thermokarst lakes on the arctic coastal plain of alaska: spatial and temporal variability in summer...

PERMAFROST AND PERIGLACIAL PROCESSESPermafrost and Periglac. Process. (2012)Published online in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/ppp.1743

Thermokarst Lakes on the Arctic Coastal Plain of Alaska: Spatial and TemporalVariability in Summer Water Temperature

Kenneth M. Hinkel,1* John D. Lenters,2 Yongwei Sheng,3 Evan A. Lyons,3 Richard A. Beck,1 Wendy R. Eisner,1 Eric F. Maurer,4

Jida Wang3 and Brittany L. Potter2

1 Department of Geography, University of Cincinnati, Cincinnati, OH, USA2 School of Natural Resources, University of Nebraska, Lincoln, NE, USA3 Department of Geography, University of California, Los Angeles, CA, USA4 Department of Biology, University of Cincinnati, Cincinnati, OH, USA

* CoUnivE-ma

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ABSTRACT

In summer 2010, water temperature profile measurements were made in 12 thermokarst lakes along a 150-km longnorth–south transect across the Arctic Coastal Plain of northern Alaska. In shallow lakes, gradual warming of the watercolumn to 1–4�C begins at the lake bed during decay of the ice cover in spring. Rapid warming follows ice-off, withwater temperature responding synchronously to synoptic weather variations across the area. Regionally, ice-off occurs2–4 weeks later on lakes near the coast. Inland lakes are warmer (13�C) in mid-summer than those near the coast (7�C),reflecting the regional climate gradient and the maritime effect. All lakes are well mixed and largely isothermal, withsome thermal stratification (< 2�C) occurring during calm, sunny periods in deeper lakes. In deep (6–9 m) lake-beddepressions that are likely ice-wedge troughs, water cools by conduction to the colder sediments below, while concur-rent warming occurs in the upper water column. A spatially dense sample of near-surface temperature measurementswas collected from one lake over a short period and shows warmer (2–3�C) temperatures on the upwind, sheltered endof the lake. This study demonstrates that climatic gradients, meteorological conditions and basin characteristics impactlake temperature dynamics. Copyright © 2012 John Wiley & Sons, Ltd.

KEY WORDS: Alaska; thermokarst lakes; water temperature; limnology; monitoring networks

INTRODUCTION

About one-fourth of the lakes on Earth are found at highlatitudes in the northern hemisphere (Lehner and Döll,2004), and many of these developed in the permafrost ofRussia, Canada and Alaska. In the continuous permafrostregion of northern Alaska, tens of thousands of lakes andponds are found on the Arctic Coastal Plain (ACP). Termed‘thaw’ or ‘thermokarst’ lakes, they began forming on thetundra surface following climate amelioration during thePleistocene-Holocene transition.In addition to being primary landscape elements, the

ubiquitous lakes of northern Alaska are an importantresource. Ponds and lakes support a variety of endemicecosystems, provide a varied aquatic habitat for plants, fishand animals, and are crucial to waterfowl that migrate into

rrespondence to: Kenneth M. Hinkel, Department of Geography,ersity of Cincinnati, Cincinnati, OH 45221–0131, USA.il: [email protected]

right © 2012 John Wiley & Sons, Ltd.

the region during the brief summer (Alerstam et al., 2001).Indigenous people utilise lakes for subsistence hunting andfishing (Eisner et al., 2009) and, more recently, as a sourceof fresh water for municipal and industrial applications(Berkes and Jolly, 2001; Jones et al., 2009). Thermokarstlakes are also a significant source of CO2 and CH4 to theatmosphere (Kling et al., 1991; Zimov et al., 1997; Walteret al., 2006, 2007), and evaporation from lake surfacescontributes to the regional cycling of moisture on the tundraduring summer.

A recent satellite-based survey of lakes (Schneider andHook, 2010) indicates that lakes are warming rapidly on aglobal basis in response to changing climate. Palaeolimnolo-gical records from circumpolar Arctic lakes show dramaticshifts in the benthic communities since ~ 1850 (Smol et al.,2005; Kaufman, 2009). Satellite-based studies also suggestthat thermokarst lakes are generally increasing in sizeand number (Smith et al., 2005; Riordan et al., 2006),although trends may vary regionally (Riordan et al., 2006;Jones et al., 2011) or temporally (Arp et al., 2011). Global

Received 5 September 2011Revised 17 June 2012Accepted 6 July 2012

K. M. Hinkel et al.

observations indicate that lake ice is melting out earlierand freeze-up is beginning later in the season (Magnusonet al., 2000; Arp et al., 2011); lengthening of the ice-freeseason creates a longer time period for absorption of solarradiation, which enhances heat flux into the basal sediments.Despite the importance of thermokarst lakes, there has

been little long-term monitoring of water temperature orother fundamental limnological parameters. A number ofstudies have been conducted on Arctic lakes and ponds(Hobbie, 1980; Kling et al., 1992; Bradley et al., 1996;Gibson et al., 1996; Rouse et al., 1997, 2003; Lamoureuxet al., 2002; Maher and Malm, 2003; Kettle et al., 2004;Kokelj et al., 2005; Mesquita et al., 2010) but have tendedto focus on individual lakes over relatively short timeperiods. Despite the value of these previous studies, they arestill inadequate for evaluating diurnal, synoptic, seasonal,interannual or long-term changes, assessing inter-lakevariation, or analysing lake response across large geographicregions. Although baseline data from Arctic lakes in NorthAmerica (McDonald et al., 1996; Hershey et al., 1999;MacIntyre et al., 2009) are being collected, there havebeen few concerted long-term efforts to systematically moni-tor Arctic lakes to assess lake response to local or regionalclimatic gradients, across different landscape-vegetationunits, or to direct human activity. Notable exceptions includea multi-year study of lake water temperatures across thetaiga-tundra boundary in the Mackenzie Delta region ofnorthwestern Canada (Burn, 2002, 2005), a large-scale surveyof lakes across the Canadian Arctic archipelago (Hamiltonet al., 2001), as well as a recent study comparing lake temper-ature and freezing dynamics of three lakes in interior Alaskato three lakes on the ACP (Arp et al., 2010). The design ofthe latter study includes an evaluation of water temperatureswithin individual lakes and between groups of lakes indissimilar environments.To date, limitations to limnological monitoring have been

largely due to the difficulty in accessing these remotelocations, the lack of rugged, affordable sensors that canbe deployed en masse and loss or damage to equipmentduring the dynamic ice-out period. This paper reports on alake survey programme designed to address some of theseissues. The objectives were: (1) to collect summer tempera-ture time series from the water column for groups of lakesalong a 150-km long north–south transect across the ACP;and (2) to explore the relation between lake temperaturedynamics, meteorological factors, climatic controls and lakebasin characteristics. Given regional warming and a longerice-free season, warmer summer water temperatures wouldpromote greater evaporative water loss, as well as enhancethe flow of heat into the lake basal sediments. Measure-ments of water temperature can be used to model heat flowand the potential impact on permafrost thaw (e.g. Burn,2002; Ling and Zhang, 2003, 2004; West and Plug, 2008;Plug and West, 2009). Near-surface water temperaturescan also be used to calibrate thermal bands from satelliteremote sensing platforms, which allows for extrapolation ofwater skin temperature to the multitude of lakes across theACP. The spatially extensive sampling scheme of this study

Copyright © 2012 John Wiley & Sons, Ltd.

is designed to represent the regional climatic gradient whileaccounting for variations in lake size and morphology.

STUDY AREA AND BACKGROUND

The climate of the ACP is polar continental, which grades topolar maritime near the coast owing to the strong seasonalclimate gradient induced by the Arctic Ocean (Haugen andBrown, 1980; Hinkel et al., 2004). The mean annual airtemperature at the coastal village of Barrow is around �11�C(National Climate Data Center, 2008), with the maximummonthly temperature in July (~ 5�C) andminimum in February(�27�C). Average winds are strong (10 knots or ~ 5 m/s) andtypically from the east or ENE, although westerly winds dooccur. Most of the 12 cm of annual precipitation falls as snow,which typically reaches depths of 0.3–0.6 m.

A 9-year record of air temperature collected along a 100-kmnorth–south transect from coastal Barrow to interior Atqasukdemonstrates significantly warmer spring and summer tem-peratures further inland, while coastal locations are relativelycool (Hinkel et al., 2012a). At inland sites, thawing degree-days in June are greater by a factor of two to three, whileaverage July temperatures are 3–6�C warmer than the coastalzone. By contrast, average February temperatures are slightlywarmer (1–2�C) near the coast due to the influence of therelatively warm water body. An analysis by Zhang et al.(2000) examined patterns of surface albedo with AVHRRimagery and concluded that spring snowmelt occurs progres-sively from the foothills of the Brooks Range towards theArctic Coast. Lakes are ice covered for 8–9 months of theyear, with maximum ice thickness of 1–2 m (Jefferies et al.,1996; Kozlenko and Jeffries, 2000; Jones et al., 2009), but itis not uncommon for deeper lakes within 25–40 km of thecoast to retain their ice cover some 2–6 weeks later thaninterior lakes owing to cooler and cloudier conditions (Hinkelet al., 2012a). Details of the regional geomorphology and lakeformation processes can be found in the companion paper(Hinkel et al., 2012b).

METHODOLOGY

Work for this project was concentrated at three locationsrepresenting distinctly different geomorphic surfaces andclimatic conditions. One group of studied lakes was nearthe coastal village of Barrow. A second group was locatedinland about 100 km near the village of Atqasuk, and thethird was 65 km further south near an abandoned reindeercamp along the ACP-Arctic Foothills transition zone. Lakeselection was based on a number of criteria; for details,see the companion paper (Hinkel et al., 2012b). About 15potential lakes were identified for each study area prior tothe field season to obtain a range of lake sizes and depths.Most of the lakes selected were closed basins lacking signif-icant inlets or outlets. In each of the three areas, one represen-tative lake (‘focus’ lake) was selected for additional intensive,

Permafrost and Periglac. Process., (2012)

Arctic Lakes Water Temperature

year-round detailed energy and water balance studies (Potteret al., 2011). Near each of the three focus lakes a basic terres-trial meteorological station (Onset 4-channel HOBO MicroStation Data Logger, H21-002) was installed to monitor airtemperature, air pressure, wind speed and direction, and solarradiation on an hourly basis.Sensor strings were deployed in many lakes to measure

near-surface and near-bed water temperatures at hourlyintervals throughout the summer season. Sensors wereaffixed to a nylon rope (1-cm diameter) that had a 22-cm (900)float at the top and a sandbag anchor at the bottom. HOBOPro V2 water temperature loggers (U22-01: accuracy0.2�C, resolution 0.02�C) were used for sensors higherin the water column. A HOBO water-level logger (U20-001-01: resolution of 2 mm) or Solinst unit (resolution of1 mm) was deployed at the base of the sensor string sincethese units record both water pressure and temperature(resolution 0.1�C).In late April 2010, instruments were deployed near the

centre of two lakes near Barrow (L100 and L107; Figure 1).Holes were augured through the 1.4-m ice cover, the sensorstring anchor lowered to the lake bed, and the float submergedand displaced to the side of the hole beneath the ice. Withsurface temperatures around �15�C, the hole quickly frozeover. Each sensor string had two water temperature loggers,

Figure 1 Shaded relief map of northern Alaska showing topography, phys-iographic regions, villages and lake locations. There is one focus lake of inten-sive study in each of the three regions: Focus A (L100) near Barrow, Focus B(L200) near Atqasuk and Focus C (L300) near Reindeer Camp, with associ-ated terrestrial meteorological stations. This figure is available in colour onlineat wileyonlinelibrary.com/journal/ppp


with one just above the sand bag anchor and one beneaththe float. Since both lakes are shallow, the lower sensor wasat a depth of 200 cm and 130 cm for lakes L100 and L107,respectively. During spring, as the ice cover melted andthinned, the float rose towards the water surface. This strategykept the sensor string from being frozen into the ice, therebypreventing it from being rafted about or destroyed by movingice during breakup.

Lakes further south were instrumented in late June 2010when the ice cover was actively decaying. Using an inflatableboat, instrument strings were deployed near the centre of thelake, although the shifting ice cover made this problematicin some cases. In one of the focus lakes (L300; Figure 1),two instrument strings were installed at either end of the lake.As noted above, most lakes used two sensors to measurenear-surface and near-bottom temperatures. However, onlyone sensor was used in very shallow lakes (< 1 m), while insome deeper lakes three sensors were deployed at near-equalvertical spacing to detect thermal stratification.

RESULTS AND DISCUSSION

Seasonal and Spatial Patterns of Near-SurfaceWater Temperature

Hourly near-surface (30 cm) time series of water temperaturefor some of the lakes instrumented in 2010 are shown inFigure 2. Coastal lakes near Barrow (100 series) are shownin red for the period after 15 June. A lake near Atqasuk(L201, black trace) was instrumented in 2010 as soon as therewas adequate open water to land a float plane (26 June). Theremaining time series are for lakes near Reindeer Camp(300 series) and include lakes of a variety of sizes and depths.In late June, only the shallowest and smallest lakes werepartially ice-free, as is typical, so these were instrumentedfirst. Lake L301 (not shown, see Table 1), for example, is only1.0 m deep near the centre and a sensor string was deployedon 21 June. Other nearby lakes were instrumented as soonas they were sufficiently open to allow for safe access,although the mobile ice cover was still largely intact nearthe basin centre. In many lakes, the instrument string wasoverrun by shifting ice pans without incurring damage orbeing dragged to a new location. At L200 (focus lake B),however, the instrument string was displaced more than 200m from the basin centre to the shore and ended up in 25 cmof water. Although the sensors continued to operate, the dataare not representative of mid-lake conditions and are notpresented here.

The impact of residual lake ice on water temperatureis apparent in Figure 2. All lakes near Reindeer Camp(300 series) begin to warm shortly after monitoring began,which was roughly coincident with ice-out. By 1 July, thelakes were largely ice-free and most lakes had warmed toa near-surface temperature of 12–14�C; this rapid warmingfollowing ice-out has also been noted by Brewer (1958)and Burn (2005). The temperature of the lakes remained


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Figure 2 Near-surface (30 cm) temperatures for all lakes instrumented in summer 2010 using hourly measurements. The 100 series lakes are near Barrow, the200 series near Atqasuk and the 300 series near Reindeer Camp.

Table 1 Summary statistics of hourly lake water and air temperature measurements (�C) collected over the 57-day standardisedperiod (27 June to 22 August 2010) at the 12 lake instrumentation sites and three nearby meteorological stations.

Site and depth Latitude Longitude Mean Median Minimum Maximum Range Std Dev.

Barrow lakesL100 (30 cm) 71.2416 �156.7745 6.59 6.26 �0.03 15.41 15.45 5.01L100 (200cm) 7.46 6.08 0.63 15.39 14.76 3.94L107 (30cm) 71.2739 �156.4973 6.47 6.10 0.19 15.44 15.25 4.80L107 (130cm) 6.96 6.08 0.58 15.20 14.62 4.19

Focus A met. station 71.2220 �156.7834 5.97 5.02 �1.13 19.03 20.16 4.31Atqasuk lakesL201 (30cm) 70.3294 �156.8378 11.60 11.05 4.35 18.41 14.06 2.99L201 (120cm) 11.41 10.91 4.27 17.37 13.10 2.95L201 (240cm) 11.22 10.80 4.30 16.80 12.50 2.82Focus B met. station 70.4668 �156.9389 9.25 8.40 �0.73 23.21 23.94 5.13

Reindeer Camp lakesL300-N (30cm) 69.9654 �156.5431 13.16 12.78 7.97 17.65 9.68 2.13L300-N (350cm) 12.90 12.58 8.64 17.27 8.63 2.09L300-N (700cm) 12.50 12.30 8.00 16.80 8.80 1.89L300-S (30cm) 69.9549 �156.5463 13.00 12.49 8.92 17.25 8.33 2.20L300-S (140cm) 12.90 12.41 8.74 17.08 8.34 2.18L301 (80cm) 70.0437 �156.9105 12.31 11.92 5.66 19.38 13.72 3.20L302 (30cm) 70.0487 �156.8001 12.75 12.27 6.89 19.58 12.69 2.97L302 (100cm) 12.36 12.01 6.67 17.95 11.28 2.79L306 (30cm) 69.9964 �156.5294 12.99 12.41 8.05 18.58 10.54 2.51L306 (170cm) 12.91 12.40 8.08 17.76 9.68 2.47L308 (30cm) 69.9864 �156.4252 12.60 12.17 7.17 17.20 10.03 2.12L308 (350cm) 12.25 12.05 6.79 16.37 9.58 1.88L308 (700cm) 11.67 11.40 6.10 16.20 10.10 1.72L309 (30cm) 70.0244 �156.5671 12.90 12.41 8.44 18.30 9.85 2.46L310 (30cm) 70.0144 �156.7026 12.76 12.20 7.85 19.60 11.76 2.63L310 (180cm) 12.57 12.01 7.28 17.57 10.29 2.54L311 (30cm) 69.9955 �156.6895 12.53 12.12 7.97 17.18 9.21 1.93L311 (230cm) 11.79 11.60 7.30 15.40 8.10 1.64L312 (30cm) 69.9533 �156.6389 12.66 12.22 8.00 17.89 9.90 2.18L312 (230cm) 12.42 12.01 7.68 16.52 8.84 2.06Focus C met. station 69.9984 �156.5504 10.38 9.41 0.05 24.82 24.77 5.32

K. M. Hinkel et al.

Copyright © 2012 John Wiley & Sons, Ltd. Permafrost and Periglac. Process., (2012)


similar and fluctuated synchronously throughout most ofthe summer (Figure 2). Lake L201, which is 65 km furthernorth near Atqasuk, shows a similar pattern but has a laterice-off date and remains slightly cooler across most of therecord. In all lakes, water temperature fluctuations corre-spond closely with variations in mean daily air temperatureas demonstrated in Figure 3. As shown in Table 1, themean air temperature of the nearest meteorological stationis 0.5 to 2.8�C cooler than the near-surface lake watertemperature for the period of record. In the relatively flatand treeless ACP, the strong winds are largely unimpeded,which promotes effective heat transfer across the lakesurface-air interface as well as strong mechanical mixingof the water column.Clear, calm warming events in late June, and around

9 July and 16 August, are apparent in both air and watertemperature records, as are periods of rapid cooling. Apersistently warm 3-week period occurred in late July andearly August, and elevated air temperatures across theregion caused water temperatures to rise to their seasonalmaximum. Following this seasonal peak, all traces show arapid cooling followed by a second, weaker warm periodin mid-late August.Initial warming of water at the two lakes near coastal

Barrow was delayed nearly a month compared to interiorlakes. The ice cover was present on these lakes untilmid-July, demonstrating the maritime impact at coastallocations. Cooler temperatures and persistent cloud coverand fog inhibit ice-cover ablation and warming of thewater column. Once the lakes are ice-free, water tempera-tures in all lakes along the 150-km transect demonstratesimilar temporal patterns, and vary synchronously inresponse to synoptic events. However, the coastal lakesconsistently remain several degrees cooler than thosefurther inland.

Figure 3 Average daily air temperatures for meteorological sites along the transectSite T4 is a long-term weather station between the Focus A and Focus B (Atqasujournal/ppp


Following ice-off, most lakes show diurnal fluctuationsof roughly 1 to 2�C in response to solar forcing and airtemperature variations. The daily fluctuations are mostpronounced in very shallow interior lakes (e.g. L301) andmost attenuated in coastal lakes around Barrow. In general,the lakes are well mixed and essentially isothermal duringmost of the ice-free period.

Thermal Stratification during Ice Decay

Only the lakes near Barrow have a water temperature recordthat precedes spring ice decay and ice-out. Figure 4 showsthe time series of near-surface and near-bottom watertemperatures for lake L107; the patterns and magnitudesfor nearby L100 are nearly identical. Note that, beginningin the first week of June, the 30- and 130-cm traces beginto diverge from the ice point. Steady warming is observednear the bottom of the water column, reaching a maximumof around 4�C by the middle of July. The upper sensor alsoshows gradual and less pronounced warming at this time,and both traces demonstrate a very low-amplitude diurnalcycle that is more apparent at the upper sensor. After severalweeks, there was a 2�C temperature difference.

Time-lapse photographs of lake L107 (Lyons et al.,2010) show that trace divergence corresponds to the forma-tion of a near-shore ice-free zone (i.e. a ‘moat’). Since waterreaches maximum density at around 4�C, it is possible thatwater warmed in the shallow moat sank and flowed beneaththe ice slab towards the deeper basin centre. We canenvision a layer of warmer, denser water at the base of theice-covered water column that gradually warms toward4�C as it mixes with cooler resident water, with the warminglayer progressively thickening. Vigorous ice-pan move-ment began on 4 July, and is associated with increased

, arranged from north (Barrow, Focus A) to south (Reindeer Camp, Focus C).k) sites. This figure is available in colour online at wileyonlinelibrary.com/




Figure 4 Water temperatures for L107 near Barrow using hourly data showing warmer temperatures at the base of the water column during ice cover decay.Ice meltout occurred in mid-July.

K. M. Hinkel et al.

fluctuations in the 30-cm trace. After the near-completedecay of the ice cover around 12 July, mechanical mixingby wind destroyed the thermal stratification, with the tem-perature traces remaining nearly identical for the rest ofthe summer record. Similar under-ice advective mecha-nisms have been proposed by Bilello (1968) and Hill(1967) to explain anomalously warm temperatures beneathan ablating ice cover, and Burn (2005) also notes the possi-bility that warmer, denser water originates on the littoralshelf of thermokarst lakes. Measurements by Hill (1967)indicated that density stratification was induced by solute-enriched water, which overwhelmed the temperature effecton density.Other studies have argued for the potentially significant

role of radiational heating of the lake bed in affectingunder-ice water temperatures. Such effects are generallylimited to periods when the ice slab is thin, snow islargely absent and solar radiation is sufficient. In a studyof thermokarst lakes near Barrow, Brewer (1958)observed a rapid increase in the water column temperatureimmediately after ice-over in September. At the time, thewater column was isothermal near 0�C. Warming of 2�Cover 11 days in late September was followed by a gradualdecline back to pre-warming levels. Eliminating heat flowfrom the basal sediments as inadequate to cause rapidwarming of the water column, and rejecting biogenicheating from microbial action, Brewer concluded thatsolar radiation was transmitted through the relatively thinice slab and absorbed by the water column during thisnear-equinox period. Warming was terminated when snowaccumulated on the ice, lending credence to the radiativeexplanation. Williams (1969) observed similar warmingin the springtime immediately beneath the ice cover of atemperate lake and invoked a comparable explanation.Burn (2005) also noted post ice-over warming of bottomwater temperatures by 1–2�C in the central basins oftwo lakes in the Mackenzie Delta area, but he attributed


this to upward conductive heat flow from warmer bottomsediments.

These processes are probably widespread in thermokarstlakes in spring as the basal sediments warm from absorbedsolar radiation. The time-lapse photographs show that theice was snow covered during the breakup period, but after4 July most of the snow had melted and the ice wasconsiderably darker. The 130-cm time series (Figure 4)shows a very slight (0.1 to 0.2�C) diurnal fluctuation intemperature during this period that increases in magnitudeas the snow melts and the ice thins, suggesting an influencefrom solar forcing.

Whatever the causative mechanism, warming at thebase of the water column is likely to occur beneath theice of most shallow lakes. Without a spatially extensivenetwork of sensor strings arrayed across the lake, it isdifficult to identify the precise mechanism since thespatial and temporal dynamics of the process are largelyunknown. This example demonstrates that it is unrealisticto assume that the water column is responding only tosimple conductive processes during the prolonged icedecay period (e.g. Zhou and Huang, 2004), since warmerwater at depth will impact not only the rate of basal icemelt, but also the magnitude of the heat flux into thebasal sediments.

Vertical Temperature Profiles during Summer

The majority of lakes monitored in 2010 show a consistentvertical and temporal pattern of water temperature; thisincludes L201, L302, L306, L310 and L311. Lake L201 isshown as an example in Figure 5 since three sensors weredeployed at depths of 30, 120 and 240 cm. Note that thetemperatures at all depths are very similar for this lake,and that they fluctuate rapidly and synchronously. Bothdiurnal cycles and synoptic-scale events are apparent inthe three traces. Over the entire period of record, the mean


Figure 5 Water temperature time series for L201 near Atqasuk using hourly data from depths of 30, 120 and 240 cm. Except during periods of warming, thethree traces are nearly identical. This figure is available in colour online at wileyonlinelibrary.com/journal/ppp


temperatures were 11.43, 11.25 and 11.06�C at depths of30, 120 and 240 cm, respectively. This indicates, on aver-age, a weak vertical temperature gradient of ~ 0.2�C/m.However, closer examination of the traces demonstratesnearly identical temperature throughout the water columnmost of the time; it is only during episodic events thatthermal stratification develops. This is especially noticeablein all records during a prolonged warming period in thelatter half of July and the first week of August. The wateris steadily warming during this period, and diurnal cyclesare apparent. This suggests relatively calm and sunnyconditions during this time, which was verified by datafrom nearby meteorological stations along the transect.During late July, maximum temperature differences acrossthe 2.5-m deep water column in lake L201 were around2�C. Similar events occur earlier and later in the record,but are of reduced duration and magnitude. In all cases,however, they are associated with warming of the watercolumn. These patterns were observed for all interior lakeswhere depths ranged from 1 to 3 m.The impact of these events is reflected in the mean water

temperature and gradients noted above. Table 1 lists theaverage temperatures for all sensors for the lakes instrumen-ted in 2010. Because the record length varies somewhat, therecord duration was standardised to a common 57-dayperiod from 27 June to 22 August.The 200 series lakes near Atqasuk have an average

surface temperature around 11�C, while those of the 300series, 65 km further south, averaged around 13�C. Thesedata show a consistent pattern of reduced average meantemperature with depth. Excluding measurements from thedeep pools, lakes that are 1.0–3.5 m deep have watertemperatures that are 0.1 to 0.5�C cooler near the base ofthe water column during this 57-day period in summer.Further, we note that the coastal lakes (100 series) deviatefrom this pattern. Average near-surface water temperatures


during this standardised period are around 6.5�C, withnear-bed temperatures being 0.5–0.9�C warmer due to theeffects of the persistent ice cover as discussed previously.

Vertical Temperature Patterns in Lakes with Deep Pools

Several lakes (L300, L308, L311 and L312) had deep poolsnear actively eroding bluffs (Hinkel et al., 2012b). In two ofthese lakes (L300 and L308), a three-sensor thermistorstring was deployed in the pool. Figure 6 shows temperaturetime series for L308, with sensors at 30, 350 and 700 cm.For most of the record, the temperatures at all three depthstrack closely, with the upper two traces being nearlyidentical. Again, however, there are warming periods whenstratification occurs (S in Figure 6). This is most noticeableduring the period mid-July to 5 August (stratification eventsS6–S8). While the near-surface water quickly warmedto ~ 12–16�C in a matter of days, the temperature increasedto only 13�C at 350 cm and actually declined to ~ 11�C atthe 700-cm depth. Although the middle part of the watercolumn gradually warmed, the water was actually coolingnear the lake bed. This was the most significant stratificationevent that occurred in all the records for 2010. A mixingevent (M in Figure 6) occurred on 1 August that affectedthe entire water column and resulted in isothermal condi-tions down to at least 3.5 m; temperatures at the 7-m depthalso warmed rapidly, but not enough to result in completemixing. This mixing event was generated by strong winds.During the week prior to the event, hourly wind velocitiesranged from 1–3 m/s as measured at the nearby ReindeerCamp meteorological station. On 1 August and again on3 August, seasonal-high wind velocities of 8 m/s and 10 m/swere recorded, respectively. The stronger wind event on3 August completely mixed the 7-m water column, leadingto an isothermal condition. Although the water in the deeppool again began to cool for several days during a warm and



Figure 6 Hourly water temperatures for L308 from depths of 30, 350 and 700 cm (deep pool), and hourly wind speeds measured at the nearby Reindeer Campmeteorological station. M and S refer to water mixing and stratification events, respectively.

K. M. Hinkel et al.

calm period, strengthening winds and cooling temperaturesbegan on 4 August and persisted until 11 August. During thisfinal mixing event, the air temperature dropped from16 to 2�C, winds ranged from 3–6 m/s and the entire watercolumn cooled from 16 to 10�C.The temperature patterns at the base of the water column

in these pools are unusual. During the three strong mixingevents, the deep water initially warmed rapidly, but thengradually cooled – even as the overlying water columnwas warming. A number of these deep-pool stratificationevents occurred during the summer. They are of varyingduration and magnitude, with events S6–S8 being the mostwell defined. The cooling rates in the lower water columnare typically around 0.01–0.05�C/h, which is equivalent toan outward heat flux of roughly 12–58 W/m2 for a 1-m layerof water. The cooling rates also appear to be independent ofthe rate at which water is warming in the upper watercolumn. Event S6 is of the longest duration (~ 7 days),and is characterised by exponential cooling towards anequilibrium temperature of ~ 11�C. Although the overallrate of cooling during this entire period is 0.009�C/h (i.e.~ 10 W/m2 per metre of water), the rate was initially muchhigher at ~ 0.025�C/h during the first 24 h.This pattern of exponential cooling rates in deeper water

suggests that these pools are largely sheltered from theeffects of waves and currents under calm conditions, andthat the water is losing heat to colder surroundings througha Newtonian cooling process (i.e. one in which the rate ofheat loss is proportional to the temperature gradient betweenthe deep water and a colder heat sink). The only candidatefor such a colder surrounding is the lake bed sediment. Heatloss to a lake bed that is at a lower and relatively constanttemperature would result in the exponential coolingobserved in the record. These pools are significant localdepressions in the lake bed where water is typicallydecoupled from the water above except during windy


events; the deep pools are likely associated with ablatingice-wedge troughs (Hinkel et al., 2012b).

The pattern in L300 is very similar to that described forL308, although there is slightly less thermal stratificationat all three sensor levels (30, 350 and 700 cm) during warm-ing periods. This is also apparent from the summer meanvertical temperature gradients shown in Table 1. In the finalweek of July, temperatures in the upper part of the watercolumn were 16–18�C, while they were 12–14�C at thebase of the deep pool. Further, during this period watertemperatures gradually warmed, and there is a 0.5�C diurnalrange apparent in the record. Stratification was destroyed on1 August, and the isothermal water column cooled to 10�Cby 9 August.

Spatial Pattern of Near-Surface Water Temperaturein L300

In L300 (focus lake C), two sensor strings were deployed:one at the north end (L300-N) over a deep pool, as discussedabove, and one at the south end (L300-S). From Table 1, itcan be seen that the average and median temperatures weresomewhat cooler (0.2�C) at the southern site over thestandardised period.

Lake bathymetric data were collected for L300 on24 June 2010 using a GPS-enabled sonar unit (Lowrance/Eagle SeaCharter 502cDF iGPS) to record water depth,water temperature and location (see Hinkel et al., 2012b,for details). Because weather conditions were favourable,the entire lake was surveyed in about 2 h during earlyafternoon. As shown in Figure 7, the sonar track starts nearthe shore and spirals progressively inward so as to createnested ellipses. The zigzag pattern was used where deeppools were observed in the northern part of the lake. Duringthis data collection period, winds were from the NE and


Figure 7 Near-surface (30 cm) water temperatures at L300 (Focus C) col-lected by a sonar unit over a 2-h period on 24 June 2010; the black tracesare sonar tracks. Note location of the instrument strings at both ends ofthe lake. This figure is available in colour online at wileyonlinelibrary.com/journal/ppp


NNE at velocities of around 5 m/s, and the air temperatureranged from 10 to 11�C. Winds had driven a very smallamount of remnant candle ice onto the southwestern shore,which melted by the end of the day. At the base of the bluff,a late-lying snow bank was releasing meltwater into thenortheastern corner of the lake.Near-surface temperature data (n = 7336) from the sonar

track were interpolated using a natural neighbour algorithm,and are mapped as an isothermal surface in Figure 7. On thisday, there was pronounced spatial pattern in surface temper-ature, with a 2–3�C temperature difference among differentportions of the lake. Cooler temperatures were found in thesouthern, downwind side of the lake, with warmest tempera-tures found near the sheltered northern and shallow easternshores. Although some of the variation might have beencaused by diurnal heating over the 2-h collection period,the overall pattern is consistent with the data in Table 1.The temperature difference between the northern and

southern instrument sites (30-cm depth) does not remainconstant over the period of record (Figure 8). If delta T iscalculated as (Tnorthern site minus Tsouthern site) on an hourlybasis, the resulting temperature difference varies between2.10�C (northern site warmer) and �1.74�C (southern sitewarmer) over the period of record. The data show strongtemporal autocorrelation; there are multi-day periods when

Figure 8 Time series of hourly temperature difference between northe


the northern site is consistently warmer for 2–7 days. Further,diurnal cycles are apparent early in the record (Figure 8).

In summer 2010, the wind was from the northern quadrant(315� to 45�) 39 per cent of the time, southerly 26 per cent ofthe time, and from the west and east 18 per cent and 17 percent, respectively. Analysis of the data does not reveal signif-icant causal correlation between delta T and wind speed, winddirection or solar radiation.

CONCLUSIONS

Based on the variety of lakes that was monitored in the threestudy areas, we make the following conclusions.

Analysis of temperature time series from two lakes nearthe Arctic coast indicates warmer (1–4�C) water near thelake bed during the period of ice-cover decay in spring, withnear-freezing temperatures immediately beneath the ice.This is likely driven by a combination of factors, includingpenetration of solar radiation through lake ice and absorp-tion by bottom sediments, as well as differential warmingin the shallower littoral zone. We hypothesise that thelower albedo and preferential warming of the near-shoreice-free moat lead to higher density water at temperaturesnear 4�C, which sinks and flows laterally beneath theice slab towards the basin centre. There, it mixes withresident water and causes gradual warming at the baseof the water column beneath the ice. After 3–4 weeks,the ice slab decays, and the water is mixed by winds. Itis likely that under-ice warming during the decay seasonis typical of thermokarst lakes that do not freeze to thebottom in winter.

Following ice-off, all lakes demonstrate rapid warming inresponse to abundant solar radiation. In accordance withregional climatic gradients, inland lakes to the south meltout2–4 weeks earlier and experience warmer (by ~ 6�C)summer water temperatures than coastal lakes affectedby cool, cloudy maritime conditions. All lakes along the150-km transect respond synchronously to synoptic weathervariations, and mimic the pattern of air temperature.

Vertical temperature profiles indicate that the lakes arewell mixed throughout most of the summer. On averageduring a 2-month period, the mean bottom water tempera-ture in lakes 1.0 to 3.5 m deep are 0.1 to 0.5�C cooler thannear-surface water, indicating weak stratification. Most of

rn and southern instrument sensors at a depth of 30 cm on L300.




K. M. Hinkel et al.

the time, however, the lakes are isothermal. During calmand sunny periods that lasted from hours to days, strongerthermal stratification of up to 2�C can occur. Strengtheningwinds quickly destroy stratification in these shallow andexposed lakes on the relatively flat, treeless tundra.Measurements from the deep pools of lakes in the

ACP-Arctic Foothills transition zone show periods of gradualcooling at the base of the water column, with concurrentwarming above. These events persist for varying durationslasting from hours to days, and occur during relatively calm,sunny periods. The data suggest that basal water is coolingby conduction to the colder sediments below, even while theupper water column warms through radiative heating. Weenvision deep, sheltered pools associated with melting icewedges beneath the lake bed where the water is largelydecoupled from convective and wind-induced mixing duringcalm, warm periods.Spatially extensive, near-surface temperature data for one

lake (L300) were collected over a short period during abathymetric survey. The resulting map shows considerablevariation in temperature across the lake, with warmer(2–3�C) temperatures on the upwind, sheltered end of thelake. Time series of intra-lake near-surface temperaturemeasurements demonstrate that temperature differences ofup to 2�C are common in the summertime, indicating thatassumptions of spatially uniform lake surface temperaturesare not always realistic.


The data collected in this network provide informationand insights that are useful for extrapolation to otherthermokarst lakes in the region. Near-surface watertemperature measurements can be used to accuratelycalibrate high-resolution satellite thermal bands by usingsmall pixels nearest the sensor string, and to develop classi-fication methods to characterise lakes according to tempera-ture patterns. The results presented here demonstratethe utility of collecting baseline data from lakes of varyingarea, depth and basin morphology, and across climaticgradients. Currently, a long-term effort is underway tomonitor year-round conditions in these and other lakes inArctic Alaska as part of National Science Foundation’s(NSF) Arctic Observing Network.

ACKNOWLEDGEMENTS

This work was supported by grants from the NSF (KMH:1107607, 0713813; JDL: 0713822; YS: 0013903; WRE:0548846). Any opinions, findings, conclusions or recom-mendations expressed in this material are those of theauthors and do not necessarily represent the views of theNSF. We are grateful for the logistical support of BarrowArctic Science Consortium, CH2MHill Polar Services andthe Ukpeagvik Inupiat Corporation, and for the helpfulremarks from the reviewers and Editor.

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