Controls on the autochthonous production and respiration of organic matter in cryoconite holes on High Arctic glaciers

Article (PDF Available)inJournal of Geophysical Research Atmospheres 117 · March 2012with34 Reads
DOI: 10.1029/2011jg001828
Abstract
There is current debate about whether the balance of photosynthesis and respiration has any impact on the net accumulation of organic matter on glacier surfaces. This study assesses controls on rates of net ecosystem production (NEP), respiration, and photosynthesis in cryoconite holes during the main melt season (June-August 2009) on three valley glaciers in Svalbard. Cryoconite thickness and organic matter content explained 87% of the total variation in rates of respiration (in units of volume), and organic matter (but not sediment depth) was a significant (p < 0.05) control on photosynthesis (by volume). The average rates of respiration and gross photosynthesis within the cryoconite holes were overall closely balanced, ranging from net autotrophic to heterotrophic. Sediment depth explained over half the variation of NEP, with net autotrophic rates typical only in sediment < 3 mm thick. The measured rates of NEP were not sufficient to account for the organic matter which has likely accumulated in the cryoconite on timescales of less than decades, suggesting three alternatives for the source of the organic matter. First, the glacier surface may have received windblown allochthonous organic material from surrounding environments. Second, cryoconite may consist of in-washed autochthonous material from the glacier surface which has comparable organic carbon content. Third, much of the organic matter may have accumulated in the hole during a nascent period, when rates of NEP were much higher. The cycling of autochthonous labile carbon produced by phototrophs may sustain a significant proportion of the total in situ microbial activity within cryoconite holes.

Figures

Controls on the autochthonous production and respiration
of organic matter in cryoconite holes on high Arctic glaciers
Jon Telling,
1
Alexandre M. Anesio,
1
Martyn Tranter,
1
Marek Stibal,
1
Jon Hawkings,
1
Tristram Irvine-Fynn,
2
Andy Hodson,
3
Catriona Butler,
1
Marian Yallop,
4
and Jemma Wadham
1
Received 10 August 2011; revised 14 December 2011; accepted 17 December 2011; published 18 February 2012.
[1] There is current debate about whether the balance of photosynthesis and respiration
has any impact on the net accumulation of organic matter on glacier surfaces. This
study assesses controls on rates of net ecosystem production (NEP), respiration, and
photosynthesis in cryoconite holes during the main melt season (JuneAugust 2009) on
three valley glaciers in Svalbard. Cryoconite thickness and organic matter content
explained 87% of the total variation in rates of respiration (in units of volume), and organic
matter (but not sediment depth) was a significant (p < 0.05) control on photosynthesis
(by volume). The average rates of respiration and gross photosynthesis within the
cryoconite holes were overall closely balanced, ranging from net autotrophic to
heterotrophic. Sediment depth explained over half the variation of NEP, with net
autotrophic rates typical only in sediment <3 mm thick. The measured rates of NEP were
not sufficient to account for the organic matter which has likely accumulated in the
cryoconite on timescales of less than decades, suggesting three alternatives for the source
of the organic matter. First, the glacier surface may have received windblown
allochthonous organic material from surrounding environments. Second, cryoconite may
consist of in-washed autochthonous material from the glacier surface which has
comparable organic carbon content. Third, much of the organic matter may have
accumulated in the hole during a nascent period, when rates of NEP were much higher. The
cycling of autochthonous labile carbon produced by phototrophs may sustain a significant
proportion of the total in situ microbial activity within cryoconite holes.
Citation: Telling, J., A. M. Anesio, M. Tranter, M. Stibal, J. Hawkings, T. Irvine-Fynn, A. Hodson, C. Butler, M. Yallop,
and J. Wadham (2012), Controls on the autochthonous production and respiration of organic matter in cryoconite holes on high
Arctic glaciers, J. Geophys. Res., 117, G01017, doi:10.1029/2011JG001828.
1. Introduction
[2] The origin of organic matter on glacier surfaces is a
matter of current debate. One explanation is that a significant
fraction of the organic matter is autochthonous; that is,
produced in situ by photosynthesis on glaciers [Anesio et al.,
2009]. The autochthonous organic matter may then be an
important source of organic carbon and nutrients for in situ
and downstream ecosystems [Anesio et al., 2009; Hood et al.,
2009]. The alternative possibility is that the organic matter
is derived dominantly from allochthonous (i.e., external)
sources [Stibal et al., 2008a], and the role of microbial activity
is relegated to the transformation of preexisting allochtho-
nously derived organic matter [Hodson et al., 2010a]. In both
cases, organic matter produced by photosynthesis may cause
a decrease in the albedo (and hence melting) of glacier surfaces
either directly by the production of dark microbial pigments
(e.g., UV-protective compounds such as scytonemin and
mycosporine-like amino acids) [Quesada et al., 1999] or
indirectly through the trapping of dark debris via the produc-
tion of exopolysaccharides [Takeuchi et al., 2001; Hodson
et al., 2010a].
[
3] Microbial activity on the surfaces of glaciers and ice
sheets is concentrated in surface sediment, known as cryoco-
nite [Säwström et al., 2002; Anesio et al., 2009]. Cryoconite is
primarily composed of inorganic debris, derived either from
aeolian dust deposition or locally derived debris from
moraines or basal ice [Takeuchi et al., 2001; Bøggild et al.,
2010]. The remaining organic fraction is typically <5% by
weight [Takeuchi et al., 2005; Hodson et al., 2010a],
although values as high as 1320% have been reported on
the Greenland Ice Sheet [Gerdel and Drouet, 1960]. A variety
of studies have demonstrated that cryoconite contains a wide
1
Bristol Glaciology Centre, School of Geographical Sciences,
University of Bristol, Bristol, UK.
2
Centre for Glaciology, Institute of Geography and Earth Science,
Aberystwyth University, Aberystwyth, UK.
3
Department of Geography, University of Sheffield, Sheffield, UK.
4
School of Biological Sciences, University of Bristol, Bristol, UK.
Copyright 2012 by the American Geophysical Union.
0148-0227/12/2011JG001828
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, G01017, doi:10.1029/2011JG001828, 2012
G01017 1of10
diversity of prokaryotic and eukaryotic microorganisms,
including both heterotrophs and phototrophs. The latter
includes both cyanobacteria and green algae [Christner et al.,
2003; Stibal et al., 2006; Edwards et al., 2011].
[
4] In this study we measured in situ rates of net ecosystem
production (NEP), respiration and gross photosynthesis in
cryoconite holes on three valley glaciers in Svalbard during
the main summer melt season in 2009. Potential environ-
mental controls on microbial productivity rates are identified
by multivariate statistical analyses of site, sediment thick-
ness, organic carbon and chlorophyll a data. Further batch
experiments were carried out to directly quantify the effects
of cryoconite thickness on rates of microbial activity. The
significance of photosynthesis for producing organic matter
within cryoconite holes and supporting in situ microbial
activity is assessed.
2. Methods
2.1. Study Sites
[
5] Incubation experiments were conducted with cryoco-
nite from three neighboring, north-facing valley glaciers,
Midtre Lovénbreen (ML), Vestre Brøggerbreen (VB) and
Austre Brøggerbreen (AB), situated in the NW of the Sval-
bard archipelago, close to the scientific base at Ny-Ålesund at
79°N 12°E (Figure 1). All three glaciers have exhibited sus-
tained negative mass balance since the 1930s resulting in
retreat and ice thinning with large proportions of the ice area
below the long-term mean snowline elevation [Hagen et al.,
2003; Nuth et al., 2007]. Incubations took place between
13 July to 25 August 2009. All incubations were carried out
on the surface of the glaciers, either within cryoconite holes
(to estimate in situ rates of net ecosystem production (NEP),
gross photosynthesis and respiration) or on the surface of
the glaciers (for experimental manipulations on cryoconite
thickness). Therefore, all incubations were conducted under
in situ light and temperature conditions. All of the studied
cryoconite holes were open to the atmosphere (i.e., the
holes were not covered by slush or ice). The majority of
cryoconite holes had a measurable flow through of water
(i.e., stream cryoconite holes). Three samples of moraine
debris were also sampled from a lateral moraine on the
eastern periphery of ML on 25 August 2009 (Figure 1).
2.2. In Situ Rates of Net Ecological Production, Gross
Photosynthesis, and Respiration
[
6] Rates of NEP, gross photosynthesis and respiration
were measured at a total of 40 cryoconite holes on the three
different glaciers (AB, ML, VB; see Figure 1). All manip-
ulations and analyses were carried out on the surface of the
glaciers, and closed bottle incubations carried out within the
respective cryoconite holes. Cryoconite was removed from
each hole using a plastic scoop, and immediately added to
two glass bottles with tapered glass stoppers (65 mL BOD
bottles, Wheaton). Care was taken to replicate the average in
situ cryoconite thickness in the incubation bottles. All bottles
were completely filled with in situ supraglacial water using a
60 mL plastic syringe, leaving no headspace in the bottles.
One of the two bottles at each site was covered with alu-
minum foil (dark bottle). The remaining bottle was left
uncovered (light bottle). In addition, at 26 sites two bottles
(one light, one dark) were filled completely with supraglacial
Figure 1. Map showing location of cryoconite holes used for in situ incubations, moraine samples, and
cryoconite thickness experimental sites on Austre Brøggerbreen (AB), Midtre Lovénbreen (ML), and
Vestre Brøggerbreen (VB). Latitude, 79°N; longitude, 12°E. The map is projected in universal transverse
Mercator (UTM) World Geodetic System (WGS 84) with contours displayed at 100 m intervals.
TELLING ET AL.: CARBON PRODUCTION ON ARCTIC GLACIERS G01017G01017
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water without sediment, to act as water only controls. The
bottles were then completely submerged in water within their
corresponding cryoconite holes and incubated for 24 (3) h.
[
7] Start and end concentrations of dissolved inorganic
carbon (DIC) and oxygen were measured in incubation
bottles using the method of Telling et al. [2010]. Measure-
ments were made on the surface of the glaciers immediately
after removing each bottle from its respective cryoconite
hole. Rates of NEP and respiration in incubation bottles
were calculated from the change in DIC (DCO
2
method) and
oxygen (DO
2
method) in light and dark bottles, respectively.
Rates of gross photosynthesis were calculated by subtracting
the change in DIC (or oxygen) of dark bottles from the
change in DIC (or oxygen) of light bottles, after correcting
for any differences in dry weights of cryoconite in the dark
and light bottles. The detection limits (defined as two
standard error of the mean of six supraglacial water samples)
were 18 mgCL
1
d
1
(DCO
2
method) and 96 mgCL
1
d
1
(DO
2
method) for rates of NEP and respiration. The detec-
tion limit for gross photosynthesis was 26 mgCL
1
d
1
(DCO
2
method) and 136 mgCL
1
d
1
(DO
2
method)
combining errors for the light and dark bottles using the least
squares method. The precision of the methods was assessed
in a previous study on eleven cryoconite holes on the same
three glaciers in the same field season [Telling et al., 2010].
The mean coefficient of variation (CV) was 16.5%
7.8% (1s), 32.0 18.8%, and 32.8 22.0% for rates of
respiration, photosynthesis, and NEP, respectively, using the
DCO
2
method. The mean CV was 29.7 21.7%, 19.2
7.9, and 48.6 54.7% for rates of respiration, photosyn-
thesis, and NEP, respectively, using the DO
2
method.
2.3. Cryoconite and Moraine Sampling and Analysis
[8] At each incubation site (Figure 1), the physical
dimensions of the cryoconite holes (length, width, depth of
water column, thickness of cryoconite debris) were measured
using a graduated rule. Cryoconite hole areas were estimated
by assuming all holes were circular, using the mean of width
and length to estimate the hole circumference. Cryoconite at
each site (and moraine debris from the lateral moraine on
ML; see Figure 1) was placed into sterile polypropylene
centrifuge tubes using plastic scoops. Samples were trans-
ported chilled back to the field laboratory at Ny-Ålesund
within 8 h of collection, and subsequently frozen (20°C)
and transported back to Bristol for later analysis.
[
9] For total organic carbon analysis (TOC) cryoconite
and moraine debris was dried in an oven at 70°C for 2 days.
Duplicate samples were then analyzed for total carbon (TC)
on a Eurovector EA3000 Elemental Analyzer and inorganic
carbon (IC) on a Coulomat 720 analyzer. total organic carbon
(TOC) was defined as the difference between TC and IC.
Duplicate TOC values for cryoconite samples were all within
10% of each other, duplicate moraine samples were between
10% and 19% of each other. The detection limit was
100 mgCg
1
dry sediment.
[
10] Chlorophyll a was analyzed following the method of
Thompson et al. [1999]. Blanks (ethanol only) were run
every 10 samples. Absorption values were standardized
against purified chlorophyll a from cyanobacteria (Sigma).
The detection limit was 100 mgL
1
chlorophyll a. After
normalizing to dry cryoconite weight (mgCg
1
d
1
), the
precision (CV) of duplicate chlorophyll a analyses was
9.3 6.7% (1s).
2.4. Multivariate Statistical Analysis of Field Data
[
11] Constrained ordination analysis allows direct assess-
ment of the relationship between known environmental
variables and variation in the multivariate data, and was used
here to assess the effect of locality, cryoconite sediment
thickness and the concentrations of organic carbon and
chlorophyll a in the sediment on the measured microbial
activity (NEP, respiration, photosynthesis).
[
12] The data were transformed prior to analysis as fol-
lows: glaciers (AB, VB, ML) were used as dummy variables
in the analysis; the TOC and chlorophyll a data were ln(x + 1)
transformed; the sediment thickness data were left untrans-
formed. Since the NEP data contained both negative and
positive numbers, they were first multiplied by 1 and then a
constant was added resulting in all numbers being positive.
These data were then ln transformed, as were the photosyn-
thesis and respiration data. All data were standardized and
centered. Samples with any missing data were removed from
analysis.
[
13] Detrended canonical correspondence analysis (DCCA)
was used first to determine the length of the gradient along
the first ordination axis, in order to select the appropriate
method for constrained ordination of the data [ter Braak and
Šmilauer, 2002]. Redundancy analysis (RDA) was then used
to evaluate the significance of the environmental variables
(glacier, sediment thickness, TOC, chlorophyll a) as controls
for biological processes, using the approach of Kaštovská
et al. [2005]. RDA is a constrained ordination technique,
based on principal component analysis (PCA), in which
ordination axes are constrained to be linear combinations of
environmental variables. The significance of the relationship
is tested with the Monte Carlo permutation test [ter Braak
and Šmilauer, 2002]. The following RDA setup was used:
focus on sample distances, 499 unrestricted Monte Carlo
permutations, and manual forward selection. In forward
selection, the construction of the regression model starts with
the environmental variable that explains the most variation in
the dependent variables. What remains of the variation to
explain after fitting the first variable is then used to choose
the second environmental variable. The process of selection
goes on until no more variables significantly explain the
residual variation. This allows to select the most significant
variables and to avoid the problem of multicollinearity
[Legendre and Legendre, 1998; Ramette and Tiedje, 2007].
[
14] All the analyses were performed in the multi-
variate data analysis software CANOCO 4.5. The program
CANODRAW 4.0 [ter Braak and Šmilauer, 2002] was used
for graphical presentation of ordination results. The results
of RDA were summarized using a biplot diagram, in which
the relative length and position of arrows show the extent
and direction of response of the selected dependent variables
to the environmental factors.
2.5. Cryoconite Thickness Experiments
[
15] Five experiments (ML Expt 13, VB Expt 12; see
Figure 1) were set up to specifically investigate the depen-
dence of rates of NEP and gross photosynthesis on cryoconite
thickness. Bottle incubations were set up exactly as described
TELLING ET AL.: CARBON PRODUCTION ON ARCTIC GLACIERS G01017G01017
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above for the in situ measurements, with the exceptions
that (1) a range of different cryoconite thicknesses were used
(0.5/1.0 to 8.0 mm, with the former values representing one
grain thick cryoconite layers) and (2) the bottles were incu-
bated on the glacier surface rather than submerged at the
bottom of cryoconite holes. As before, all manipulations and
analyses were performed on the surface of the glaciers. In
each experiment, a light and dark bottle was set up for each
cryoconite thickness, along with water only controls.
3. Results
3.1. Microbial Activity
[
16] There were significant positive correlations between
the DO
2
and DCO
2
methods for NEP (r = 0.786; p < 0.001;
n = 40; Spearmans correlation; two-tailed t test), respiration
(r = 0.882; n = 40; p < 0.001; Spearmans correlation; two-
tailed t test) and photosynthesis (r = 0.793; n = 40; p <
0.001; Spearmans correlation; two-tailed t test; see
Figure 2). The mean respiratory quotient (RQ, defined as the
molar DCO
2
/ DO
2
) and photosynthetic quotient (PQ; defined
as the molar DO
2
/ DCO
2
) were 0.85 0.24 (1s) and 1.24
0.53 (1s), respectively. The mean RQ and PQ were similar to
those established earlier in the season on the same glaciers of
0.80 0.17 (1s) and 1.24 0.20 (1s), respectively [Telling
et al., 2010].
[
17] Rates of gross photosynthesis and respiration in water
only incubations were close to the detection limit and lower
than those of bottles with cryoconite added (Figure 3). Rates of
gross photosynthesis and respiration in incubations amended
with cryoconite were 588.9 327.4 (1s) mgCl
1
d
1
(18.7
10.3 mgCg
1
d
1
) and 655.7 392.0 mgCl
1
d
1
(1s)
(18.7 9.1 mgCg
1
d
1
), respectively (Figure 4). Rates
of NEP in bottles amended with cryoconite ranged from
net autotrophy (maximum of 170.5 mgCL
1
d
1
,or
10.6 mgCg
1
d
1
) to net heterotrophy (minimum of
997.9 mgCL
1
d
1
,or11.7 mgCg
1
d
1
). The
mean rate of NEP in all bottles amended with cryoconite
was close to balance at 45.7 216 0.2 mgCL
1
d
1
(1s)(0.12 4.1 mgCg
1
d
1
; see Figure 4).
3.2. Cryoconite Hole Physical Dimensions and Organic
Chemistry
[
18] The mean cryoconite water volume: cryoconite sedi-
ment volume of cryoconite holes on AB, ML and VB ranged
between 31:1 and 42:1 (Table 1), within a factor of two of
the typical 60:1 ratio used in the bottle incubations. The
mean thickness of cryoconite used in the in situ incubations
was 2.9 1.8 mm (1s), with a range from 1 mm up to 8 mm
(Figure 4d). The TOC of cryoconite ranged from 5.0 to
44.5 mg C g
1
sediment (0.5 to 4.5% by sediment weight;
see Figure 4e). Mean TOC values for AB, ML, and VB
cryoconite were 17.3, 20.1, and 33.9 mg C g
1
, respectively.
The three moraine samples from ML had TOC concentra-
tions of 7.7 10.0 mg C g
1
(1s), 4.2 0.8 mg C g
1
(1s)
and 2.9 10.2 mg C g
1
(1s), with a mean value for all
three samples of 4.9 2.8 mg C g
1
(1s). The chlorophyll a
content of cryoconite ranged from 2.1 to 23.0 mgCg
1
(Figure 4f), while no chlorophyll a was detected in any of
the moraine samples. Chlorophyll a and TOC of cryoconite
samples were significantly positively correlated (r = 0.602;
p < 0.001; n = 40; Spearmans correlation; two-tailed t test).
3.3. Multivariate Analysis
[
19] The length of the gradient along the first ordination
axis of the data determined by DCCA was 0.54 SD. Since
Figure 2. Scatterplots showing correlations between DCO
2
and DO
2
measurements in cryoconite holes
for (a) net ecological production (NEP), (b) respiration, and (c) photosynthesis. Here r values are deter-
mined by Spearmans correlation, and p values are determined from two-tailed t tests.
Figure 3. Respiration versus photosynthesis for all data.
Water only, cryoconite <3 mm thick, and cryoconite 3+ mm
thick are plotted separately. There is a good correlation
between photosynthesis and respiration if all cryoconite-
amended data are considered (r = 0.814; p < 0.001; n = 40;
Spearmans correlation; two-tailed t test) and a stronger cor-
relation if only shallow (<3 mm) are considered (r = 0.948;
p < 0.001; n = 25; Spearmans correlation; two-tailed t test).
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linear ordination methods are recommended when the gra-
dient length is <3 SD and absolute values are analyzed
[ter Braak and Šmilauer, 2002; Ramette and Tiedje, 2007],
the linear constrained ordination method RDA was employed
to explain the variation in the microbial activity data.
[
20] Figure 5 shows the results of RDA using glaciers
and the sediment thickness, TOC and chlorophyll a data as
independent (explanatory) variables and the photosynthesis,
respiration and NEP data (in units of volume) as dependent
(explained) variables. The analysis explained a total of 58.7%
of the data variation and identified three significant (p < 0.05)
controls on microbial production: sediment thickness
accounted for 36.3% (p = 0.002; F = 17.65), TOC concen-
tration for 14.6% (p = 0.004; F = 8.88) and VB for 5.6%
(p = 0.046; F = 3.70).
[
21] Table 2 shows the results of three RDAs that used
photosynthesis, respiration and NEP on a volume basis, as
the only dependent variable and all the environmental vari-
ables as independent variables. While TOC was the only
variable significantly explaining the variation in the photo-
synthesis data, respiration was significantly affected by the
concentration of TOC and sediment thickness and also the
locality of VB explained a significant portion of the varia-
tion. The only significant control of NEP in our analysis was
sediment thickness, accounting for more than half of the
variation within the data (Table 2).
3.4. Cryoconite Thickness Experiments
[
22] In each of the five cryoconite thickness experiments
NEP was net autotrophic (rates of photosynthesis greater
than respiration) at cryoconite thicknesses <3 mm, with
trends toward net heterotrophy with increasing cryoconite
thickness (Figure 6). Slopes of best fit linear regression
lines for NEP were significantly less than zero in all cases
(p < 0.05; two-tailed t test). This further corroborates the
inverse relationship between cryoconite thickness and NEP
documented for in situ cryoconite holes (Figure 5). The mean
cryoconite thickness where NEP = 0 for all five experiments
was 3.0 mm 1.2 mm (1s).
4. Discussion
4.1. Controls on Rates of Respiration and
Photosynthesis on Svalbard Valley Glaciers
[
23] There were significant (p < 0.05) correlations between
rates of respiration (in units of volume) with both sediment
thickness and organic carbon, explaining a combined total of
72.7% of total variation in the respiration rates (Table 2).
The significant increase in respiration with increasing sedi-
ment depth is likely due to increases in both the amount of
labile organic matter and number of microbial cells within
the incubations, since cryoconite samples were added to the
bottles in bulk. The simplest explanation for the significant
correlation between TOC and respiration is that a relatively
constant fraction of TOC between sites is bioavailable, and
hence increasing TOC increases the amount of available
carbon for respiration. The reason for the small (6.8% of
total variation; see Table 1) but significant negative rela-
tionship of VB site to respiration (Figure 5) is unclear. One
possibility is that cryoconite holes on VB contain a higher
Figure 4. Box plots of data used in multivariate analysis: (a) net ecological production (NEP), (b) respi-
ration, (c) photosynthesis, (d) sediment thickness, (e) total organic carbon (TOC), and (f) chlorophyll a.
Horizontal line shows the median; bottoms and tops of shaded boxes show the 25th and 75th percentiles,
respectively; error bars show the 90th percentiles; and solid dots show individual outliers. AB, Austre
Brøggerbreen; ML, Midtre Lovénbreen; VB, Vestre Brøggerbreen.
Table 1. Physical Dimensions of Studied Cryoconite Holes
a
Glacier
Cryoconite
Hole Area
(cm
2
)
Cryoconite Hole
Water Depth
(cm)
Cryoconite
Thickness
(mm)
Volume Water:
Volume
Cryoconite
AB (n = 8) 48.4 19.4 8.3 2.3 3.0 2.0 40.6 22.9
ML (n = 24) 51.1 34.7 5.8 3.2 2.7 0.9 31.4 26.6
VB (n = 8) 65.0 56.0 6.1 6.0 2.8 3.0 42.0 61.1
a
Values are means 1s .
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proportion of refractory to labile carbon, although we have
no data to further test this hypothesis.
[
24] The lack of any significant relationship (p < 0.05)
between photosynthesis (in units of volume) and sediment
thickness is consistent with light limitation within deeper
sediment layers. Incident PAR is typically completely atten-
uated at thicknesses of <1 mm in silty sediments restricting
photosynthesis to the upper submillimeter depth of sediment
grains [Jorgensen and DesMarais, 1986; Garcia-Pichel and
Bebout, 1996]. The significant correlation (p < 0.05) of TOC
with photosynthesis, accounting for 26.1% of the total vari-
ation of photosynthesis (Table 2), is unlikely to be because
in situ photosynthesis within the cryoconite holes forms the
majority of TOC. We show in section 4.3 that the measured
rates of NEP are unlikely to be able to produce more than a
small fraction of TOC within the studied cryoconite holes
on a reasonable timeframe. An alternative explanation for
the positive correlation between TOC and photosynthesis
is that the breakdown of organic matter allochthonous to the
cryoconite holes (although potentially produced autoch-
thonously elsewhere on the glacier) is supporting photo-
autotrophic growth, perhaps by the supply of essential
nutrients such as nitrogen or phosphorus [Tranter et al.,
2004; Stibal et al., 2008b, 2009].
[
25] Less than half of the variation in rates of gross pho-
tosynthesis could be explained by the measured variables in
this study (Table 2), indicating that additional factors are
likely important. One key missing variable measured in this
study for explaining rates of gross photosynthesis is the
availability of photosynthetically active radiation (PAR),
which changes with season, cloud cover, and shading effects
by cryoconite hole walls and local topography [Hodson et al.,
2010b]. The availability of nutrients such as phosphorus and
nitrogen may also be an important factor for microbial
growth and activity [Stibal et al., 2008b; Telling et al., 2011].
Phosphorus has previously been shown to be limiting in the
water phase of cryoconites [Mindl et al., 2007]. Further,
some cryoconite holes on AB, ML and VB can become
depleted in available aqueous and sediment-bound nitrogen
resulting in microbial nitrogen fixation [Telling et al., 2011].
Microbial nitrogen fixation could feasibly have a negative
impact on rates of microbial growth and activity since it is an
energetically costly process relative to the uptake of aqueous
nitrogen species [Gutschick, 1978; Telling et al., 2011].
[
26] The strong positive correlation between respiration
and photosynthesis in cryoconite holes, with a tendency to
net autotrophy within <3 mm thick sediments (Figure 3), is
consistent with a closely coupled microbial carbon loop
where labile carbon produced by phototrophs supports the
majority of the measured rates of respiration [Hodson et al.,
2010a]. Therefore while the majority of organic carbon
within Svalbard cryoconite holes is not likely formed in situ
within the holes via photosynthesis (see section 4.3), con-
versely the recycling of labile organic carbon produced by
photosynthesis within cryoconite holes may support a sig-
nificant fraction of the total microbial activity within the
holes.
4.2. Controls on Net Ecosystem Production
on Svalbard Valley Glaciers
[
27] The only significant (p < 0.05) measured environ-
mental control on NEP was sediment thickness, accounting
for 55.7% of total NEP variation (Table 2). The strong
Table 2. Results of RDAs With Manual Forward Selection Using
Photosynthesis, Respiration, and NEP as the Only Dependent
Variable
a
Variation
Explained (%) p Value F Value
Photosynthesis 40.2
1 TOC 26.1 0.002 10.95
Respiration 72.7
1 TOC 41.8 0.002 22.22
2 sediment thickness 24.1 0.002 21.16
3 Västre Brøggerbreen 6.8 0.010 7.16
Net ecosystem production 63.2
1 sediment thickness 55.7 0.002 39.02
a
Independent variables were included in analysis according to their
percentage of variation explained. Only significant controls (p < 0.05) are
shown.
Figure 5. Results of RDA using glaciers and the sediment
thickness, total organic carbon (TOC), and chlorophyll a data
as independent (explanatory) variables (solid lines) and the
photosynthesis, respiration, and NEP data as dependent
(explained) variables (dotted lines). VB, Vestre Brøggerbreen.
Chlorophyll a and the glaciers Midtre Lovénbreen and Austre
Brøggerbreen were not significant (p < 0.05) explanatory
variables. Note that during the data transformation prior to
RDA, the sign of NEP was changed; hence the positive cor-
relation between sediment thickness and NEP is actually
negative.
TELLING ET AL.: CARBON PRODUCTION ON ARCTIC GLACIERS G01017G01017
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control of sediment thickness on NEP is clearly shown in
Figures 3 and 6 and indicates that NEP in cryoconite holes
tends to net autotrophy at sediment thicknesses of <3 mm
(where rates of photosynthesis are typically greater than rates
of respiration) and toward net heterotrophy at sediment
thicknesses of >3 mm (where rates of respiration typically
exceed those of photosynthesis). The relative increase in
respiration over photosynthesis at sediment depths >3 mm
(Figures 3 and 6) may be explained by light limitation in
thicker sediments owing to shading by cryoconite grains.
[
28] The mean rate of NEP on the valley glaciers (0.12
4.1 mgCg
1
d
1
) was slightly net heterotrophic, although
close to the detection limit. Although there appears to be little
or no overall accumulation of autochthonous organic carbon
in the cryoconite holes of this study, there are likely to be loci
of net autotrophy and net heterotrophy which are strongly
controlled by sediment thickness. Importantly, cryoconite
debris in hydrologically isolated cryoconite holes on Arctic
glaciers tend to form uniform one grain layers owing to the
effect of lateral as well as vertical melting of ice by cryoco-
nite grains heated by solar radiation [Cook et al., 2010].
There may therefore be a tendency for net autotrophy in more
hydrologically stable regions with lower slopes and less
water flow, allowing time for cryoconite to equilibrate to a
one grain thick layer. Conversely, cryoconite holes with
higher slopes and more rapid streamflow may have a ten-
dency toward net heterotrophy owing to greater flushing of
cryoconite holes and the subsequent piling up of thicker
sediment. For example, a previous glacier wide survey of
cryoconite coverage on ML toward the end of the main melt
season demonstrated that cryoconite holes on the lower half
of ML (where cryoconites in this study were focused; see
Figure 1) were dominated by stream cryoconite holes with
relatively high rates of supraglacial flushing, while the upper
half of ML was dominated by relatively isolated cryoconite
holes with relatively low rates of supraglacial flow [Hodson
et al., 2007]. The potential for organic carbon from autoch-
thonous origin to accumulate at higher and less hydrologi-
cally disturbed parts of the Greenland Ice Sheet ablation
zone has been recently demonstrated [Stibal et al., 2012]. We
hypothesize that rates of significant net organic production
on ML may therefore be focused in the upper half of ML
where thin one grain layer cryoconite may dominate. Fur-
thermore different surface regions on valley glaciers may
switch between overall net autotrophy to net heterotrophy
throughout the melt season owing to changes in the hydro-
logical regime.
4.3. Sources of Organic Matter in Svalbard
Cryoconite Holes
[
29] We assess the potential importance of net autotrophy
for producing organic carbon in thin (1 to <3 mm thick)
cryoconite layers using equation (1):
Melt seasons ¼
CryconiteorganiccarbonðÞmoraineorganiccarbonðÞ
NEP timeðÞ
ð1Þ
where melt seasons is the number of melt seasons to produce
the TOC or estimated phototroph biomass of the cryoconite
hole, cryoconite organic carbon (in units of mgCg
1
)is
either TOC or estimated phototroph biomass (the latter
estimated by converting the measured concentrations of
chlorophyll a to phototroph carbon biomass using a ratio of
1:47) [Riemann et al., 1989], moraine organic carbon is the
mean TOC value of moraine debris on ML (4900 mgCg
1
),
NEP is in units of mgCg
1
d
1
for holes with autotrophic
growth only, and time is the typical length of a melt season on
ML (60 days) [Hodson et al., 2007]. Although the moraine
Figure 6. Results of cryoconite thickness experiments carried out on Midtre Lovénbreen (ML1, ML2,
and ML3) and Vestre Brøggerbreen (VB1 and VB2). Graphs show the change in rates of gross photosyn-
thesis, respiration, and net ecosystem production (NEP) as a function of cryoconite thickness. The smallest
grain sizes (0.5 to 1 mm) represent the thickness of a single cryoconite grain.
TELLING ET AL.: CARBON PRODUCTION ON ARCTIC GLACIERS G01017G01017
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organic carbon values in equation (1) are based on just three
samples of a lateral moraine from ML, their low TOC values
(relative to cryoconite) are consistent with previous reports of
moraine from the glaciers. A value of 3000 mgCg
1
has
been reported from lateral moraine on ML [Borin et al.,
2010], while the TOC content of barren soils and subglacial
derived sediment in the forefields of VB, ML and AB has
been reported as typically below detection (<1% dry weight)
while cryoconite debris TOC on the three glaciers was typi-
cally several percent dry weight [Kaštovská et al., 2005].
[
30] The mean time needed to form the estimated photo-
troph biomass in thin (1 to <3 mm sediment thickness)
autotrophic cryoconite holes is 3.3 3 years (1s), with a
minimum of 1 year and maximum of 14 years (Figure 7). The
mean time required to form the mean TOC in the same holes
is 105 91 years (1s), with a minimum of 20 years and a
maximum of 348 years (Figure 7). The typical residence time
of cryoconite in stream washed holes on Svalbard valley
glaciers has been estimated to on the order of <1 year to
several years [Stibal et al., 2008a; Hodson et al., 2010a].
Thin (1 to <3 mm) cryoconite sediments on Svalbard valley
glaciers may therefore have sufficiently high rates of
autochthonous carbon production to support the growth of a
relatively small (compared to TOC) phototrophic biomass,
but likely not more than a fraction of their TOC. More
hydrologically stable (isolated) cryoconite holes elsewhere
however could have substantially greater residence times on
the ice surface, and hence a greater potential for autochtho-
nous organic matter accumulation. For example, cryoconite
on sections of the Greenland Ice Sheet may have a residence
time of >100 years on the ice surface [Nobles, 1960], and
autochthonous organic carbon production provides one
explanation for the higher TOC content of cryoconite debris
documented on parts of the Greenland Ice Sheet (e.g.,
1320% TOC by dry weight) [Gerdel and Drouet, 1960]
relative to Svalbard valley glaciers (Figure 4).
[
31] The above calculations indicate that at the time of
sampling overall rates of net autochthonous organic carbon
production within the cryoconite holes were unable to account
for the majority of organic matter in the cryoconite. This dis-
crepancy in the organic carbon budget of the cryoconite holes
can be explained in one of three ways. First, there may be
allochthonous inputs of organic carbon from environments
external to the glacier into the cryoconite holes. Second, there
may be autochthonous organic carbon production in alter-
native supraglacial habitats on the glacier surface which is
subsequently washed into the cryoconite holes. Third, NEP
rates in nascent cryoconite holes may have been substantially
higher than in the holes at the time of sampling.
[
32] Input of allochthonous organic carbon into cryoconite
holes is likely, although the magnitude of this flux is cur-
rently unknown. Windblown organic matter from adjacent
tundra [Stibal et al., 2008a] is perhaps the most likely source.
There may also be input of organic carbon near the terminus
of the glaciers however from subglacial debris pushed up to
the surface in the form of pressure ridges, although reported
values of for subglacial debris samples at the front of AB, ML
and VB are low (<1% TOC dry weight) [Ka štovská et al.,
2005].
[
33] The autochthonous production of organic carbon in
alternative habitats on the glacier surface, and subsequent
washing into cryoconite holes, is also feasible. The snow-
pack overlies the entire surface of Svalbard valley glaciers at
the start of the season and retreats upslope as the melt season
progresses [Hodson et al., 2008], while dispersed cryoconite
lying on the ice surface can constitute up to half of the total
mass of cryoconite on Svalbard valley glaciers [Hodson et al.,
2007]. The NEP of the snowpack and dispersed cryoconite
environments is currently unknown [Hodson et al., 2008] but
could potentially be significant [Takeuchi, 2002; Hodson
et al.,2007].
[
34] The third explanation for the source of organic carbon
in cryoconite holes is that rates of NEP are greater in nascent
cryoconite holes than in more mature cryoconite holes. This
mechanism appears plausible but is currently untested.
Microimaging of cryoconite grains indicates that cryoconite
in cryoconite holes is typically composed of individual small
grains of 100 mm or less bound together in larger aggregates
by exopolysaccharides [Takeuchi et al.,2001;Hodson et al.,
2010a; Langford et al., 2010]. Given the negative relation-
ship between cryoconite thickness and NEP (Figures 5 and 6),
it is plausible that rates of photosynthesis could dominate
over respiration in the initial phase of aggregation resulting in
relatively high rates of NEP. In contrast the more mature
cryoconite grains in the cryoconite holes of this study had a
relatively close balance between photosynthesis and respira-
tion (Figure 3). The lack of cryoconite <0.5 mm thick in the
cryoconite holes of this study (Figures 4d and 6) tends to
suggest however that either the initial aggregation of cryo-
conite debris is extremely rapid, or that the initial aggregation
does not occur within the cryoconite holes themselves but
instead within either the snowpack and/or dispersed cryoco-
nite debris on the ice surface [Langford et al., 2010]. Some
support for the latter comes from a previous study on an
Antarctic glacier that demonstrated a threshold aggregation
size is necessary before cryoconite can absorb sufficient
solar energy to melt into the ice to form cryoconite holes
Figure 7. Estimates of the number of melt seasons required
to produce organic carbon in thin (<3 mm thick) autotrophic
cryoconite sediment: (a) time required to form total organic
carbon and (b) time required to form the estimated photo-
troph carbon biomass of cryoconite. These time estimates
were calculated using equation (1) (see section 4.3). Photo-
troph biomass was estimated by assuming a 1:47 ratio between
the measured chlorophyll a concentrations of cryoconite and
phototroph carbon biomass (after Riemann et al. [1989]; see
section 4.3).
TELLING ET AL.: CARBON PRODUCTION ON ARCTIC GLACIERS G01017G01017
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[MacDonell and Fitzsimons, 2008]. It is possible that a
similar threshold aggregation size mechanism operates on
Arctic valley glaciers.
5. Conclusions
[35] Cryoconite thickness and organic matter were signif-
icant controls on rates of respiration (in units of volume) in
cryoconite holes on three Svalbard valley glaciers. Organic
matter (but not sediment depth) was a significant control on
photosynthesis. Sediment depth explained over half the
variation of net ecosystem production (NEP), with net auto-
trophic rates typical only in sediment <3 mm thick. The
measured rates of NEP were not sufficient to account for the
organic matter which has likely accumulated in the cryoco-
nite holes on timescales of less than decades, suggesting that
either (1) the glacier surface receives allochthonous organic
material from surrounding environments, or (2) organic
matter is derived from in-washed autochthonous material
from alternative habitats (e.g., the snowpack and dispersed
cryoconite) on the glacier surface, or (3) organic carbon
accumulated in the hole during a nascent period, when rates
of NEP were much higher. The cycling of autochthonous
labile carbon produced by phototrophs within cryoconite
holes may sustain a significant proportion of the total in situ
microbial activity within cryoconite holes.
[
36] Acknowledgments. This work was funded by grants awarded to
A.M.A. and A.H. from NERC (NE/G00496X/1 and NE/G006253/1). We
are grateful to three anonymous reviewers and the Associate Editor, whose
comments greatly strengthened this paper. We would like to thank Nick
Cox for his logistical help at NERC Arctic station Ny-Ålesund.
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    • "Biocryomorphic evolution of cryoconite holes 9 is an important and hitherto overlooked regulator of ice surface biogeochemistry. Net carbon sequestration necessitates accumulation of autochthonous OC, which has previously been identified in the interior zones of the Greenland Ice Sheet (Stibal et al., 2012a; Telling et al., 2012). This promotes granule growth and stabilization by entangling allochthonous organic and inorganic matter and by supporting heterotrophy, resulting in the production of humic and extracellular polymeric 'cements' (Langford et al., 2010; Takeuchi et al., 2010). "
    [Show abstract] [Hide abstract] ABSTRACT: Microbial photoautotrophs on glaciers engineer the formation of granular microbial-mineral aggregates termed cryoconite which accelerate ice melt, creating quasi-cylindrical pits called 'cryoconite holes'. These act as biogeochemical reactors on the ice surface and provide habitats for remarkably active and diverse microbiota. Evolution of cryoconite holes towards an equilibrium depth is well known, yet interactions between microbial activity and hole morphology are currently weakly addressed. Here, we experimentally perturbed the depths and diameters of cryoconite holes on the Greenland Ice Sheet. Cryoconite holes responded by sensitively adjusting their shapes in three dimensions ('biocryomorphic evolution') thus maintaining favourable conditions for net autotrophy at the hole floors. Non-targeted metabolomics reveals concomitant shifts in cyclic AMP and fucose metabolism consistent with phototaxis and extracellular polymer synthesis indicating metabolomic-level granular changes in response to perturbation. We present a conceptual model explaining this process and suggest that it results in remarkably robust net autotrophy on the Greenland ice sheet. We also describe observations of cryoconite migrating away from shade, implying a degree of self-regulation of carbon budgets over mesoscales. Since cryoconite is a microbe-mineral aggregate, it appears that microbial processes themselves form and maintain stable autotrophic habitats on the surface of the Greenland ice sheet. This article is protected by copyright. All rights reserved.
    Full-text · Article · Apr 2016
    J.M. CookJ.M. CookA. EdwardsA. EdwardsM. BullingM. Bulling+1more author...[...]
    • "Processes of carbon cycling have long been documented in cryoconite holes (Tranter et al., 2004;Bagshaw et al., 2007;Foreman et al., 2007;Stibal and Tranter, 2007), but the extent of the dependence of photosynthetic organisms concentrated at the surface of the sediment layer on heterotrophic processes deeper in the sediment has not been fully discussed in the literature to date. This inter-dependence may also help explain the link between sediment depth and P:R balance previously observed in short term (6-24 hr) cryoconite incubations (Telling et al., 2012). The heterotrophic activity in the deeper sediment serves to relocate organic matter, fixed as CO 2 , from the deeper layers to the surface of the sediment, and to dissolve carbonate minerals. "
    Full-text · Article · Jan 2016 · Environmental Research Letters
    • "The causes and likely distribution of net heterotrophy also deserves equal attention. One of the greater uncertainties in this context is the so-called bacterial growth efficiency, which Foreman et al (2013) suggest is likely to be as low as 1—2% in a range of ice surface habitats and thus capable of explaining a large proportion of the CO 2 transfer to the atmosphere that is inferred from incubation studies (e.g. Telling et al 2012). However, the empirical approaches used to calculate this crucial parameter might not be appropriate for supraglacial ecosystems. "
    [Show abstract] [Hide abstract] ABSTRACT: The fourteen letters that contributed to this focus issue on cryospheric ecosytems provide an excellent basis for considering the state of the science following a marked increase in research attention since the new millennium. Research letters from the focus issue provide significant insights into the biogeochemical and biological processes associated with snow, glacier ice and glacial sediments. This has been achieved via a significant, empirical effort that has given particular emphasis to glacier surface habitats. However, far less is known about aerobiology, glacial snow covers, supraglacial lakes and sub-ice sedimentary habitats, whose access for sampling and in-situ monitoring remains a great challenge to scientists. Furthermore, the use of models to explore key fluxes, processes and impacts of a changing glacial cryosphere are conspicuous by their absence. As a result, a range of process investigations and modelling studies are required to address the increasing urgency and uncertainty that is associated with understanding the response of cryospheric ecosystems to global change.
    Full-text · Article · Nov 2015
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