Introduction
Long cold winters, continuous permafrost, and expansive, but shallow, lakes and wetlands characterize many coastal plains of the Arctic. These characteristic physio-climatic conditions are thought to protect subaqueous permafrost from talik development (
Lachenbruch et al. 1962;
Burn 2002) and limit availability of perennial liquid water for overwintering aquatic habitat, as well as water supply for human activities (
Jones et al. 2009;
Arp et al. 2019). Historically, freshwater ice in northern Alaska could grow to 2 m thick or more by winter's end, resulting in the majority of shallow lakes, wetlands, and streams freezing solid (
Brewer 1958;
Sellmann et al. 1975;
McNamara and Kane 2009).
Maximum ice thicknesses (MIT) exceeding 2 m was often reported during the 1960s and 1970s for lakes on the Barrow Peninsula (
Bilello 1980) and north of Teshekpuk Lake (
Weeks et al. 1981). As recently as 1992,
Jeffries et al. (1994) also reported MIT exceeding 2 m for Arctic Coastal Plain (ACP) of northern Alaska lakes. These and other ice thickness records have been used for long-term models of MIT suggesting an average of 1.9 m between 1947 and 1997 with simulated ice growth up to 2.5 m thick (
Zhang and Jeffries 2000). In the northern Canadian Arctic, MIT records have similarly often exceeded 2 m for numerous lakes during the 1960s and 1970s (
Bilello 1980), including a High Arctic lake with a MIT of 2.4 m as recently as 1978 (
Woo 1980) and lakes with MIT exceeding 2 m along the western Arctic coast as recently as 1997 (
Burn 2002). Lake ice growth simulations for northern Canada suggest MITs ranging from 1.4 to 2.0 m from 1960 to 1990 (
Brown and Duguay 2011) and simulated ice growth regularly exceeding 2 m thickness on Baker Lake in northern Canada up until 2002 (
Dibike et al. 2011). Both modeling and field studies of Arctic freshwater ice suggest that snow depth rather than air temperature exerts the dominant control on lake ice growth (
Zhang and Jeffries 2000;
Brown and Duguay 2010;
Dibike et al. 2011;
Arp et al. 2018).
Pronounced winter warming in the Arctic, particularly along northern Alaskan coastlines (
Wendler et al. 2014;
Box et al. 2019), follows anticipated Arctic amplification in relation to sea ice decline (
Serreze and Barry 2011) and corresponding responses in freshwater ice growth (
Alexeev et al. 2016). Although observations of MIT in Arctic Alaska have been temporally and spatially patchy up until the last decade (
Arp et al. 2018), simulated MIT for Barrow Peninsula lakes suggests a significant thinning trend of 0.14 m/decade since 1976 (
Surdu et al. 2014). Decreasing MIT over recent decades has been closely linked to increasing extents of lakes that no longer freeze solid with bedfast ice according to late winter imaging using synthetic aperture radar (SAR) on the Barrow Peninsula (
Surdu et al. 2014) and the central ACP (
Arp et al. 2012). Model experiments suggest that both thinning lake ice and decreasing bedfast ice are responding to more early winter ocean-effect snowfall with declining sea ice extent along the Beaufort Sea coast (
Alexeev et al. 2016). Yet, more recent late winter SAR analysis over a longer period (1992–2016) than other ACP SAR studies by
Surdu et al. (2014) and
Arp et al. (2012) suggests only high interannual variability in bedfast-ice extents with few significant trends (
Engram et al. 2018). Recent field studies also confirm this high variability in both MIT and lake ice regimes (bedfast vs. floating ice) by examining contrasting winters of 2015–2016 (1.8 m MIT) with 2016–2017 (1.2 m MIT) (
Arp et al. 2018). However, in the following two winters on the Barrow Peninsula (2017–2018 and 2018–2019), lake MIT was even thinner, less than 1.2 m (
Arp et al. 2020), potentially suggesting an even more rapid decline in freshwater ice thickness or perhaps widening ranges and new patterns of Arctic winter variability.
Here, we report on unusually thick-ice conditions observed during the winter of 2020–2021 in the Fish Creek Watershed (FCW) in the National Petroleum Reserve in Alaska (NPR-A), much more typical of MIT recorded several decades ago (i.e., before the 1990s). Ice thickness observations on lakes and rivers in this recent extreme winter are compared with past observed and simulated records, as well as climate data, to place this winter into a longer-term context. Updated late winter SAR analysis of bedfast-ice extent following
Engram et al. (2018) is used to understand how this unusually thick-ice winter may have impacted freshwater habitat and winter water supply. In this region of the NPR-A, both industrial development and subsistence activities are increasingly sensitive to how these natural resources interact with freshwater ice and are responding to climate change, including conditions perhaps more like winters of the past.
Results
Before any river or lake ice was drilled in late April 2021, thin wind-swept snow conditions were strikingly apparent on lakes with much bare ice, along river corridors with exposed sand bars and shrubs, and across wide expanse of tundra with visible sedges, cottongrass, and dwarf birch (
Fig. 2). For comparison, in many if not most years during late winter fieldwork, almost all ACP surfaces are uniformly white and the shadows and orientation of
sastrugi snow drifts are the main features visible on otherwise flat topography. However, a newly installed ice thickness buoy that transmitted data via satellite daily at Richard's Pond, approximately 10 km west of Nuiqsut, had indicated that the lake ice had reached nearly 2 m thickness (
Fig. 3). Still, these new snow–ice buoy data were not yet calibrated nor had the technique been well tested, such that these readings appeared suspect in comparison to many recent years of physical measurements when MIT barely surpassed 1 m thickness by late April.
Measurement of ice at 15 floating-ice lakes revealed an average thickness of 1.8 m, excluding three lakes that were unexpectedly frozen solid with bedfast ice (
Fig. 1). Ice ranged from 1.57 m thick on a small oxbow lake with 12 cm average snow depth to 1.93 m thick on two lakes with an average snow depth of 3 cm, including abundant bare, snow-free ice. The average ice thickness on the Ublutouch River was 1.91 m. Using local air temperature data to standardize these late April field observations to MIT, which was estimated to occur in mid-May (
Fig. 3), suggested that ice grew to 1.91m thickness in the easterly Fish Creek area and 1.81m thickness in the westerly Inigok area (
Fig. 4). For comparison, estimated MIT for the previous 14 years (excluding 2020) in the Fish Creek area averaged 1.46 m and ranged from 1.18 m in 2018 to 1.83 m in 2013. A shorter MIT record from the Inigok area (2011–2019) showed an average of 1.47 m that ranged from 1.32 m in 2018 to 1.79 m in 2013 (
Fig. 4).
Ice growth model estimates of MIT were simulated for low- and high-snow scenarios from 1990 to 2021 to provide context for the 2020–2021 winter of interest, as well as expected maximum ranges of variation over the past 30 years (
Fig. 4). High-snow simulations ranged from 1.18 m MIT in 2018 to 1.50 m MIT in 1992 and low-snow simulations ranged from 1.71 m MIT in 2018 to 2.17 m MIT in 1992. Average winter (October–April) air temperatures recorded at Utqiaġvik for thin- and thick-ice years were −13.7 °C (2017–2018) and −22.0 °C (1991–92), respectively, compared with −17.2 °C in 2020–2021 (
Fig. 5). Observed MIT for Barrow Peninsula lakes in 1992 was 202 cm with average snow depths (
Jeffries et al. 1994); however, we are not aware of any ice thickness measurements from late winter of 2021 in this region for comparison. For observed MIT in the Fish Creek area, over half (60%) of years were close to or below the high-snow simulations, 33% fell in the middle range, and only the year 2021 tracked the low-snow simulations closely (
Fig. 4). A similar distribution was apparent for the shorter record of MIT observations from the Inigok area. In the years when observed MIT closely matched high-snow simulations (2006–2009, 2011–2012, 2014, and 2018–2019), late winter (April) snow depth in Utqiaġvik averaged 31 cm (±9 cm s.d.) compared with 18 cm (±5 cm s.d.) of snow for years when MIT was closer to low-snow simulations of ice growth (
Figs. 4 and
5). Late winter snow depth on lakes recorded in the FCW are much more relevant to ice growth conditions than tundra snow and these shorter records only show slightly more snow (21 ± 3 cm s.d.) during years matching high-snow simulations, compared with snow depths (16 ± 4 cm s.d.) in years matching low-snow simulations. End-of-winter snow observed in 2021 on lakes in the FCW averaged 10 cm depth, while tundra snow conditions in the same year in Utqiaġvik were not reported.
The landscape-scale impact of ice thickness on the ACP may be best seen by analysis and classification of late winter SAR imagery to document the extent of lakes that freeze solid with bedfast ice. In the Fish Creek area in April 2021, 63% of lake surface area had bedfast ice compared with an average extent of 51% for the previous 28 years (
Fig. 6A). Observed MIT in the Fish Creek area explained 73% (
P < 0.01) of the variation in lake bedfast-ice extent with the winter of 2008 removed from this analysis (
Fig. 6B). At the end of 2007, lake levels were notoriously low on the ACP (
Jones et al. 2009), which can have an important impact on lake ice regimes in the subsequent winter (
Arp et al. 2018). In the Inigok area, where lakes are deeper, but with wide shallow shelves (
Fig. 7A), 55% of lake surface area had bedfast ice in April 2021, which was very similar to conditions observed with SAR in April 2013 and higher than the long-term average of 46% lake bedfast-ice extent (
Fig. 6A). MIT explained a slightly higher portion of the interannual variation in bedfast-ice extent in the Inigok area compared with the Fish Creek area (
Fig. 6B). This slight difference in relationship strength may be because Inigok area lakes are deeper and there are many more lakes in the Fish Creek area that freeze entirely solid in thick-ice years and may support mostly floating ice in thin-ice years. Classification of ice regimes as either bedfast-ice lakes (>95% of surface area frozen solid) or floating-ice lakes (>5% of surface area with liquid water below ice) suggested that of 3618 lakes analyzed in the FCW (both ROIs), 27% had floating-ice regimes in the winter of 2020–2021. This is compared with the recent thin-ice year of 2017–2018 when 54% of the lakes were classified as having floating-ice regimes.
Taking advantage of the widely varying MIT and bedfast-ice extents for recent late winters allowed us to estimate the extent of lake area available for overwintering habitat and liquid water extraction in a warm, snowy winter (2018) vs. a very cold, low-snow winter (2021) (
Fig. 7). This analysis suggests that an approximate additional 78 km
2 of lake surface area (12%) was potentially available for overwintering fish habitat and liquid water extraction over the full winter of 2017–2018 compared with the recent winter of 2020–2021 in the Inigok area. In the smaller Fish Creek SAR analysis area, approximately 49 km
2 more lake surface area (27%) was potentially available for overwinter fish habitat and liquid water extraction in 2017–2018 compared with 2020–2021. The majority of this transient winter aquatic habitat in the Inigok area is along shelf margins of large deep lakes and a few very small lakes and ponds that freeze entirely with bedfast in some years and retain some liquid water below floating ice in others (
Fig. 7A). In the Fish Creek area, however, many relatively large lakes were entirely bedfast with no potential overwinter habitat in 2020–2021, while these same lakes maintained liquid water through the entire winter in 2017–2018 (
Fig. 7B), as well as the majority of previous years.
Differences in winter water availability between the extremely thick-ice winter of 2020–2021 and lower ice growth winter of 2017–2018 were also estimated based on the same SAR classifications of bedfast vs. floating ice in both ROIs, though variation in lake bathymetry or shallow-zone depth distributions among lakes and region will have some impact on estimation accuracy. In the Inigok area, we estimated that end-of-winter water availability was 0.49 m greater per lake in 2018 compared with 2021, equivalent to 7800 hectare -meters (20.6 billion US gallons or 78 million m
3). In the smaller Fish Creek ROI where differences in MIT were greater between these years, we estimated that end-of-winter water availability was 0.74 m greater per lake in 2018 compared with 2021, equivalent to 3626 hectare-meters (9.6 billion US gallons or 36 million m
3). Winter water use from lakes in the northeastern NPR-A varies greatly from year to year depending on industrial exploration activities ranging from <50 hectare -meters (132 million US gallons or 500 000 m
3) in 2010–2016 to 200 hectare-meters (528 million US gallons or 2 million m
3) in 2017 and 2018 (
Arp et al. 2019).
Discussion
Large interannual variations in freshwater MIT and its consequences for key components of the Arctic system (habitat, water supply, and permafrost) have long been recognized in northern Alaska (
Zhang and Jeffries 2000). Despite a long-term trend toward declining MIT (
Arp et al. 2012;
Surdu et al. 2014), along with several winters of record thin MIT and low extents of bedfast ice (
Arp et al. 2018), widely varying winter air temperatures, and particularly snow accumulation, still appear to make high interannual variability in MIT the dominant pattern.
A number of studies in northern Alaska have documented the importance of MIT relative to water depth for predicting summer fish (
Haynes et al. 2014;
Laske et al. 2016;
Heim et al. 2019) and plankton (
Beaver et al. 2019) communities based on classifying waters as freezing solid (bedfast ice) or maintaining liquid water through the winter (floating ice). What is less certain is how aquatic biota respond to changing winter habitat from year to year in a particular lake or stream, watershed, or regionally. Management of winter water supply for industry on Alaska's North Slope is primarily based on fish communities present during summer inventories and water available under static MIT and lake levels (
Arp et al. 2019). Understanding whether unusually thin- or thick-ice conditions and corresponding expansion and contraction (or elimination) of overwintering habitat have a significant effect on fish communities and other aquatic biota is warranted. Do species of long-standing subsistence value, such as broad whitefish (
Coregonus nasus), thrive in thick-ice winters and do important forage-based species such as ninespine stickleback (
Pungitius pungitius) explode following warm snowier winters because of greatly expanded overwintering habitat? Pacific salmon (
Oncorhynchus spp.), which may be expanding their range into North Slope rivers (
Nielsen et al. 2013), may find suitable spawning habitat in multiple winters with thin ice and then have a subsequent cohort limited by a thick-ice winter in following years.
Perhaps an even more interesting example of a range-expanding aquatic organism and ice is the North American beaver (
Castor canadensis), which has expanded into NW Arctic Canada (
Jung et al. 2017) and has been hypothesized to be advancing toward Alaska's North Slope (
Tape et al. 2018). Thick winter ice may have historically excluded beavers from many regions of the Arctic, but recently these limitations may be relaxing—with the exception of the winter of 2020–2021 on Alaska's North Slope. Another aquatic mammal that relies on under-ice habitat, the muskrat (
Ondatra zibethicus), has in fact been observed increasingly along the Colville River in northern Alaska in recent years (
Fish Creek Traditional Ecological Knowledge Workshop 2016). During the summers of 2018 and 2019, we made several surprising first-time muskrat sightings in lakes, streams, and rivers of the FCW, yet none were observed during the summers of 2021 and 2022. These anecdotal observations only serve to open broader questions about how to monitor and manage aquatic organisms, habitats, and water supply with ongoing climate change, which includes occasional winters more typical of historically thick-ice conditions.
Our report here on the very thick freshwater ice conditions in response to a cold and, likely more importantly, unusually low-snow winter helps to provide a context for better understanding how long-term trends in winter climate, including perhaps increasingly extreme variability, may impact resources of management concern in the NPR-A and the Arctic more broadly. For example, impacts to overwintering fish and winter water supply during sudden thick-ice years will be magnified in regions with many shallow lakes versus regions with deeper lakes or lakes of varying depth. Incorporating both fundamental techniques, such as traditional ecological knowledge, and emerging technologies, such as eDNA to detect fish presence, may provide a path forward to understanding the role of new Arctic winters on aquatic natural resources. Continued tracking of winter ice conditions, in addition to the weather and climate factors that regulate ice growth, particularly snow, and the biological response to winter conditions, particularly fish abundance and distribution, will be crucial to such efforts.