Beaver Engineering: Tracking a New Disturbance in the Arctic

K. D. Tape¹, J. A. Clark¹, B. M. Jones², H. C. Wheeler³, P. Marsh⁴, and F. Rosell⁵

¹Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK, USA
²Water and Environmental Research Center, Institute of Northern Engineering, University of Alaska Fairbanks, Fairbanks, AK, USA
³School of Life Sciences, Anglia Ruskin University, Cambridge, UK
⁴Department of Geography, Wilfrid Laurier University, Waterloo, ON, Canada
⁵Department of Natural Sciences and Environmental Health, University of South-Eastern Norway, Bø, Norway

Highlights

• Recent satellite imagery and older aerial photography show that North American beavers (Castor canadensis) are colonizing the Arctic tundra of Alaska, with over 12,000 ponds thus far counted in western Alaska, a doubling of ponds since 2000.
• In Canada, beaver pond mapping is underway, complemented by scattered observations of recent changes. Eurasian beavers (C. fiber) are rebounding in Asia but remain south of the Arctic tundra in most locations.
• The Arctic Beaver Observation Network was established in 2020 to help integrate, guide, and disseminate information concerning beaver range expansion into tundra regions and implications for ecosystems and resources.

Introduction

Research on North American beaver (Castor canadensis) engineering in the Arctic has made great strides in recent years, but most of the work lies ahead. Over the last several decades, people in remote Alaska communities have observed an influx of beavers (ADF&G Reports 1965-2017). We quantified this trend by using satellite imagery to detect beaver pond formation in Alaska tundra regions, mapping approximately 12,000 beaver ponds (e.g., Fig. 1, right panel), including a doubling in most areas during the last 20 years (Tape et al. 2021). We showed that new beavers are controlling surface water increases, which affects underlying permafrost (Jones et al. 2020). Fieldwork is underway to characterize the impacts of beaver ponds on aquatic and terrestrial Arctic ecosystems, starting with hydrology and permafrost, and continuing downstream to methane flux, fish populations, and aquatic food webs. As a result of these efforts, most of the questions surrounding beaver engineering in the Arctic are presently being examined but are unanswered. To coordinate research and action among stakeholders in the circumarctic region, the Arctic Beaver Observation Network (A-BON) was formed in 2020 and a synthesis effort is underway to identify knowledge gaps and support the integration of different approaches and perspectives.

Tundra be dammed

In 2016 we imagined that beaver distribution and possible dispersal in the Arctic could be identified by mapping beaver ponds in satellite imagery and older aerial photography through time. Initial studies confirmed our suspicions (Tape et al. 2018), as did the observations of local people in northwest Alaska, who have been observing the influx of beavers for a half-century (ADF&G Reports 1965-2017). Yet the scale and magnitude of this new disturbance regime was unknown in Alaska and the circumarctic. We have since mapped approximately 12,000 beaver ponds (using 2015-19 imagery) in Arctic tundra of Alaska (e.g., Fig. 2). This mapping excludes southwest Alaska tundra and thus provides an underestimate of the total number of beaver ponds. Most areas show a doubling of beaver ponds in the last 20 years (Tape et al. 2021). In 1949-55 aerial photography covering coastal areas of western Alaska, there are no detectable beaver ponds. Stream by stream and floodplain by floodplain, beavers are transforming lowland tundra ecosystems. Increased vegetation productivity (see essay Tundra Greenness) and the expansion of woody shrubs (Myers-Smith et al. 2015) due to climate change has created more forage and dam construction materials, translating to more favorable habitat. The increase in winter stream discharge (St. Jacques and Sauchyn 2009) also implies greater aqueous habitat. Finally, the earlier end of winter and onset of spring (see essay Terrestrial Snow Cover), when beavers can again begin foraging, effectively shortens what is presumably the most challenging time in the annual life cycle of beavers. It remains unclear whether beaver colonization of the Arctic is occurring due to climate change ameliorating habitat or a decrease in trapping pressure, or some combination of both.

In Canada, beaver distribution changes have also been observed both by local people and scientists. Concern over rising numbers of beavers in the Inuvialuit settlement region in the Northwest Territories in northwestern Canada was sufficient to instigate a harvesting incentive scheme in 2017. Although publications of academic studies of beavers in northern Canada have been sparse to date, there are reports of beavers north of the previously known range (Jung et al. 2016).

In Europe, Eurasian beavers (C. fiber) were widespread from the Arctic to the Mediterranean, before being substantially reduced around the twelfth century, and almost extinct by the sixteenth century. Today, the Eurasian beaver has restored a large area of its original range, and increased in numbers from around 1200 beavers a century ago to an estimated 1.5 million individuals today; beavers distribution reaches the northern coast throughout most of Europe (Halley et al. 2021). In Asia, beaver distribution remains well south of Arctic tundra regions, though recent northward range extensions have been observed (Halley et al. 2021). In general, research on beavers in Arctic tundra regions is in its early stages. A coordinated circumarctic beaver pond mapping effort is underway, which will hopefully establish the footprint, if not the nature, of this new disturbance regime in the Arctic.

Implications of beaver engineering in the Arctic

Beavers are a keystone species whose engineering is known to heavily influence streams, rivers, riparian corridors, and lakes in North America, Eurasia, and South America (Whitfield et al. 2015). Beavers are known to dramatically change the landscapes they inhabit by harvesting shrubs, saplings, and trees, which they use to construct dams, inundate the surrounding landscape, and create their watery world. Beavers build lodges of mud and vegetation in water that is deep enough for an underwater entrance that remains unfrozen and permits access for them, but not predators. By constructing dams, beavers severely alter the stream flow regime, which facilitates the arrival of new species, including riverine plants, invertebrates, and fish (Bunn and Arthington 2002). Beaver ponds in temperate ecosystems enhance aquatic habitat complexity and biodiversity.

It remains unclear how these impacts will be manifest in the Arctic, where low water temperatures inhibit stream productivity and biodiversity, and where permafrost holds much of the soil together. People living in remote communities are concerned for resources such as fish, water quality, and boat access (Moerlein and Carothers 2012). In an area of northwest Alaska with exceptional satellite imagery coverage, we discovered that beavers are the dominant factor (66%) controlling increases in surface water extent (Jones et al. 2020), which thaws underlying permafrost as it inundates tundra vegetation. Beaver dams divert flow, sometimes catastrophically when they fail, and can thaw and destabilize the landscape (Lewkowicz and Coultish 2004) through fluvio-erosional and thermokarst processes (Fig. 3). Thawing of permafrost associated with new beaver ponds would initially release carbon and methane stored in permafrost, though the magnitude and fate of these fluxes are complex and unknown. Permafrost thaw, thermokarst, and the inception of a more dynamic lowland Arctic ecosystem suggest an exacerbation of effects due to warming air temperatures. As beavers create thermal and biological oases by the thousands, they could provide a foothold for boreal aquatic species, including fish and aquatic invertebrates. For now, however, these remain hypotheses that will spawn downstream studies involving field measurements and local knowledge to answer.

Arctic Beaver Observation Network (A-BON): Tracking a new disturbance

Recognizing an impending need to understand the scale, dynamics, and effects of beaver engineering in the tundra, individuals and organizations joined forces across Alaska, Canada, and Eurasia to identify key questions and involve stakeholders and land managers. A-BON involves natural scientists, social scientists, tribal entities and local observers, and agency land managers across the Arctic. A-BON has working groups in Alaska, Canada, Europe, and Asia.

The initial goals of A-BON are to (1) include diverse backgrounds and establish working relationships, (2) identify key questions for study, (3) align study designs and observation methods, (4) facilitate co-production of knowledge between scientists and local observers, (5) understand stakeholder land/resource management objectives, (6) assemble an expert steering committee to advise other Arctic science bodies (e.g., Conservation of Arctic Flora & Fauna, Circumpolar Biodiversity Monitoring Program), and (7) synthesize, archive, and disseminate relevant and co-produced data. These goals are actively being refined.

A synthesis effort underway within A-BON aims to identify key questions and knowledge gaps surrounding beaver colonization in the tundra across natural and social scientists, Indigenous organizations and observers, and land managers. Preliminary results demonstrate the breadth of interests and concerns, spanning beaver ecology, biophysical and socio-cultural impacts, local and Indigenous knowledge, management, and adapting to the evolving relationships with beavers. A-BON will discuss these and other efforts at the first meeting in March 2022 in Fairbanks, Alaska (could be virtual). We welcome participation in A-BON from anyone with interest in the topic.

Acknowledgments

K. Tape, J. Clark, and B. Jones acknowledge NSF OPP #1850578 and #2114051. H. Wheeler acknowledges the UK-Canada Arctic partnership bursary (NERC/BEIS) and Anglia Ruskin Next Steps QR fund.

References

ADF&G, 1965-2017: ADF&G (Furbearer) Reports. Alaska Department of Fish & Game, Division of Wildlife Conservation, Juneau, Alaska.

Bunn, S. E., and A. H. Arthington, 2002: Basic principles and ecological consequences of altered flow regimes for aquatic biodiversity. Environ. Manage., 30(4), 492-507, https://doi.org/10.1007/s00267-002-2737-0.

Halley, D. J., A. P. Saveljev, and F. Rosell, 2021: Population and distribution of beavers Castor fiber and Castor canadensis in Eurasia. Mammal Rev., 51, 1-24, https://doi.org/10.1111/mam.12216.

Jones, B. M., K. D. Tape, J. A. Clark, I. Nitze, G. Grosse, and J. Disbrow, 2020: Increase in beaver dams controls surface water and thermokarst dynamics in an Arctic tundra region, Baldwin Peninsula, northwestern Alaska. Environ. Res. Lett., 15(7), 075005, https://doi.org/10.1088/1748-9326/ab80f1.

Jung, T. S., J. Frandsen, D. C. Gordon, and D. H. Mossop, 2016: Colonization of the Beaufort coastal plain by Beaver (Castor canadensis): a response to shrubification of the Tundra? Can. Field Nat., 130(4), 332-335, https://doi.org/10.22621/cfn.v130i4.1927.

Lewkowicz, A. G., and T. L. Coultish, 2004: Beaver damming and palsa dynamics in a subarctic mountainous environment, Wolf Creek, Yukon Territory, Canada. Arct. Antarct. Alp. Res., 36, 208-218, https://doi.org/10.1657/1523-0430(2004)036[0208:bdapdi]2.0.co;2.

Moerlein, K. J., and C. Carothers, 2012: Total environment of change: impacts of climate change and social transitions on subsistence fisheries in northwest Alaska. Ecol. Soc., 17(1), 10, https://doi.org/10.5751/es-04543-170110.

Myers-Smith, I. H., and Coauthors, 2015: Climate sensitivity of shrub growth across the tundra biome. Nat. Climate Change, 5(9), 887-891, https://doi.org/10.1038/nclimate2697.

St. Jacques, J. -M., and D. J. Sauchyn, 2009: Increasing winter baseflow and mean annual streamflow from possible permafrost thawing in the Northwest Territories, Canada. Geophys. Res. Lett., 36(1), L01401, https://doi.org/10.1029/2008gl035822.

Tape, K. D., B. M. Jones, C. D. Arp, I. Nitze, and G. Grosse, 2018: Tundra be dammed: Beaver colonization of the Arctic. Glob. Change Biol., 24(10), 4478-4488, https://doi.org/10.1111/gcb.14332.

Tape, K. D., J. A. Clark, and B. M. Jones, 2021: Beaver Pond Locations in Arctic Alaska, 1949 to 2020. Arctic Data Center. Accessed 15 September 2021, https://doi.org/10.18739/A2QR4NR6D.

Whitfield, C. J., H. M. Baulch, K. P. Chun, and C. J. Westbrook, 2015: Beaver-mediated methane emission: The effects of population growth in Eurasia and the Americas. Ambio, 44(1), 7-15, https://doi.org/10.1007/s13280-014-0575-y.

December 2, 2021

Ocean Acidification

J. N. Cross¹, A. Niemi², N. Steiner³, and D. J. Pilcher^1,4

¹Pacific Marine Environmental Laboratory, NOAA, Seattle, WA, USA
²Freshwater Institute, Fisheries and Oceans Canada, Winnipeg, MB, Canada
³Institute of Ocean Sciences, Fisheries and Oceans Canada, Sidney, BC, Canada
⁴Cooperative Institute for Climate, Ocean, and Ecosystem Studies, University of Washington, Seattle, WA, USA

Highlights

• Recent work has shown that the Arctic Ocean is acidifying faster than the global ocean, but with high spatial variability.
• A growing body of research indicates that acidification in the Arctic Ocean could have implications for the Arctic ecosystem, including influences on algae, zooplankton, and fish.
• Cutting-edge tools like computational models are increasing our capacity to understand patterns, trends, and impacts of ocean acidification in the Arctic region.

The uptake of anthropogenic carbon dioxide (CO₂) causes a cascade of chemical reactions that decreases ocean pH and carbonate ion concentrations, a process known as ocean acidification (OA). While OA is a global process, some of the fastest rates of ocean acidification around the world have been observed in the Arctic Ocean (e.g., Qi et al. 2017, 2020). These extremely rapid rates of acidification reflect the Arctic’s natural vulnerability to changes in pH, caused by cold temperatures, naturally higher baseline CO₂ concentrations resulting from global circulation processes, seasonal processes that rapidly concentrate CO₂ in some water masses, as well as unique land-sea interactions and hydrological mechanisms (circumpolar perspective broadly reviewed by AMAP 2018). Surface waters in some parts of the Arctic Ocean are already undersaturated with respect to some biologically important calcium carbonate minerals (e.g., aragonite and calcite) and most regions of the Arctic are likely to become corrosive (able to dissolve biologically important carbonate minerals) by the end of the century (AMAP 2018). These changes could have serious implications for the regional ecosystem, including detrimental impacts on local wildlife, cultural assets and practices, and subsistence resources.

Robust sampling programs that prioritize the collection of OA data (e.g., pH, partial pressure of CO₂, dissolved inorganic carbon (DIC), and total alkalinity (TA)) are extremely difficult to implement in the Arctic. The coastal sub-Arctic seas exhibit a highly dynamic spatial and temporal variability as the underlying biogeochemistry is impacted by a range of land, ocean, and atmosphere processes. Accordingly, mature OA monitoring systems must be highly resolved in both space and time to provide adequate information for decision support. Given the expansive area, the remote geographic location, and harsh winters, traditional monitoring tools are also challenging to deploy consistently in the Arctic region, although some of these time series are starting to mature (e.g., Beaufort Gyre: Zhang et al. 2020; Canadian Archipelago: Beaupré-Laperrière et al. 2020; Eurasian Basin: Ulfsbo et al. 2018; Fram Strait: Chierici et al. 2019; Svalbard: Jones et al. 2021).

Despite these advances, we do not have a synoptic understanding of OA across the pan-Arctic system. Accordingly, computational models grounded in observable data have emerged as a useful tool to help explore spatial-temporal variability due to their much finer spatial and temporal resolution. Using these outputs, researchers are better able to explore the intensity, duration, and extent of ecosystem exposure to OA processes. In recent years, regional and global modeling studies have been used to explore both long- and short-term aspects of OA in the Arctic (e.g., Bering Sea: Pilcher et al. 2019; pan-Arctic, Terhaar et al. 2020), as well as the processes leading to these trends that are notoriously difficult to observe (e.g., pan-Arctic sea-ice related impacts: Mortenson et al. 2020). However, there is substantial regional and seasonal variability especially where land processes can influence OA, highlighting potential problems with interpolating sparse measurements (e.g., Chierici et al. 2019; Jones et al. 2021). Better regional to local climate projections may provide key improvements. Model studies continue to be refined and will likely form a pivotal part of future Arctic OA research.

As the observational record of OA in the Arctic continues to grow, research on the possible impact of OA on Arctic ecosystems continues to progress both in the laboratory and in the field (Fig. 1). The primary concern is that the short food web linkages so characteristic of the Arctic may lead to widespread impacts of OA across the ecosystem, creating both winners and losers. This is evident at the very base of the food chain: for example, OA negatively affects the calcification of some Arctic phytoplankton (pan-Arctic: Ardyna and Arrigo 2020) and may shift the community toward smaller species (western Arctic: Sugie et al. 2020). Some primary producers may experience little impact; research syntheses indicate that OA likely has a limited effect on sea ice algae, given that the biogeochemistry of the ice matrix itself naturally undergoes extreme fluctuations that result in evolutionary resilience (central Arctic: Torstensson et al. 2021).

At the zooplankton trophic level, the quintessential species for detrimental OA impacts is the pteropod (sea snail). These organisms are extremely sensitive to ocean pH and are often used around the world as indicators that can inform OA conditions. Both laboratory and field observations have shown that pteropod responses to OA include reduced juvenile survival, reduced shell growth and condition, as well as costly metabolic regulation. Arctic population connectivity and morphological characteristics of pteropods is a growing area of research. For example, recent studies of natural populations indicate a high occurrence of severe shell dissolution in the Bering Sea, Amundsen Gulf, and Svalbard margin (Niemi et al. 2021; Bednaršek et al. 2021; Anglada-Ortiz et al. 2021, respectively). While pteropods are an important biological indicator, research on other organisms specific to Arctic ecosystems will also support regional relevance. For example, fish show sensitivities to OA, including important species such as Arctic cod (e.g., western Arctic cod populations: Steiner et al. 2019; eastern Arctic cod: Hänsel et al. 2020). However, key questions remain to fully understand the mechanisms that produce individual and population-level responses to OA. Across species (fish, benthic and pelagic invertebrates) repair, adaptability, and associated tolerance have been linked to resource availability (e.g., Niemi et al. 2021; Hänsel et al. 2020; Duarte et al. 2020; Goethel et al. 2017), indicating the importance of a holistic ecosystem approach to understand OA biological responses (Fig. 2).

Seals, walrus, and marine birds may be impacted by the inherent vulnerability of their preferred foods to acidification. Calcifying bivalves are particularly sensitive to acidified conditions; weakly acidified waters can reduce growth, while severely corrosive waters can eventually begin to dissolve shells. Although laboratory experiments have identified resilience to acidification in Arctic invertebrates (Goethel et al. 2017), some model research suggests that they are likely to be among the most negatively impacted invertebrate populations in the world (Tai et al. 2021). Though the specifics remain uncertain, it is likely that the consequences of continuing OA will be detrimental for parts of the marine food web over the next decade. Warming and acidification are likely to become compounding stressors for the Arctic ecosystem by the end of the century. More research will be necessary to determine how acidification-stressed marine invertebrate populations may influence the Arctic ecosystem. This work will be especially important given that invertebrates as well as their predators are important commercial, cultural, and subsistence resources across the region.

Building ecosystem and human resilience to OA in the Arctic in part will require global solutions, given that OA is primarily caused by global carbon dioxide emissions. However, regional decision makers are likely to benefit from continually improving resolution of both data collection and regional modeling, which will provide additional support for Arctic decision makers and ecosystem management. Measurements from novel sensors, especially those collecting surface data are likely to improve the resolution of regional CO₂ flux products and provide a better understanding of the surface CO₂ sink for carbon across the Arctic. Additionally, short-term forecasting applications are likely to be developed for regional and pan-Arctic models in support of ecosystem management systems. Through this process, development of connections between biogeochemical and ecosystem models, based on empirical laboratory and in-situ data, are likely to allow researchers to continue to explore the impact of OA on regional ecosystems. The ultimate goal is to develop an interdisciplinary, hybridized approach that will allow the scientific community to develop annual OA products of the type now produced for temperature or sea ice extent for the Arctic Report Card. Given the high connectivity of processes leading to OA and its downstream effects, trans-national data access and collaboration will be essential for success.

In addition to improvements in data collection, it also seems likely that coastal marine CO₂ removal may scale inside coastal waters in the Arctic. The IPCC has acknowledged that achieving climate goals requires substantial changes to atmospheric greenhouse gas concentrations. While emissions-reduction approaches are an essential component of addressing this challenge, negative emissions strategies will be necessary for keeping global temperatures at recommended levels (IPCC 2021). Negative emissions strategies refer to a portfolio of techniques that are used to manually remove greenhouse gases from the atmosphere and store them away from the atmosphere. Carbon dioxide removal (CDR) specifically references techniques that remove legacy emissions of CO₂ from the atmosphere. Where these techniques involve the coastal oceans, there may be opportunities to remediate acidified ocean conditions in some instances. However, these marine CDR techniques are in their infancy and will require additional study to limit the risks associated with deployment, despite the potential benefits of atmospheric carbon sequestration and OA mitigation. It will also be essential to consider the ethical and climate justice ramifications of these techniques, especially regarding Arctic communities and peoples (Cassotta 2021).

References

AMAP, 2018: AMAP Assessment 2018: Arctic Ocean Acidification. Arctic Monitoring and Assessment Programme (AMAP), Tromsø, Norway. Vi+187 pp.

Anglada-Ortiz, G., K. Zamelczyk, J. Meilland, P. Ziveri, M. Chierici, A. Fransson, and T. L. Rasmussen, 2021: Planktic foraminiferal and pteropod contributions to carbon dynamics in the Arctic Ocean (North Svalbard Margin). Front. Mar. Sci., 8, 661158, https://doi.org/10.3389/fmars.2021.661158.

Ardyna, M., and K. R. Arrigo, 2020: Phytoplankton dynamics in a changing Arctic Ocean. Nat. Climate Change, 10(10), 892-903, https://doi.org/10.1038/s41558-020-0905-y.

Beaupré-Laperrière, A., A. Mucci, and H. Thomas, 2020: The recent state and variability of the carbonate system of the Canadian Arctic Archipelago and adjacent basins in the context of ocean acidification. Biogeosciences, 17, 3923-3942, https://doi.org/10.5194/bg-17-3923-2020.

Bednaršek, N., and Coauthors, 2021: Integrated assessment of ocean acidification risks to pteropods in the northern high latitudes: Regional comparison of exposure, sensitivity and adaptive capacity. Front. Mar. Sci., 8, 671497, https://doi.org/10.3389/fmars.2021.671497.

Cassotta, S., 2021: Ocean acidification in the Arctic in a multi-regulatory, climate-justice perspective. Front. Climate, 3, 713644. https://doi.org/10.3389/fclim.2021.713644.

Chierici, M., M. Vernet, A. Fransson, and K. Y. Børsheim, 2019: Net community production and carbon exchange from winter to summer in the Atlantic Inflow to the Arctic Ocean. Front. Mar. Sci., 6, 528, https://doi.org/10.3389/fmars.2019.00528.

Duarte, C. M., A. B. Rodriguez-Navarro, A. Delgado-Huertas, and D. Krause-Jensen, 2020: Dense mytilus beds along freshwater-influenced Greenland shores: Resistance to corrosive waters under high food supply. Estuar. Coast., 43(2), 387-395, https://doi.org/10.1007/s12237-019-00682-3.

Goethel, C. L., J. M. Grebmeier, L. W. Cooper, and T. J. Miller, 2017: Implications of ocean acidification in the Pacific Arctic: experimental responses of three Arctic bivalves to decreased pH and food availability. Deep-Sea Res. Pt. II, 144, 112-124, https://doi.org/10.1016/j.dsr2.2017.08.013.

Hänsel, M. C., J. O. Schmidt, M. H. Stiasny, M. T. Stöven, R. Voss, and M. F. Quaas, 2020: Ocean warming and acidification may drag down the commercial Arctic cod fishery by 2100. PLoS One, 15(4), e0231589, https://doi.org/10.1371/journal.pone.0231589.

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Niemi, A., N. Bednaršek, C. Michel, R. A. Feely, W. Williams, K. Azetsu-Scott, W. Walkusz, and J. D. Reist, 2021: Biological impact of ocean acidification in the Canadian Arctic: Widespread severe pteropod shell dissolution in Amundsen Gulf. Front. Mar. Sci., 8, 600184, https://doi.org/10.3389/fmars.2021.600184.

Pilcher, D. J., D. M. Naiman, J. N. Cross, A. J. Hermann, S. A. Siedlecki, G. A. Gibson, and J. T. Mathis, 2019: Modeled effect of coastal biogeochemical processes, climate variability, and ocean acidification on aragonite saturation state in the Bering Sea. Front. Mar. Sci., 5, 508, https://doi.org/10.3389/fmars.2018.00508.

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Sugie, K., A. Fujiwara, S. Nishino, S. Kameyama, and N. Harada, 2020: Impacts of temperature, CO₂, and salinity on phytoplankton community composition in the western Arctic Ocean. Front. Mar. Sci., 6, 821, https://doi.org/10.3389/fmars.2019.00821.

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November 23, 2021

River Discharge

R. M. Holmes¹, A. I. Shiklomanov^2,3, A. Suslova¹, M. Tretiakov³, J. W. McClelland⁴, L. Scott¹, R. G. M. Spencer⁵, and S. E. Tank⁶

¹Woodwell Climate Research Center, Falmouth, MA, USA
²University of New Hampshire, Durham, NH, USA
³Arctic and Antarctic Research Institute, St. Petersburg, Russia
⁴Marine Science Institute, University of Texas at Austin, Port Aransas, TX, USA
⁵Florida State University, Tallahassee, FL, USA
⁶University of Alberta, Edmonton, AB, Canada

Highlights

• In 2021, the combined discharge (January through October) from the six Eurasian rivers was 1850 km³, which was 81 km³ or ~5% greater than during the 1981-2010 reference period.
• In 2020, the combined discharge of the eight largest Arctic rivers was 2623 km³, ~12% greater than the average over the 1981-2010 reference period.
• In 2019, the combined discharge of the eight largest Arctic rivers was 2233 km³, 5% less than the 1981-2010 average.
• In 2020, an extraordinarily high May discharge from Eurasian rivers of 443 km³ (96% above average) was followed by an extraordinarily low June discharge of 432 km³ (21% below average), indicating a shift of the freshet to earlier in the season.
• The long-term observations for Eurasian and North American Arctic river discharges demonstrate an upward trend, providing evidence for the intensification of the Arctic hydrologic cycle.

Introduction

Arctic river discharge is a key indicator reflecting changes in the hydrologic cycle associated with widespread environmental change in the Arctic. It is the most accurately measured component of the Arctic water cycle (Shiklomanov et al. 2006). Records of Arctic river discharge since the early 1930s reveal a long-term increase of freshwater flux to the Arctic Ocean, providing compelling evidence of intensification of the Arctic water cycle (Peterson et al. 2002; McClelland et al. 2006). This hydrologic and associated biogeochemical change has significant ramifications for the Arctic Ocean, which contains only about 1% of global ocean water yet receives 11% of the global river discharge (Aagaard and Carmack 1989; McClelland et al. 2012).

Of the eight largest Arctic rivers by annual discharge, six lie in Eurasia (Kolyma, Yenisey, Lena, Ob’, Pechora, and Severnaya Dvina) and two are in North America (Mackenzie and Yukon). Collectively, the watersheds of these eight rivers cover approximately 70% of the pan-Arctic drainage area and account for the majority of river water input to the Arctic Ocean (Fig. 1). In this report we present river discharge values for these eight rivers for 2019 and 2020, and for the Eurasian portion of these same rivers for the first ten months of 2021, updating the 2018 Arctic Report Card (Holmes et al. 2018). 2021 data are not available for the two North American rivers at the time of this report. Here, we use a common baseline period of 1981-2010 to compare and contextualize recent observations.

Discharge records

In 2021, the combined discharge (January through October) from the six Eurasian rivers was 1850 km³, which was 81 km³ or ~5% greater than during 1981-2010 reference period. The majority of this increase was driven by the Yenisey River. The Pechora and Severnaya Dvina showed below average discharge, 26% and 28%, respectively (Fig. 2).

In 2020, the combined annual discharge of the eight largest Arctic rivers was 2623 km³, which was 272 km³ or ~12% greater than the 30-year average. This increase is greater than the annual average discharge of the Yukon River. Discharge from the two North American rivers combined was 630 km³, ~28% greater than their 1981-2010 average. Discharge from the six Eurasian rivers combined was 1992 km³, ~7% greater than the average over the 1981-2010 reference period, or ~10% greater than average for whole period of record from 1936 to 2020 (Table 1).

High annual discharge of the North American rivers in 2020 was primarily driven by the high discharge values in July, August, and September (+2.1, +2.6, +2.8 std. dev. above average, respectively; Fig. 3). This is attributed to an unusually wet summer, the wettest summer since 1985 based on analysis of precipitation aggregated over the Mackenzie and Yukon watersheds (Hersbach et al. 2020).

For the Eurasian rivers in 2020, extraordinarily high May discharge (+3.1 std. dev. above average) was followed by extraordinarily low June discharge (-2.3 std. dev. below average; Fig. 3). This pattern observed across the Eurasian rivers is consistent with the observed high terrestrial snow cover and snow water equivalent during winter 2019/20, followed by a remarkably warm spring in 2020 (Ballinger et al. 2020; Mudryk et al. 2020). This led to an early melt of a large snowpack, shifting more of the freshet runoff period from June to May. Discharge for May and June combined was 13% higher in 2020 compared to the baseline period.

In contrast to 2020, 2019 was a relatively low-discharge year. The combined discharge of the eight largest Arctic rivers was 2233 km³, 118 km³ or 5% less than the 1981-2010 average (Fig. 4). Discharge from the two North American rivers and the six Eurasian rivers was ~9% and ~4% less than average, respectively.

Low annual discharge in 2019 from the North American rivers was driven by low May, June, and July discharge (-0.8, -0.9, -1.4 std. dev. below average, respectively; Fig. 3). Similarly, Eurasian rivers had lower than average discharge in May and June (-0.5, -0.9 std. dev. below average, respectively; Fig. 3). These low summer discharge observations are consistent with the below-average snow water equivalent in April 2019 in both the Eurasian and North American Arctic (Mudryk et al. 2019).

The 85-year time series available for the Eurasian Arctic rivers demonstrates a positive linear trend. Their combined annual discharge is increasing by 2.5 km³ per year. For the North American Arctic rivers, the increase over the period of record (1976-2020) was 1.1 km³ per year (Fig. 4). These long-term observations indicate that Arctic river discharge continues to trend upward, providing powerful evidence for the intensification of the Arctic hydrologic cycle (Shiklomanov et al. 2021).

Methods and data

Discharge values are based on observational discharge data from the downstream-most stations listed in Table 2. Discharge measurements for the six Eurasian rivers began in 1936, whereas discharge measurements did not begin until 1973 for the Mackenzie River and 1976 for the Yukon River. Discharge data for the Kolyma at Srednekolymsk are not available for 2019 and 2020; they were calculated based on monthly correlations with the next downstream station, the Kolyma at Kolymskoe. Average monthly values for 1978-2001 were used to calculate the correction factor. The Yukon is missing discharge values from October-December 2020. We therefore used long-term average values for those three months, which account for less than 17% of the mean annual discharge. All discharge data reported here are available through the Arctic Great Rivers Observatory at arcticgreatrivers.org/discharge/.

Acknowledgments

We thank the United States Geological Survey (Yukon), Water Survey of Canada (Mackenzie) and Roshydromet (Severnaya Dvina, Pechora, Ob’, Yenisey, Lena, and Kolyma) for the discharge data used here. This work was supported by grants from the National Science Foundation in support of the Arctic Great Rivers Observatory (NSF 1602615, 1603149, 1602680, 1602879). The processing and analysis of near-real time data for Russian Arctic rivers were supported in part by the Russian Foundation for Basic Research (grant 18-05-60192).

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November 17, 2021