Global warming has a significant impact on the regional scale on the Arctic Ocean and surrounding coastal zones (i.e., Alaska, Canada, Greenland, Norway and Russia). The recent increase in air temperature has resulted in increased precipitation along the drainage basins of Arctic rivers. It has also directly impacted land and seawater temperatures with the consequence of melting permafrost and sea ice. An increase in freshwater discharge by main Arctic rivers has been clearly identified in time series of field observations. The freshwater discharge of the Mackenzie River has increased by 25 % since 2003. This may have increased the mobilization and transport of various dissolved and particulate substances, including organic carbon, as well as their export to the ocean. The release from land to the ocean of such organic material, which has been sequestered in a frozen state since the Last Glacial Maximum, may significantly impact the Arctic Ocean carbon cycle as well as marine ecosystems.
In this study we use 11 years of ocean color satellite data and field observations collected in 2009 to estimate the mass of terrestrial suspended solids and particulate organic carbon delivered by the Mackenzie River into the Beaufort Sea (Arctic Ocean). Our results show that during the summer period, the concentration of suspended solids at the river mouth, in the delta zone and in the river plume has increased by 46, 71 and 33 %, respectively, since 2003. Combined with the variations observed in the freshwater discharge, this corresponds to a more than 50 % increase in the particulate (terrestrial suspended particles and organic carbon) export from the Mackenzie River into the Beaufort Sea.
The Arctic Ocean plays an important role in the global carbon cycle as it
contributes to up to 14 % of the global ocean uptake of atmospheric carbon
dioxide (Bates and Mathis, 2009). Observations over the last 20 years have
revealed significant impacts of climate change at high latitudes, notably in
the Arctic Ocean and surrounding coastal zones (Serreze et al., 2000;
Macdonald et al., 2005). Air temperature has increased by 1
Variations in air temperature and precipitation anomalies observed
in the Mackenzie drainage basin from 1973 to 2013
More than 10 years ago, Syvitski (2002) used a stochastic model to estimate
the impact of climate change (warming of Arctic regions) on sediment
discharge by Arctic rivers. His study predicted a 22 % increase in the
flux of sediment exported by rivers for every 2
While field observations have been and are still scarce in such remote regions (e.g., along the drainage basins of North American and Siberian rivers), a methodology has been recently developed to remotely sense the variations in suspended particulate matter (SPM) concentrations at the mouth of Arctic rivers using ocean color radiometry (Doxaran et al., 2012). Since ocean color satellite data that cover more than a decade (see, for instance, Doxaran et al., 2009) are available, this method can be combined with field measurements of the river discharge and particulate organic content to estimate the actual mass of terrigenous particles (suspended particulate matter and particulate organic carbon (POC)) supplied to the ocean by any Arctic river and to study its seasonal and interannual variations.
The present study focuses on the mouth and turbid plume of the Mackenzie River in the Beaufort Sea (Canadian Arctic Ocean). This river is the largest single source of terrestrial particles entering the Arctic Ocean. The regional ocean color algorithm developed by Doxaran et al. (2012) for this area is based on a large bio-optical in situ data set collected in 2009 during the MALINA oceanographic campaign. It has been successfully tested on a selection of cloud-free and sea-ice-free ocean color satellite images recorded during the 2009, 2010 and 2011 summer periods. It is here first improved to efficiently discriminate the floating sea ice, clouds, haze and highly turbid waters near the mouth of the Mackenzie River by tuning processing flags and visually inspecting every single pass. It is then applied to an 11-year-long data set of ocean color observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Aqua satellite platform. Results are used to estimate the monthly fluxes of SPM and terrestrial POC delivered by the Mackenzie River to the Beaufort Sea during the melting season in order to reveal possible trends resulting from the observed increase in freshwater discharge since 2003 (Fig. 1). The evolution of the floating sea-ice cover and extension of the Mackenzie River plume are also analyzed and discussed.
Quasi-true-color MODIS-Aqua image recorded on 24 June 2004 (250 m spatial resolution). The study area includes the (i) Mackenzie River mouth west and east branches (red box) and delta zone (green box) and (ii) the Beaufort Sea in the Canadian Arctic Ocean. During winter time the connection between the river delta and adjacent coastal waters is closed by the stamukha while drifting sea ice develops offshore.
The southeast of the Beaufort Sea is characterized by the presence of a large
and shallow continental shelf bordered to the east by the Amundsen Gulf, to
the west by the Mackenzie canyon, to the south by the delta of the Mackenzie
River, and to the north by the Beaufort Sea and Canada Basin (Fig. 2, Carmack
and MacDonald, 2002). Two river mouths characterize the shallow delta zone:
one in the west, where both the river flow and water turbidity are usually
large, and one in the east, with a low river flow. During the winter period,
sea ice accumulates north of the delta zone resulting in the formation, along
the 20 m isobath, of a ridged ice barrier of considerable thickness
(
As in Doxaran et al. (2012), the selected ocean color satellite data are those recorded by the MODIS sensor onboard the Aqua platform. This sensor has provided at least one image of the study area every day since 2002 and has several bands in the near-infrared (NIR) and shortwave infrared (SWIR) spectral regions which are required for atmospheric corrections over turbid coastal waters (Wang and Shi, 2007). A single satellite sensor was used in this study in order to generate a consistent time series of SPM concentration.
MODIS-Aqua level-1 data were downloaded from the National Aeronautics and
Space Administration (NASA) OceanColor website (
Removal of the atmospheric contribution to the total signal was performed
using the NIR–SWIR algorithm of Wang and Shi (2007); this correction method
was proved to be the most appropriate for the highly turbid waters (high SPM
load) present in the mouth the Mackenzie River (Doxaran et al., 2012). Due to
high loads of SPM in the Mackenzie Delta, a number of marine pixels were
often classified as clouds or ice-covered in the standard SeaDAS processing
(i.e., using the default mask threshold values) due to high reflectance
values in the near-infrared spectrum. This issue was tackled by increasing
the cloud albedo threshold for cloud flagging in the atmospheric correction
procedure from the initial value of 0.027 to 0.4 in a small area bounded in
the east from
Example of processing (atmospheric corrections to retrieve the
remote-sensing reflectances at 555 and 748 nm, then inversion of the
748
From an initial total of about 10 000 images taken over the study area and covering the period from 2003 to 2013, only 562 images were ultimately used in the present study despite the large number of MODIS-Aqua revisits at high latitudes. This emphasizes the need for a constellation of low-earth-orbit ocean color satellites to increase the number of observations in such areas, which are highly affected by clouds and sea ice. Due to the very high sea-ice and cloud covers, the data recorded during the months of April and May ultimately proved to be unusable for SPM retrieval and were discarded from our analysis. Interestingly, the number of images per month increases with time over the observation period (2003–2013). This is consistent with the receding of sea-ice concentration over the last decade. The highest number of images in 1 month was reached in July 2012 (Fig. 4), a year of record-low sea-ice extent (Perovich et al., 2013). While the minimum of sea-ice extent occurs in September (usually a very cloudy month in this region), the highest number of satellite observations occurs in July, when daylight lasts for more than 20 hours, with higher sun zenith angles compared to the month of September allowing a greater number of observations per day in July than in September. The number of available images for each month was recorded to weight the linear regression of monthly SPM concentration against time during time series analysis.
The suspended particulate matter (SPM) concentration varies over 4 orders of
magnitude (typically from 0.1 to more than 100 g m
Number of Level-2 images available for each month of the time series (images selected after visual inspection).
The use of two linear relationships is needed as the remote-sensing
reflectance signal in the NIR linearly increases with increasing SPM
concentration up to about 90 g m
The semiempirical relationship was established based on field measurements
collected during the 2009 summer period. It is assumed here to be valid for
the entire period of satellite observations (2003–2013). The uncertainty
associated with the remotely sensed SPM concentrations derived from our
algorithm is estimated as
The SPM flux was computed each month, from June to September, simply by
multiplying the monthly-averaged SPM concentration (in grams per cubic meter)
by the monthly-averaged freshwater discharged by the river (in cubic meters
per second) and by the duration (in seconds) of the month. This approach is
simplistic but gives a robust estimate of average SPM fluxes in the very
shallow river mouth and delta zones. It does not require measurements of
vertical profiles of current velocities. The estimates were computed for two
specific geographical zones: (i) the river mouth defined as a box with the
boundaries 68.7–69.5
Satellite-derived sea-ice concentration, expressed in percent (%), corresponds to the area of a pixel covered by sea ice relative to the total area of that pixel, such that a value of 100 indicates a pixel that is totally covered by sea ice and a value of 0 indicates a pixel free of sea ice. It is computed using a linear combination of the ratios of brightness temperatures measured in the microwave spectrum using the NASA Team 2 algorithm (Markus and Cavalieri, 2000).
Daily sea-ice concentration data were downloaded from the ftp server of the
Zentrum für Marine und Atmosphärische Wissenschaften (ZMAW,
Typical SPM and sea-ice concentration (in grams per cubic meter and percent, respectively) maps obtained over the study area for selected days in June, July and August 2004. From June to July the breaking of the stamukha results in the discharge of turbid freshwater from the Mackenzie River into the Beaufort Sea. Even during the summer period the delta zone remains the most turbid area (maximal SPM concentrations).
Figure 5 illustrates the resulting observations made regarding each
individual MODIS satellite image when combined with the daily mapped sea-ice
concentration data. This example shows the spectacular spring ice breakup
event (typically occurring in early or mid-June) and highlights the rapid
stamukha breakdown, which allows the turbid freshwaters of the Mackenzie
River initially constrained in the delta zone to spread over the Beaufort
Shelf. Figure 5 also highlights the contrast between the spring and the
midsummer period (early August), by which time the coastal zone is totally
free of ice. On 12 June 2004, a wide ice barrier extends all along the
coast with sea-ice concentrations still close to 100 % in the Mackenzie
Delta zone, in the Amundsen Gulf and over most of the Beaufort Sea. Note that
SPM concentrations in the ice-free waters (between the stamukha and floating
sea ice) are already high (about 10 g m
These four daily snapshots provide an initial overview of the SPM and sea-ice seasonal dynamics over the study area. However, performing a multiyear and seasonal analysis of these two parameters requires the use of monthly-averaged composites.
Two main areas of interest are distinguished in the present study: (i) the
river delta, as defined in Fig. 2, and (ii) the river plume defined as the
area where SPM was greater than 10 g m
The limited number of months with conditions suitable for ocean color
observations due to ice cover and the polar night (i.e., about 4 months a
year) prevents the use of common time series decomposition tools since it is
not possible to obtain an entire annual cycle. We therefore preferred to use
linear regressions of each satellite product (SPM, sea-ice concentration)
against time to infer its trend. When performing the linear regression of
monthly SPM concentration against time, the number of pixels available for
each month, i.e.
We first look at the “seasonal” dynamics of SPM and sea ice in the region of interest. Here, the term “seasonal” refers to the monthly variations over the 4-month period (June to September) which corresponds to the maximal Mackenzie River discharge (Fig. 1). Also before analyzing the multiyear variations and potential trends, we highlight how different successive years can be in terms of sea-ice coverage and SPM dynamics.
Similar situations were observed in 2003 and 2004: in June, the ice extends
all along the coast except in front of the west branch of the Mackenzie River
mouth (Fig. 6). The water is highly turbid in the delta zone and also on the
continental shelf, bounded in the north by the offshore sea-ice edge
(
Monthly (June to September) composites of sea-ice and surface water SPM concentrations in 2003 and 2004.
Same as Fig. 6 but for 2006 and 2008.
The situation is much more unusual in June 2006 as the whole Beaufort Sea is covered by sea ice except along the coastline and in the Mackenzie Delta. These unusual June conditions return to the average conditions in July as the floating sea ice has moved northwards: extremely turbid waters then concentrate in the delta zone, while turbid plumes extend along the continental shelf. Once again in August and September, SPM concentrations progressively decrease on the shelf while remaining high in the delta zone. Opposite conditions are observed at the beginning of the 2008 summer period, with a minimum floating sea-ice cover observed from June to September (Fig. 7). SPM concentrations gradually decrease along the shelf during this period while remaining very high in the delta zone.
Same as Fig. 6 but for 2011, 2012 and 2013.
A high number of cloud-free days in 2011, 2012 and 2013 (Fig. 4) yield a
maximum of satellite observations (Fig. 8). In 2011, the stamukha zone can
still be observed in June as it progressively breaks, resulting in the
presence of floating sea ice along the coast. High SPM concentrations extend
from the delta zone to the continental shelf with the main river plume
extending towards the Mackenzie Canyon. In August and September, high SPM
concentrations are only detected in the delta zone, while clear waters cover
most of the continental shelf. The minimum sea-ice cover is observed during
the 2012 summer period, allowing turbid plumes with high SPM concentrations
to extend up to 72.5
The month of June is probably the most interesting to consider regarding the
extent of sea ice in the Beaufort Sea. The sea-ice cover depends primarily on
the atmospheric conditions (air temperature and wind) during the long winter
period, when ice has formed and extended south from the very high latitudes
(greater than 80
In June 2003, 2004 and 2005, similar conditions of sea ice and stamukha
extent were observed, with (i) floating sea ice spreading over most of the
southern Beaufort Sea up to 70.5
It should be also noted that the number of MODIS-Aqua satellite observations over the study area, and thus the number of cloud-free and sea-ice-free days, significantly increased from 2003 to 2013. Therefore, before examining in detail the variations in SPM concentrations over this 11-year period, our analyses already reveal significant changes in both the sea ice and cloud cover in the Beaufort Sea region.
Data on the freshwater discharge of the Mackenzie River (for the station
10LC014 (
The analysis of the monthly SPM maps generated for 2003 to 2011 provides a qualitative overview of the spatial and temporal variations in SPM concentrations in the Mackenzie mouth and delta, as well as in the Beaufort Sea. In order to quantify these variations and highlight a potential trend, these monthly-averaged SPM concentrations were plotted as a function of time, first only considering the Mackenzie Delta zone (see Fig. 2 for detailed geographical location).
Multiyear (2003–2013) variations in and trend of the monthly-averaged
SPM concentration at the Mackenzie River mouth
For the 11 years of the time series, the 4-monthly-averaged SPM
concentrations (June to September) revealed the SPM variations during the
summer period (i.e., when the Mackenzie River mouth is directly connected to
the Beaufort Sea), which gives a first overview of the load of SPM exported
from the river to the coastal ocean (Fig. 9). These SPM concentrations vary
from about 70 to 100 g m
To further confirm this assumption, we examine the relationship between the
monthly-averaged (i) Mackenzie freshwater discharge (in m
Having observed and quantified the increase of the Mackenzie freshwater
discharge and SPM concentration at the river mouth, the next logical step is
to look at the resulting variation in the SPM flux delivered by the river to
the Arctic Ocean. The result is an estimation of the mass of SPM in grams
(g) transported downstream through the river mouth. The SPM flux obtained
corresponds to the solid discharge of the Mackenzie River into the Beaufort
Sea, i.e., the mass of SPM delivered by the river each month during the June
to September summer period. The same SPM flux calculation is made in the
delta zone but cannot be considered as horizontal downstream transport as
this zone is also affected by SPM vertical dynamics (particle settling and
resuspension of bottom sediments). Note that here we call the
downstream limit of the Mackenzie River the river mouth, i.e., the geographical zone defined
as the box with the boundaries 68.7–69.5
The results obtained in the river mouth emphasize the large monthly
variations in the SPM flux (by a factor of 6) during the summer period
(Fig. 10a). The SPM flux is typically higher in June and July than in August
and September, which can be explained by the variations in the Mackenzie
River freshwater discharge. A strong interannual variability in the SPM flux
is also observed, which is directly related to the interannual variability in
the freshwater discharge. The timing of the breakup of the stamukha probably
plays an important role in the interannual variability of the SPM flux by
slowing down the river flow into the delta zone. As for the SPM
concentrations (Fig. 9), a trend of increasing SPM flux is observed from 2003
to 2013. Fitting the time series using a linear regression reveals a
significant positive trend (
Monthly-averaged SPM flux (in g) estimated at the river mouth
(geographical zone defined as a box with the boundaries
68.7–69.5
Similar observations are made in the delta zone (Fig. 10b). It is interesting
to note that the SPM fluxes in the delta zone and at the river mouth are
linearly correlated (
Multiyear (2003–2013) trends in the variation in the
monthly-averaged SPM concentration in the Mackenzie River plume
We finally analyze the impact of the increase in SPM concentrations and
fluxes in the river mouth and delta zone on the Mackenzie continental shelf
(Fig. 11). Only the SPM concentrations (estimated using satellite data within
the first few meters below the air–water interface; Doxaran et al., 2012)
are considered here. Given the data available in this study, it is
impossible to compute the SPM flux as we do not have access to the surface
current velocity and direction. The extent of the plume over the study area
is defined as the area where SPM concentrations are higher than 10 g m
The extent of the Mackenzie River plume mainly experienced strong month-to-month variations, being typically at a maximum in June and then progressively decreasing until August and September. This clearly reflects the observations made on the SPM maps (Figs. 6 to 8): the river plume spreads out over the shelf during the breakup of the stamukha and regresses to the coast, mainly in the delta zone, during the summer period as the river discharge and thus freshwater transport decrease. There is no significant interannual trend observed concerning the plume extent as it is mainly dependent on the sea-ice coverage and the regional hydrodynamics (Ehn et al., 2015). While SPM concentrations increase within the surface waters of the plume, its extension into the Beaufort Sea remains unchanged due to the presence of a pack of floating sea ice. A direct impact of the decrease in sea-ice extent on the primary productivity of Arctic waters has been observed (Arrigo et al., 2008; Tremblay et al., 2011). However, it is not understood yet whether this increase in terrestrial substances along the continental shelf contributes to the development of phytoplankton blooms under the Arctic sea ice (Arrigo et al., 2012).
An important result presented in the previous section concerns the significant increase of SPM flux at the mouth of the Mackenzie River observed over the last 11 years, with a potential effect not only on the marine ecosystem but also on the northern community. The erosion of the shoreline has dramatic consequences for the local community and an increase in sediment transport and deposition along the Beaufort Shelf could mitigate the impact of the erosion. However, a numerical model of ocean circulation would be required to properly assess the fate of the extra sediment exported. Another important aspect of our findings deals with the mass of terrestrial SPM (and subsequent POC) exported from the Mackenzie River to the Beaufort Sea and the balance of this budget with the sedimentation rates in the delta zone and continental shelf. Taking into account the degradation processes of the organic material within the water column (Bélanger et al., 2006) and the fate of bottom sediments (Chaillou et al., 2007), it is necessary to quantify the percentage of terrestrial organic matter that will be buried in marine sediments and attempt to explain it by taking into account the highly refractory state of POC transported by rivers (Hedges et al., 1997; Keil et al., 1997).
At the mouth of the Mackenzie River, water masses are transported downstream
to enter the delta zone to supply it with dissolved and particulate
materials. As a first approximation, the SPM flux estimated in the present
study corresponds to the mass of SPM delivered by the river to the delta
during the 4-month summer period (June to September). On average, from 2003
to 2013, the total mass of SPM estimated during this period is about 20
(
It is important to attempt to explain such differences. On the one hand,
estimates based on field measurements by Macdonald et al. (1998) relied on
data collected at least 100 km upstream of the delta zone (i.e., at the
Arctic Red River station). The complex network of secondary streams branching
off the river in the upper Mackenzie Delta may act as an efficient trapping
for SPM, which would imply that an annual delivery of
120
For the first time, to our knowledge, ocean color satellite data at a moderate spatial resolution (1 km) have been routinely processed over the recent 11-year-long period (2003–2013) to estimate and map the SPM concentrations at the mouth and along the plume of the Mackenzie River in the Canadian Arctic Ocean. The regional algorithm developed by Doxaran et al. (2012), initially applied to selected cloud-free MODIS scenes in 2009, 2010 and 2011, was here optimized to minimize the effects of sea ice, cloud and haze masks while also retrieving the seawater reflectance over the highly turbid waters of the Mackenzie Delta zone. This improved algorithm was applied to MODIS-Aqua satellite data to extract a maximum of information on SPM dynamics in the study area. As a result, SPM maps were produced each year for the period from the beginning of June to the end of September; the remaining months were discarded due to a lack of observations, a result of the combined effect of low solar light and of sea ice covering most of the study area. It was interesting to note that, probably due to effects of Arctic warming on the receding of sea-ice cover at high latitudes, a significant increase in the number of images with valid pixels was recorded in May and October after 2010. Lastly, sea-ice cover and SPM concentrations were superimposed to discard ocean color data possibly contaminated by the presence of sea ice and to study the combined dynamics of sea ice and SPM over the study area.
The monthly-averaged SPM and sea-ice maps produced were used to analyze the
“seasonal” and interannual dynamics of SPM at the river mouth, in the delta
zone and in the river plume. The highest SPM concentrations and largest
extension of the plume were systematically observed in June, following the
breakup of the stamukha and usually corresponding to the annual peak of the
river freshwater discharge. As the river flow progressively declined from
August to September, SPM concentrations gradually decreased in offshore
waters but remained high in and around the delta zone where SPM did
accumulate. This finding probably results from other processes involved in
the transport of SPM, such as erosion due to permafrost thawing or extreme
rain events. In addition to strong interannual variations, a trend was
observed in both the delta zone and river plume, respectively corresponding
to 50 and 35 (
The observed increase in the river discharge of SPM and turbidity (SPM concentrations), combined with changes in other key environmental factors in the coastal Arctic Ocean (Tremblay et al., 2011), will certainly have a rapid impact on the fate of the terrestrial organic carbon and on the marine ecosystems: ocean color satellite observations have already suggested an increase in the annual primary production in the Arctic Ocean (Arrigo et al., 2008).
D. Doxaran actively contributed to in situ bio-optical measurements in the study area and designed the regional SPM algorithm used in this study; he also contributed to its routine application to ocean color satellite data. E. Devred downloaded and processed the full time series of ocean color satellite observations and analyzed the trends observed in the variations in SPM concentrations and fluxes. M. Babin initiated the whole study and was the P.I. of the MALINA project (and oceanographic campaign). D. Doxaran prepared the manuscript with contributions from all coauthors.
This study was funded by ANR, the French Agence Nationale de la Recherche
(MALINA project, P. I. M. Babin). We acknowledge the NASA GSFC for providing
free access to MODIS satellite data and SeaDAS software and Environment Canada,
specifically the Water Office