estimated with remote sensing data for the year 2007. The means are
listed in the lower left corner for all grid boxes (All), along with subsets from the
Tropics (latitudes <20°). Northern Hemisphere Extra Tropics (NHET:
latitudes >20°N) and Southern Hemisphere extra-tropics (latitudes below 20°S
(SHET) (Joiner et al., 2018; Yoshida, Y.; Zhang et al., 2018) republished under
creative commons.
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Pollard Global Freshwater CO
2
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(Le Quéré et al., 2017). The Northern latitudes dominate, even
though the rates of CO
2
emissions from tropical and subtropical
from lakes and rivers were high for their latitudes (e.g., the
Amazon, Congo and Sub Sahara). Twice as much CO
2
is
being emitted (mostly above the latitude of 45°N) from the
Northern compared to the Southern Hemisphere. This begs the
question “What are the sources of the DOC driving such high
rates of BR CO
2
emissions?”
Organic Carbon Sources of the Dissolved
Organic Carbon Pool
There are large scale and diffuse sources and fluxes of DOC in
freshwater (Mulholland, (2003)) driving the BR and CO
2
emissions from freshwater. Possible sources include DOC
mineralised in lake sediments mobilising organic carbon
deposited in the present and the distant past; Aeolian organic
matter transported from any and every corner of the global not to
mention the anthropogenic inputs from agriculture, land clearing
and urbanisation, However, identifying and Quantifying these
DOC sources remain elusive. As diffuse sources of DOC, they are
not simple to individually quantify on a global scale.
Because freshwater ecosystems are generally net heterotrophic
(Cole, 2013), by definition, their source of DOC is allochthonous
(defined here as organic carbon from elsewhere, in either space or
time). Indeed, when you consider lake and river sources of DOC
from the surrounding landscape, you are hard pressed to find any
system without a watershed/catchment inputting terrestrial organic
carbon (Figure 4 cf Figure 8). Bacteria readily mineralise
terrestrially derived macromolecules, considered refractory, like
lignin and phenolic compounds in freshwater (Ward et al., 2013).
Freshwater sediment bacterial mineralization processes are a
major source of the DOC driving high rates of surface water DOC
inputs (Pace and Prairie, 2005;Cardoso, et al., 2013). This also
produces the dissolved inorganic carbon (DIC) supersaturating
freshwater reaching pCO
2
concentrations as high as 1,500 ppm
(Cardoso, et al., 2013). Freshwater sediments may not be the sinks
of terrestrial organic carbon we thought (Cole et al., 2007).
McCallister and del Giorgio (2012) elegantly demonstrated
how bacteria respire ancient carbon from lake sediments
considered permanently stored (ancient 1,000–3000 BP). Cole
and Caraco (2001) also showed that highly
14
C-depleted carbon
of ancient terrestrial origin (1,000–5,000 years old) were also
important sources of labile DOC supporting BR in the
Hudson River (NY United States). In Quabbin (Northern
Hemisphere) and Wivenhoe (Southern Hemisphere) reservoirs
both showed bacterial respiration rates were highest closest to the
sediment suggesting bacterial mineralisation processes are
sources of DOC (Figure 8) as others have also shown
(Cardoso, et al., 2013). Hence, the distant past and present
organic carbon are allochthonous sources of DOC connected
to today’s labile DOC pool of Figure 8.
Major sources of terrestrial DOC input, ie terrestrial GPP,
appear to contribute to the pattern presented in Figure 5. For
example, between 30–45°NinFigure 8 the large terrestrial GPP of
the Northern temperate forests seen on the East Coast of North
America align with some of the highest estimates of rates of CO
2
outgassing in the Northern hemisphere (Figure 5).
While there is a healthy debate over whether “fish eat trees”or
“not”, it is safe to say both views are correct. Freshwater food webs
use aquatic and terrestrial primary production; which one
dominates depends on the environmental conditions (Cole
et al., 2007;Cole et al., 2011;Pollard and Ducklow 2011;Cole,
2013;Carpenter et al., 2016;Brett et al., 2017).
Pace and Prairie, (2005) estimated the gross primary production
(GPP)forglobalfreshwaterlakesas0.65PgCy
−1
.GeneLichens
(1973) presented a global review of the total net primary
production (NPP) in freshwater as 1.3 Pg C y
−1
.Thiscomparesto
>58.5 Pg C y
−1
respired globally into the atmosphere through
bacterial respiration in this study. Taking into account losses of
primary production to higher trophic groups, freshwater primary
production is not a major source of organic carbon entering the
DOC pool (Figure 9). There will never be enough freshwater
primary production to support the high rates of bacterial
respiration. Thus, freshwater primary production globally cannot
be considered the major source of the DOC pool of Figure 9.
TABLE 3 | DOC concentrations in freshwater lakes and rivers in different biomes
from low to high Latitudes of Northern and Southern Hemispheres (Adapted
from Mulholland, (2003) additional data from^Pollard and Ducklow, 2011;*Oliver
et al., 2017).
Freshwater Biomes DOC mg.L
-1
(Mean)
Tundra 2
Boreal Forests 7
Temperate 4
Temperate Northern rainforest* 6 to 11
Semi-arid 1
Wet Tropics 8
Dry Tropics 3
Dry-subtropics^5
Humid climates 4 to 13
FIGURE 9 | Model of how bacteria use carbon from the DOC pool to
respire the sources of DOC to emit CO
2
from freshwater (DOC) in the pool
remains constant—input = output. Viral lysis of the bacteria facilitates bacterial
respiration by recycling organic carbon through the DOC pool.
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Pollard Global Freshwater CO
2
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Viral lysis of bacteria motivating high DOC
turnover—not a DOC source
Viral lysis of the bacteria is not a fresh or new source of organic
carbon entering the DOC pool (Figure 9). They do, however,
facilitate the emission CO
2
from freshwater. In 2001 Wetzel
described how freshwater viruses lysed bacteria. This process
releases bacterial carbon back into the DOC pool (Figure 9)
(Weinbauer et al., 2002). Viral control of high rates of bacterial
growth in freshwater ‘short circuits’the food chain as high
rates of bacterial production are lost to viral lysis (Pollard and
Ducklow, 2011). In a freshwater reservoir, Pradeep Ram et al.
(2016) also showed higher viral lysis of bacteria is
accompanied by higher bacterial respiration rates and leads
to a significant loss of organic carbon to the atmosphere
through bacterial lysis. Indeed, the viral lysis of bacteria
shifted their reservoir ecosystem to net heterotrophy. Thus,
viral lysis of bacteria ensures the DOC in freshwater is
efficiently respired and returned to the atmosphere as
shown in Figure 8. Ecologically this helps explain why we
see such high rates of BR and DOC turnover in freshwater
Figure 5.
[DOC] Pool (Concentration) Versus DOC
Turnover—Flux
Many studies of freshwater follow changes in the concentration
of DOC. Yet, globally there is little difference between these
concentrations in rivers and lakes amid a range of latitudes and
vastly different biomes (Table 3). There is a fundamental
difference between DOC concentrations and DOC turnover
that is not readily appreciated. Quantifying the turnover of the
DOC pool (Figure 9) is a precondition for modelling the
organic carbon entering the DOC pool and CO
2
being
emitted into the atmosphere via freshwater bacterial
respiration.
The high bacterial respiration rates in freshwater measured
here (Figure 3) are coupled with a low and stable concentration
pools of DOC (2–13 mg/L) (Table 3). This requires that the rate
of input of organic carbon to the DOC pool of Figure 9 must also
be high and equal to the rate of bacterial respiration. Hence there
are major sources DOC supporting the rapid turnover of the
DOC pool as discussed above.
Bacterial respiration as a function of
temperature
Water temperature plays a major role in determining the rate of
freshwater BR (Apple et al., 2006). Freshwater temperatures have
the greatest impact on bacterial physiology—increasing BR,
decreasing bacterial production and lowering bacterial growth
efficiencies (Price and Sowers, 2004;Scofield et al., 2015). The
microbial mineralisation of organic material is most often
described as a simple exponential relationship (Bridgham and
Ye, 2013), as I have used in Figure 3.
In this study, the Q
10
(temperature coefficient) of 2.1
determined using Eq. 2 (n= 326) (Figure 3) was as expected
for bacterial respiration.
Q10 BR2/BR110+
/(T2−T1)(2)
Others have found a similar Q
10
value for natural and cultured
populations of bacteria, describing Q
10
values of around 2
(Carignan et al., 2000;Apple et al., 2006;Berggren et al., 2010).
The observed dependence of BR on temperate in freshwater
(Sobek and Transvik, 2005;Apple et al., 2006) also suggests there
is no shortage of external DOC sources to freshwater ecosystems
(Figure 9)(Oliver et al., 2017); as does the prevalence of net
heterotrophy in freshwater lakes (Cole et al., 2000;Pace and
Praire, 2005). Global freshwater temperatures are, justifiably,
substituted into Eq. 1 to determine BR rates across the globe
to generate Figure 4.
Bacterial Respiration Rates in Context
We see tropical freshwaters emitting carbon at rates of 1 Pg C y
−1
for African inland waters and were 0.9–2.9 Pg C y
−1
for the
Amazon (Borges et al., 2015;Sawakuchi et al., 2017;Ward
et al., 2017). These estimates were made using evasive fluxes
of CO
2
into floating chambers and gas transfer co-efficient. Their
estimates are consistent with the tropical biomes I have estimated
on both sides of the equator Panama Canal cf Amazon River
(Figure 5). Others have also found freshwater lakes can be
responsible for a quarter of the carbon in the atmosphere
(Tanentzap, et al., 2019).
Pace and Prairie reviewed BR methods in 2005 and provided
an overview of BR in freshwater lakes. Globally, estimates of
planktonic respiration (using a respiration quotient of 1.0) that
ranged from 0.7 to 162 mmol C m
−3
d
−1
. They estimated 0.83 Pg
C.y
−1
was emitted from freshwater lakes globally. This compares
to nearly 4 Pg C. y
−1
that Ward et al. (2017) estimated. The rate of
BR measures in this study averaged 2.46 ± 0.32 mol C.m
−3
.d
−1
(
xμ± SE, n= 326). This study results fits within these ranges.
Freshwater CO
2
Emissions and the 2021
Global Carbon Budget
Global greenhouse emissions from fossil fuels and industry are on
track to grow by 2% in 2017, reaching a new record high of
9.9 Pg y
−1
(Le Quéré, et al., 2017). This study found global surface
freshwater CO
2
emissions are 6 times this rate —>58.5 PgC.y.
−1
The Intergovernmental Panel on Climate Change (IPCC)
Sixth Assessment Report (IPCC, (2021)) assessed the global
CO
2
fluxes. The atmosphere stores 871 PgC, 283 of which is
the result of anthropogenic inputs. This is increasing by 4 PgC.
y
−1
(IPCC, (2021)). The report shows freshwater CO
2
outgassing
as a mere 0.3 PgC. y
−1
. However, this study found global
freshwater lakes are outgassing CO
2
at a rate of 58.5 PgC. y
−1
(Figure 5).
These differences are likely due to the indirect methods used in
the past to measure freshwater BR, compared to the in situ
measures applied in this study and others who used direct
CO
2
flux methods (Ward et al., 2017). The evasive fluxes of
CO
2
methods used in the sub-Sahara, Congo in Africa and South
America are also in situ-based techniques, and they align with my
in situ global estimate of BR and CO
2
emissions.
Frontiers in Environmental Science | www.frontiersin.org June 2022 | Volume 10 | Article 9049559
Pollard Global Freshwater CO
2
Emissions
RETRACTED
Using 423 km
3
mean volume of the Amazon (Grace et al., 2002)
and multiplying by my mean BR for the same areas of the Amazon,
I estimate CO
2
emissions from the Amazon to be 3.3 ± 1 (
xμ±SE,
n= 20) PgC. y
−1
.Thisresultfit well with the 2.29 PgC. y
−1
that
Ward et al. (2017) estimated for the Amazon. This is independent
supporting evidence, with another direct method, that my global
freshwater emission data are indeed related to the real world.
Today we are seeing major sources of DOC from lake sediment
organic matter both today and from biomes thousands of years in
the past. Add to that the sources of DOC from almost every sphere
in the present day—Atmosphere, Biosphere, Hydrosphere
(excluding Oceans) and Lithosphere (Figure 9). Little wonder
freshwater CO
2
emission is such a big part of the global carbon
budget. However, this is not recognised in the latest IPCC’ssixth
report (2021).
Future of Freshwater Emissions
O’Reilly et al. (2015) estimated (from 1985 to 2009) that global
lake surface water temperatures are rising by 0.34 °C per decade.
Based on my relationship between surface water temperature and
rates of BR in Eq. 1 (depth of 10 m), each decade will deliver
and extra 1.1 ± 0.2 PgC. y
−1
to the atmosphere from the Northern
Hemisphere and 0.5 ± 0.1 PgC. y
1
from the Southern
Hemisphere. The average anthropogenic increase in
atmospheric carbon is around 4 PgC y
−1
(IPCC, (2021)).
Freshwater emissions will account for 3% of this increase per year.
The higher freshwater emissions in the Northern Hemisphere
cf the Southern Hemisphere are also consistent with the
conclusions of Weyhenmeyer et al. (2015). They connected
land use and climate temperature increases with diffuse
sources of DOC. They conclude emissions from boreal lakes
(Northern Hemisphere) are approaching those of lakes in warmer
latitudes closer to the equator. Sound familiar? See Figure 5.
CONCLUSION
I have shown here that global freshwater returns carbon to the
atmosphere at a momentous rate! Yet it is either not considered in
current climate models or is shown at rates that are two orders of
magnitude lower than what I have measured in this global study.
Conspicuously absent or very low estimates of freshwater CO
2
emissions, such as in the latest IPCC (2022) report.
Given the magnitude of the freshwater carbon return to the
atmosphere that I have measured here, gauging future responses
of our climate to warming demands we quantitatively connect the
land—freshwater - atmosphere into today’s climate change
predictions. While the overall climate change jigsaw picture
will not change, how the puzzle pieces are arranged will
change. We need to find and include the freshwater puzzle
piece that fell off the table. Only when we work together to
globally identify and quantify the diffuse sources of DOC entering
freshwater that is drive bacterial respiration can we even dream of
managing freshwater CO
2
emissions. Doing so, hopefully, will
give the human race another opportunity to change our current
climate change trajectory.
‘THE END’of ‘The BEGINNING’
DATA AVAILABILITY STATEMENT
The raw data supporting the conclusion of this article will be
made available by the authors, without undue reservation.
AUTHOR CONTRIBUTIONS
The author confirms being the sole contributor of this work and
has approved it for publication.
ACKNOWLEDGMENTS
I wish to thank editors and the referees for their insightful
comments. Thanks also to ‘The Queensland-Smithsonian
Fellowship Award”and all those who enabled my freshwater
incubations across the world—Stuart Davies and William Tootle
at the Smithsonian Institute at Harvard Herbaria, Harvard
University, Cambridge, Mass., and Helene Muller-Landau and
Joe Wright at the Smithsonian Institute in Panama for getting me
started me on my quest; and, the Managers and staff of Quabbin
Reservoir (MA, United States) and Wivenhoe dam (SEQ, Qld),
the community of Fair Haven (NY, United States) on Lake
Ontario and the many others.
SUPPLEMENTARY MATERIAL
The Supplementary Material for this article can be found online at:
https://www.frontiersin.org/articles/10.3389/fenvs.2022.904955/
full#supplementary-material
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Conflict of Interest: The author declares that the research was conducted in the
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2
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