Contrasting the ecological effects of decreasing ice cover versus accelerated glacial melt on the High Arctic's largest lake

Abstract

Lake Hazen, the High Arctic's largest lake, has received an approximately 10-fold increase in glacial meltwater since its catchment glaciers shifted from net mass gain to net mass loss in 2007 common era (CE), concurrent with recent warming. Increased glacial meltwater can alter the ecological functioning of recipient aquatic ecosystems via changes to nutrient budgets, turbidity and thermal regimes. Here, we examine a rare set of five high-resolution sediment cores collected in Lake Hazen between 1990 and 2017 CE to investigate the influence of increased glacial meltwater versus alterations to lake ice phenology on ecological change. Subfossil diatom assemblages in all cores show two major shifts over the past approximately 200 years including: (i) a proliferation of pioneering, benthic taxa at approximately 1900 CE from previously depauperate populations; and (ii) a rise in planktonic taxa beginning at approximately 1980 CE to present-day dominance. The topmost intervals from each sequentially collected core provide exact dates and demonstrate that diatom regime shifts occurred decades prior to accelerated glacial inputs. These data show that diatom assemblages in Lake Hazen are responding primarily to intrinsic lake factors linked to decreasing duration of lake ice and snow cover rather than to limnological impacts associated with increased glacial runoff.

1. Introduction

Glaciers are in retreat on a global scale and the rates of early twenty-first century mass loss are historically unprecedented [1]. An initial consequence of rapidly melting glaciers is an increase in meltwater discharge, which can affect the ecology and functioning of recipient aquatic ecosystems via physical and chemical processes [2]. Examples include alterations to thermal structure [3], turbidity [4], nutrient concentrations [5] and the mobilization and release of stored pollutants such as metals (e.g. mercury) and organic contaminants [6–8].

Subfossil diatom (Bacillariophyceae) assemblages preserved in sediment cores from lakes in glacierized catchments are useful bioindicators to assess the timing and magnitude of ecological impacts from increased glacial runoff. Diatom assemblages respond sensitively to limnological changes associated with enhanced glacial inputs. For example, diatoms have effectively tracked the effects of nutrient subsidies [9] and disruptions to thermal stratification [3] from increased glacial meltwater. To date, most research on the ecological effects of glacial meltwater on lakes has focused on alpine regions with only a few examples from the Arctic [10].

Here, we assess how accelerated glacial melt has affected the ecology of an iconic lake in the Canadian High Arctic, a region where glaciers and icecaps have recorded marked mass losses in recent years [11]. Specifically, we use a unique series of high-resolution sediment cores (accumulation rates of less than 5 yr cm⁻¹ in near-surface sediments among the five cores), collected over the period from 1990 to 2017 common era (CE), to examine the influence of enhanced glacial inputs on diatom assemblages in Lake Hazen (Ellesmere Island, High Arctic Canada), the largest lake by volume (51.4 km³) north of the Arctic Circle (figure 1).

Figure 1.

Location of sediment coring sites in Lake Hazen and the glaciers within its catchment. (Online version in colour.)

Lake Hazen's hydrological regime is predominantly regulated by summer melt of its catchment glaciers [12]. Modelled annual glacier runoff for the Lake Hazen watershed dates to 1949 CE [13]. Although there is always glacial melt during the summer months, the shift from net mass gain to loss beginning in 2007 CE has caused an approximately 10-fold increase in the delivery of glacial meltwater, sediment, organic carbon, nutrients (P and N), and contaminants to the lake [13]. Modelled net annual mass balance from Lake Hazen's catchment glaciers show a 2 year return to net mass gain for 2014–2015 CE, followed by a reversion to net loss in 2016 CE (figure 2). Glacial meltwater accounts for greater than 70% of all hydrologic inputs (on average) over the duration of the record, and approximately 85% of all inputs since 2007 CE. Accelerated melting of Lake Hazen's catchment glaciers has reduced water residence times by approximately 60%, from 146 years for the period spanning 1949–2006 CE, to 62 years for the period spanning 2007–2016 CE. If the years with the lowest runoff values are used (1949–2006 CE, excluding 1950, 1957, 1960, 1962 which were abnormally warm years; figure 2) the mean residence time was 174 years. Likewise, during peak runoff years (2007–2012 CE), the mean residence time was only 49 years (table 1).

Figure 2.

Recent cryosphere history across the Lake Hazen watershed in relation to the sediment core series. (a) Lake ice-free conditions from 1985 to 2017 (based on [13,14] and in situ observations (2012–2017)). (b) Modelled net annual mass balance (bars) and cumulative mass balance (line) for glaciers in the Lake Hazen watershed. All values are in Gt, but note that the annual and cumulative mass balances are plotted on different y-axis scales. (c) Modelled glacial runoff (annual and 5 year running mean) from 1948 to 2017 (adapted from [13], with additional data for the post-2013 CE period). (Online version in colour.)

Table 1.

Mean water residence times for Lake Hazen calculated from the sum of the various hydrologic inputs to the lake, including glacial runoff, snowmelt runoff and direct precipitation on the lake surface. (The different time periods provide comparative estimates for before and after the onset of accelerated glacial inputs in 2007 CE. The years of 1950, 1957, 1960 and 1962 CE are statistical outliers and therefore the baseline water residence time (1949–2006 CE) was calculated both with and without those years. Details of methods used for calculating water residence times are given in the electronic supplementary material, S4.)

time period	water residence time (years)
	mean	median
1949–2006	146	189
1949–2006 (excl. 1950, 1957, 1960, 1962)	174	196
2007–2012	49	53
2007–2016	62	72

We present fossil diatom records from two dated sediment cores, spanning the past approximately 200 years, collected in May 2017 CE from two separate locations in Lake Hazen (figure 1). The two coring sites, located approximately 13 km apart, experience different levels of glacial influence with the ‘main’ site being located in closer proximity to several glacier-fed rivers than the ‘blister’ site (figure 1). We contrast these data with three additional diatom profiles from Lake Hazen cores, all collected from the ‘main’ site in 1990 [15], 2005 [16] and 2013 CE [13]. This unique approach allows for the comparison of fossil assemblages at various points in time before and after the onset of glacial net mass loss in 2007 CE. The utility of coring a specific lake over many years is that the age of the undisturbed surface sediments is unquestionable, namely the date of core collection. This is especially beneficial when assessing impacts from new and emerging stressors that post-date earlier core collections. Such an approach is particularly advantageous in High Arctic regions where dating surface sediments with ²¹⁰Pb is notoriously difficult owing to extended ice cover, permafrost suppression of ²²²Rn (the parent nuclide of ²¹⁰Pb), and reduced atmospheric scavenging owing to low precipitation [17]. Our unique set of high-resolution core collections allows us to unequivocally determine whether diatoms responded to accelerated glacial runoff via the various ways that meltwater can affect lakes (e.g. nutrient enrichment, increased turbidity, disruption to thermal regimes), or whether they responded more closely to warming-related declines in ice cover duration and attendant limnological changes related to longer growing seasons and increased pelagic habitat [18,19].

2. Material and methods

Sediment cores were obtained during the spring of 2017 CE from two locations, ‘blister’ (81.79171°, −71.46929°; depth = 258 m) and ‘main’ (81.82493°, −70.71498°, depth ≈ 260 m), near Johns Island in the north-central region of Lake Hazen [20]. The three sediment cores from previous studies using the same high-resolution approach [13,15,16] were recovered near the ‘main’ core location, which is the deepest portion of Lake Hazen (depth = 267 m). The ‘blister’ coring site (depth = 258 m) is located approximately 13 km west-southwest of the ‘main’ site (figure 1). Cores with well-preserved sediment–water interfaces were recovered using a UWITEC gravity corer (8.6 cm internal diameter) and sectioned into 0.5 cm intervals.

Radioisotopes for geochronology, specifically ²¹⁰Pb and ¹³⁷Cs, for the 2017 CE cores were counted at Queen's University on a digital high-purity germanium spectrometer (DSPEC, Ortec^®) with a well-type gamma detector consisting of a germanium crystal with lithium diffused electrodes. In both cores, dates generated from ²¹⁰Pb models did not correspond to distinct peaks in ¹³⁷Cs activity that denote the 1963 period of maximum atmospheric fallout (electronic supplementary material, S2 and S3). The excess ²¹⁰Pb profiles showed low activities (less than 100 Bq kg⁻¹) and several non-monotonic features (electronic supplementary material, S2 and S3) and were deemed less reliable for geochronology compared to ¹³⁷Cs. Age-depth models were based on peak ¹³⁷Cs activity (electronic supplementary material, S2 and S3) using ScienTissiME dating software [21]. When possible, 300 diatom valves, and always a minimum of 100 valves, were identified and enumerated for each sediment interval. Diatoms were identified to the lowest taxonomic level possible following the taxonomy of [22–26]. Diatom data from [15], presented in figure 3a, were obtained using WebPlot Digitizer v. 4.2.

Figure 3.

Summary of the relative abundances of benthic Fragilaria sensu lato taxa (Staurosirella pinnata, Pseuduostaurosira brevistriata, Staurosira construens, Staurosira construens var. venter) relative to planktonic Cyclotella sensu lato taxa (Discostella stelligera, Lindavia bodanica, Lindavia rossii and Lindavia comensis) for five Lake Hazen sediment cores. In deeper sediment intervals, fragilarioid taxa were present and well preserved, but so rare that quantitative enumeration was not practical. Grey arrows on the x-axis timescale indicate the year of core collection, including (a) 1990 CE [15]; (b) 2005 CE [16]; (c) 2013 CE [13]; (d) ‘blister’ core from 2017 CE; and (e) ‘main’ core from 2017. See text for details. (Online version in colour.)

3. Results

The two sediment cores retrieved in 2017 CE, ‘main’ and ‘blister’, recorded similar fossil diatom profiles (figure 3d,e). Following a period of low diatom abundance (i.e. valves present but so rare that quantitative enumeration was not practical), starting at approximately 1900 CE the assemblages became populated primarily by early-colonizing, benthic Fragilaria sensu lato taxa including Staurosirella pinnata Ehrenb., Pseudostaurosira brevistriata Grunow, Staurosira construens Ehrenb., and Staurosira construens var. venter (Ehrenb.) Hamilton. We note that diatom preservation was excellent, and we recorded no evidence of valve dissolution in the sediments with low diatom abundances. Also, dilution by autochthonous or allochthonous sediment cannot explain the low abundances as both organic carbon flux (g m⁻² yr⁻¹) and sedimentation rates (kg m⁻² yr⁻¹) remained low and stable between the late 1800s to 1940 CE when the transition occurred from low to high abundances of fragilarioids [13]. The benthic fragilarioids dominated until approximately 1980 CE, after which there was a rise in the relative abundances of planktonic Cyclotella sensu lato taxa including Discostella stelligera (Cleve & Grunow) Houk & Klee, Lindavia bodanica (Eulenst. ex Grunow) Nakov et al., Lindavia rossii (Håk.) Nakov et al., and Lindavia comensis (Grunow) Nakov et al. (figure 3d,e). These cyclotelloids continued to increase in relative abundance and eventually superseded benthic fragilarioids as the dominant diatoms in Lake Hazen. Detailed relative abundance species data for the 2017 CE main and blister cores are provided in the electronic supplementary material, S1.

Sediment cores collected in 1990 [15], 2005 [16] and 2013 CE [13] recorded similar diatom assemblage shifts to the 2017 CE cores, including a gradual rise in benthic fragilarioids at approximately 1900 CE followed by increasing abundances of cyclotelloids in the late portion of the twentieth century (figure 3).

4. Discussion

The 2017 Lake Hazen cores record two large-scale ecological reorganizations over the past approximately 200 years. First is the proliferation of benthic Fragilaria sensu lato taxa from near-negligible abundances, beginning in the early 1900s CE. Second is the rise in planktonic cyclotelloids at approximately 1980 CE, which rapidly replace benthic fragilarioids as the dominant diatom taxa in the lake (figure 3). Both major assemblage shifts occurred prior to the acceleration of glacial inputs in 2007 CE, when catchment glaciers shifted from net mass gain to net mass loss [13]. This has been evident since the first cursory palaeolimnological study of Lake Hazen [15] performed in 1993 CE, which recorded the onset of obligate planktonic taxa in the uppermost sediments of a core collected in 1990 CE (figure 3a), 17 years before catchment glaciers began to shrink. Subsequent sediment cores collected in 2005 CE ([16]; figure 3b), 2013 CE ([13]; figure 3c), as well as the two 2017 CE cores in this present study (figure 3d,e), confirm that major diatom assemblage shifts began several decades prior to the onset of increased glacial runoff in 2007 CE.

An extensive body of Arctic palaeolimnological studies, performed on lakes in glacier-free catchments, have documented diatom compositional shifts similar to those recorded in Lake Hazen, including several from Ellesmere Island [27–30]. These studies ascribed the assemblage shifts to decreased duration of snow and ice cover and longer growing seasons associated with warming [18,31–34]. Specifically, the initial rise in benthic fragilarioids occurs in response to the amelioration of diatom growing conditions with longer ice-free conditions in littoral environments. In Arctic regions, these benthic fragilarioids are often the first to colonize newly formed lakes immediately following deglaciation [35] in perennially ice-covered lakes that begin to melt with rising temperatures [28,32]. With continued warming, and more extensive ice-melt, the deep lakes with sufficient pelagic environments will become ice-free for extended periods, allowing planktonic taxa to flourish and eventually surpass benthic species as the dominant diatom algae (figure 4). Recent shifts from benthic fragilarioids to planktonic cyclotelloids have also been recorded with the onset of warming and enhanced periods of thermal stratification in hundreds of lakes in glacial-free catchments throughout the Northern Hemisphere [19,31–35,37], as well as Andean lakes in South America [38,39].

Figure 4.

Schematic illustration showing how warming temperatures affect summer ice cover and diatom assemblage shifts in Lake Hazen. The effects of changing ice cover on diatoms is discussed in detail in [18,31]. Arctic temperature anomaly data are from [36]. Mean annual temperature (MAT) data from the two nearest meteorological stations to Lake Hazen (Alert and Eureka, Ellesmere Island) are presented as an average for the years 1951–2018 (https://climate.weather.gc.ca/historical_data/search_historic_data_e.html, accessed 8 November 2019). Diatom data are from [13]. (Online version in colour.)

The sediment cores collected in 1990 and 2005 CE from Lake Hazen show incontrovertibly that the major diatom assemblage shifts pre-date the accelerated glacial inputs that began in 2007 CE (figure 3a,b). Nonetheless, this is not to say that glacial effects cannot ultimately overpower intrinsic lake factors associated with longer growing seasons and changes in habitat availability. For example, rapid deglaciation in the catchment of a Svalbard lake resulted in high turbidity and reduced light penetration, which caused a rapid decrease in the abundance of planktonic diatoms [3]. Similarly, two other lakes on Svalbard recorded the disappearance of planktonic diatoms associated with enhanced catchment erosion and a subsequent increase in turbidity [40]. Even during periods when catchment glaciers are accruing ice, Lake Hazen receives glacial inputs during the summer months. However, the 10-fold increase in turbid meltwater post-2007 CE has not, as of yet, hindered the growth of planktonic diatoms as recorded by the continued dominance by Cyclotella sensu lato taxa in sediment cores (figure 3c,d,e). Decreasing ice cover remains the dominant driver of limnetic assemblage composition. Satellite imagery of Lake Hazen from 1985 to 2016 CE shows completely ice-free conditions during summer months in all but seven of those years (1985–1987, 1992, 1996, 2005 and 2009; figure 2).

Increases in Cyclotella sensu lato taxa, particularly Discostella stelligera, have at times been linked to nitrogen enrichment from glacial meltwater [9], but this is not a viable hypothesis at Lake Hazen. First, as mentioned earlier, the onset of Cyclotella sensu lato increases predate the onset of accelerated glacial runoff (figure 3). Second, Cyclotella sensu lato taxa continue to dominate to the present day, despite declines in glacial inputs (and thus nutrients) post-2013 CE, as inferred from the modelled net annual mass balance of catchment glaciers (figure 2). Third, a comprehensive limnological survey of Lake Hazen shows that phytoplankton cell densities, biomass and chlorophyll a concentrations remained extremely low throughout the summer growing season, explained by particle-bound nutrients being transported below the photic zone via dense turbidity currents, and thus largely unavailable to the aquatic biota [12]. Likewise, research from the Northern Cascades Mountains (USA) has shown that melting glaciers do not always result in increased nitrate levels and altered algal abundance and composition in receiving lakes [41]. Finally, we note that similar diatom assemblage shifts have occurred in many other Arctic regions in lakes with declining ice covers but lacking glaciers in their catchments [19,33].

In Lake Hazen, the timing and nature of the diatom shifts reflect within-lake limnological changes associated with decreasing duration of lake snow and ice cover [18,31]. The approach of using multiple sediment cores over several decades negates any uncertainty surrounding dating in near-surface sediments and demonstrates unequivocally that large-scale ecological changes in the High Arctic's largest lake began prior to the onset of accelerated glacial inputs. The long-term ramifications of this major ecological regime shift in Lake Hazen remain unknown but will probably cascade throughout the ecosystem.

Data accessibility

Additional data are available in the electronic supplementary material.

Competing interests

We declare we have no competing interests.

Funding

This research was funded via Natural Science and Engineering Research Discovery grants to J.P.S., I.L. and V.L.S.L.; ArcticNet and Polar Continental Shelf Program grants to I.L. and V.L.S.L.; and Kenneth M. Molson Foundation and Northern Scientific Training Program grants to I.L.