PUBLICATIONS
Geophysical Research Letters
RESEARCH LETTER
10.1002/2015GL067599
Basin scale controls on CO2 and CH4 emissions
from the Upper Mississippi River
Key Points:
• Upper Mississippi River is a recurrent
CO2 sink
• Upper Mississippi River is a consistent
CH4 source
• River biology, physics, and sediments
control gas emissions
John T. Crawford1, Luke C. Loken2,3, Emily H. Stanley3, Edward G. Stets1, Mark M. Dornblaser1,
and Robert G. Striegl1
Supporting Information:
• Supporting Information S1
Abstract
Correspondence to:
J. T. Crawford,
[email protected]
Citation:
Crawford, J. T., L. C. Loken, E. H. Stanley,
E. G. Stets, M. M. Dornblaser, and R. G.
Striegl (2016), Basin scale controls on
CO2 and CH4 emissions from the Upper
Mississippi River, Geophys. Res. Lett., 43,
doi:10.1002/2015GL067599.
Received 28 DEC 2015
Accepted 6 FEB 2016
Accepted article online 10 FEB 2016
Published 2016. This article is a US
Government work and is in the public
domain in the United States of America.
CRAWFORD ET AL.
1
National Research Program, U.S. Geological Survey, Boulder, Colorado, USA, 2Wisconsin Water Science Center, U.S.
Geological Survey, Middleton, Wisconsin, USA, 3Center for Limnology, University of Wisconsin-Madison, Madison,
Wisconsin, USA
The Upper Mississippi River, engineered for river navigation in the 1930s, includes a series
of low-head dams and navigation pools receiving elevated sediment and nutrient loads from the mostly
agricultural basin. Using high-resolution, spatially resolved water quality sensor measurements along 1385
river kilometers, we show that primary productivity and organic matter accumulation affect river carbon
dioxide and methane emissions to the atmosphere. Phytoplankton drive CO2 to near or below atmospheric
equilibrium during the growing season, while anaerobic carbon oxidation supports a large proportion of
the CO2 and CH4 production. Reductions of suspended sediment load, absent of dramatic reductions in
nutrients, will likely further reduce net CO2 emissions from the river. Large river pools, like Lake Pepin, which
removes the majority of upstream sediments, and large agricultural tributaries downstream that deliver
significant quantities of sediments and nutrients, are likely to persist as major geographical drivers of
greenhouse gas emissions.
1. Introduction
River networks process large quantities of carbon resulting in direct emissions of CO2 [Butman and Raymond,
2011; Raymond et al., 2013; Abril et al., 2014] and CH4 to the atmosphere [Borges et al., 2015; Stanley et al.,
2015], which has a significant impact on the Earth’s greenhouse gas budget [Bastviken et al., 2011]. Typically,
river ecosystems respire more organic matter than they fix through photosynthesis [Wetzel, 2001; Hotchkiss
et al., 2015] and are thus oversaturated in CO2 with respect to the atmosphere. This excess gas quickly emits
to the atmosphere due to turbulent mixing of the water column. However, the current paradigm of aerobic
respiration exceeding photosynthesis may not apply to highly disturbed rivers such as the Mississippi, which
has been dammed and receives excess nutrients from a mostly agricultural basin. Damming decreases water
velocities, promoting sedimentation and improved water clarity which may increase primary production by
lifting light limitation. In essence, these nutrient-rich impoundments may act more similar to eutrophic lakes,
having high rates of primary production that ultimately decrease dissolved CO2. Major alterations to sediments
and the physical structure in disturbed rivers could also have consequences for other facets of the carbon cycle
such as promoting anaerobic metabolism [Stanley et al., 2015] and CH4 formation.
The current navigable portion of Upper Mississippi River (UMR) begins above Minneapolis, MN, and extends to
the confluence with the Ohio River, near Cairo, IL (drainage area ~490,000 km2). Many of the backwaters, side
channels, and isolated lakes prior to construction of the 27 locks and dams in the early twentieth century are still
present and are seasonally connected with the main channel. Damming of the river created large but relatively
shallow impoundments that slow water velocities and aid in commercial navigation. The UMR receives significant water discharge from a number of tributaries draining agricultural watersheds throughout the basin,
forested watersheds in Wisconsin and Minnesota, and urban areas. The location of these tributaries is a major
driver of longitudinal nutrient and sediment patterns [Wasley, 2000]. Downstream of the final navigation
structure near St. Louis, Missouri, the addition of the Missouri River with the Mississippi River creates large
changes in flow velocities, sediments, and geomorphic structure. Downstream of the impounded portion of
the UMR, floodplains are disconnected and the river is mostly isolated to a deep channel via levees and flow
control structures. We therefore hypothesized that the large ecosystem shifts throughout the length of the
UMR would result in significant changes to the metabolic function of the river, with consequences for production and emission of greenhouse gases.
MISSISSIPPI RIVER GREENHOUSE GASES
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Geophysical Research Letters
10.1002/2015GL067599
We assessed patterns and controls of dissolved CO2 and CH4 concentrations by (i) generating a high-resolution
map of chemical and physical conditions of the river using a high-speed water quality mapping platform
[Crawford et al., 2015]. We supplemented this one time survey with (ii) repeated water quality mapping in a
large river section to address seasonality and variations in river flow. We also explored the role of anaerobic
carbon metabolism and geomorphic controls on CH4 concentrations. In all, we collected > 400,000 measurements of CO2, CH4, algal fluorescence, turbidity, and supporting water chemistry parameters across 1385 km
of the Upper Mississippi River, including backwater habitats and the confluences of major tributaries.
2. Methods and Measurements
2.1. Sensor Platform
We used the FLAMe water quality mapping platform [Crawford et al., 2015] to generate large-scale maps of the
UMR. A water intake system (Figure S1) delivered water to onboard sensors including a YSI EXO 2 water quality
sonde that recorded water temperature, turbidity, and phytoplankton fluorescence (chlorophyll a) and other
basic parameters at 1 Hz. Water was then stripped of dissolved gases using a sprayer-type equilibrator [Santos
et al., 2012] that were analyzed by a Los Gatos Research ultraportable greenhouse gas analyzer (CO2 and CH4,
cavity ringdown spectroscopy). Time series data were converted to spatial data by combining with onboard
GPS data (Wide Area Augmentation System enabled). Data were quality checked and corrected using procedures given in Crawford et al. [2015], but using new mathematical constants established for a newer version
of the intake system (Table S1). Briefly, in order to match individual sensor response characteristics and to obtain
more accurate spatial data, we applied sensor-specific corrections using equation (1) [Fofonoff et al., 1974].
X o ¼ X c þ τs
dX
dt
(1)
where Xo is the τ s-corrected (the sensor time constant) value at time t and dX
dt is the instantaneous rate of
change of sensor output and Xc is the observed value. Use of equation (1) should ideally lead to a step
response to a step change input. We note that this is the same strategy used to correct oceanographic
conductivity and temperature instruments [see Fozdar et al., 1985]. In experiments where we rapidly switched
between distinct water sources, the τ s-corrected data show good responses to step change inputs and
indicate that this is a suitable technique for generating high accuracy spatial data.
We used two approaches that varied in their spatial and temporal extent. Our first approach was to map the
entire navigable portion of the Upper Mississippi River (~1300 km) during summer base flow in August 2015.
The majority of observations in this data set were made in the navigation channel, but we also chose sections
of the river for additional, more extensive mapping (side channels, impounded areas, and backwaters).
Second, we examined spatial patterns of greenhouse gases and other parameters in Navigation Pool 8
beginning in October 2014 and repeated this sampling during April 2015, June 2015, and August 2015.
During each survey, we sampled the water column at 1 Hz while underway. Survey tracks were nearly
identical during each replicate and covered the entire main channel, side channels, impounded areas, and
backwater lakes.
2.2. Statistical Analysis of Spatial Data
We used least squares regression to assess the relationships between greenhouse gases and water quality
parameters, as well as geomorphic factors (sediment composition). The spatial data generated by high-frequency
instruments exhibit significant spatial autocorrelation. Such correlation among points violates a key assumption
of least squares regression. To avoid the effects of spatial autocorrelation on model results (particularly the
residuals), we used a simple bootstrapping procedure where 10% subsamples of the full data set were used in
least squares regression 1000 times (following log transformation). Results of the bootstrapping procedure
indicate that there was no substantial impact of spatial autocorrelation on model results.
3. Results and Discussion
3.1. Sediments and Eutrophication
Impacts of land use, tributaries, and infrastructure were clearly evident in high-resolution water quality maps
(Figure 1). Our survey showed significant transitions in turbidity (sediments and other light scattering particles) associated with important sediment sinks and tributaries. For example, high turbidity above Lake Pepin
CRAWFORD ET AL.
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Geophysical Research Letters
10.1002/2015GL067599
Figure 1. (left) Maps of turbidity, (middle) carbon dioxide concentration expressed as multiple of saturation (e.g., 2× saturation),
(right) and methane concentration expressed as multiple of saturation in the Upper Mississippi River observed in August 2015
using a high-speed water quality mapping platform; n = 270,000; background is 2011 land cover (National Land Cover Database)
with brown and yellow denoting agricultural lands.
dropped from ~25 units to below 6 between the upper portion of the lake and the confluence with the
Chippewa River. High turbidity was not encountered again until > 400 km downstream, where major agricultural tributaries elevated the sediment load in the channel. The upper river basin has experienced massive
historical increases in sediments that are consistently reduced by Lake Pepin [Engstrom et al., 2009] and other
geographic sediment sinks [Houser et al., 2010; Wasley, 2000]. Long-term records support a strong sediment
sink in the river (despite increasing terrestrial loading) which has led to decreasing exports of sediments
downstream during recent periods, likely due to trapping behind dams and other infrastructure [Heimann
et al., 2011]. Along with elevated sediment loading, the Mississippi River receives enormous quantities of
nutrients (nitrogen and phosphorus), primarily from agriculture [Turner and Rabalais, 1991; Turner and
Rabalais, 2003], resulting in severe eutrophication. For example, average total phosphorus concentrations
in the upper portion of the river are between 0.16 and 0.19 mg L1 during summer months, while nitrate
typically exceeds 1.5 mg L1 NO3-N [Houser et al., 2010], both far above theoretical thresholds for nutrient
limitation [Uehlinger, 2006; Houser et al., 2015]. The elimination of nutrient constraints on primary production
[Houser et al., 2015; Bukaveckas et al., 2011] has likely altered the ecology of the river and has even led to
seasonal blooms of potentially toxic cyanobacteria [Huff, 1986]. In line with the predictions for eutrophic, high
water residence time rivers [Hilton et al., 2006], the UMR also showed a pattern of increasing algal concentrations with distance downstream (Figure S2 in the supporting information) which were evident as algal scums
and dark green coloration throughout the river survey. Such elevated sediments and nutrients delivered to
the river have significant consequences for greenhouse gases.
3.2. Carbon Dioxide Concentrations
Nearly every publication documenting river and stream CO2 concentrations has shown ubiquitous CO2
supersaturation [e.g., Butman et al., 2016; Raymond et al., 2013; Borges et al., 2015], and freshwater CO2 supersaturation is a widely accepted paradigm. The UMR extensively violated this paradigm during the 2015 growing season (April–August). Nearly the entire length of the impounded portion of the UMR was undersaturated
in CO2 during our 2015 August survey (median CO2 = 12.3 uM, 85 % saturation; Figure 1) and was thus a sink
CRAWFORD ET AL.
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2
0.8
Figure 2. Plots of (left) chlorophyll a fluorescence versus CO2 concentration (p < 0.001; r = 0.57; y = 2.72×
) and turbid2
0.52
ity versus (right) CO2 concentration expressed as percent of saturation (p < 0.001; r = 0.24; y = 1.36×
) from the Upper
Mississippi River during August 2015; colored contours indicate the density of points in a region; n = 245,007.
of atmospheric CO2. This was confirmed by drifting chamber measurements showing CO2 uptake during periods of undersaturation (supporting information). Seasonal surveys of the river in Navigation Pool 8 showed
recurrent undersaturation, with CO2 concentrations closely tracking water discharge during summer months
but not during fall (seasonal median = 90% of saturation, undersaturation 54% of the year, Figure S3). Such
widespread CO2 undersaturation implies a strong biologically driven sink. High-resolution spatial data support the conclusion that CO2 concentrations are greatly reduced by intense primary production in the river
[Houser et al., 2015; Bukaveckas et al., 2011] but only when turbidity is low and phytoplankton are abundant.
Primary productivity in the UMR is thought to be light limited [Houser et al., 2015; Bukaveckas et al., 2011;
Owens and Crumpton, 2006], as is common for metabolism in rivers and streams, in general [Bernot et al.,
2010]. In support of this conclusion, the logarithm of CO2 saturation was negatively correlated with the logarithm of turbidity (a strong predictor of light availability) [Giblin et al., 2010] (Figure 2). CO2 saturation was also
highly correlated with algal fluorescence (Figure 2). Turbidity and algal fluorescence explained > 70% of the
total variance in CO2 concentrations across the entire length of the UMR (multiple linear regression,
p < 0.0001, variance inflation factor < 2). Moderate CO2 sink behavior has also been observed in highly
eutrophic lakes in the agricultural Midwestern U.S. [Balmer and Downing, 2011; Pacheco et al., 2013].
However, other eutrophic rivers such as the turbid Hudson River do not illustrate CO2 uptake [Raymond
et al., 1997]. Meanwhile, the Columbia River (the second largest river in the U.S.) has experienced a pattern
of “greening” due to high nutrient loads, decreased turbidity, and increased phytoplankton production
[Prahl et al., 1998; Sullivan et al., 2001], but the potential impacts on CO2 exchanges have not yet been
addressed. This raises a major question regarding how CO2 in rivers will respond to agricultural
expansion/intensification and infrastructure changes in other parts of the world. Will damming alleviate light
limitation through sedimentation [e.g., Wang et al., 2015] and draw down CO2 when nutrients are abundant,
or will systems remain heterotrophic?
3.3. Methane and Anaerobic Carbon Cycling
Despite the fact that the UMR was a weak CO2 sink during the summer due to intense photosynthesis, the
river was always supersaturated with CH4, regardless of physical location (Figure 1). Median CH4 concentrations in August were 0.12 uM, over 400 times greater than atmospheric equilibrium. From > 100 direct
measurements of gas exchange (supporting information), we calculated an average CH4 emission rate of
4.22 μmol m2 d1, which was 4 times greater than the global average for rivers and streams [Stanley et al.,
2015]. These estimates of concentration and emissions of CH4 do not currently include ebullition (bubbling),
which is likely present throughout the river. Elevated CH4 concentrations and emissions in the UMR are a
result of anaerobic conditions in organic-enriched river sediments [Strauss et al., 2006] as organic matter
CRAWFORD ET AL.
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Geophysical Research Letters
Figure 3. Plot of CO2 concentration versus O2 concentration in the
Upper Mississippi River expressed as percent of saturation; dotted
lines denote atmospheric equilibrium, and solid line denotes the 1:1
relationship representing aerobic metabolism; colored contours
indicate the density of points in a region; least squares regression of
log transformed data indicated strong correlation, n = 245,007,
2
0.29
p < 0.0001, r = 0.75, y = 2.68×
.
10.1002/2015GL067599
content was a strong predictor of CH4
concentrations (Figure S6). This relationship partially confirms findings from small
agricultural streams in the basin [Crawford
and Stanley, 2015]. Relationships between
organic-rich river environments and reducing conditions are not unique to the
Mississippi, as similar results have been
reported for the Amazon, Congo, and
Zambezi Rivers [Richey et al., 1988; Borges
et al., 2015], as well as for run-of-river
impoundments in Germany [Maeck et al.,
2013]. Yet high CH4 concentrations were
unexpected given that the UMR has much
greater O2 concentrations and a limited
backwater extent relative to the Amazon
[Richey et al., 2002], and extremely small
wetland contributions relative to some
African rivers [Borges et al., 2015].
Anaerobic carbon cycling was also an
important source of dissolved CO2, not
just CH4. Comparing CO2 and O2 stoichiometry allows for the assessment of nonaerobic processes such as anaerobic carbon oxidation (see supporting
information). Despite the conclusion that river primary production is responsible for much of the spatial variability in CO2 concentrations, aerobic reactions such as photosynthesis and autotrophic + heterotrophic
respiration of organic matter cannot account for all dissolved CO2, as indicated by CO2 concentrations plotting to the right of the 1:1 line in Figure 3. Our calculations of anaerobic CO2 are constrained to periods when
CO2 was oversaturated, and estimates varied widely. Yet we found that median CO2 concentrations not
attributable to aerobic processes were approximately 71 μmol L1 (or ~4× saturated concentrations, see
supporting information). In many locations, the majority of CO2 is derived from nonaerobic processes. This
finding suggests a new paradigm for the inland water carbon cycle that must begin to address metabolic
processes not involving O2 [Stanley et al., 2015]. A lack of coupling between O2 and CO2 could partially
explain the divergence between modeled/measured O2 metabolism and CO2 fluxes in rivers [Hotchkiss
et al., 2015]. While Hotchkiss et al. [2015] attributed excess CO2 in the Mississippi River and elsewhere to both
internal and external sources, our data suggest that a major CO2 fraction is being generated through
anaerobic processes. Given that methane is present (and generally 10–100 times saturated), and positive
denitrification has been documented throughout the river [Strauss et al., 2006], locally reduced conditions
are likely ubiquitous in the UMR. Methanogenesis and other anaerobic reactions (e.g., denitrification, sulfate
reduction, and manganese reduction) can produce CO2 independent of O2 which is not detectable with
contemporary metabolism techniques. With technological improvements in coupled O2 and CO2 sensors,
we may be able to tease apart aerobic and anaerobic respiration and develop a better understanding of
the pertinent carbon reactions in this and other aquatic ecosystems. Additionally, it is possible that carbonate
precipitation (also known as “reverse” weathering) could be a CO2 source to the water column that is
decoupled from oxygen dynamics [Tobias and Böhlke, 2011]. This alternative hypothesis will require further
analysis of geochemical kinetics and equilibria in the UMR.
3.4. Conceptual Model of Modified River Greenhouse Gas Production
Our data corroborate previous predictions regarding large river eutrophication [Hilton et al., 2006], and
potentially enhanced CH4 production in river impoundments [Maeck et al., 2013], leading us to propose a
model of river greenhouse gas production under anthropogenic influences (Figure 4). First, when dams
and other navigation structures increase water residence times to greater than the average replication time
of algal cells, algal biomass (chlorophyll a) increases downstream (greening). This phenomenon has been
observed in the heavily impounded Columbia and other rivers [Prahl et al., 1998; Sullivan et al., 2001;
CRAWFORD ET AL.
MISSISSIPPI RIVER GREENHOUSE GASES
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Geophysical Research Letters
10.1002/2015GL067599
Figure 4. A conceptual model of (top) river greening, (middle) sediment trapping and tributary loading, (right) and their predicted cumulative effects on water clarity and greenhouse gas concentrations in large eutrophic and modified river systems.
Skidmore et al., 1998], as well as in our data for the UMR (Figure S2). As a result of elevated algal biomass,
water column photosynthesis becomes a significant control on riverine CO2 levels (at least during the summer). Second, settling and storage of sediments improves water clarity thus removing light constraints on
photosynthesis. But large tributary sediment loads can override this effect, consequently reestablishing light
limitation. Finally, the trapping of fine and potentially organic-rich sediments throughout the river creates an
environment that promotes methanogenesis [Maeck et al., 2013; Stanley et al., 2015] and other anaerobic
reactions. From this model, we can expect that large nutrient-rich rivers that undergo damming and other
modifications will follow a similar trajectory of greening and perhaps an increase of anaerobic carbon cycling,
with potential consequences for the atmospheric greenhouse gas budget.
4. Conclusions
Acknowledgments
This work was supported by the U.S.
Geological Survey’s Land Carbon program. L.C.L. and E.H.S. were also supported by DEB-1440297, NTL LTER. We
thank Stephen Powers, Doug Halm, and
Peter Turner for their assistance in the
field. The University of Wisconsin
Physical Sciences Lab designed and
manufactured the FLAMe water intake
system. J. Finlay and two anonymous
reviewers provided important feedback
on the manuscript. Any use of trade or
product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. All data
are available by contacting the corresponding author.
CRAWFORD ET AL.
The UMR carbon cycle is highly altered and may represent the state of other rivers experiencing the dual
pressures of agriculture and hydrologic modification. This major river is a consistent source of CH4 to the
atmosphere and a recurrent sink of CO2 during the summer growing season due to high nutrient loading
and intense primary productivity. Despite efforts to reduce nutrient loads to the Mississippi River, little progress has been made. In fact, some regions show increasing nitrate loads over time [Sprague et al., 2011]
potentially due to high nutrient groundwater discharge and the enormous buffering capacity of agricultural
soils [Turner and Rabalais, 2003]. Thus, patterns of high primary productivity in the river can be expected to
persist for decades to come. Absent of significant reductions in nutrient loading, the trend of decreasing suspended sediments [Heimann et al., 2011] will likely reduce the CO2 source strength of the river by further
relieving light limitation of primary production. On the other hand, significant contemporary CH4 emissions
from highly productive depositional zones such as backwaters and large impounded areas, as well as the
main channel, indicate that the river will persist as a greenhouse gas source.
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