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Macroalgal habitats support a sustained flux of floating biomass but limited
carbon export beyond a Greenland fjord
Thomas Gjerluff Ager
a,b,
⁎,Dorte Krause-Jensen
b,c
,Birgit Olesen
a,c
, Daniel F. Carlson
d
,
Mie Hylstofte Sichlau Winding
e
, Mikael K. Sejr
b,c
a
Department of Biology, Aarhus University, 8000 Aarhus C, Denmark
b
Department of Ecoscience, Aarhus University, 8000 Aarhus C, Denmark
c
Arctic Research Center, Aarhus University, 8000 Aarhus C, Denmark
d
Optical Oceanography, Institute of Carbon Cycles, Helmholtz-Zentrum Hereon, 21502 Geesthacht, Germany
e
Greenland Climate Research Centre, Greenland Institute of Natural Resources, 3900 Nuuk, Greenland
HIGHLIGHTS GRAPHICAL ABSTRACT
•Abundant macroalgal communities sup-
port a flux of floating macroalgae in
Greenland fjord.
•Temporal variation in floating macroalgal
biomass and composition.
•The majority (80 %) of the floating bio-
mass is retained in the fjord system.
•Only about 6.92 t C yr
−1
or 0.02 % of
estimated NPP was exported beyond the
fjord.
ABSTRACTARTICLE INFO
Editor: Martin Drews
Keywords:
Blue carbon
Macroalgae
Arctic
Carbon export
Floating macroalgae
Despite growing attention on the contribution of macroalgae to carbon cycling and sequestration (blue carbon), more ob-
servational data is needed to constrain current estimates. In this study, we estimate the floating macroalgal carbon flux
within and beyond a large sub-Arctic fjord system, Nuup Kangerlua, Greenland, which could potentially reach carbon
sinks. Our study estimates 1) the fjord-scale area with macroalgal coverage and barrens caused by sea urchin
grazing, 2) the floating macroalgal biomass in the fjord, and 3) the annual export flux of floating macroalgae out of
the fjord system. ROV surveys documented that macroalgal habitats cover 32 % of the seafloor within the photic zone
(0-30 m) with an average coverage of 39.6, 22, and 7.2 % in the depth intervals 0–10, 10–20, and 20-30 m, respectively.
15 % of the area suitable for macroalgae was denuded by sea urchin grazing. Floating macroalgae were common with an
average biomass of 55 kg wet weight km
−2
. Densities and species composition varied seasonally with the highest levels
after storms. The floating biomass was composed of intertidal macroalgal species (58 %) (Fucus vesiculosus, Fucus distichus,
and Ascophyllum nodosum) andkelps(42%)(Saccharina longicruris, S.latissima, and Alaria esculenta).Wedeployedsurface
GPS drifters to simulate floating macroalgal trajectories and velocity. Data indicated that 80 % of the floating biomass is
retained in the fjord where its fate in relation to long-term sequestration is unknown. Export beyond the fjord was limited
and indicated an annual floating macroalgal export beyond the fjord of only 6.92 t C yr
−1
,whichisequalto~0.02%of
the annual net primary production. Our findings suggest that floating macroalgae support a limited blue carbon potential
beyond this fjord and that future research should focus on the fate of retained floating macroalgae and subsurface export
to resolve the connectivity between macroalgal habitats and long-term carbon sinks.
Science of the Total Environment 872 (2023) 162224
⁎Corresponding author at: Department of Biology, Aarhus University, 8000 Aarhus C, Denmark.
E-mail address: 201708279@post.au.dk (T.G. Ager).
http://dx.doi.org/10.1016/j.scitotenv.2023.162224
Received 1 September 2022; Received in revised form 30 January 2023; Accepted 9 February 2023
Available online 15 February 2023
0048-9697/© 2023 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Contents lists available at ScienceDirect
Science of the Total Environment
journal homepage: www.elsevier.com/locate/scitotenv
1. Introduction
Macroalgal habitats are globally the most widespread and productive of
coastal vegetated ecosystems covering about seven million km
2
(Duarte
et al., 2022;Pessarrodona et al., 2022)andextendingintotheArcticregion
where climate change may open new habitats (Assis et al., 2022). These
habitats support a rich diversity of coastal life and food webs while also
playing a significant, although poorly constrained role in carbon cycling
and marine carbon sequestration (Krumhansl and Scheibling, 2012;
Krause-Jensen and Duarte, 2016;Filbee-Dexter et al., 2018;Ortega et al.,
2019;Frigstad et al., 2020). The role of macroalgae and other marine
vegetation in global carbon sequestration was introduced in 1981 (Smith,
1981) and re-emphasized in a 2009 framework dubbed ‘Blue Carbon’
(Nelleman et al., 2009), which, however, has mainly addressed
angiosperm-based habitats (i.e. mangrove, salt marshes, and seagrasses)
which build carbon deposits in the sediments below them (Duarte et al.,
2013). Macroalgae, on the other hand, mainly grow attached to hard sub-
stratum where local carbon burial is not possible. However, increasing evi-
dence of considerable export of macroalgal carbon, in the form of both
particulate organic carbon (POC) and dissolved organic carbon (DOC), to
sinks beyond their habitat has sparked the discussion of the role of
macroalgae in the global carbon cycle (Krause-Jensen and Duarte, 2016;
Krause-Jensen et al., 2018;Santos et al., 2019;Smale et al., 2017).
Macroalgal photosynthesis removes CO
2
from the water column and builds
it into biomass, but in the process also produces DOC, which can be
exported from the macroalgae bed, sustaining lateral carbon flows
(Watanabe et al., 2020). Furthermore, part of the biomass can be exported
as detritus (Krumhansl and Scheibling, 2012), which may also support car-
bon flux into the seabed (Queirós et al., 2019). Various fingerprinting
methods are available to identify macroalgal carbon in marine sediments
(Geraldi et al., 2019). Stable isotopes have e.g. been used to document
and quantify macroalgal carbon in seagrass sediments and in the deep
ocean (e.g. Fischer and Wiencke, 1991;Garcias-Bonet et al., 2019). Envi-
ronmental DNA (eDNA) hasin recent years been used to document the pres-
ence of macroalgae in marine surface sediments (Ørberg et al., 2021;
Ortega et al., 2020;Queirós et al., 2019;Queirós et al., 2023), in deep ma-
rine sediments (Frigstad et al., 2020), and in the water column as far as
5.000 km from coastal areas (Ortega et al., 2019), further highlighting
the potential for lateral export of macroalgae carbon, although without di-
rectly quantifying the contribution to carbon sequestration. Sequestration
potential is highly dependent on the lability of the algae, both during export
and when reaching a sink habitat, a factor that varies markedly between
species and depends on environmental conditions (Trevathan-Tackett
et al., 2015). One study has documented that 20–30 % of the detritus
from the kelp Laminaria hyperborea is decomposed extremely slowly or
not at all under anaerobic conditions over a 300-day period, while other
studies have identified that 2–9 % of released macroalgae POC was seques-
tered within sediments (Hardison et al., 2010;Pedersen et al., 2021;
Queirós et al., 2019). At global scale, a first-order estimate suggests that
macroalgae contribute 173 Tg carbon annually to sequestration (Krause-
Jensen and Duarte, 2016). However, this estimate includes several impor-
tant uncertainties (Krause-Jensen and Duarte, 2016) and additional obser-
vational data is needed to constrain both quantification and key processes
determining the fate of macroalgal carbon.
The Greenlandic coastline has 44,000 km of shoreline, which supports ex-
tended macroalgal habitats (Krause-Jensen and Duarte, 2014), with kelp for-
ests typically extending to 15–40 m depth and in some areas down to 60 m
(Krause-Jensen et al., 2012, 2019). Macroalgal carbon has been documented
in food webs and sediments off the Greenlandic coast using isotope signatures
and eDNA (Gaillard et al., 2017;Ørberg et al., 2021), which suggests a poten-
tial for macroalgal carbon sequestration in the region. The distribution of
macroalgae is influenced by a variety of biological and physical factors. Sea
ice limits the light availability on the seafloor while melting glaciers provide
high inputs of silt, which increases light attenuation in the water column
(Murray et al., 2015), thus decreasing depth distribution of macroalgae
(Bartsch et al., 2016). Furthermore, ice calved from marine-terminating gla-
ciers, combined with seasonal sea ice, can cause scouring of the coastline,
negatively impacting macroalgal distribution (Sejr et al., 2021). Sea urchins
are present throughout the Greenlandic coastline (Blicher et al., 2007), and
although they may extensively graze on macroalgae creating seabed barrens
denuded of standing biomass (Filbee-Dexter and Scheibling, 2014), such reg-
ulation has not been quantified in Greenland.
One of several proposed mechanisms supporting macroalgal export to car-
bon sinks is the surface drift of dislodged macroalgae with buoyant structures
and subsequent sinking to carbon sinks in fjord and shelf sediments or the
deep sea (Krause-Jensen and Duarte, 2016). This study sets out to quantify
Fig. 1. Study site.
Map of Greenland showing the Nuup Kangerlua study area in south-west Greenland (A) as well as details on study sites (B). Red dots indicate location of ROV (Remotely
Operated Vehicle) studies. Blue squares indicate deployment of drifters. Dotted lines indicate sailed transects with numbers referring to transect information in Table S1
(repeated transects are highlighted). The red area indicates the inner fjord, and the green area indicates the outer fjord of the main branch. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of this article.)
T.G. Ager et al. Science of the Total Environment 872 (2023) 162224
2
the annual carbon export of floating macroalgae in a large sub-arctic fjord sys-
tem, Nuup Kangerlua, in Greenland in terms of biomass, species composition,
and export trajectories. We do this based on quantification of 1) coverage of
coastal macroalgal habitats in relation to depth and sea urchin density,
2) floating biomass, and 3) export trajectoriesandvelocitiesbasedonGPS-
tracked surface drifters. By assessing floating biomass, we obtain information
on surface densities of macroalgal-based carbon in the fjord. Combining this
information with data on surface currents velocities and direction derived
from drifters, we estimate a net export of the floating macroalgal carbon
out of the fjord system. We compare our estimate of floating biomass and
beyond-fjord export with estimates of macroalgal standing stock and net pri-
mary production (NPP) in the fjord and discuss the results in the context of
the potential for carbon sequestration.
2. Materials and methods
2.1. Study area
This studywas conducted in the subarctic fjord systemNuup Kangerlua,
southwest Greenland (64°N, 51°W, Fig. 1), from April 2021 to August 2021
(Table S1). A branch of the fjord system extends beyond the inner fjord but
is periodically full ofice and therefore excluded from the study (Fig. 1). The
Nuup Kangerlua has approximately 2013 km
2
of surface area and 1200 km
of coastline, characterized by rocky substrates and with extensive
macroalgae habitats (Krause-Jensen et al., 2012;Mortensen et al., 2011).
The intertidal macroalgal community is dominated by Ascophyllum
nodosum,Fucus vesiculosus,andFucus distichus, while the subtidal zones
are dominated by kelps within the order Laminariales such as Saccharina
latissima, S. longicruris, Alaria esculenta,andAgarum clathratum (Krause-
Jensen et al., 2012;Ørberg et al., 2018). The dominant canopy-forming in-
tertidal species all have buoyant structures, and the same is the case for the
dominant kelp S. longicruris which has a long hollow stipe. In case of dis-
lodgement, these buoyant features allow the macroalgae to drift with sur-
face currents. By contrast, A. clathratum, A. esculenta, and S. latissima do
not have such buoyant structures but were nevertheless occasionally ob-
served entwined within other floating macroalgae.The fjord mouth consti-
tutes a narrow strait spanning 5.6 km that floating macroalgae would need
to pass in order to be exported beyond the fjord. Skerries are located both
within and beyond the fjord and are potential locations for beaching.
Water circulation in the fjord is affected by freshwater supply from three
marine-terminating glaciers, three land-terminating glaciers, rivers, and
snowmelt during spring. Furthermore, the fjord system is subject to semi-
diurnal tides with a maximum amplitude of 5.5 m (Richter et al., 2011).
The bathymetry of the fjord varies with maximum depth reaching 625 m,
while three sills are present at 170, 250, and 277 m depths (Mortensen
et al., 2011).
2.2. Carbon export of floating macroalgae
To estimate the biomass of floating macroalgae and the associated car-
bon export out of Nuup Kangerlua, we quantified three parameters: 1) the
distribution of macroalgal habitats and coverage using a Remotely Oper-
ated Underwater Vehicle (ROV), 2) the amount of floating biomass in the
fjord system from ship observations and collection and quantification of
the floating biomass, and 3) the direction and velocity of surface currents
from GPS-tracked drifters to provide a first estimate of the net export of
floating macroalgae out of the fjord.
2.2.1. Quantifying floating macroalgae biomass
To quantify floating macroalgae biomass, we sailed 34 cross-fjord tran-
sects with a small research boat (“Aage V. Jensen II”, Greenland Institute of
Natural Resources”). The transects' rangedin length from 3.2 to 8.7 km. To
identifyfloating macroalgae, one person scanned the surface from each side
of the boat while covering the transect at a speed <15 knots. Observed
macroalgae were collected using nets and transported to the Greenland In-
stitute of Natural Resources (GINR) forsorting and weighing.In a few cases,
where patches of floatingmacroalgae wereeither very dense withvery high
biomass or loosely distributed over larger areas, only a fraction of the ob-
served macroalgae was collected. The missed fraction of macroalgae bio-
mass was then estimated visually. Before sailing each transect, the sea
state was determined using the Beaufort Scale Sea State Chart. Based on
the sea state and general weather conditions, the maximum distance at
which floating macroalgae could be observed was estimated from tests.
The sight range constituted the transect width and only macroalgae within
the set distance were collected.
In the lab, collected macroalgae were sorted into three groups: 1) Fucus
species (F. distichus &F. vesiculosus) from now on referred to as ‘Fucus’,
2) Ascophyllum nodosum, and 3) Laminariales (Saccharina longicruris, S.
latissima, and Alaria esculenta). Laminariales were treated as a single group
due to few observations of A. esculenta and S. latissima. Subsamples of Fucus
(n= 10), Ascophyllum (n = 10), Laminariales stipes (n= 5), and
Laminariales blades (n = 5) were weighed (wet weight, ww) and subse-
quently dried in an oven at 60 °C until the dry weight (dw) was constant. Fur-
thermore, carbon (C) and nitrogen (N) content of dried Ascophyllum, Fucus,
and Laminariales (stipes and blade) were analyzed for samples collected in
April and August.
To account for seasonal variation of floating macroalgae three transects
(Transect 1a, 2, 10, Fig. 1B) were repeated in April, June, and August. Fur-
thermore, the transect located in the fjord system outlet (Transect 1b,
Fig. 2. Observed floating macroalgae.
A) Summed observed floating wet weight of macroalgal species made along transects. ‘April Total’refers to the total biomass identified in April. Bars withindashed polygons
indicate observations in the repeated transects (1a, 2, 10) in the different months. n is the number of macroalgal observation. B) Densities of floating macroalgal species.
T.G. Ager et al. Science of the Total Environment 872 (2023) 162224
3
Fig. 1B) was surveyed 3 times on different days during April. Macroalgal
biomass observations were converted into surface densities (kg/area)
using the following equation
ρ¼WW
RoS LðÞ (1)
Where ρis the density, WW is the wet weight of macroalgae, RoS is the
range of sight in kilometers and L is the length of transects in kilometers.
The average total biomass of floating macroalgae in the fjord system at
a given sampling time was assessed as ρ∗total fjord surface area
(2013 km2). Furthermore, the macroalgal observations were used to
interpolate and create a map of floating macroalgae in Nuup Kangerlua.
Mapping was conducted using the Inverse Distance Weighting method in
QGIS v3.10.
2.2.2. Macroalgal habitat distribution and density
Macroalgal habitat distribution in relation to depth, substrate, and den-
sity of sea urchins was quantified in real-time usinga FiFish pro V6 plus ROV
(red dots in Fig. 1B). The ROV was launched manually from the vessel and
steered to the coastline at the start and end of each transect. The ROV was
tilted in a 90° degrees position to film directly atop the seabed and operated
in a straight line perpendicular to the coastline at 0–30 m depth. The ROV
was fitted with two laser lights 15 cm apart, which allowed the area cov-
ered to be estimated. All variables were determined in 10 m depth intervals
(0–10 m, 10–20 m, and 20–30 m). The 0–10 m interval consisted of 49 ob-
servations, while the 10–20 m and 20–30 m intervals only consisted of 46
observations due to ROV engine breakdown, making it impossible to go
deeper than 10 m. Macroalgal cover was estimated from the video in 5 %
intervals, and substrate cover was divided into two categories, (1) suitable:
hard substrate suitable for macroalgae growth, and (2) not suitable: small
surfaces not suitable for macroalgae growth (i.e. sand and gravel) and
also estimated in 5 % cover intervals. A Kruskal-Wallis test followed by a
pairwise Wilcoxon rank sum t est was conducted to test for significant differ-
ences in macroalgal and substrate cover, respectively, between depth inter-
vals. Moreover, sea urchin density was quantified to estimate the potential
impact of sea urchins on total macroalgalcover in the Nuup Kangerlua fjord
system. Sea urchin cover was divided into four categories: None (0 m
−2
),
Few (up to 1 m
−2
), Moderate (1–3m
−2
), and High (>3m
−2
). The ROV-
sites where sea urchins had turned macroalgae cover into barrens were de-
fined by the following criteria: macroalgae cover ≤10 %, high sea urchin
density (>3m
−2
), and suitable substrate ≥50 %.
The total area of macroalgal habitat in Nuup Kangerlua was estimated
for each depth interval (0–10 m, 10–20 m, and 20–30 m). This was done
by multiplying the average algal cover of each depth interval based on all
observations from ROV surveys in that interval, with the total ground
area of the corresponding depth interval within the study area. The area
of each interval was extracted from the Bed Machine v3 Ocean Bathymetry
Model as this is currently the only available model.
Thefractionofsiteswithseaurchinbarrenswasusedtogenerateanes-
timate of reduced macroalgal habitat distribution due to grazing in each
depth interval:
Reduction in macroalgae distribution
¼Macroalgae distribution
100 fraction of barren states 100
Macroalgae distribution (2)
2.2.3. Trajectories and velocities of surface drift
To estimate floating macroalgal trajectories, velocities, likelihood of
beaching, and export from the fjord system, GPS-tracked drifters (Carlson
et al., 2020) were deployed within the fjord system. Initially, 12 drifters
were deployed in the middle of the northernmost branch of the system,
with 3 of the drifters being deployed within floating macroalgae patches.
The main fjord branch was split up into two zones; the inner fjord (red
area, Fig. 1B) and outer fjord (green area, Fig. 1B) near the mouth of the
fjord, to help identify variation in the surface water movement. Beached
drifters were repeatedly retrieved and redeployed throughout the study pe-
riod resulting in a total of 20 deployments (Fig. 1B). Some beached drifters
refloatedon their own during high tide, andthis was consideredas a contin-
ued singledeployment. The drifters reported their position(incl. sea surface
temperature) in 30 min intervals, enabling calculations of their average ve-
locities and their general trajectories. In the outer fjord region, the major
and minor axes of the ellipse encompassing drifter positions were oriented
at 62° and 331°, respectively. The major axis aligned with the main axis of
the fjord at the mouth. A bounding box was then created in the outer fjord
system (Fig. 5C). Subsets of the trackers passing through the box were ex-
tracted and velocities were estimated using forward differencing. The ve-
locity components were rotated to align with the major and minor axes
and a mean velocity was then calculated.
2.2.4. Annual floating macroalgae carbon export from Nuup Kangerlua
calculations
To estimate the annual floating macroalgae carbon export from Nuup
Kangerlua to the open ocean, the area of surface water leaving the fjord
was computed by multiplying the fjord mouth width by the average veloc-
ity of trackers. By multiplying the species-specificobservedfloating
macroalgal densities by the area exported, the floating macroalgae export
per time was determined. Subsequently, the macroalgal export was con-
verted intocarbon export using the wet-to-dry weight ratio and carbon con-
tent. Lastly, the beaching ratio of trackers was taken into account by
multiplying the results by the fraction of trackers leaving the system
(Table S3).
3. Results
3.1. Floating macroalgae biomass
Floating macroalgae were sampled along 34 cross-fjord transects
(Fig. 1B) with a combined length of 151 km covering an area of
28.24 km
2
of sea surface. Observations were only made during calm condi-
tions (sea state 0–2 for 30 transects) or wind driven chop (sea state 3–4for4
transects; see Table S1 for transects information). The cross-fjord transects
revealed a consistent presence of floating macroalgae throughout the Nuup
Kangerlua fjord system. In the repeated transects (transect 1a, 1b, 2, 10 in
Fig. 1B), floating macroalgae were continuously observed, indicating a con-
stant transport. Biomass distribution ranged from areas with only a few kg
km
−2
to ‘hot spots’with densities of 155 kg wet weight km
−2
(Fig. 3). The
average density of floating algae in the fjord system was 55 kg ww km
−2
(Fig. 2B). Up-scaled tothe total surface areaof the fjord (2013 km2) the av-
erage total biomass of floating algae was ~110 tons. A total of 1545 kg of
floating macroalgae (Fig. 2A) was collected from 83 observations with a
sample size ranging 0.04–147.69 kg. Laminariales and Ascophyllum ac-
counted for the majority of the floating macroalgal biomass, and Fucus con-
stituted a smaller fraction (Fig. 2A). Findings within the Laminariales group
were heavily dominated by S. longicruris. All groups of macroalgae were ob-
served frequently throughout the fjord system, often inlong (100 s meters)
bands perpendicular to the shoreline. The bands were composed mainly of
whole fresh thalli of Ascophyllum and Fucus, with older stipes of
Laminariales intertwined. Furthermore, larger patches/areas of floating
macroalgae were observed corresponding with topographic eddies in satel-
lite imagery (see supplementary material). Two observations in June con-
tributed heavily to the total biomass of floating Laminariales. The nature
of these observations differed from the other observations of Laminariales,
with the presence of fresh thalli and many individuals entangled very
densely. These observations contributed 81.92 and 109.53 kg, respectively.
Floating biomass densities and algal group composition exhibited
temporal variations over the study period. Densities varied from
2.46 kg km
−2
in August and 58.39 kg km
−2
in April to 124.39 kg km
−2
in June (Fig. 2B). Ascophyllum was the most common algal group observed
in April. Due to the majority of the transects being sailed in this period,
Ascophyllum is over-represented in the total distribution of floating biomass
T.G. Ager et al. Science of the Total Environment 872 (2023) 162224
4
across algal groups. Algal group composition shifted in June with
Laminariales becoming the most dominant, followed by Fucus.
Laminariales was also dominant during August measurements when only
a few observations were made of Fucus and Ascophyllum (Fig. 2A).
The wet weight to dry weight ratios varied between subtidal and inter-
tidal algal groups with Laminariales having a higher water content than
Fucus and Ascophyllum while the carbon content was similar among groups
(Table 1,ANOVA,P<0.05).
As a result, each kg of wet-weight macroalgae biomass exported out of
the fjord is equal to a net amount of carbon export of 0.08 kg, 0.05 kg,
and 0.09 kg from Fucus, Laminariales, and Ascophyllum, respectively. Only
four driedsamples of Ascophyllum were analyzed due to one single finding
hereof in August.
3.2. Macroalgal cover and habitat extent
ROV surveys showed a significant decrease in macroalgal cover toward
deeper water, whereas the suitable substrate only decreased significantly
between 0 and 10 m and deeper intervals (Fig. 4A, Kruskal-Wallis,
P<0.001). The average macroalgal cover decreased from 39.6 (±23.6)%
in the 0–10 m interval to 22 (±20.8)% in the 10–20 m interval, and 7.2
(±7.4)% in the 20–30 m interval. The bathymetry extracted from the
Bedmachine v3 model yielded an overall seabed area of 297.37 km
2
be-
tween 0 and 30 m of depth where macroalgae are expected to grow
(Fig. 4B, blue areas). The largest area, 205.16 km
2
, was located in the
0–10 m interval, while the 10–20 m and 20–30 m intervals constituted
57.67 km
2
and 34.54 km
2
, respectively (Fig. 4B, blue areas). The total
area of macroalgal habitat distribution was estimated at 81.22 km
−2
in
the 0–10 m interval, 12.66 km
2
in the 10–20 m interval, and 2.48 km
−2
in the 20–30 m interval. This yielded a potential macroalgal habitat of
96.36 km
2
at 0–30 m of depth in Nuup Kangerlua out of a total area of
297.37 km
2
in the 0–30 m depth zone, i.e. 32 % of this depth zone was cov-
ered. Based on the average per-area standing stock (3.26 kg ww m
−2
,
Ronowicz et al., 2020;Filbee-Dexter et al., 2019;Filbee-Dexter et al.,
2022;Malavenda, 2021;Sharp et al., 2008) and NPP(0.299 kg C
m
−2
yr
−1
,Pessarrodona et al., 2022) of relevant macroalgal species in Arc-
tic this would correspond to a total standing stock of 314,113 t ww with a
NPP of 28,812 t C yr
−1
.
3.3. Surface water movement
Drifter data showed overall complex surface water circulation patterns
compared to a simplified two-layer estuarine circulation. However, some
of the drifter trajectories showed an outwards transport pattern in the sur-
face waters. The drifters traveled an average distance of 67.4 (±62.9) km
while remaining in transit for 515.9 (±716.4) hours. The average travel
speed of the drifters was 0.09 (±0.06) m/s. Drifters traveled the greatest
distances and at the highest speeds in the inner fjord, with both measures
decreasing toward the mouth of the fjord. Time spent in transit was greater
near the fjord mouth. The 7 drifters deployed in the inner fjord (red area,
Fig. 5A) all stayed near the north shore of the fjord before beaching aside
from two drifters diverging deeper into the fjord before beaching. Drifters
deployed near the mouth (blue area, Fig. 5B&5C) showed a greater varia-
tion in transport patterns. Two drifters beached on the northern coastline,
while seven drifted south-east and beached on the skerries. Some
drifters refloated on several occasions before beaching again. Four
drifters exited the fjord system (passing the red line, Fig. 5B) without
beaching on the skerries (Fig. 5C). After exiting, one drifter moved
north along the coast and was then stranded. Another drifter stopped
transmitting while in open waters. The two remaining drifters transited
across the Labrador Sea, and the latest observations were sent off the
coast of Newfoundland, documenting a traveling distance of 4578 -
5739 km in 197 days (Fig. 5D). The average velocity in the outer fjord
system was 0.05 (±0.31) m/s outwards based on 148 measurements
of 9 drifters passing through the bounding box created in the area
(Fig. 5Band5C).
Fig. 3. Floating macroalgal densities in Nuup Kangerlua.
Contours indicate interpolated floating macroalgal density estimated from macroalgal observations in April, June, and August 2021.
T.G. Ager et al. Science of the Total Environment 872 (2023) 162224
5
Fig. 4. Cover of Macroalgae and Suitable Substrate of Nuup Kangerlua.
A) Average percent coverage of macroalgae and substratum in three depth intervals estimated from ROV observations (0-10 m (n= 49), 10-20 m and 20-30 m (n= 46)) at
start and end of eachtransect. Significant differencesare displayed using compact letter displaying (p<0.05). B) Map of bathymetry between 0 and 30 min Nuup Kangerlua
in 10 m intervals including the inner branch which was not surveyed (red). Blue areas indicates where macroalgae is expected to grow and therefore included within the
study. Grey areas show the Greenland ice sheet, and stars represent marine-terminating (red) and land-terminating (black) glaciers. Bathymetry originates from
Bedmachine v3 model. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. Tracker trajectories.
Trajectories for trackers A) deployed in the inner fjord, B) Deployed near the fjord mouth, C) that exited the fjord. D) Shows trajectories of trackers exiting the fjord systems
including potential sink sites of the different macroalgae groups as estimated based on literature-information on their longevity/lability (see Section 4.2). + indicate the
deployment location of drifters. Dark-blue area indicates the carbon sequestration horizon (1.000 m depth). The black polygon in C) and D) indicates the area used for
calculating average velocity out of the fjord. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
T.G. Ager et al. Science of the Total Environment 872 (2023) 162224
6
3.4. Floating macroalgal carbon export
We quantified the annual export flux of floating macroalgal carbon from
Nuup Kangerlua as a first estimate of the potential export of macroalgae to
sites beyond the fjord system. The estimate should be treated with caution
since we did not resolve the full annual variation in all processes such as
surface circulation patterns. Furthermore, interannual variability in these
processes, which can only be resolved by multi-year studies, mayalso affect
macroalgal carbon export rates.
The exported water surface area per time (1.01 km
2
/h) was calculated
by multiplying the total average tracker velocity (0.179 km/h) by the
width of the mouth (5.636 km). Multiplication by the individual algae
groups' surface densities yielded the annual export of macroalgal wet-
weight biomass. Based on wet weight to dry weight ratios, and carbon
Fig. 6. Sea urchin ‘barrens’in Nuup Kangerlua.
A) Distribution of seaurchins catergories in the three depth intervals (0–10 m (n= 47), 10–20 m (n=46),20–30 m (n = 46)) in Nuup Kangerlua. B) Relation between
macroalgal cover, suitable substrate, and sea urchin densities across ROV sites at 0–10 m, 10–20 m, and 20–30 m of depth. ROV observation sites have been split into
four categories marked by the dashed lines. These categories are introduced in the first panel.Y-axis dashed line indicate the upper limit of macroalgae cover for barrens.
X-axis dashed line indicate lower limit of suitable substrate to be considered a ‘barren’.
T.G. Ager et al. Science of the Total Environment 872 (2023) 162224
7
content (Table 1), the algal groups' respective carbon export was calculated.
Beaching was taken into account by multiplying the exported carbon with
the fraction of trackers exiting the fjord (0.2). The estimated contribution
to carbon export of each macroalgae group was: Fucus 1.31 t C yr
−1
,
Laminariales 1.93 t C yr
−1
,andAscophyllum 3.18 t C yr
−1
totalinga net car-
bon export of 6.92 t C yr
−1
from Nuup Kangerlua (For details see Table S4).
3.5. Sea urchin distribution
ROV surveys revealed a considerable variability in sea urchin densities
(Fig. 6A). In the 0–10 m depth interval, six sites met our criteria of a barren
state (macroalgae cover ≤10 %, high sea urchin density (>3m
−2
), and
suitable substrate ≥50 %) representing 12.8 % of the area with macroalgal
cover. A reduction of 12.8 % in the 0–10 m interval corresponds to a de-
crease of 11.89 km
2
macroalgal habitat (see eq. 2). In the 10-20 m depth in-
terval, 13 of 46 observations exhibited barren characteristics, indicating a
reduction of 26.1 % of the area with macroalgal coverin this depth interval,
corresponding to a decrease of macroalgal habitat of 4.47 km
2
.Inthe
20–30 m depth interval, 9 of 46 observations showed a barren state,
indicating a reduction of 19.6 % of the total macroalgal cover, correspond-
ing to a decrease in macroalgal habitat of 0.8 km
2
(Fig. 6B). This indicates
a total macroalgal habitat reduction of 17.16 km
2
due to grazing by sea
urchins.
Chi-squared test of urchin densities between depth intervals showed no
significant differences (p = 0.530). However, further statistical analysis of
Pearson residuals revealed the ‘none’category of sea urchins tended to be
more common in the 0–10 m depth, while the ‘high’category was more com-
moninthe10–20 m depth interval. There was almost no difference in Pearson
residuals in the 20-30 m depth interval. This indicates sea urchin grazing is
mostly an important factor in the 10–20 m depth interval, which aligns with
our observation of reduction in total percentage of macroalgal cover.
4. Discussion
4.1. Floating macroalgal carbon export
Field estimates of C-stocks and fluxes of macroalgal ecosystems are im-
portant to fill the gap in understanding the role of macroalgae in the carbon
cycle and delimiting their blue carbon potential.This knowledge gap is par-
ticularly large in the Arctic where only limited quantification of macroalgal
distribution and associated carbon fluxes are available. We estimated a total
carbon export by floating macroalgae of 6.92 t C annually out of the Nuup
Kangerlua fjord system corresponding to ~0.02 % of the annual NPP of
macroalgae within the fjord (Pessarrodona et al., 2022). The exported car-
bon can potentially support sequestration at distal carbon sinks on the shelf
and in the deep sea if the exported carbon accumulates there. To provide
context for our estimate, we scaled down the most current global estimate
of macroalgal carbon sequestration (Krause-Jensen and Duarte, 2016)to
Nuup Kangerlua, based on the macroalgal habitat area, resulting in a hy-
pothesized total POC export of 956.26 t C annually from Nuup Kangerlua
to the deep sea (Supplementary material, Table S2). Our estimateof carbon
export via floating macroalgae is therefore 2–3 orders of magnitude lower
than what expected from downscaling the global estimate. Adding to this,
we do not consider the loss of biomass after exiting the fjord system and po-
tentially reaching the deep sea, further extending the difference between
the two estimates. Combined with the low fraction of NPP being exported,
this suggests that export of floating macroalgae from Nuup Kangerlua to the
deep sea is relatively negligible as the carbon exported out of the fjord is
roughly equal to the CO
2
-emissions of a single American household (Song
et al., 2019).It should benoted that both estimates are associated with con-
siderable uncertainties. In our estimate, the modelled bathymetry of the
fjord is attached with large uncertainties, and personal observations from
the fjord shows that many of the coves in the fjord have depth >30 m.
This would result in a decrease in macroalgal habitat extent, and the asso-
ciated standing stock and NPP. However, even a large decrease in the
macroalgal habitat extent would not affect conclusions regarding the signif-
icance of the floating macroalgal export. The global estimate includes an
additional potential pathway of macroalgae POC export, which is subsur-
face transport (Krause-Jensen and Duarte, 2016), since macroalgae lacking
structural buoyant components can also be exported as bed load or
entrained within the water column. This fraction of POC export is not ac-
counted for in our study and can potentially add to the estimated POC ex-
port, especially for the negatively buoyant Laminaria species. This has
been observed in previous studies where L. hyperborea sustained a substan-
tial detritus production and subsurface export (Filbee-Dexter et al., 2018;
Smale et al., 2022). Therefore, subsurface transport represents another
and likely more important pathway for carbon export out of the fjord sys-
tem, which needs to be evaluated to determine the full export of carbon
via macroalgal POC beyond the fjord.
We identified temporal variations in floating macroalgal biomass densi-
ties, and therefore also the carbon export. This probably reflected variation
in physical conditions. Peak densities in June coincided with strong winds
during the days prior to sampling and in the dominance of floating
Laminariales with fresh thalli including holdfasts, indicating they had
been dislodged recently. High wind speeds have previously been associated
with increased whole-thalli dislodgement of macroalgae and an increase in
floating macroalgal biomass (Duggins et al., 2003;Gilson et al., 2021;
Rothäusler et al., 2021). In contrast, observations made in April after a pe-
riod of calm weather conditions consisted of old stipe material, which could
either be remains from earlier dislodgement of entire individuals or the dis-
lodgement of old individuals (S. latissima typically has a lifespan of
3–5yearsinGreenland;Borum et al., 2002). Therefore, the timing of
field days is important to consider, as surveys before the storm would likely
have resulted in lower levels of floating macroalgal biomass. Kelps have
been in focus in many studies of macroalgae in the blue carbon framework
(Filbee-Dexter et al., 2018;Pedersen et al. , 2021). However, thisstudy high-
lights the importance of the intertidal brown algal speciesregarding export
of floating macroalgal carbon, which represented 2.5-fold the amount of
carbon compared to Laminariales. This can likely be linked to the presence
of pneumatocysts on the intertidal species. The high densities of floating in-
tertidal macroalgal biomass in April and June could be related to coastal
scouring by glacial ice and breakup of seasonal ice. The marine-
terminating glaciers of Nuup Kangerlua calve ice in the deepest section of
the fjord system (Carlson et al., 2017). On its journey from the deep fjord
to the open ocean, the ice travels along the coastlines causing severe scour-
ing, which can dislodge macroalgae growing in the intertidal zone (Sejr
et al., 2021). Previous studies have also linked the seasonal dynamics of
floating macroalgal densities, with high densities during spring and sum-
mer and a decline toward fall and winter, to the seasonal benthic growth
cycle (Rothäusler et al., 2021). However, as we do not have observations
during this entire period, we have not been able to identify this trend.
Strong variations in the amplitude of semi-diurnal tides can also potentially
affect dislodgement of macroalgae, but thallus morphology and plasticity
render macroalgae in areas of high tidal currents resistant to re-occurring
mechanical stress (Duggins et al., 2001;Duggins et al., 2003). We therefore
evaluate that sampling uncertainties of our study were limited regarding
quantification of the floating biomass densities on individual sampling
occasions while scaling these estimates to anannual export is more suscep-
tible to error.
Table 1
Wet Weight to Dry Weight Ratio.
Wet Weight to Dry Weight ratios (n= 10 per group) and carbon content (% of dry
weight) (Fucus n= 6, Laminariales n= 9(stipes n=3,bladen=6),Ascophyllum
n= 4) of the algae groups Fucus, Laminariales &Ascophyllum (±std). Significant
differences are displayed using compact letter display.
Algae Group Wet weight/dry weight Carbon content (%)
Fucus 4.35 (±0.81)
a
0.36 (±0.05)
c
Laminariales 7.32 (±0.80)
b
0.35 (±0.05)
c
Stipes 6.78(±0.79)
b
0.37(±0.02)
c
Blades 7.85(±0.28)
b
0.33 (±0.06)
c
Ascophyllum 3.89 (±0.28)
a
0.36 (±0.01)
c
T.G. Ager et al. Science of the Total Environment 872 (2023) 162224
8
To convert the floating macroalgae densities into a carbon export rate,
we deployed surface drifters in the fjord system. The drifter data revealed
a complex surface circulation pattern reflecting the impacts of tides and
winds. Though 20 trajectories provide insufficient data for modeling sur-
face movement and beaching rates (Blanken et al., 2020;Pawlowicz,
2021), they provide a simple and inexpensive indicator of transport trajec-
tories, velocities, and fate of floating macroalgae. However, drifter data re-
sults should be interpreted with caution due to the complexity of the
system. In this study we computed an Eulerian mean of the outer fjord sys-
tem surface water movement using a relatively small number of trajectories
compared to similar studies (Blanken et al., 2020;Pawlowicz, 2021). Ide-
ally, many more Lagrangian trajectories would be used to characterize
transport in the fjord system, emphasizing the need for continued drifter
observations in Nuup Kangerlua. Despite having 148 independent observa-
tions computing the velocity in the outer fjord system, the mean velocity
does not resolve the full seasonal dynamics at the fjord mouth so any ex-
trapolation can only be applied to similar conditions (e.g., stratification,
winds, tides). This is reflected in the large standard deviation of the export
velocity. Furthermore, drifters are not able to mimic the longevity of float-
ing macroalgae species, because they do not sink unless broken. The plastic
box containing the trackers protrudes a little above the surface water mak-
ing them more susceptible to wind and therefore beaching (Perry et al.,
2018), whereas macroalgae have a higher proportion of submerged bio-
mass likely making them more influenced by surface currents.
4.2. Fate of macroalgae carbon from Nuup Kangerlua
Our results demonstrate a continuous flux of floating macroalgal-based
carbon from the macroalgae habitats of Nuup Kangerlua sustaining an average
floating biomass of 110 tons. Despite only constituting 0.03 % of the standing
stock, the cycling of the floating biomass pool is likely much higher, making it
relevant in fjord carbon pools. Part of this carbon can potentially reach carbon
sinks in fjord sediments, shelf sediments, or the deep ocean, where it can be
stored over significant time scales or support secondary production. The like-
lihood that the floating biomass reaches potential carbon sink sites and is se-
questered there depends on a number of factors including the persistence of
the floating macroalgae, which is related to their lability. The algae can only
reach a potential carbon sink if the degradation of the biomass is slower
than the time it takes to float and sink to that site, and long-term sequestration
at the sink site will depend on the further degradation of the biomass.
To address the possibility of exported macroalgae POC reaching sink sites,
we extended our analysis of trajectories exiting the fjord system (Fig. 5D) by
combining them with literature information on the persistence and lability
of the floating biomass. Floating persistence of intertidal species observed in
this study (A. nodosum,F. distichus, and F. vesiculosus) has previously been in-
vestigated both in microcosm experiments (Vandendriessche et al., 2007)
andinsitu(Ingólfsson, 1995;Ingólfsson, 1998). The in-situ studies showed
that the intertidal macroalgae can float for a minimum of 20 days
(Ingólfsson, 1998). Lability and, hence, floating time is affected by tempera-
ture as shown in a microcosm experiment, where the macroalgae stayed afloat
much longer (211 days) at 5 °C than at 10 °C (~50 days for F. vesiculosus,
and~100daysforA. nodosum)(Vandendriessche et al., 2007). No data is
available for the local subtidal species, but larger, subtidal, temperate kelp spe-
cies (Durvillaea antarctica &Macrocystis pyrifera) rafting longevity has been
documented for a maximum of 65 and 53 days, respectively (Fraser et al.,
2011;Graiff et al., 2016). Other subtidal kelps in the Laminaria genus
(L. hyperborea) have high initial decay rates under aerobic conditions, with
40–60 % of biomass being lost within 4–6 weeks at 10 °C, followed by a de-
crease in decay rates (Pedersen et al., 2021). Drifter data from this study
followed off-shore trajectories crossing the Labrador Sea, where sea surface
temperatures ranged from 0.9 to 9.9 °C. The trackers reached areas with
depth >1000 m after approximately 10 days indicating that floating
macroalgal biomass originating in the Nuup Kangerlua can reach the deep
sea (Fig. 5D). Sinking speed of macroalgae is also a relevant factor for evaluat-
ing whether exported macroalgae may reach the deep sea before they are
decomposed. The downward flux of macroalgae is related to detritus size,
with smaller macroalgal pieces sinking more slowly (Filbee-Dexter et al.,
2020). Sinking rates are largely unknown, but observations of swift transpor-
tation have been made (Dierssen et al., 2009). Recent studies have used eDNA
toidentifythepresenceofmacroalgaewithinthesedimentsalongthecoastof
Greenland confirming the carbon pathway from kelp beds to the deep sea
(Ørberg et al., 2021).
While a fraction of the drifting macroalgae POC was exported out of the
fjord system, the majority remained within the fjord system corresponding
to our observation that 80 % of the deployed drifters ended up on the shore,
suggesting that a large fraction of floating macroalgae may end up as beach
wrack. On the rocky shorelines of Nuup Kangerlua, there are several possi-
ble fates of macroalgae debris. A proportion of the stranded macroalgae
would be remineralized as oxic conditions support high turnover rates
(Pedersen et al., 2021), leaving less carbon available for sequestration
within the beach sediments. Drifter data also revealed the possibility that
macroalgae refloat during springtides when tidal amplitudes are greatest,
to again be transported in surface currents and potentially exported.
Another possible fate for macroalgal carbon in Nuup Kangerlua is se-
questration within the deep coastal sediments of the fjord. Current esti-
mates suggest that 4.6 % of exported macroalgae is buried within shelf
sedimentsglobally (Krause-Jensen and Duarte, 2016), but for fjord systems,
this number is likely higher. Fjord systems are known to behot spots of or-
ganic carbon burial, representing 11 % of annual marine carbon burial
globally (Smith et al., 2015). A recent study documented sequestration of
macroalgal carbon 13 km off the English coast at only 48 m depth, and es-
timated that 4–9 % of macroalgal POC detritus from nearby shores was se-
questered therein (Queirós et al., 2019). C/N analysis of sediment cores in
Nuup Kangerlua indicates that sequestered total organic carbon (TOC) is
mainly of marine origin and that TOC content is 46-fold higher in the
outer fjord compared to the inner fjord (Oksman et al., 2022). Thus, some
sequestration of autochthonous carbon from the habitats must occur within
the fjord system from either phytoplankton or macroalgae. Our observation
of abundant floating biomass but low likelihood ofout-of-fjord export high-
lights the need to identify the fate of kelp detritus within Greenland fjords
before the carbon sequestration potential can be quantified.
4.3. Blue carbon potential of Greenland macroalgae
The Greenland coastline represents 44,000 km of potential macroalgae
habitats, typically extending to 15–40 m, and occasionally as deep as 61 m,
which likely sustain macroalgae carbon export. However, very little infor-
mation is available on kelp distribution in Greenland (Krause-Jensen
et al., 2019;Krause-Jensen and Duarte, 2014). Modeling of intertidal and
subtidal brown macroalgae distribution in the upper 30 m suggests they
cover ~77,500 km
2
in Greenland (Assis et al., 2022) although only
900 km
2
may be characterized as dense kelp forests (Kvile et al., 2022).
We estimated that macroalgae cover 94.4 km
2
in Nuup Kangerlua, which
has a coastline of 1200 km. If we assume that this estimate is representative
for Greenland and upscale it to 44,000 km coastline, we get an area of
3531 km
2
covered by macroalgae. This number is likely overestimated be-
cause large areas of Greenland fjords unlike Nuup Kangerlua have substan-
tial seasonal sea ice cover, which reduces kelp coverage. It suggests that
current model estimates should be treated with caution.
The Nuup Kangerlua system supports a substantial macroalgal produc-
tion, which is related to the limited sea ice cover (Krause-Jensen et al.,
2012). Combined with high levels of nutrients being transported to the sur-
face by subglacial discharge (Meire et al., 2017), Nuup Kangerlua repre-
sents optimal conditions for macroalgae growth in the outer fjord. In the
inner fjord, glacial meltwater with high sediment loads deteriorates the un-
derwater light climate in summer with negative impact on macroalgae pro-
ductivity and distribution (Bartsch e t al., 2016;Hop et al., 2016;Pehlke and
Bartsch, 2008). Silt accumulates in the areas just beyond the mouth of the
river, decreasing the availability of suitable substrate for macroalgal growth
(Hop et al., 2016). Three land-terminating glaciers are present in the inner-
most part of Nuup Kangerlua and sea ice extends from three marine-
terminating glacial fronts in winter, further limiting macroalgae growth.
T.G. Ager et al. Science of the Total Environment 872 (2023) 162224
9
Therefore, we did not conduct surveys in the areaof the very innermost part
of the fjord near the glaciers as we believe this area would not support
macroalgal growth. Nor did we include it in our calculations of macroalgae
habitat distribution. Despite macroalgal growth potentially being limited in
the inner fjord, Nuup Kangerlua represents a highly productive system.
Sea urchins are present throughout the Greenland coastline (Blicher
et al., 2007) and are known to exert a strong control on macroalgal cover
in rocky subtidal reefs being able to cause regime shifts from lush kelp for-
ests to ‘barrens grounds’(Filbee-Dexter and Scheibling, 2014). We esti-
mated that sea urchin grazing could potentially decrease macroalgae
habitats, and its associated blue carbon potential, in the Nuup Kangerlua
fjord system by ~15 %, and thus be a substantial factor in limiting blue car-
bon potential. In this estimate, we do not take other physical factors into
consideration and only account for the grazing of sea urchins occurring at
high densities in barren areas. Sea urchinshave been hypothesized to func-
tion as a mediator of macroalgae export through mobilization and transfor-
mation of kelp detritus under moderate grazing pressure, leaving the role of
sea urchins in regard to macroalgae blue carbon potentially ambiguous
(Duggins et al., 2001;Filbee-Dexter et al., 2020).
In the future Arctic, higher temperatures and a decrease in sea ice cover
are estimated to drive the expansion of macroalgae habitats along the
Greenlandic coastline, which would also increase the blue carbon potential
of Greenland (Assis et al., 2022;Krause-Jensen and Duarte, 2014). Com-
bined with spatial variations in physical and biological factors characteris-
tic of Greenland, this highlights the need for more studies on the role of
macroalgae in the carbon cycle of Greenland.
5. Conclusion
Our study shows that floating macroalgae were abundant in Nuup
Kangerlua. Intertidal macroalgae contributed significantly to the floating
biomass, especially in spring, and melting of the ice-foot and ice scouring
was considered important for their contribution. Floating subtidal kelp
was especially abundant following storms. Trajectories from surface drifters
showed that the likelihood of out-of-fjordexport was low as most (80 %) of
the floating biomass was retained in the fjord. Combining the observed
floating biomass with drifter data resulted in an estimated carbon export
from floating macroalgae of 6.92 t C yr
−1
,representing<0.1 % of estimated
macroalgae NPP. This indicate that floating macroalgae do not provide a sub-
stantial pathway to distal sinks for carbon beyond Greenland fjords. We esti-
mate a total vegetated area in the Nuup Kangerlua system of 96.36 km
2
,
corresponding to 0.07 km
2
per km of coastline, and estimated that sea ur-
chins likely reduced macroalgal distribution by 15 %. The fate of macroalgae
detritus retained within the fjord system and subsurface macroalgal export
are major knowledge gaps to be addressed in future studies.
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.scitotenv.2023.162224.
Credit authorship contribution statement
TGA, MKS and DKJ conceived and planned the study. TGA, MKS, BO
and MHSW conducted field work. TGA analyzed data and prepared a first
draft of the manuscript and figures. All authors contributed to the interpre-
tation of data and to improving and editing the manuscript.
Funding
This research was supported by the Independent Research Fund
Denmark (8021-00222 B, ‘CARMA’). The study is also a contribution to
the Greenland Ecosystem Monitoring Program (http://g-e-m.dk/).
Data availability
Data will be made available on request.
Declaration of competing interest
We have no competing interests.
Acknowledgement
We are thankful to the Greenland Institute of Natural Resources who
provided us with laboratory facilities during the fieldwork in Green-
land. A special thanks to the education coordinator of the ASSP course,
Thomas Juul Pedersen, who made that possible. We also thank the com-
petent boatsmen Carl Isaksen and Flemming Heinrich who assisted in
the fieldwork. WorldView imagery contained within this manuscript
was obtained from European Space Imaging via the European Space
Agency (ESA) Third Party Missions (TPM) programme. We would like
to thank both European Space Imaging and ESA for making this imagery
available to us.
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