The role of inland waters in the
carbon cycle at high latitudes
Erik Lundin
Department of Ecology and environmental Sciences
Umeå 2014
This work is protected by the Swedish Copyright Legislation (Act 1960:729)
© Erik Lundin 2014
ISBN: 978-91-7459-781-3
Cover: The west part of the Stordalen catchment with lake Torneträsk in the
background. The Norwegian border on the horizon. Photo: Erik Lundin
Printed by: KBC Service center
Umeå, Sweden 2014
Table of contents
Table of contents
List of papers
Abstract
Introduction
Aims
Study area
Methods
Sampling program and data collection
GIS analyses
Flux calculations
Results and discussion
Winters contribution to annual lake emissions
Integrating streams and lakes in aquatic C emission estimates
The importance of CH4 vs. CO2
Sediment burial
Integrating aquatic systems in the catchment C balance
Conclusions
Author contributions
Thanks!
References
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List of papers
This thesis is a summary of the following papers that are referred to in the text by
their Roman numerals:
I.
Lundin, E. J., R. Giesler, A. Persson, M. S. Thompson, J. Karlsson (2013).
Integrating carbon emissions from lakes and streams in a subarctic
catchment. Journal of Geophysical Research: Biogeosciences. 118: 1 – 8.
II.
Karlsson, J., R. Giesler, J. Persson, E. J. Lundin (2013). High emissions of
carbon dioxide and methane during ice thaw in high latitude lakes.
Geophysical Research Letters. 40: 1-5.
III.
Lundin, E. J., J. Klaminder, D. Bastviken, C. Olid, S. V. Hansson, J.
Karlsson. Strong climate impact on the carbon emission – burial balance in
high latitude lakes. Submitted manuscript.
IV.
Lundin, E. J., T. Christensen, R. Giesler, M. Heliasz, J. Klaminder, D.
Olefeldt, A. Persson, J. Karlsson. A weak C sink at high latitudes: support
from an integrated terrestrial – aquatic carbon balance. Manuscript.
Papers I and II are reproduced with kind permissions from John Wiley & Son Inc.
ii
Abstract
Understanding the drivers of climate change requires knowledge about the global
carbon (C) cycle. Although inland waters play an important role in the C cycle by
emitting and burying C, streams and lakes are in general overlooked in bottom-up
approached C budgets. In this thesis I estimated emissions of carbon dioxide (CO2)
and methane (CH4) from all lakes and streams in a 15 km2 subarctic catchment in
northern Sweden, and put it in relation to the total catchment C exchange. I show
that high-latitude aquatic systems in general and streams in particular are hotspots
for C emission to the atmosphere. Annually, the aquatic systems surveyed in this
study emitted about 10.8 ± 4.9 g C m-2 yr-1 (ca. 98 % as CO2) which is more than
double the amount of the C laterally exported from the catchment. Although the
streams only covered about 4% of the total aquatic area they emitted ca. 95% of the
total aquatic C emission. For lake emissions, the ice break-ups were the most
important annual events, counting for ca. 45% of the emissions. Overall, streams
dominated the aquatic CO2 emission in the catchment while lakes dominated CH4
emission, 96 % and 62 % of the totals, respectively.
When summing terrestrial and aquatic C fluxes together it showed that the
aquatic emissions alone account for approximately two thirds of the total annual
catchment C loss. The consequence of not including inland waters in bottom-up
derived C budgets is therefore a risk of overestimating the sink capacity of the
subarctic landscape. However, aquatic systems can also act as C sinks, by
accumulating C in sediment and thereby storing C over geological time frames.
Sediment C burial rates were estimated in six lakes from a chronology based on
210
Pb dating of multiple sediment cores. The burial rate ranged between 5 - 25 g C
m-2 yr-1, which is of the same magnitude as lake C emissions. I show that the
emission:burial ratio is about ten times higher in boreal compared to in subarcticarctic lakes. These results indicate that the balance between lakes C emission and
burial is both directly and indirectly dependent on climate. This process will likely
result in a future increase of C emissions from high-latitude lakes, while the C burial
capacity of these same lakes sediments weaken.
iii
Introduction
One of the big challenges of our time is to understand the drivers and effects of climate
change [IPCC, 2007]. Understanding the climate system requires knowledge of the
global carbon (C) cycling, including magnitude and control of various sources and sinks
in coupled land-water-atmospheric systems. As climate warming is predicted to be
amplified towards high-latitudes, subarctic-arctic regions are recognized as areas of
special interest [Callaghan et al., 2010; Serreze and Francis, 2006]. Arctic and
subarctic terrestrial ecosystems have across the Holocene been atmospheric C net sinks
[Harden et al., 1992; Hayes et al., 2011; McGuire et al., 2010]. Low rates of C
mobilization and degradation relative to inputs and incorporation into permafrost have
resulted in the establishment of large C stocks in high-latitude peatlands, soils and
sediments [Ferland et al., 2012; Tarnocai et al., 2009; Zimov et al., 2006a]. There are
now concerns that higher temperatures and increased exposure and export of soil
organic C leads to increased respiration rates in both terrestrial and aquatic systems
[Dorrepaal et al., 2009; Vonk and Gustafsson, 2013; Yvon-Durocher et al., 2010].
Although it is likely that the terrestrial productivity increases due to climate warming
[Melillo et al., 1993], increased respiratory losses could cause an offset in the balance
between C sequestration and release [Hayes et al., 2011; Maberly et al., 2013].
However, whole catchment studies are rare. Efforts are mainly focused on terrestrial
systems and do normally not consider the lateral loss and fate of terrestrial C in aquatic
systems [Christensen et al., 2012; Houghton, 2003; Sarmiento et al., 2010].
In 1994, Cole et al. found a general super saturation of CO2 in surface waters of
lakes, indicating net losses of CO2 to the atmosphere. Since then, several studies have
shown that inland waters not only are transport medium of inorganic and organic C, but
also are an active component in the global C cycle by both storing and emitting C
during its passage from land to sea [Aufdenkampe et al., 2011; Battin et al., 2009; Cole
et al., 2007]. Global estimates suggest that more than half of the terrestrial C export is
lost in inland waters, mainly as gas evasion to the atmosphere, before reaching the
oceans [Aufdenkampe et al., 2011; Tranvik et al., 2009]. The atmospheric loss of CO2 is
direct via evasion of CO2 imported in supersaturated soil water [Hope et al., 2004;
Kling et al., 1991; Walter et al., 2007] or indirect after mineralization of terrestrial
organic C by biological and photochemical processes inside the aquatic systems
[Bertilsson and Tranvik, 2000; Karlsson, 2007]. For the CH4 emission, an important
source is decomposition of organic C in lake sediments [Kelly and Chynoweth, 1981;
Walter et al., 2008]. Independent of the relative importance of the pathways leading to
C emission from inland waters, these losses must be accounted for and integrated with
terrestrial C fluxes for a complete land-atmosphere C exchange estimate [Battin et al.,
2009].
1
The quantitative importance of aquatic systems for the release of C to the
atmosphere at high-latitudes remains unclear, largely due to a lack of studies integrating
CO2 and CH4 fluxes for complete aquatic networks at a relevant time scale and a lack of
comparisons of such aquatic fluxes with terrestrial C fluxes. The few existing highlatitude environment C balance studies all identify aquatic systems as significant
sources of atmospheric C [Christensen et al., 2007; Karlsson et al., 2010; Kling et al.,
1991]. None of them however include atmospheric C exchange of first-order streams.
Recent research shows high CO2 concentrations and gas transfer velocities in streams
versus lakes, resulting in significant emission from streams despite only covering a
small portion of the aquatic area of the catchment studied [Koprivnjak et al., 2010;
Striegl et al., 2012; Teodoru et al., 2009]. Further, it is shown that the winter respiration
significantly contributes to the annual respiration in lakes and to the production of
dissolved inorganic C (DIC) [Karlsson et al., 2008]. Despite these indications, the CO2
and CH4 accumulated in ice covered lakes over winters and released at ice thaw in
spring is commonly overlooked [Riera et al., 1999; Striegl et al., 2001]. In theory, the
relative importance of respired C accumulated under ice should be large at highlatitudes since winters are long and ice covers lakes during a large proportion of the
year. Since CH4 is a very potential greenhouse gas relative to CO2 [Reisinger et al.,
2010; Reisinger et al., 2011], it is of interest to compare the proportion of their
emissions. It could be assumed that lakes compared to streams are of generally larger
importance for CH4 emissions due to favorable conditions for CH4 production in lake
sediments and that this production is high compared to the sum of external input and
internal production in streams. However, it is also possible that such differences are
counteracted by relatively higher CH4 oxidation in the water column of lakes versus
streams [Bastviken et al., 2004].
Simultaneously, inland waters have been recognized to bury significant amounts
of C in sediments where it accumulates over geological time-scales [Anderson et al.,
2009; Ferland et al., 2012; Kortelainen et al., 2004]. The C burial in inland water
sediments is three-fold higher than the burial in ocean sediments, making inland water
sediments comparable to the C stocks of high-latitudes peatland soils and biomass, and
constitutes the second to third largest C pool in high-latitude environments [Anderson et
al., 2009; Ferland et al., 2012; Kortelainen et al., 2004]. The proportion of organic C
getting buried in sediments is determined by the burial efficiency, which is controlled
by substrate quality, oxygen exposure time and temperature [Gudasz et al., 2010; Sobek
et al., 2009]. This suggests that bioclimatic conditions affect burial efficiencies and
thereby the ratio between emissions and burial. Despite their relative importance,
studies including both C emission and burial estimates are rare and therefore is it not
known how C sequestration varies across different climate regimes.
2
Aims
The overall aim of this thesis was to assess the role and importance of inland waters in
the C cycle at high-latitudes. To address this topic, I investigated the following factors:
•
•
•
•
•
The relative importance of stream versus lake C emissions.
How winter C accumulation and ice-thaw emissions contribute to the total
emissions from inland waters.
The relative importance of CO2 vs. CH4 emissions from lakes and streams.
The importance of sediment C burial and how it does relate to C emissions.
The relative importance of aquatic systems for the total catchment net C
exchange.
Figure 1. The Stordalen catchment with the lake Torneträsk in the background. The photo is taken towards
the north.
Study area
All the studies included in this thesis were carried out in the Stordalen catchment (68°N,
19°E) in subarctic Sweden (Figure 1), ca. 10 km east of the Abisko Scientific Research
Station. During the last decades, several hydrological and C balance studies have been
carried out in this catchment [Christensen et al., 2012; Hasan et al., 2012; Heliasz et
al., 2011; Jackowicz-Korczynski et al., 2010; Jonasson et al., 2012; Karlsson et al.,
3
2010; Olefeldt et al., 2012]. The 15 km2 large catchment includes alpine terrain
dominated by heaths and dwarf shrubs (e.g. Empetrum hermaphroditum, Vaccinium sp.
and Betula nana) at higher altitudes (600 – 770 m a.s.l.), subalpine terrain covered with
mountain birch forest (Betula pubescens ssp. czerepanovii) and mires dominated by
Sphagnum mosses at lower altitudes (360 - 600 m a.s.l.). The catchment is located in the
zone of discontinuous permafrost [Johansson et al., 2006] and the mires contain areas
of palsa [Malmer et al., 2005]. The active layer is approximately 60 cm [Johansson et
al., 2006]. The catchment contains 27 lakes (area: 0.001 – 0.145 km2, max depths: 0.5 –
8.5m) of which one is alpine and the others are subalpine and surrounded by mire and
birch forest (Figure 2). Most of the lakes are connected by a network of streams. The
streams all start as small (< 1 m wide, < 0.5 m deep) systems in the relatively steep,
upper reaches of the catchment, but subsequently flow through flatter mires where they
grow deeper, wider and less turbulent. The catchment outlet of the lake L1 (Figure 2).
Mean annual air temperature between 2000 – 2009 was 0.6 ± 0.4 °C. Hence, the mean
annual temperature during the last decade presently exceeds the 0°C [Callaghan et al.,
2010]. The coldest and warmest months were February (-9.5 ± 3.1 °C) and July (12.5 ±
1.2 °C), respectively. Average annual precipitation (2000 – 2009) was 340 ± 56 mm of
which about 40% normally precipitates between November and May when the lakes in
the catchment typically are ice-covered.
Figure 2. Map of the Stordalen catchment showing the distribution of vegetation types. Coordinates are given
in decimal degrees (WGS 84). Reproduced from Paper IV.
4
Methods
Sampling program and data collection
Field work was carried out winter and summer seasons between 2009 – 2013 to cover
the full annual cycles. Partial pressures of CO2 and CH4 were measured in all lakes
(monthly) and streams (biweekly) during the ice-free season of 2009 (Papers I, II, and
IV), to obtain a high spatial resolution. In the ice-free seasons of 2010 and 2011, partial
pressures were continuously measured in-situ (probe + logger) to obtain a high temporal
resolution (Paper III and IV). During the winters of 2009 – 2011, gas concentrations
were sampled under the lakes ice (Papers I, II, and IV). Measurements of CH4 fluxes
were performed in six lakes during the ice-free season of 2010, by using floating
chambers (Papers III and IV). Sediments were cored in six lakes between 2011 and
2013 (Papers III). Published data was also compiled from literature to supplement the
data collected from Stordalen. In Paper III sediment C burial and annual C emission
data from boreal and subarctic-arctic lakes were compared. In paper IV, C exchange
data from the alpine tundra, forest and peatlands together with estimates of the
catchment C export were collected.
GIS analyses
Areas of lakes and streams were obtained by digitizing an ortho photo. Due to a low
image resolution and dense surrounding vegetation, it was not possible to estimate the
area of stream segments with a width smaller than approximately 1.5 m using remote
sensing. Instead we assumed these stream segments to be uniform channels with a width
estimated from field measurements at each sample location. Slope gradient calculations
were obtained by a rise-over-run calculation for all stream segments. The elevation and
stream lengths were obtained using a high resolution (1 m) LiDAR-derived digital
elevation model (DEM) [Hasan et al., 2012]. The catchment area was modeled in a 10
m resolution DEM using the hydrology tools available in ArcGIS 9.3.1. Volumes of the
winter sampled lakes were determined from interpolations of integrated GPS and echo
sounding depth measurements. The interpolations were performed in the Arc GIS 9.3.1
geostatistical analysis package using the ordinary Kriging method. Areas of each
ecosystem component (vegetation classes) were estimated by a classification analysis of
an aerial imagery (Paper IV).
Flux calculations
The momentum diffusive gas flux between water surface and atmosphere from partial
pressures and piston velocities (k) was estimated using Fick’s law [Jonsson et al.,
2008]. Lake fluxes were determined from a wind-dependent k function, developed in
small lakes [Cole and Caraco, 1998]. The k for streams was calculated by using the
5
relationship between CO2 reaeration coefficients and stream slope, depth and width, as
done by Wallin et al. [2011]. Total fluxes of CH4 into the floating chambers were
estimated from concentration increases over time (Paper III). Estimations of lake
emissions at spring ice thaw (Papers I, II, III) were carried out as the difference in mass
of CO2 and CH4 before and after ice break-up. It was assumed that a minor fraction of
the CH4 storage under ice was oxidized in the water column [Michmerhuizen et al.,
1996].
The sedimentation rate of the sediments was based on 210Pb dating to establish
accurate and precise chronologies of sedimentary deposits accumulated over the past
100-150 years [Appleby and Oldfield, 1992; Goldberg, 1963]. Sediment accumulation
rates were determined using the Constant Flux – Constant Sedimentation (CF:CS)
model [Krishnas.S et al., 1971]. This generates estimates on sedimentation rates into
older sediments and thus, provides rates representative for inputs to sediment layers
where decomposition is progressing at a very slow rate [Galman et al., 2008]. The C
accumulation rates were calculated by multiplying the inferred sediment accumulation
rates with the measured C concentration of each core (Papers III and IV).
Results and discussion
Winters contribution to annual lake emissions
Despite the reduction of the ice-cover period during the last decades [Callaghan et al.,
2010], the frozen period of the studied lakes in this study still extended from midOctobers until mid-May. Paper II shows that both CH4 and CO2 consistently accumulate
during the winters in the lake systems of Stordalen (Figure 1 in Paper II). On average, 4
g C m-2 yr-1 CO2 and 0.8 g C m-2 yr-1 CH4 were annually emitted from lakes during a
relative short time frame after ice thaw. Lake depth was the main explaining factor for
the magnitude of CO2 emission (Figure 2 in Paper II). This is logical given that depth
integrated pelagic respiration would increase with depth in the water column. Further, a
substantial proportion of the sediments might freeze in shallow lakes, thereby
constraining sediment respiration. The control of CH4 accumulations in winter was
however less dependent of lake morphology (Figure 2 in Paper II), but more on the
surrounding landscape, for example, higher values in lakes surrounded by mires.
Peatlands are considered hotspots for CH4 production and the CH4 produced by these
deep soil layer are known to enter surface water transported via the ground water
[Backstrand et al., 2008; Walter et al., 2006].
Ice break-up emissions played an important quantitative role in an annual cycle.
The proportion of dissolved CO2 and CH4 accumulated under ice and emitted in spring
were substantial relative to annual emissions in all lakes. By including early ice thaw
6
fluxes in annual emissions, many lakes switched from apparent C sinks to C sources.
Overall, emissions of accumulated C during ice thaw corresponded to ca. 12 – 100% of
annual C emissions, which is consistent to what Karlsson et al. [2010] and Striegl et al.
[2001] reported in two similar studies. Also streams initially followed the same patterns
as lakes and showed increasing CO2 concentrations during early winters (Figure 1 in
Paper II, unpubl. data for 4 additional streams). However, the streams in Stordalen are
shallow and commonly freeze solid in the late winters. Therefore, no enhanced flux of
C needs to be counted for due to the thawing of the streams in Stordalen. Paper II
shows the importance of including C accumulated under lakes ice when estimating
annual C emissions from small high-latitude lakes. This indicates that there is a risk that
bottom-up calculated regional C budgets underestimate C emissions from lakes, when
the winter accumulated C is overlooked, and thereby overestimate the catchment C sink
[see i.e. Battin et al., 2009; Cole et al., 2007; Tranvik et al., 2009].
Integrating streams and lakes in aquatic C emission estimates
Streams in particular and aquatic systems in general cover a low proportion of the
Stordalen catchment. About 5% of the total catchment area is comprised of aquatic
systems. Lakes dominate, accounting for 96% of total aquatic area whereas streams
account for 4% of total aquatic area. Lakes were generally supersaturated in CO2 and
CH4 during the open water season (Paper I and III). However, a minor number of lakes
showed undersaturation of CO2 during mid summer. The streams in Stordalen were at
all locations and sampling occasions supersaturated in CO2 and CH4 (except one site in
August 2009, being undersaturated in CH4, Paper I and IV). High gas concentrations
but also high gas transfer velocities resulted in C fluxes that were of about two
magnitudes higher in streams than in lakes (see Figure 2 in Paper I). In the ice free
season of 2009, average annual fluxes of CO2 and CH4 were 15600 ± 8000 (median:
10300 ± 5400) and 190 ± 30 (median: 100 ± 20) mg C m-2 d-1, respectively. Stordalen is
a relatively steep catchment compared to the catchments where similar studies have
been carried out. Small, steep and turbulent first order streams entail high gas transfer
velocities, resulting in high estimated C fluxes relative to those previously reported
from high-latitude streams [Koprivnjak et al., 2010; Reeburgh et al., 1998; Striegl et al.,
2012; Teodoru et al., 2009]. The estimated average lake CO2 and CH4 fluxes were 180
± 110 and 9 ± 6 mg C m-2 d-1, respectively. The average diffusive fluxes of CO2 and
CH4 from lakes during the ice-free season was comparable to earlier reports from
Alaska, Canada and northern Sweden [Jonsson et al., 2003; Kling et al., 1992; Laurion
et al., 2010; Reeburgh et al., 1998]. Although the streams only covered about 4% of the
total aquatic area they emitted ca. 95% of the total aquatic C emission from the
catchment, accounting for 8.6 ± 5.3 g C m-2 yr-1 (expressed by unit catchment area). Of
the emission from lakes (0.51 ± 0.22 g C m-2 yr-1), 45% (0.23 ± 0.08 g C m-2 yr-1) was
7
emitted during a short period after ice break-up. Comparative studies between streams
and lakes are rare but these results compiles with the general view of high C fluxes from
streams relative to lakes, and that small streams in particular are hotspots for C emission
in high-latitude catchments [Wallin et al., 2013].
The importance of CH4 vs. CO2
In accordance with other studies, the flux of CH4 from streams was small compared to
the flux of CO2 [Hope et al., 2001; Striegl et al., 2012], i.e. CH4 only accounted for ca.
1% of total stream emissions (Paper I). In lakes, the proportion of CH4 was larger but
still small compared to emissions of CO2 (Paper I and Supplementary materials in Paper
III). In 2010, when both ebullition and diffusive fluxes were estimated, CO2 represented
up to ca. 90% of total C emissions (Paper III). The contribution of ebullition was highly
variable, accounting for between 10 – 90% of total CH4 emissions. Although, the CH4
chamber measurements carried out in 2010 were designed to cover a high spatial
resolution rather than high temporal resolution, the average daily fluxes were in range
of what was found by high using temporal resolution measurement over several years in
a similar lake in the Stordalen catchment [Wik et al., 2013]. Overall, streams dominated
the aquatic CO2 emission in the catchment (96 % of total) while lakes dominated CH4
emission (62 % of total). However, the results from Stordalen might not be
representative to the subarctic-arctic region as a whole. A majority of the north Siberian
lakes are underlain by organic rich Pleistocene – age loess permafrost, so called yedoma
[Zimov et al., 1997; Zimov et al., 2006b]. In thermokarst lakes from that region, Walter
et al. [2006] measured annual CH4 emissions to be ca. 24.9 ± 2.3 g CH4 m-2 yr-1. These
emissions are magnitudes higher than most fluxes measured in non-yedoma surrounded
lakes [Bastviken et al., 2011; Casper et al., 2000; Reeburgh et al., 1998] and are about
nine times higher than the average CH4 emission from lakes in Stordalen. Such
emissions would equate CH4 with CO2 emissions and eight fold the radiative forcing
potential (GWP100) of lake emissions in Stordalen [Reisinger et al., 2010; Reisinger et
al., 2011]. Regardless, streams would still be the overall most important aquatic
component, responsible for 90% of the total aquatic C emission.
Sediment burial
The C burial in the six surveyed lakes ranged between 5 – 25 g C m-2 yr-1 which is in the
range of earlier published estimates from subarctic-arctic and boreal lakes (see Table S3
in Paper III, Supplementary Information). No significant mass loss was detected in any
of the sediment samples after acidification, indicating negligible accumulation of
inorganic C in the sediments. Combining the burial rates and annual C emissions from
Stordalen with data from individual lakes in previously published studies (see Table S4
in Paper III, Supplementary Information), suggests that there is a linear relationship
8
between emission and burial (R2 = 0.61, P = 0.007) in lakes (Figure 1 in Paper III). Also
C emissions and burial rates in boreal lakes showed a linear relationship (R2 = 0.87, P <
0.001, Figure 1 in Paper III) which is consistent with Kortelainen et al. [2013].
However, compared to subarctic-arctic lakes the regression slope is steeper in boreal
lakes. Hence, the average emission:burial ratio was substantially higher (F1,27 = 72.6, P
< 0.001) in boreal lakes (mean = 32 ± 26) than in subarctic-arctic lakes (mean = 2.4 ±
1.7). These findings are in accordance with data from other studies. The data obtained
from the literature search (see Table S2 and S3 in Paper III, Supplementary
Information) showed that boreal lakes have significantly higher C emissions than
subarctic-arctic lakes (F1,73 = 22.2, P < 0.001, Figure 2a in Paper III) while C burial
rates are comparable across biomes (Figure 2b in Paper III). The difference between the
emission:burial ratio of these biomes can be explained by a number of potential
mechanisms. The organic C rich and thereby dark colored waters of boreal lakes
[Laudon et al., 2012; Melillo et al., 1993] enhance heterotrophic respiration and
suppress the fixation of CO2 [Ask et al., 2012] which results in higher C emissions in
boreal relative to in subarctic-arctic lakes. Additionally, higher water temperatures of
boreal lakes further enhance CO2 losses by stimulating heterotrophic respiration
[Gudasz et al., 2010; Yvon-Durocher et al., 2012]. However, as CO2 fixation is often
constrained by poor light conditions or availability of nutrients, the increased CO2
fixation induced by higher water temperatures, can be expected to be relatively small
[Karlsson et al., 2009]. Altogether, this leads to lower C burial capacity and a higher
emission loss in warm boreal lakes in comparison to subarctic-arctic systems. Evidently,
the ratio of emission:burial cannot be assumed to be constant but instead exhibits
pronounced differences among biomes that are linked with catchment characteristics
such as their soils and vegetation.
Integrating aquatic systems in the catchment C balance
The incorporation of terrestrial C into aquatic systems translocate C and emissions from
where it first was sequestered downstream the catchment. Over three years (2009 –
2011), mean annual aquatic C emissions, normalized to the catchment area, were 10.8 ±
4.9 g C m-2 yr-1 (ca. 98 % as CO2) which is more than two-fold larger than annual mean
C lateral catchment export (3.8 ± 1.9 g C m-2 yr-1, Paper IV). On a catchment scale,
these losses are relatively large and their variability is rather low compared to the C
exchange of terrestrial ecosystems (see Table 2 in Paper IV). In fact, the aquatic
emissions alone constitute approximately two thirds of the total annual catchment C
loss. In agreement with global estimates [Aufdenkampe et al., 2011; Battin et al., 2009],
the C balance of the Stordalen catchment is offset when C fluxes from the aquatic
systems are excluded (Figure 2 in Paper IV). If aquatic emissions and export are
ignored in the Stordalen catchment, the total catchment C losses would be
9
underestimated and hence push the C balance over the steady-state threshold (Figure 2
and Table 3 in Paper IV). The consequence of not including inland waters in bottom-up
derived C budgets is therefore a risk of overestimating the sink capacity of the subarctic
landscape. My results indicate that Stordalen is not a strong sink but rather a source of
C. Thereby, the view of high-latitude landscapes as C sinks may be questioned.
Although, there are a fair number of lake C emission estimates made (see Table S2
in Paper III, Supplementary Information) high-latitude C emission estimates that
integrate both lakes and streams are rare. The results presented in this thesis is in
agreement with recent studies [Striegl et al., 2012; Wallin et al., 2013], showing high
stream emissions relative to lateral export. Considering the disproportional importance
of small streams in the landscape C balance (Paper IV), it is vital that new studies assess
stream C emissions and their variability.
Inland waters play dual roles in the catchment C cycle by also accumulating C in
sediment, where it is stored over geological time [Ferland et al., 2012; Kortelainen et
al., 2004]. Using the linear relationship presented in Paper III between lake C burial
and mean emissions, indicates that the lakes in Stordalen to accumulate 0.4 ± 0.2 g C m2
yr-1. This corresponds to ca. 13% of what annually is sequestered in the peatlands
[Olefeldt et al., 2012] and 3 – 4% of the average total annual catchment C loss.
High-latitudes are predicted to face severe climatic changes during the coming
century [IPCC, 2007; Serreze and Francis, 2006]. These changes are suggested to be
accompanied with increased terrestrial productivity, increased respiration rates and
increased lateral C export [Frey and Smith, 2005; Laudon et al., 2012; Melillo et al.,
1993; Zimov et al., 2006b]. If true, high-latitude aquatic systems will receive increased
loads of inorganic and organic C [Hope et al., 2004; Laudon et al., 2012; Maberly et al.,
2013]. As Paper III shows, such a development would result in enhanced aquatic C
emissions while no expected increase in burial rates. This further suggests that the
importance of including inland waters in bottom-up approached C budgets will increase
in a future warmer climate scenario.
Conclusions
Inland waters play a dual role in the C cycle at high-latitude by both emitting CO2 and
CH4 to the atmosphere and by accumulating C into sediments. The results presented in
this thesis show that high-latitude aquatic systems in general and streams in particular
are hotspots for C emissions to the atmosphere. Although the streams only covered
about 4% of the total aquatic area they emitted ca. 95% of the total aquatic C emission.
Ice break-up was an important event accounting for almost half the annual lake C
emissions. Mean annual aquatic C emissions, normalized to the catchment area, were
10.8 ± 4.9 g C m-2 yr-1 (ca. 98 % as CO2) which is more than two-fold larger than annual
lateral C export. On a catchment scale, these losses are relatively large compared to the
10
C exchange of terrestrial ecosystems, approximately constituting two thirds of the total
catchment C loss. Hence, in bottom-up derived catchment C budgets, the terrestrial C
sink is likely to be overestimated if annual emissions from inland water are not
included. Further, these results show that the balance between lake C emission and
burial is both directly and indirectly dependent on climate. Due to a changing climate,
high-latitude inland waters are predicted to receive increasing loads of C, while the
relative C burial capacity of the aquatic systems weakens. These processes will most
likely result in an increase of C emissions from high-latitude lakes to the atmosphere,
while no increase in sediment burial rates can be expected.
11
Author contributions
Authors are referred to by their initials
I.
J.Ka. planned the measurements of dissolved gases in streams and lakes. E.L.
and A.P. preformed the GIS analyses. E.L. led the field and laboratory work,
calculated fluxes, did the statistical analysis and wrote the paper. J.Ka., M.T.
and R.G. commented on the analyses and the manuscript.
II.
E.L. and J.Ka. designed the study and the developed the conceptual idea. E.L.
led the field and laboratory work with major contributions from J.P. E.L.
performed the GIS analyses. J.Ka. and E.L. analyzed the results. J.Ka. wrote
the paper with contributions from E.L. R.G. commented on the manuscript.
III.
E.L. and J.Ka. designed the study and the developed the conceptual idea. The
gas flux measurements were led by E.L. The sediment coring was carried out
by E.L., S.H. and J.Kl. The CO2 flux calculations were performed by E.L. E.L.
analyzed the CH4 flux data with inputs from D.B. J.Kl. and C.O. dated the
sediment cores. J.Kl., E.L. and C.O. calculated the burial rates. The statistical
analyses were performed by E.L. E.L. wrote the manuscript with contributions
from J.Ka., J.Kl. and D.B. S.H. and C.O. commented on the manuscript.
IV.
E.L. J.Kl. and J.Ka. designed the study and the developed the conceptual idea.
E.L., T.C., M.H., and A.P. contributed with data. A.P. preformed most of the
GIS analysis with contributions from E.L. E.L. analyzed the data with
contributions from J.Ka. and J.Kl. E.L. wrote the most of the manuscript with
contributions from J.Kl. and J.Ka. T.C., R.G., and A.P. commented on the
manuscript.
Authors: A.P. = Andreas Persson, C.O. = Carolina Olid, D.B = David Bastviken, E.L.
= Erik Lundin, J.Ka. = Jan Karlsson, J.Kl. = Jonatan Klaminder, J.P. = Jenny Persson,
M.H. = Michal Heliasz, M.T. = Megan Thompson, R.G. = Reiner Giesler, S.H. =
Sophia Hansson, T.C. = Torben Christensen
12
Thanks!
First of all to my supervisors Janne and Reiner and of course my unofficial supervisor
Jonatan: Thank you for introducing me to what we call science. With supervisors like
you, it is at least never boring!
All my other co-authors; David Bastviken, Megan Thompson, Andreas Persson,
Sophia Hansson, Carolina Olid, Jenny Persson, Torben Christensen and Michal Heliasz.
Also a lot of thanks to Keith, Eva, and Lars at the KBC printing service for all the help
with finishing this thesis. All people that have been working in the field and in the lab
with me. Freezing, snowed on, rained on, getting eaten on: Tyler (I still feel bad for
bringing you jetlagged on short walk 500m verticals throw the arctic jungle with a raft
on your back), Thomas, Marina, Patricia, Daniel, Jakob, Hugo, Anders, Gerard, Erik.
Students at “Arctic Geoecology” (Yes, I do forgive you for dropping the ruttner sampler
at 120m).
I never have as much fun as when I do science, and I never do as much science
as when I’m out skiing. Therefore, to all my skiing friends that have being out with me
during my PhD studies (Mattias, Aron, Tyler, Katie, Eva, Jono, Makoto ”Mackan”
Kobayashi, Frida, Hendrik, Gerard, Sophia, Anna*2, Boel, Jakob, Gabriel, Kalle, Isak,
Dorothée, Petter, Kristian, “The Kiruna crew”, Signe, Rob, Gesche, Laurenz, Martin,
Yve, Ellen, Christian, Akki, Pär, Cathy, Krister, Andreas, Jane, Mathias, Jonas, Janne,
Christian Gudasz, Peter, Herco, Yvette, Marie, Fredrik, Lasse, Viggo, Daniel, “The
Njulla Night Riders”, empty slot for putting forgotten names in): Thanks!
The CIRC family; Tyler, Katie, Makoto, Hendrik, Megan & Mike et al., Peter,
My, Niak, Frida, Jono, Keith, Siggi, Ellen, Christian, Ann, Yve, Stern, Gesche,
Laurenz, Signe, Rob, Reiner, Janne & My, Jonatan, Marina & Gustaf et al. All frequent
guests and CIRC and ANS; Daniel, Jonas, Johan O, Helene, Gustaf T., Peter M., “The
SLU crew” (Maja, Andres, et al.), “The VU crew”, “The Crill crew”, Tom P., Jon,
Carsten, and everyone else spending their summers in Abisko. A special thanks to
Makoto for introducing me into NMA and Jonas, Tyler, Hendrik, Gerard and Jonatan
for good fights. ANS staff: Bengt, Linnea, Lillian, Noella, Annika, Majlis, Anders,
Thomas, Crister, Ola, Rob, Niclas, Magnus, for all the nice coffee breaks.
All PhD students, post docs, professors, teachers, staff, pub visitors, innebandy
players, etc. at EMG.
Understanding and less understanding friends; Jakob & Anna-Clara, Johan &
Jessica, Gabriel & Linn, Hanna, Klas, Sofia & Kim, Mathias & Milly with family, et
al.), wondering when it is time for me to get a real job. My loving family; Mum, dad,
my brother Mattias and “faster” Sollan!
Eva for making me a bit decent. Kus!
13
14
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