Watershed warming and declining lake ice cover

The Lake Hazen watershed has warmed intensely since the turn of the century. Mean (±SD) summer (June, July, August (JJA)) land surface temperatures of glacier-covered regions of the Lake Hazen watershed increased by 0.21 ± 0.05 °C y−1 from 2000 to 2012, representing a 2.6 °C warming over that time period (Fig. 1). Most of this warming occurred from 2007 to 2012 when summer temperatures were 0.9 °C warmer than the period mean (−4.9 °C). Between 1994 and 2010, temperatures in the upper 1 m of soils on the desert landscape near the Lake Hazen base camp rose by 0.14 ± 0.11 °C y−1 (Fig. 2), with most warming occurring from 2007 onward. Soil warming was most pronounced in spring. May–June soil temperatures were 4 ± 1 °C warmer in 2007–2012, relative to 1994–2006, likely due to warmer near-surface air temperatures (Fig. 2) driving an earlier onset of snowmelt. Furthermore, the number of days during which soil temperatures were above freezing at 50 cm depth increased by 24 days in July and 22 days in August (Fig. 2), resulting in permafrost soils that, prior to 2007, were frozen year-round now being part of the seasonally thawed active layer for most of the summer, a trend recorded across most northern permafrost sites16. Between 2000 and 2012, the monthly mean lake surface temperatures for May (snow-covered ice), June (bare ice following snowmelt), July (moating and breaking up of ice) and August (up to full absence of ice) increased by, on average, 0.16, 0.07, 0.14 and 0.10 °C y−1 (Fig. 3), which is significantly greater than the increase in summer surface temperature observed for other seasonally ice-covered lakes around the world (median = 0.048 °C y−1; ref. 17). More rapid warming in spring advanced the onset of ice break-up by an average of 0.9 d y−1, while less intense warming in August delayed freeze-up by 0.3 d y−1. On average, the mean ice-free area (5 May to 5 September) of the lake increased by 3 km2 y−1 (or 0.5% y−1) after 2000. Annual daily ice-free area (%) was significantly related to annual August lake surface temperatures (Pearson's correlation r = 0.867, p = 0.0001). However, annual daily ice-free area (%) was not related to annual glacier runoff volume (see below) (Pearson's correlation r = 0.023, p = 0.94), suggesting that thermal inertia induced by inputs of relatively warm glacial meltwaters from the watershed in JA (e.g., mean (±SD) 1997–2012 water temperatures in the Abbé, Very and Turnabout rivers were 5.3 ± 1.2, 9.3 ± 1.8 and 9.3 ± 2.3 °C; Environment and Climate Change Canada) was not the primary driver of lake surface warming in a given year. Full ice-off on Lake Hazen became more frequent in recent decades as the lake went ice free for periods of a month or more in 60, 80 and 88% of the years, between 1985 and 1995, 1996 and 2005 and 2006 and 2012, resulting in the progressive loss of multiyear ice (this study and refs.18,19).

Fig. 2 Changes in air and soil temperatures in the Lake Hazen watershed. a Mean annual change in monthly soil temperature (°C y−1) for the period 1994–2010. b Difference in mean monthly soil temperature (°C) between the periods of 2007–2010 and 1994–2006, indicating that soil temperatures have primarily increased since 2007. c Increase in the number of days mean daily soil temperature was above freezing during 2007–2010 compared with the 1994–2006 baseline, showing a recent thickening of the soil active layer. Depth 0 is shielded air temperature at 1 m above the soil surface Full size image

Fig. 3 Temporal trends in surface temperature, ice phenology and ice cover at Lake Hazen. a Monthly mean (±SE) lake surface temperatures (°C) measured at 30 sites on Lake Hazen. b Onset dates (day of year) of melt and freeze-up. c Mean daily ice-free area (% of total lake area) on Lake Hazen between May 5 and September 5. For illustrative purposes, linear trend lines are shown, though none are significant (p > 0.05) Full size image

Hydrological changes in the Lake Hazen watershed

The hydrological regime of Lake Hazen is primarily, and strongly, controlled by the glaciers within its catchment. Modeled annual glacier mass balances (annual accumulation minus annual ablation) for the period 1949–2012 showed a distinct shift from net mass gain to net mass loss beginning in 2007–2008 (Fig. 4). This change was coincident with rising air temperatures20,21 and amplified by positive temperature–albedo feedbacks (whereby higher surface temperatures drive albedo declines that enhance surface warming and/or melt, leading to additional reductions in surface albedo) that together increased both the duration and intensity of meltwater production in summer. This warming caused intense melting events within a short 6–8-week period in June-August, even in the interior high-elevation regions of the glaciers where melt previously occurred infrequently22. Modeled mean rates of annual glacier runoff reached 660 kg m−2 or 1.8 km3 y−1 over the entire Lake Hazen catchment in 2011 (Fig. 4). Modeled runoff rates agree well with direct measurements of river discharge at the Lake Hazen outflow (Ruggles River; Fig. 4, Supplementary Fig. 1). For the period 2007–2012 relative to 1996–2006, increased glacier runoff raised water levels in Lake Hazen by 0.8 m on average (Supplementary Fig. 2), and increased mean annual discharge from its outflow by 370%, from 0.49 to 1.8 km3. Using modeled glacier runoff values to extend the time series provided by the instrumental record from the Ruggles River, we find that the last time runoff rates were comparable to those in 2007–2012 was during a brief warm period in the 1950s (Fig. 4) that was recorded by the few weather stations operating in the High Arctic during that time23 and which is evident in the earliest records of glacier mass balance from this region24. Annual runoff in excess of 1 × 109 m3 occurred in three of the five years between 2007 and 2012, but only twice in the previous 58 years. The large input of glacier meltwaters into Lake Hazen has reduced the residence time of water in the lake from its historical average of ~89 years10 to only 25 years.

Fig. 4 Changes in glacier mass balance, glacial runoff and Lake Hazen discharge. a Modeled net annual mass balance (bars) and cumulative mass balance (line) for glaciers in the Lake Hazen watershed for 1948–2012. All values are in Gt, but note that the annual and cumulative mass balances are plotted on different y-axis scales. b Modeled glacial runoff (annual and 5-year running mean, 1948–2012) compared with measured daily discharge from Lake Hazen at the outflow (Ruggles River) from 1996 to 2012 Full size image

Recent changes informed by the paleolimnological record

We used a multi-proxy paleolimnological approach to place the recent warming and its impacts on Lake Hazen within a longer-term context. Analyses of lacustrine sediments (Fig. 5) show that Lake Hazen has recently undergone a major regime shift in response to climate warming. The magnitude of the impacts is unprecedented and exceeds anything observed in the past 300 years, including during the period of warming at the end of the Little Ice Age. Sediment accumulation rates since 2007 (4.2 kg m−2 y−1) are on average 8 times higher relative to the pre-1948 baseline period (0. 5 kg m−2 y−1) (Fig. 5), mirroring recent trends in glacial runoff (Fig. 4), which is the main driver of sediment delivery to the lake. Elevated discharge of glacier-fed rivers into the lake has resulted in dense, oxygen-rich turbid underflows25 facilitating mixing and oxygenation of bottom waters (Supplementary Fig. 3). This recent summertime ventilation of anoxic bottom waters likely marks a departure from the stable low redox conditions inferred to be historically prevalent at the bottom of the lake when glacial runoff and sedimentation rates were low. Increased sediment delivery has also resulted in increased sequestration of anthropogenic contaminants, such as mercury (Hg) and legacy organochlorine pesticides (OCPs), into lake sediments (Fig. 5). Rising concentrations of legacy OCPs in sediments post 2000 (Supplementary Fig. 4), after a decline from maxima in the 1980s, reflect remobilization of OCPs previously deposited and stored in glaciers26, increasing exposure of arctic aquatic foodwebs to legacy contaminants.

Fig. 5 Sediment record of diatom abundance, geochemical parameters, contaminants and sedimentation rates. Diatom, geochemical and contaminant analyses were completed on three separate sediment cores collected in close proximity in May 2013 from the deepest location in Lake Hazen10. One of these cores was used for 210Pb radiometric dating (also see Supplementary Fig. 7) and calculation of sedimentation rates. This same core was analyzed for organic matter geochemistry and multi-element concentrations. OC organic carbon, C carbon, N nitrogen, P phosphorus, THg total mercury, OCP organochlorine pesticides. See Supplementary Fig. 4 for sediment concentration profiles of N, P and contaminants. The horizontal lines demarcate when diatoms first appear in the paleolimnological record in significant numbers (bottom), when the relative abundance of planktonic diatom species first began to increase (middle) and then surpassed that of benthic species (top) Full size image

Lake Hazen sediments contain low concentrations of organic carbon (OC) (<2.5%; Fig. 5). However, recent increases in sediment inputs have resulted in a parallel 1000% increase in OC accumulation rates (Fig. 5), which is much higher than the 50% increase, on average, in OC accumulation that North American boreal lakes have experienced since 195027. Recent (2007–2012) accumulation rates in Lake Hazen (14–71 g OC m−2 y−1) are now similar to or greatly exceed those found in boreal and northern temperate lakes (15 ± 9.4 g OC m−2 y−1)27. Profiles of δ13C, δ15N and C:N ratios in bulk sediments are consistent with gradual organic matter (OM) diagenesis28, and show that the source of OM accumulating in Lake Hazen sediments is, and always has been, primarily terrestrially derived. Likely sources of terrestrial OM in this low-productivity High Arctic ecosystem29 include vegetation and soils destabilized by increased flow in glacial river channels and deltas, and vegetation and old soil OM previously overrun by glacier advances following the warmer hypsithermal period 9000 to 5000 years bp. Sedimentary profiles of δ13C and δ15N have been used to evaluate changes in autochthonous algal productivity28; however, recent changes in Lake Hazen productivity cannot be assessed in this manner. The δ13C and δ15N values of contemporary particulate OM in the upper water column (−29.4 and 4.1‰), where productivity is highest, differed from those of older, pre-2000 sedimentary OM (−25.9 and 2.4‰). Intermediate δ13C and δ15N values (−26.5 and 2.7‰) were observed only in very surface sediments (Fig. 5). Together, these data demonstrate that autochthonous OC in this ultra-oligotrophic lake is rapidly decomposed at the sediment–water interface, rather than accumulated. In fact, rates of decomposition have likely increased with recent increases in OC inputs and summer bottom water oxygenation (Supplementary Fig. 3). Therefore, the sediment archive is not particularly sensitive to changes in algal productivity, which may already be increasing in response to decreased ice cover and increased nitrogen and phosphorus inputs from glacial rivers (Fig. 5).

Ecological shifts in Lake Hazen

To determine how the ecology of primary producers in the lake has changed as a result of climate-driven changes to the Lake Hazen watershed, algal (diatom) community assemblages were reconstructed from microfossil counts in dated sediments30. Prior to ~1890, diatom fossils, although well preserved, were rare (Fig. 5, Supplementary Fig. 5), indicating that algal growth was severely restricted by extensive ice cover on Lake Hazen31,32,33. Subsequently, when temperatures began to rise ca. 1890 as indicated by the ice core record from the Canadian Arctic34, taxa common to nearshore habitats flourished31,32,33, specifically Fragilaria sensu lato species such as Staurosirella pinnata, Staurosira construens and Staurosira venter (Supplementary Fig. 5), suggesting development of greater ice-free areas along the lake’s shoreline. The most recent large-scale ecological reorganization began in the late 1980s when planktonic Cyclotella sensu lato35 increased in relative abundance and eventually supplanted benthic species ca. 1998 (an exceptionally warm summer in the Canadian Arctic36) as the dominant taxa in the diatom community (Fig. 5). This reorganization was likely driven in large part by the observed earlier onset of ice break-up, an increase in the growing season ice-free area of the lake and Lake Hazen becoming mostly ice free in late summer, all of which enhanced light penetration in the pelagic environment.

Climate-related changes are also impacting the only fish species in Lake Hazen, Arctic Char, the physiological condition of which has declined significantly in recent years (Fig. 6). Although assessing the exact causes of this decline is beyond the scope of this study, one contributing factor may be increased turbidity in the lake, arising from increased discharge of sediment-rich glacier-fed rivers, which impacts the feeding efficiency of this visual predator whose main prey are chironomid midges and young Arctic Char (cannibalism)37. As climate warming continues in the region, resulting in accelerated net glacial mass loss and increasing glacier-fed river discharge, we predict that the physiological condition of Arctic Char will progressively decline further. Any decline in the physiological condition in these long-lived, slow-growing fish could threaten what was already thought to be an ecologically sensitive population9 of one of the Arctic’s most economically and culturally important species.

Fig. 6 Physiological condition of Arctic Char (Salvelinus alpinus) in Lake Hazen. Fulton’s condition factor was calculated for Arctic Char with a mass of 200 g or greater collected between 1981 and 2014. Small open circles are condition factors for individual Arctic Char, whereas larger blue circles are mean condition factors for a given year. A quadratic trend line was fitted to all the data (p < 0.001). Arctic Char image credit: Kativik Ilisarniliriniq Full size image

In conclusion, the current biogeochemical, limnological and ecological conditions in Lake Hazen have no precedent within the last ~300 years. Although other lakes around the world may respond differently to a warming climate38, we show that, because of tight coupling with the cryosphere, changes to the Lake Hazen ecosystem were mediated primarily by increasing glacial melt and loss of lake ice cover. Rising inputs of glacial runoff into Lake Hazen altered the lake’s hydrology and increased the delivery of sediment, OC, nutrients and contaminants, likely enhancing in-lake processes such as net ecosystem productivity and contaminant bioaccumulation. A decrease in seasonal ice cover resulted in warming of surface waters and, more importantly, allowed planktonic algae to fill a niche which was previously climatically inaccessible, re-organizing the ecology of the lake at the base of the foodweb. Collectively, rising air temperatures, increasing glacial melt and runoff, decreasing summer lake ice cover, shifts in primary producer communities and declining fish condition demonstrate the coupling between watershed changes and in-lake conditions and processes. This vast, deep lake, the High Arctic’s largest freshwater ecosystem, has experienced drastic changes in the last decade, despite its volume, thermal inertia and hypothesized resilience to climate change. Such changes, and their consequences, are certain to increase further as warming of northern latitudes continues into the future, undoubtedly jeopardizing the security of traditional freshwater foods and other ecosystem services for northern Indigenous peoples throughout the Arctic.