Spatial and temporal variability of lake ontogeny in south-western Greenland

Quaternary Science Reviews 126 (2015) 1e16

Contents lists available at ScienceDirect

Quaternary Science Reviews journal homepage: www.elsevier.com/locate/quascirev

Spatial and temporal variability of lake ontogeny in south-western Greenland A.C. Law a, *, N.J. Anderson a, S. McGowan b, c a

Department of Geography, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK School of Geography, University of Nottingham, University Park, Nottingham NG7 2RD, UK c School of Geography, University of Nottingham Malaysia Campus, 43500 Semenyih, Selangor Darul Ehsan, Malaysia b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 September 2014 Received in revised form 20 July 2015 Accepted 6 August 2015 Available online xxx

Holocene palaeolimnological records of diatoms and b carotene (a proxy for aquatic production) from four lakes in the low Arctic region of south-western Greenland were used to investigate the role of climate on lake ontogeny. Two of the lakes are located in the maritime, coastal region near Sisimiut and two inland close to the head of Kangerlussuaq fjord, where there is a more continental climate. Diatom records from the four lakes (AT1, AT4, SS1381, SS8) had similar long-term ontogeny trends, independent of climatic setting and the changes are interpreted as responses to first order weathering controls on catchment/lake chemistry. Short-term excursions from these broad trends occurred in one coastal site (AT4) caused by intense erosion of the steep catchment, and at inland sites where temporary hydrological closure and lake level decline occurred during the mid-Holocene (~8000 e 5000 cal a BP). Algal production (as b carotene) was more closely and consistently correlated with climatic changes; it peaked during the mid-Holocene, the warmest period of the Holocene, at all sites and there were transient increases in production in inland lakes during the Medieval Climate Anomaly and Little Ice Age because of fertilization through increased aeolian dust deposition. A synthesis of seven palaeolimnological records from this region identified that only the mid-Holocene was correlated with diatom stratigraphic zones and there was considerable among-site variability in later Holocene lake response to climate forcing in this area. Comparable long-term trends in species assemblage turnover (DCA/CA axis 1 scores) clearly demonstrate that lakes have predictable ontogeny trends in this region, characterised by maximum alkalinity and nutrient availability in the first few millennia followed by progressive oligotrophication and alkalinity loss. However, individual lake and catchment characteristics (lake morphology, catchment geomorphology), when modified by climatic change (vegetation cover, erosion, weathering rates, aeolian dust deposition, lake level) can diverge from this ontogeny template leading to complex ecological transitions in lakes from this region. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Low Arctic Greenland Holocene Diatom Ontogeny Climatic variability

1. Introduction The direction and pace of ecological change in lakes is driven by a hierarchy of environmental controls which can change over time, thus interpretation of biotic proxies is complex (Fritz and Anderson, 2013). Climate may influence lake ecology directly or through a range of catchment-mediated processes which “filter” the ecological response (Battarbee, 2000; Leavitt et al., 2009). Direct climate effects include modifying the duration of ice-cover and thermal

* Corresponding author. Present address: Geography, Geology and the Environment, William Smith Building, Keele University, Keele, Staffordshire ST5 5BG, UK. E-mail address: [email protected] (A.C. Law). http://dx.doi.org/10.1016/j.quascirev.2015.08.005 0277-3791/© 2015 Elsevier Ltd. All rights reserved.

stratification through changes in temperature and influencing hydrological budgets and nutrient delivery. Climate may also alter lake catchment processes and thus indirectly influence lake ecological structure through modification of vegetation cover, and weathering or microbial activity in soils in turn influences the delivery of sediments and solutes into lakes (Fritz and Anderson, 2013). Lakes located in previously glaciated regions are thought to undergo a predictable trajectory of ecological development, controlled by soil and vegetation succession following deglaciation, in a process termed ontogeny (Battarbee, 1991; Engstom et al., 2000; Fritz and Anderson, 2013). Many lake records from these regions demonstrate ontogeny trends characterised by high alkalinity and production in the first few millennia followed by

2

A.C. Law et al. / Quaternary Science Reviews 126 (2015) 1e16

progressive oligotrophication (Engstrom et al., 2000; Boyle, 2007; Wilson et al., 2012; Boyle et al., 2013). These time-dependant processes determine lake water chemistry, biological production and therefore ecological structure. Lake ontogeny, or successional change in lakes, therefore results in a baseline of change upon which other changes of palaeoclimatological or palaeoecological interest may be superimposed (Perren et al., 2012; Fritz and Anderson, 2013). Remote Arctic sites offer a rare opportunity where lake ontogeny trajectories can be examined in the absence of significant anthropogenic disturbance. Palaeolimnological evidence from the freshwater, open basin inland lakes of south-western Greenland indicates that production and alkalinity were highest during the early Holocene (prior to 8000 cal a BP) and then declined as the lakes underwent oligotrophication from ~6000 cal a BP at the coast and ~6500 cal a BP at the inland lakes (Fredskild, 1983; Anderson et al., 2008). The early stages (before 8000 cal a BP) were controlled by climatic amelioration and climate-independent processes set in motion by glacial retreat. However, the onset of oligotrophication was determined by time-dependent edaphic transitions (e.g., soil and vegetation development) in the catchment (Fredskild, 1983; Anderson et al., 2008; Heggen et al., 2010). Perren et al. (2012) proposed that regardless of lake age and climatic setting, freshwater, open basin lakes in south-western Greenland will demonstrate systematic ecological transitions as a result of time-dependent ontogeny processes. Lakes will also exhibit individual ecological histories depending upon how they are influenced by the environmental characteristics of their location (e.g., aeolian activity and effective precipitation). In contrast, the closed basin and sub-saline lakes of the inland region are hydrologically connected with the regional climate, these lake system demonstrated large-scale ecological, chemical and sedimentary changes in relation to periods of variable effective precipitation during the Holocene (McGowan et al., 2003; Anderson and Leng, 2004; Aebly and Fritz, 2009). More data are required to understand the controls on lake ecological development and ontogeny under different climatic and catchment settings, and in response to Holocene climatic variability. Small Arctic lakes are considered to be particularly sensitive to climatic perturbations, therefore their sediment records are widely used for climate reconstruction using biological proxies (Douglas and Smol, 1994; Rouse et al., 1997). Fossil diatoms (Bacillariophyceae) are the most commonly used proxy in palaeolimnological and palaeoclimatological research because of their sensitivity to a range of environmental parameters (e.g., pH, icecover, temperature, nutrients, light and lake depth), and their siliceous cell walls are well preserved in lake sediment records. Sedimentary pigments are also valuable proxies for investigating palaeoclimate and palaeoenvironmental and palaeoecological change. Phototrophs (e.g. algae, phototrophic bacteria, aquatic plants and terrestrial plants) use pigments as a light capturing agent for photosynthesis (e.g. chlorophylls, carotenoids and their derivatives) (Carpenter et al., 1988; Leavitt and Carpenter, 1989). Some pigments are taxonomically specific while others are ubiquitous in most algal groups therefore enabling reconstruction of past primary production (b carotene), food web dynamics and lake ecological response to climate forcing (Leavitt and Hodgeson, 2001). Adequate interpretations of lake sediment records, in terms of climate, therefore require a thorough understanding of ontogenetic change, and how it may vary among different types of lakes. A simple way to determine the reliability of biological proxies in lake sediments as palaeoclimatic records is to use multiple sites to determine among-lake variability. This paper uses multiple sites to investigate the role of lake setting and Holocene climatic variability

(the thermal maximum of the mid-Holocene, the Neoglacial cooling of the late-Holocene and the last ~1500 cal a BP including the Medieval Climate Anomaly and the Little Ice Age) on lake ecological trajectories and ontogeny. We test the hypotheses that timedependant lake ontogeny overrides local climatic factors and therefore the ecological trends of the lakes will be independent of climatic setting and Holocene climatic forcing. In order to test these hypotheses we studied two lakes from the continental inland region and two from the maritime, coastal region of south-western Greenland. We used diatom stratigraphies and b carotene (a measure of aquatic production) from four freshwater lakes to determine how lake development differed under contrasting climatic settings. 2. Regional setting 2.1. Study sites The four study sites are located in the widest ice-free area (~150 km) of south-western Greenland (66
N and 68
N) which stretches from the ice-margin to the coast (Fig. 1). There are approximately 20,000 lakes in this region (Anderson et al., 2001) which formed following deglaciation and retreat of the ice-margin from the coast at ~11,000 cal a BP and later from the inland sites at ~8000 cal a BP (van Tatenhove et al., 1996; Wagner and Bennike, 2012). Consequently, the coastal study lakes are approximately ~2000 years older (AT1 formed at 10,050 cal a BP and AT4 at 10,300 cal a BP) than the inland sites (SS8 formed at 8300 cal a BP, and SS1381 at 8400 cal a. BP) located close to the head of the fjord at Kangerlussuaq (Fig. 1). The region is characterised by a low Arctic climate gradient, with a maritime, cool and wet coast and a continental, arid and warmer interior. Inland, average annual precipitation is ~150 mm a1, half as much as received at the coast (>300 mm a1), MAT is 6
C and the annual temperature range is 29
C (18 to 10.5
C). The coastal region experiences a smaller temperature range of 19
C (12
C to 7
C) and a MAT of 3.7
C (Hasholt and Søgaard, 1978). The limnology of lakes in south-western Greenland is controlled by the regional climate and their morphometry (Anderson et al., 2001; McGowan et al., 2003; Anderson and Leng, 2004). Inland there is a negative evaporation: precipitation ratio, which causes evaporative enrichment and results in relatively more productive lakes with elevated conductivity. Conversely, the coastal lakes are more dilute and oligotrophic because of the wetter, cooler coastal climate and higher rates of hydrological flushing (Hogan et al., 2014) and allochthonous DOC input (Anderson and Stedmon, 2007). The vegetation reflects the regional climate differences. Arctic heath communities such as Ericaceae, Empetrum spp. and the dwarf shrub Salix spp. (e.g S. glauca and S. herbacea) dominate the vege€ cher, 1949; Leng and Anderson, 2003). Inland, tation at the coast (Bo the vegetation is characterised by Arctic dwarf shrub tundra including Betula nana, S. glauca, Ledum palustre, Dryas integrifolia, Vaccinium spp., Empetrum nigrum, herbs, grasses and mosses. South facing slopes have less vegetation due to water stress (Eisner et al., 1995; Anderson et al., 2001). The regional geology comprises Archaean gneiss and Quaternary till deposits (Jensen et al., 2002). Freeze-thaw weathering has produced large quantities of weathered bedrock at the coast. The close proximity to the ice-margin means that the inland area is affected by aeolian activity caused by the deflation of the ice-marginal sandur (Willemse and €rnqvist, 1999; Willemse et al., 2003). To The two inland lakes are located approximately 10 km west north-west of the airport at Kangerlussuaq at the head of Sondre Stromfjord (Fig. 1; see Anderson et al., 2012 for photographs). SS8 (67
0.666 N, 51
04.4410 W) has a small catchment to lake ratio

A.C. Law et al. / Quaternary Science Reviews 126 (2015) 1e16

3

Fig. 1. Location of the study region in south western Greenland and the four study lakes (black squares), the location of lakes SS49, SS16 and SS32 from Perren et al. (2012) and SS2 from Anderson et al. (2008) are also shown (white squares).

(13.3), one outflow and one inflow maintained by a small pond. Despite being a shallow lake (10 m), SS8 stratifies weakly; the bottom waters are anoxic during the winter under ice. SS1381 (67
00.8550 N, 51
07.1720 W) has three inflows and one outflow and stratifies strongly in the summer with an anoxic hypolimnion. Both inland lakes are freshwater, oligotrophic (TP < 10 mg L1) with slightly elevated conductivities (>350 ms cm1) and alkalinities (pH > 8) compared to the coastal lakes. The two coastal lakes are situated ~10 km of Sisimuit (Fig. 1). AT1 (66
58.051 N, 53
24.095 W) is a small (11 ha), deep (~18 m) lake with a large catchment area to lake ratio (13.6) and a heavily weathered catchment located at 475 m.a.s.l. AT1 is drained by one outflow, which is active only in the early summer months, and has numerous inflows from higher areas in the catchment and perennial snowbanks (see Anderson et al., 2012 for photographs and further details). AT4 (66
57.899 N, 53
29.9280 W) is located at 200 m.a.s.l, has a low catchment area to lake ratio (7.7) and is ~18 m deep. AT4 is drained by one active outflow and is supplied by numerous inflows from the higher areas of the catchment and perennial snowbanks (see Anderson et al., 2012 for photographs). The two lakes are dilute (conductivities <50 mS cm1), oliogtrophic (TP < 10 mg L1) and circumneutral (pH between 7 and 7.5). DOC concentration in the two lakes is low (<10 mg L1) because of high precipitation and catchment runoff at the coast. The spectral characteristics of the DOC suggest that it is produced allochthonously (Anderson and Stedmon, 2007). AT4 stratifies during the summer with minimal hypolimnetic oxygen depletion and AT1 stratifies weakly and intermittently.

2009; Bennike et al., 2010). At the coast, macrofossil and geochemical evidence suggests that the mid-Holocene was warmer and drier from ~10,000 until ~5500 cal a BP (Anderson and Leng 2004; Wagner and Bennike, 2012). Inland, colder, wetter conditions of the late-Holocene (4000e0 cal a BP), or Neoglacial period, (Fredskild, 1983; Anderson et al., 1999; McGowan et al., 2003; Aebly and Fritz, 2009) followed the mid-Holocene period. The combined evidence suggests that the Neoglacial was preceded by a pluvial period at 4600 cal a BP and that Neoglacial cooling proper started from ~4000 cal a BP (Fredskild, 1983; Anderson et al., 1999), with a second intense pluvial period at ~2000 cal a BP (McGowan et al., 2003; Aebly and Fritz, 2009). At coastal sites, Neoglacial cooling caused significant changes to catchment vegetation, weathering processes and sediment delivery into lakes. However, the dates for the onset of these changes caused by Neoglacial cooling varies between ~5800 and 4500 cal a BP at different lakes (Anderson et al., 2012; Leng et al., 2012; Wagner and Bennike, 2012). During the last ~1500 years, warming which peaked at ~1300 cal a BP (the Medieval Climate Anomaly; MCA), was followed by cooling into the Little Ice Age (LIA) (~500 cal a BP; Dahl-Jensen et al., 1998). Evidence from the inland region suggests that climate was cool, arid and windy during the LIA and was associated with two episodes of extensive € aeolian activity between 1300 and 1100 (Willemse and TOrnqvist, 1999) and at ~ 550 cal a BP (Willemse et al., 2003).

3. Methodology 2.2. Climate history of south-western Greenland

3.1. Sediment coring and correlation

Slightly different dates are published for the onset and termination of the climatically distinct periods of the mid and late Holocene in south-western Greenland. Consequently, using ice-core (Dahl-Jensen et al., 1998) and local radiocarbon-dated lake sediment records (Fredskild, 1983; Anderson et al., 1999; Kaufman et al., 2004), we define the mid-Holocene in south-western Greenland as 8000e5000 cal a BP and the late-Holocene as 4000e0 cal a BP. Evidence from the inland region suggests that the mid-Holocene (8000e5000 cal a BP) was characterised by summer temperatures ~2.5
C warmer than present (peak mid-Holocene warmth between 7000 and 6500 cal a BP) and lower precipitation totals (Fredskild, 1973; Bennike, 2000; Anderson and Leng, 2004; Aebly and Fritz,

Holocene sediment sequences were retrieved from the deepest parts of the lakes during April 2006 using overlapping drives from a 1-m Russian corer at the coastal lakes and a piston corer at the inland lakes. A HON-Kajak corer (9-cm internal diameter; Renberg, 1991) was used to collect the upper sediments and extruded in the field at 1-cm intervals. Drives were correlated based upon distinctive visible laminations confirmed by loss-on-ignition (LOI) profiles to produce a continuous record. The cores were split and subsampled for diatom and pigment analyses. The chronology was derived through radiocarbon dates, with details of the LOI analyses and the Organic Matter Accumulation Rates (OMAR) given in Anderson et al. (2012).

4

A.C. Law et al. / Quaternary Science Reviews 126 (2015) 1e16

3.2. Diatom analyses Sediment samples for diatom analysis were taken at 2-cm (coastal lakes) or 4-cm (inland lakes) intervals. Sample preparation followed Renberg (1990a) due to the large number of samples. Dried aliquots were then mounted onto microscope slides using Naphrax™ resin and 300 diatom valves were counted at 1000x magnification using a Leica DMRE microscope. A variety of general and regional floras were used to identify the diatoms (Krammer and Lange-Bertalot, 1986e1991; Foged, 1953, 1955, 1958, 1972, 1977). The regional floras and contemporary samples taken from the lakes were used to determine the habitat preferences (benthic, planktonic and tychoplanktonic) of diatom species. Dissolution of diatom valves was assessed using a two-scale system (pristine and dissolved cf. Ryves et al., 2001). Statistically significant zones in the diatom data were determined by optimal splitting using the programme Psimpoll 4.27 (Bennett, 2003e2009). 3.3. Pigment analyses Approximately 0.2 g samples of freeze-dried sediment were analysed at all levels of the sediment core (1-cm resolution). Pigments were extracted using a mixture of acetone: methanol: water (80:15:5) at 20
C for 24 h and extracts filtered using a PTFE syringe filter (0.22 mm pore size), dried under N2 gas before dissolution in an acetone and ion-pairing reagent mixture for injection into the high performance liquid chromatography (HPLC) unit (Agilent 1200 Series) (McGowan et al. 2012). Modified versions of the separation methods of Mantoura and Llewellyn (1983) and Chen et al. (2001) were used to process the coastal and inland lake samples, respectively. b carotene was identified based upon absorbance spectra and retention times and concentrations calculated by calibration against commercial standards expressed relative to organic content (from LOI) as nmol g1 organic matter. 3.4. Data analysis Correspondence Analysis (CA) with down-weighting of rare species was used to explore the down-core trends in the diatom assemblages (% abundance) of the four lakes using Canoco version ^ 4.56 (ter Braak and Smilauer 1997). The CA for lakes AT4 and AT1 revealed an arch suggesting that Detrended Correspondence Analysis (DCA) would be more appropriate to explore the down-core trends in the diatom data from these lakes. The long-term trends in axis 1 scores from these lakes are characterised by a transition from more alkaliphilus to acidophilus species (see Section 5.1); suggesting lake alkalinity is strongly associated with axis 1 scores in these lakes. Furthermore, other studies have shown that alkalinity is a strong driver of ecological change in lakes from this region (Ryves et al., 2002; Perren et al., 2009, 2012). Long-term ontogeny trends among our four lakes were compared with equivalent data available from four previously published lakes from the region (lakes SS49, SS16 and SS32; Perren et al. 2012; and SS2; Anderson et al., 2008) using correlation analysis (SPSS) of time series standardized to 100year time intervals with linear interpolation. For this analysis we used the published axis 1 scores (CA/DCA or CCA) and, where available, proxy data for aquatic production (eg., Chl a or b carotene). 4. Results 4.1. Coastal lakes 4.1.1. AT1 The diatom zones and DCA axis 1 scores indicate greater variability in the diatom assemblage record prior to 6300 cal a BP, with

three distinct zones being identified prior to 8500 cal a BP (Fig. 2). The earliest zone AT1-1 (>9950 cal a BP) was dominated by benthic Staurosirella pinnata (~50%) and planktonic Fragilaria tenera (<28%), but in zone AT1-2 (~9950 e 9200 cal a BP) the relative abundance of F. tenera (<71%) and Discostella stelligera complex increased (~30%). There were more benthic diatoms in zone AT1-3 (8400e9200 cal a BP) including Pseudostaurosira pseudoconstruens (<55%) and S. pinnata (~22%), and abundances of planktonic F. tenera declined as D. stelligera complex increased (up to 50%). Zone AT1-4 (8400 e 6300 cal a BP) was dominated by benthic species (Staurosira construens var. constrens, S. pinnata, and P. pseudoconstruens) and D. stelligera decreased gradually whilst Cyclotella rossii complex increased after ~7000 cal a BP to ~7%. In zone AT1-5 (6300e1600 cal a BP) benthic species were still common, with a slightly different assemblage to the previous zone (comprising Pseudostaurosira brevistriata, S. construens var. venter, P. pseudoconstruens and S. pinnata). In the most recent two zones (<1600 cal a BP) there was a diverse benthic assemblage dominated by Fragilarioid taxa. Planktonic Tabellaria flocculosa, Aulacoseira lirata var. alpigena and Aulacoseira lirata first occurred in Zone AT1-6 (after 1600 cal. a BP) whereas planktonic D. stelligera declined during this zone and C. rossii was present in the highest abundances throughout the core (9%). Diatoms in the upper zone, AT1-7, are distinguished by higher percentages of Sellaphora laevissima and P. brevistriata. Maximum concentrations of b carotene (~400 e 20 nmol g1) occurred during zone AT1-4, (8500 e 7500 cal a BP), remained moderately high until 5200 cal a BP and then declined to very low concentrations thereafter (Fig. 2). Benthic diatom assemblages dominated the lake throughout the Holocene, but planktonic species were present in their highest proportions for the first ~4000 years (<48%) and tychoplanktonic diatoms only occurred during the last ~1500 cal a BP (Fig. 2). 4.1.2. AT4 Four diatom zones were identified at lake AT4, and the first two zones (before 7000 cal a BP) were especially distinct from the subsequent zones, as indicated by the higher DCA axis 1 scores (Fig. 3). In the first 600 years of lake development (zone AT4-1; >9700 cal a BP) benthic taxa were overwhelmingly dominant, progressing through a sequence of P. pseudoconstruens (89%), S. construens var. venter (85%) and S. pinnata (95%). These benthic species continued to be common between 9700 and 7000 cal a BP (zone AT4-2) with the addition of Pseudostaurosira parasitica and Fragilaria robusta, and C. rossii complex increased throughout this zone. In zone AT4-3 (7000e4000 cal a BP) benthic species (e.g., P. brevistriata, P. pseudoconstruens, S. construens var. venter and S. pinnata) remained common but the diatom assemblage diversity increased as several other smaller benthic species (e.g., Kolbesia suchlandtii, Psammothidium levanderi and Rossithidium pusillum) first appeared or increased, and C. rossii complex and P. brevistriata became major components of the assemblage (each ~20%). In the last 4000 cal a BP (zone AT4-4) the diatom assemblage was dominated by planktonic taxa (C. rossii complex, D. stelligera complex and Aulacoseira lacustris and Aulacoseira leavissima). The overall pattern is therefore a decline in relative abundance of benthic diatoms from ~100% in the early Holocene to ~66% in recent times. Maximum b carotene concentrations (~90 nmol g1) occurred during zone AT4-2 (9000 e 7000 cal a BP) and then declined to low concentrations until ~700 cal a BP, when they increased again. The lake comprised only benthic diatom assemblages for the first ~1300 years, after which planktonic and tychoplanktonic diatom species increased steadily throughout the Holocene. The maximum abundances of planktonic (<60%) and tychoplanktonic (28%) species occurred during the last ~4000 years.

A.C. Law et al. / Quaternary Science Reviews 126 (2015) 1e16 Fig. 2. The relative abundances of diatoms (>5%) from lake AT1 sorted by weighted averaging. Cyclotella rossii complex total counts include counts for C. comensis, C. rossii and C. ocellata. Discostella stelligera complex includes total counts for D. stelligera and D. pseudostelligera. The benthic: planktonic: tychoplanktonic ratio (%), DCA axis 1 sample scores, b carotene concentration (nmol g1) and diatom zones are also presented.

5

6 A.C. Law et al. / Quaternary Science Reviews 126 (2015) 1e16 Fig. 3. The relative abundances of diatoms (>5%) from lake AT4 sorted by weighted averaging. Cyclotella rossii complex total counts include counts for C. comensis, C. rossii and C. ocellata. Discostella stelligera complex includes total counts for D. stelligera and D. pseudostelligera. The benthic: planktonic: tychoplanktonic ratio (%), DCA axis 1 sample scores, b carotene concentration (nmol g1) and diatom zones are also presented.

4.2. Inland lakes 4.2.1. SS8 Diatom preservation in Lake SS8 was suboptimal during the early part of the record (>5000 cal a BP) and especially poor during the first 600 years (zone SS8-1 >8600 cal a BP; Fig. 4). The CA axis 1 scores indicate that diatom assemblages were most distinct after the 5000 cal a BP transition point (between zones SS8-5 and SS8-6), as indicated by higher scores after this time. Despite poor preservation in zone SS8-1, diatoms were dominated by S. construens var. venter (~70%) and P. pseudoconstruens (~20%) in the upper part of the zone. The benthic diatom assemblage became slightly more diverse during zone SS8-2 (8600e6300 cal a BP), with P. brevistriata (<80%), Pseudostaurosira subsalina (<20%) and Epithemia sorex (<30%). Benthic species remained dominant during zone SS8-3 (6300e5600 cal a BP) when Amphora pediculus was the most abundant species (<80%), but declined to negligible amounts in zone SS8-4 (5600e5000 cal a BP), when Amphora libyca and P. brevistriata were dominant. In zone SS8-5 (5000e1600 cal a BP) the diatom assemblage was more diverse with A. libyca (<55%) and Staurosira construens var. exigua (<38%), and it is the only zone where there are substantial proportions of planktonic species Cyclotella bodanica var. lemanica (<18%). Planktonic species were absent during the last 1600 cal. a BP (zone SS8-6), and benthic species included P. subsalina (<50%), Planothidium conspicuum (up to 100%) and Fragilaria robusta (<40%). b carotene was highest (~950 nmol g1) during zone SS8-2 until ~ 7500 cal a BP, when it declined to low concentrations (Fig. 4). 4.2.2. SS1381 The CA axis 1 scores indicate that most variability in diatom communities occurred prior to 5200 cal a BP, encompassing three diatom zones (Fig. 5). In the first zone SS1381-1 (before 7500 cal a BP) diatom preservation was poor, but assemblages were dominated by C. bodanica aff. lemanica, S. construens var. venter, Navicula cryptocephala and Navicula rynchocephala. Preservation of valves remained poor throughout the short-lived zone SS1381-2 (7500e7100 cal a BP) but there were high relative abundances of A. libyca (<40%), Epithemia adnata (<40%) and P. brevistriata (<20%). During zone SS1381-3 (7200e5200 cal a BP) the diatom assemblage was composed solely of benthic taxa and was more diverse, including A. pediculus (<30%) and Cocceneis placentula var. lineata (<30%). In contrast, planktonic taxa were dominant in zone SS13814 (5200e900 cal a BP) with high relative abundances of D. stelligera complex (<70%), C. bodanica aff. lemanica (<80%), and C. rossii complex (<40%). After 900 cal a BP (zone SS1381-5) C. bodanica aff. lemanica was the only planktonic taxa present in high proportions (<60%), alongside significant quantities of Achnanthidium minutissimum, Encyonopsis microcephala, A. pediculus and S. construens var venter. Maximum b carotene concentrations were observed between ~8000 and 5000 cal a BP (zones SS1381-1 to SS1381-3) and concentrations were lower thereafter. 5. Discussion 5.1. Ecological interpretation of the diatom data 5.1.1. Coastal lake AT1 The presence of planktonic F. tenera and D. stelligera for the first ~1500 years of lake formation suggests nutrient rich and ice-free conditions (Fig. 2). The autoecology of these two species remains undecided (Cremer and Wagner, 2004; Saros and Anderson, 2014), however, D. stelligera and F. tenera have been identified in mesotrophic, relatively nutrient-rich lakes in eastern Greenland (Cremer et al., 2001b; Cremer and Wagner, 2004). Alkaline and nutrient rich

7 Fig. 4. The relative abundances of diatoms (>5%) from lake SS8 sorted by weighted averaging. Discostella stelligera complex includes total counts for D. stelligera and D. pseudostelligera. The CA axis 1 sample scores, b carotene concentration (nmol g1) and diatom zones are also presented. The spotted shading highlights low F-index values (low preservation) and the grey shading indicates where no diatoms were preserved.

A.C. Law et al. / Quaternary Science Reviews 126 (2015) 1e16

8 A.C. Law et al. / Quaternary Science Reviews 126 (2015) 1e16 Fig. 5. The relative abundances of diatoms (>5%) from lake SS1381 sorted by weighted averaging. Cyclotella rossii complex total counts include counts for C. comensis, C. rossii and C. ocellata. Discostella stelligera complex includes total counts for D. stelligera and D. pseudostelligera. The benthic: planktonic: tychoplanktonic ratio (%), CA axis 1 scores, b carotene concentration (nmol g1) and diatom zones are also presented. The spotted shading highlights low F-index values (low preservation).

A.C. Law et al. / Quaternary Science Reviews 126 (2015) 1e16

conditions continued until ~6000 cal a BP, although a possible change in light regime resulted in a more benthic-dominated assemblage (P. pseudoconstruens, S. construens var. constrens and S. construens var. venter). The transition to a more oligotrophic and dilute lake is reflected in a more acidophilus, benthic assemblage (e.g., P. brevistriata) from ~6000 cal a BP. The presence of Aulacoseira species and T. flocculosa indicate more oligotrophic (Joynt and Wolfe, 2001; Cremer and Wagner, 2004; Finkelstein et al., 2014), acidic waters during the last ~1500 years. 5.1.2. Coastal lake AT4 S. pinnata, S. construens var. venter and P. pseudoconstruens characterised the first ~1000 years of lake development at AT4 (Fig. 3). These pioneer species are common in post-glacial lake sequences (Round, 1957; Howarth, 1976; Bradshaw et al., 2000) and are connected with high alkalinity and nutrient availability in Arctic € m et al., 1997). lakes (Fredskild, 1984; Pienitz et al., 1995; Weckstro S. pinnata has been associated with high quantities of minerogenic input and turbid water conditions in south-western Greenland (Perren et al., 2012) and elsewhere in the Arctic (Bouchard et al., 2004; Antoniades et al., 2005). A more diverse assemblage and the occurrence of S. construens var. constrens, which has high alkalinity and temperature optimas (Fredskild, 1984; Pienitz et al., €m et al., 1997), suggests warmer, more eutrophic 1995; Weckstro conditions until ~7000 cal a BP. Circumneutral conditions are inferred by the presence of P. brevistriata from ~7000 cal a BP. The autoecology of C. rossii complex is poorly understood (Saros and Anderson, 2014). Small Cyclotella and Discostella species are found in a range of boreal, arctic and alpine environments today and the ecological factors controlling their abundance is thought to be a complex interaction among DOC, light, thermal stratification and nutrient abundance (Saros et al., 2012, 2013; Saros and Anderson, 2014). However, the existence of C. rossii from ~7000 cal. BP may reflect more oligotrophic waters; this species was also identified during the mid-Holocene at AT1 (Fig. 2) and is found in high abundances in clear, oligotrophic lakes from eastern Greenland (Cremer and Wagner, 2004). Acidophilus Aulacoseira spp. during the last 3000 cal a BP indicate a shift to a more oligotrophic, acidic lake. High abundances of D. stelligera during the last 4000 years are interpreted as the result of Neoglacial catchment deterioration and increased nutrient input from allochthonous sediment inwash (Anderson et al., 2012). D. stelligera was associated with the nutrient rich and alkaline conditions of the early-mid Holocene at AT1 (Fig. 2) and has been found in nutrient and organic enriched waters in eastern Greenland (Cremer et al., 2001b). 5.1.3. Inland lake SS8 Diatom preservation was poor for the first ~5000 years at SS8 (Fig. 4), which may be the result of highly alkaline conditions caused by periods of lake-level lowering and evaporative enrichment in response to the warmer more arid climate during the midHolocene (McGowan et al., 2003; Anderson and Leng, 2004). The diatom assemblage during this period reflects alkaline, nutrientrich conditions (S. pinnata, S. construens var. venter and P. pseudoconstruens, and E. sorex) interspersed with periods of more circumneutral, freshwater episodes (e.g., P. brevistriata). High abundances of A. pediculus, S. construens var. venter and A. libyca between ~6500 and 5000 cal a BP suggest a period of high alkalinity, ion concentration and temperature optima possibly relating to maximum Holocene warmth and lake level lowering (Bennike, 2000; McGowan et al., 2003; Anderson and Leng, 2004; Bennike et al., 2010). Between ~5000 and 2000 cal a BP the diatom assemblage demonstrates that the lake became more dilute and less enriched with nutrients and ions than prior to ~5000 cal a BP. This diatom assemblage was characterised by high abundances of

9

species indicative of highly alkaline/conductive waters, such as A. libyca (optima >2000 mScm1) and P. subsalina (Ryves et al., 2002; McGowan et al., 2003) but also a number of species that reflect more circumneutral and meso-oligotrophic conditions (eg., S. construens var. exigua, P. subsalina and Planothidium conspicuum). The species assemblage shifted during the last ~2000 years, although the species present were still indicative of a weakly alkaline/conductive lake. This change in diatom assemblage reflects aeolian deposition, which introduced nutrients and ions into the lake. 5.1.4. Inland lake SS1381 The first ~1500 years of lake development at SS181 were characterised by high diatom dissolution, high alkalinity and nutrients, and ion-rich, stratified waters (Fig. 5; S. construens var. venter and E. adnata, C. bodanica aff. lemanica). Between ~7000 and 5100 cal a BP a benthic dominated assemblage suggests highly alkaline and nutrient rich waters (e.g., A. pediculus and C. placentula). The absence of C. bodanica aff. lemanica, as a species which thrives in deeper lakes (Saros and Anderson, 2014), during this period may reflect lake level lowering associated with the warmer, more arid conditions of the mid-Holocene (Bennike, 2000; McGowan et al., 2003; Anderson and Leng, 2004; Bennike et al., 2010). During the last ~5000 years, the diatom assemblage was characterised by species indicative of more dilute, meso-oligotrophic, circumneutral water conditions (e.g., D. stelligera and Achnanthidium minutissimum). 5.2. Controls on diatom response e ontogeny Lakes in the boreal and high latitudes commonly become more oligotrophic and acidic as they age (Renberg, 1990b; Engstrom et al., 2000; Bigler et al., 2003). During the immediate phases of deglaciation, nutrient and base rich solutes are washed into the lake from the catchment resulting in elevated water alkalinity (Pearsall, 1921; Crocker and Major, 1955). As the catchment stabilises, edaphic processes such as soil and vegetation development sequester nutrients and base cations which, combined with an increase in allochthonous DOC input, cause a decline in lake water alkalinity, nutrient availability and biological production (Whitehead et al., 1989; Engstrom et al., 2000; Fritz et al., 2004). Typically, the different stages in these long-term ontogeny trends are associated with specific diatom assemblages; Fragilarioid genera (Staurosira, Pseudostaurosira and Staurosirella spp.) dominate during the first two millennia, followed by Cyclotella and Discostella species, and eventually in some lakes, Aulacoseira species. The Fragilaria (sensu latu) > Cyclotella ± Aulacoseira sequence has been observed in many Arctic lake records including East Greenland (Cremer et al., 2001a), the Canadian high Arctic (Michelutti et al., 2007) and recently, in three lakes in south-western Greenland (Perren et al., 2012). Furthermore, over multiple interglacials, lakes will demonstrate the same diatom sequence (Wilson et al., 2012). The repeatability of this diatom sequence has led to the suggestion that it can be used as a template for ontogeny in boreal, high latitude and low Arctic settings (Perren et al., 2012). We analysed the ratio of Fragilaria (sensu latu), Cyclotella and Aulacoseira species in relation to axis 1 scores of the four study lakes and three additional lakes (SS2; Anderson et al., 2008, SS16 and S49; Perren et al., 2012) to establish whether the long-term trends in diatom succession were characteristic of timedependent ontogeny processes (Fig. 6). The long-term trends in CA/DCA/CCA axis 1 scores are comparable between all of the lakes (Fig. 7) and represent a transition from species assemblages with high nutrient and alkalinity tolerances to more oligotrophic and acidophilus assemblages (Figs. 2e5 and Section 5.1). Furthermore,

10

A.C. Law et al. / Quaternary Science Reviews 126 (2015) 1e16

Fig. 6. The ratios of Fragilaria (black shading), Cyclotella (grey shading) and Aulacoseira (white shading) (as % of the total of the three species) from coastal lakes AT1, AT4 and SS49 (Perren et al., 2012) and inland lakes SS8, S1381, SS2 (Anderson et al., 2008) and SS16 (Perren et al., 2012). In addition, diatom axis 1 scores (white line) for coastal lakes AT1 and AT4 (DCA) and SS49 (Perren et al., 2012; CCA) and inland lakes SS8 (CA), SS1381 (CA), SS2 (Anderson et al., 2008; DCA) and SS16 (Perren et al., 2012; CCA) are presented. Diatom dissolution at SS8 prevents the use of the CA axis 1 scores.

all lakes except SS2 and SS49 demonstrate a significant correlation between their CA/DCA/CCA axis one scores and lake production (b carotene) (Table 1). Taken together these results suggest that the diatom assemblages were principally responding to changes in lake water alkalinity and associated nutrient availability, controlled by time-dependant ontogeny processes (for further explanation see Section 5.3). However, the lakes have individual ecological histories; the diatom assemblages that track the long-term decline in lake water alkalinity vary spatially and temporally between the lakes (Fig. 6). Only two of the lakes, AT4 and SS16, demonstrate a transition from a dominance of Fragilaria (sensu latu) > Cyclotella ± Aulacoseira in response to long-term alkalinity loss and oligotrophication (ontogeny) (Fig. 6). The diversity of assemblages present at different stages of lake development highlights how lake characteristics (e.g., lake bathymetry, catchment geomorphology, and climatic setting) interact with ontogeny to produce individual diatom histories. For example, at the inland sites, where the lakes are closely connected to the regional climate, periods of lake level lowering associated with the warmer, drier conditions in the mid-Holocene (Fredskild, 1973; Bennike, 2000; Aebly and Fritz, 2009) caused a temporary dominance or an increase in benthic assemblages (SS1381 and SS2; Fig. 6). There is also evidence at the inland sites (SS8, SS1381 and

SS2; Fig. 6) for periodic expansions of planktonic diatoms related to established pluvial episodes at ~4600 and 2000 cal a BP (McGowan et al., 2003; Anderson and Leng, 2004; Aebly and Fritz, 2009). The coastal lakes (AT1, AT4 and SS49) highlight how Cyclotella species can dominate at different stages of lake development because of climate-induced catchment changes (Fig. 6). At AT1 (Fig. 2) Cyclotella stelligera was present when the lake was alkaline and nutrientrich in the early Holocene and declined as oligotrophication occurred in the mid-Holocene at ~6000 cal a BP. At AT4, the onset of oligotrophication at ~7000 cal a BP witnessed high abundances of Cyclotella rossii complex and an increase in Cyclotella stelligera from ~4000 cal a BP (Fig. 3). The presence of Cyclotella stelligera, which was associated with high alkalinity and nutrients at AT1, in an assemblage indicative of oligotrophication and acidification, is difficult to reconcile (Fig. 2). However, it is suggested that colder, wetter conditions of the Neoglacial (Fredskild, 1983; McGowan et al., 2003; Kaufman et al., 2004) caused vegetation dieback and soil deterioration which, because the catchment is steep, resulted in the inwash of nutrient-rich sediments allowing Cyclotella stelligera to flourish. Conversely, because AT1 has a less steep, higher altitude (475 m.a.s.l) catchment than AT4, Neoglacial cooling resulted in the development of permafrost. The effect of this climate-induced catchment change was clearer water, and reduced hydrological

A.C. Law et al. / Quaternary Science Reviews 126 (2015) 1e16

11

Fig. 7. The diatom axis 1 scores for coastal lakes AT1 and AT4 (DCA) and SS49 (Perren et al., 2012; CCA). Also presented are the axis 1 scores inland lakes SS1381 (CA), SS2 (Anderson et al., 2008; DCA) and SS16 (Perren et al., 2012; CCA).

Table 1 The correlations between the biological proxies at the four study lakes and lakes SS49, SS16, SS32 (Perren et al., 2012) and SS2 (Anderson et al., 2008).

AT1 b carotene AT4 DCA 1 AT4 b carotene SS49 DCA 1 SS49 Chl a SS8 CA 1 SS8 b carotene SS138 CA 1 SS1381 b carotene SS2 DCA 1 SS2 b carotene SS16 CCA 1 SS32 DCA 1

AT1 DCA 1

AT1 b carotene

AT4 DCA 1

b carotene

SS49 DCA 1

SS49 Chl a

SS8 CA 1

b carotene

SS138 CA 1

b carotene

SS2 DCA 1

b carotene

SS16 CCA 1

0.65* 0.88* 0.56* 0.90* 0.15 0.46* 0.48* 0.78* 0.87* 0.93* 0.08 0.76* 0.48*

0.69* 0.39* 0.60* 0.11 0.40* 0.31* 0.65* 0.81* 0.74* 0.14 0.67* 0.37*

0.56* 0.84* 0.12 0.56* 0.29* 0.63* 0.76* 0.85* 0.07 0.84* 0.63*

0.62* 0.01 0.04 0.27* 0.33* 0.47* 0.59* 0.19 0.33* 0.22

0.19 0.41* 0.44* 0.73* 0.79* 0.86* 0.08 0.71* 0.33*

0.09 0.10 0.02 0.08 0.05 0.02 0.08 0.03

0.34* 0.39* 0.35* 0.40* 0.38* 0.52* 0.42*

0.42* 0.34* 0.40* 0.08 0.34* 0.25

0.69* 0.68* 0.12 0.66* 0.20

0.87* 0.19 0.68* 0.43*

0.04 0.71* 0.29*

0.01 0.21

0.67*

AT4

SS8

SS1381

SS2

All correlations that are significant at the ¼ p 0.05 level are marked with an *.

transfer of sediments and nutrients from the catchment, supporting a purely benthic assemblage (Fig. 6). Consequently, in the low Arctic region of south-western Greenland the Fragilaria (sensu latu) > Cyclotella ± Aulacoseira sequence is not an accurate template for ontogeny; lake setting and climate can modify ecological response to long-term ontogeny.

5.3. Does climate affect lake ontogeny? To test the hypothesis that dominant Holocene trends in diatom

assemblages are independent of climate, DCA/CA/CCA axis 1 scores were plotted against time since lake formation (Fig. 7). Additional profiles from other palaeolimnological records from south-western Greenland were included in this analysis: coastal lake SS49 (Perren et al., 2012), and inland lakes SS16 (Perren et al., 2012), and SS2 (Anderson et al., 2008) (Fig. 7; Table 1). The temporal trends of the DCA/CA axis 1 scores since lake formation from lakes across the regional climate gradient are similar (Fig. 7; Table 1) and represent diatom response to long-term alkalinity and nutrient loss. These results suggest that as lakes aged in this region, they demonstrate

12

A.C. Law et al. / Quaternary Science Reviews 126 (2015) 1e16

changes in water chemistry and nutrient availability at the same stage in their development. Therefore, it is not climate forcing, but time-dependant ontogeny processes, such as catchment development and geochemical coupling between the lakes and their catchments (Whitehead et al., 1989; Engstrom et al., 2000; Fritz et al., 2004) that determines the diatom communities of these lakes. The data presented supports the results of Perren et al. (2012) who concluded that the ecological trajectory of lakes in southwestern Greenland is determined by time-dependant processes controlled by lake ontogeny. These results contrast with research in the high Arctic where, because of limited edaphic catchment processes, climate is considered the primary driver of long-term ecological development (Wolfe, 2002; Michelutti et al., 2007; Wilson et al., 2012). There are, however, differences in the rates of change among the lakes. Two of the coastal sites, AT1 and SS49 (Perren et al., 2012) show very similar linear declines in DCA axis 1 scores over the first ~6000 yrs after their formation before stabilizing, whereas two of the inland lakes (SS2, SS1381) show more rapid linear declines over ~4000 yrs (Fig. 7). Diatom dissolution prevents the application of this approach at SS8. However, the slower rate of change at AT4 (a linear decline over ~8000 years; Fig. 7) coupled with major excursions from the linear trend, suggests that this weathering controlled process was disrupted by climate-mediate catchment disturbance at this site. Similarly, SS16 exhibits a broadly linear change over 6000 yrs but there are a number of departures, although these again may be associated with poor diatom preservation (Perren et al., 2012). The trends of the DCA/CA axis 1 profiles (indicative of diatom turnover in response to changing catchment derived alkalinity; Sections 5.1 and 5.3) broadly agree with predictions of the ALLOGEN soil weathering model (Boyle, 2007), which showed that long-term trends in diatom-inferred lake water acidity due to loss of apatite is characterised by a two-step decline (Fig. 7). The rates of decline in Boyle's study are much higher than observed in the Greenland lakes but the latter sites all have substantially lower precipitation and shorter ice-free seasons. The difference in the slopes (compared to Boyle's study) of DCA versus age (Fig. 7) since lake formation in south-western Greenland may reflect catchment differences in temperature, precipitation and soil depth, which would influence weathering rates and solute transfer from land to lake. Comparison of coastal and inland sites (Fig 7), however, shows faster rates of change in the inland sites, which have less precipitation and higher temperatures, suggesting that the arid and warmer conditions accelerate lake ontogeny. Our data therefore demonstrate that the primary long-term trends in diatom assemblages are independent of direct climate forcing, but in areas where lake hydroclimate is strongly influenced by the regional climate, the rate of ontogenetic change can be modified by climate. 5.4. Climate and lake production In contrast to the main trends in the diatom assemblage data, which indicate weathering as a primary control on surface water chemistry and hence diatom succession, lake production is more closely correlated with climate (temperature). Maximum biological production (as b carotene) was observed at lakes AT1, SS8 and SS1381 during the period of maximum Holocene warmth between 8500 and 5000 cal a BP (Fig. 7), reflecting similar production responses to regional climate forcing. Although there is some among site variability in the production peaks (Fig. 8), which can be attributed to lake size and catchment characteristics, there is good agreement in the timing of production peaks. However, AT4 demonstrated maximum biological production slightly earlier than the other lakes (~8500 cal a BP), which is thought to reflect dating

uncertainties at the base of the core. At the older, coastal lakes there is an initial period of low b carotene concentration which then increased to peak values (Fig. 8), whereas at the inland lakes which were formed ~2000 years later, close to the period of maximum warmth (Bennike, 2000; Bennike et al., 2010), the initial b carotene concentrations were already high. Oligotrophication associated with Neoglacial cooling is indicated by the progressive decline in b carotene from ~5000 cal a BP at the four lakes (Fig. 8). A similar response was recorded at coastal lake SS49 (Figs. 1 and 8) where biological production (inferred as reconstructed Chlorophyll a) was high between ~9000 and 5300 cal a BP and maximal at ~8000 cal a BP (Perren et al., 2012). The rapid decline of in-lake production at AT1 from ~5000 cal a BP (Fig. 8) can be interpreted as the onset of the colder, wetter conditions of the Neoglacial (Fredskild, 1983; McGowan et al., 2003; Kaufman et al., 2004). These conditions were probably amplified at a higher altitude (475 m.a.s.l) resulting in a return to permafrost and reduced hydrological transfer of sediments and nutrients into the lake. The catchment changes resulted in lower DOC export and led to clear and weakly acidic, oligotrophic waters most suited to benthic diatoms (e.g., P. brevistriata, S. construens var. exigua, T. flocculosa; Fig. 2). Around this time at AT1 there was a decline in organic matter accumulation rate (OMAR; Fig. 8); the agreement between the OMAR and b carotene (r ¼ 0.68) suggests the organic matter is largely autochthonous at this site, in contrast to AT4 where the OMAR and b carotene records are decoupled (Fig. 8; r ¼ 0.28). Anderson et al. (2012) interpreted the OMAR at AT4 as reflecting catchment disturbance and the input of allochthonous sediments (i.e. soil) into the lake. Some support for this interpretation is provided by an increase in Cenococcum geophilum sclerotia spores and TOC at ~4500 cal a BP at SISI12, a nearby lake (Leng et al., 2012). The presence of Cenococcum spores have been linked to increased erosion related to catchment degradation during Neoglacial cooling (Wagner and Bennike, 2012). At the inland sites, the OMAR and b carotene records indicate maximum production during the thermal maximum (8500 and 5000 cal a BP) and around the period of peak Holocene warming (Fig. 8; Bennike, 2000; Bennike et al., 2010). However, this was also a period of very low effective moisture at the head of Kangerlussuaq fjord observed in the diatom-inferred conductivity record at SS6 and Braya Sø (SS4) ~ 6800 e 6000 cal a BP (McGowan et al., 2003; Aebly and Fritz, 2009) and in the isotope record ~ 7000 e 5600 cal a BP (Anderson and Leng, 2004). There would have been nutrient concentration effects associated with evaporation and lake level lowering at these sites. There was also a transient increase in production around 1600 cal a BP at SS8 (Fig. 8), possibly associated with aeolian deposits increasing nutrient inputs to the lake (Anderson et al., 2012). Aeolian activity was associated with deflation of sandur deposits from the ice margin, during the cooling episode following the MCA around 1300e1100 cal a BP and the LIA €rnqvist, 1999; Willemse et al., at ~500 cal a BP (Willemse and To 2003), when sediment input into the lakes was responsible for changes in light, nutrients and alkalinity generation. At SS2, a neighbouring lake, although there was an early Holocene maximum of in-lake production (b carotene ~ 200 nmol g1; Anderson et al., 2008) maximum b carotene occurred during the LIA, (~480 nmol g1), again presumably associated with aeolian activity (Anderson et al., 2008). The coastal lakes are located too far from the inland ice-margin to receive such significant aeolian deposits, and hence there is little evidence of increased production during the last ~1300 cal a BP. 5.5. Holocene climatic variability and diatom responses The main periods of regional Holocene climatic variability (as

A.C. Law et al. / Quaternary Science Reviews 126 (2015) 1e16

13

Fig. 8. b Carotene concentration (nmol g1 organic matter), Organic Matter Accumulation Rate (OMAR; g OM cm2 yr1) with loess smooth for the four study lakes (AT1, AT4, SS8 and SS1381) during the Holocene. Also presented are the b Carotene concentration (nmol g1) data from lake SS2 (Anderson et al., 2008) and the Chl a from lake SS49 (Perren et al., 2012).

14

A.C. Law et al. / Quaternary Science Reviews 126 (2015) 1e16

defined in Section 2.2.) are compared with statistically significant diatom zones from eight lakes in Fig. 9. The timing of some of the key transitions in the diatoms (as statistically significant biostratigraphic zones) coincides with the independently defined climate periods, namely the mid-Holocene thermal maximum, the Neoglacial of the late-Holocene and the LIA (Fig. 9). The climate period coinciding with the largest number of significant diatom zone boundaries is the peak of mid-Holocene warming, which is observed in all lakes (excluding SS32 which is too young). Given dating uncertainties, the differences between the start and the end of peak warming during the mid-Holocene should not be overinterpreted. Significant changes in the diatom assemblages are observed at the end of the mid-Holocene thermal maximum in three of the lakes (AT1, SS8, and SS16; Fig. 9), and the Neoglacial conditions of the late-Holocene are also clearly marked in three lakes (AT4, SS16, and SS32; Fig. 9). While this analysis suggests that Holocene climate variability was a driver of ecological change at some sites, mainly during the mid-Holocene thermal maximum, there is little systematic, geographic pattern from coast to ice-sheet margin after 6500 cal a BP (Fig. 9). This analysis further highlights the spatial and temporal complexity of biotic responses in individual lakes (See Reuss et al., 2013; Perren et al., 2012, Section 5.4 below). At AT1, the mid-Holocene thermal maximum (from ~8500 cal a BP) is marked by a decline in the planktonic D. stelligera and a switch in the Fragilarioid taxa, notably the expansion of P. brevistriata (Fig. 2). A similar expansion of this taxon occurred at around 7000 cal a BP at AT4, but interestingly there is a major expansion in the planktonic C. rossi complex at the same time (Fig. 3). Inland, during the mid-Holocene thermal maximum (~8000e5000 cal a BP), the presence of alkaliphilous (e.g. A. pediculus, A. libyca), and planktonic diatom species (C. bodanica aff. lemanica) combined with a low F-index (high rates of diatom dissolution) before ~ 5000 cal a BP at the two inland lakes (Figs. 4 and 5) indicates highly alkaline, ion-rich waters (McGowan et al., 2003, Ryves et al., 2006). The increased dominance of alkaliphilous C. bodanica aff. lemanica (Ryves et al., 2002) at SS1381 (<80%; Fig. 5), and alkaliphilous benthic Fragilarioid taxa at SS8 (Fig. 4) can be interpreted as reflecting increased alkalinity and nutrient inputs during the last ~1300 cal a BP, associated with aeolian activity. The diatom responses of the inland lakes (SS16 and SS32; Perren et al., 2012; SS2; Anderson et al., 2008; SS1381 and SS8: this paper) during the last ~ 1500 cal a BP and the LIA (aeolian activity) provide evidence that ecological and chemical lake development trends can be altered by climate change, but not necessarily by temperature changes per se. 5.6. Regional heterogeneity: Holocene climatic variability and lakecatchment interactions Despite the similarities in the long-term ontogeny driven trends (CA/DCA/CCA 1 scores; Fig. 7) there are differences in the individual ecological histories among the four study sites. The diatom zones also demonstrate spatio-temporal variability when compared to regional climate history inferred from other proxies (see Section 2.2; Fig. 9). These results can be explained because different catchment and lake characteristics (e.g. catchment hydrology, geology, morphometry, vegetation cover) modify or “filter” lake ecological response to known episodes of climatic variability (Leavitt et al., 2009; Fritz and Anderson, 2013). The ontogeny trajectory of some lakes is more closely coupled with climate due to their catchment geomorphology. For example, the physical setting of lakes SS1381 and SS8 dictates that under warmer, drier climatic episodes, lake level lowering may have temporarily caused the lakes to become closed basins with elevated alkalinities, predominantly benthic diatom communities and high

Fig. 9. The statistically significant diatom zones (shown by the dashed lines) for the 4 study lakes (AT1, AT4, SS8 and SS1381), lakes SS49, SS16, SS32 (Perren et al., 2012), and SS2 (Anderson et al., 2008). Periods of Holocene climatic variability (the mid-Holocene thermal maximum, the Neoglacial conditions of the late-Holocene, the MCA and the LIA) are marked by the grey shading. Peak mid-Holocene warming (Bennike, 2000; Bennike et al., 2010) is highlighted by the dark grey shading.

rates of diatom dissolution (Figs. 4e6). There is evidence for ecological change associated with lake level lowering during the mid-Holocene thermal maximum (Aebly and Fritz, 2009) from the sediment records of the oligosaline lakes (McGowan et al., 2003). Consequently, the long-term trends of oligotrophication and declining alkalinity that are exhibited by neighbouring lakes (e.g. SS2; Anderson et al., 2008; Lake Johs. Iversen Sø; Fredskild, 1983; SS16; Heggen et al., 2010; Perren et al., 2012) are modified at these sites by regional climate. Catchment morphology and climatic setting can modify climate-independent ontogeny trends. At the coast, the effect of catchment morphology on lake response is highlighted by the contrasting ecological responses of the two coastal lakes (Figs. 2 and 3). At AT4, which is located within a cirque basin, the onset of Neoglacial cooling resulted in landscape instability, increased erosion and sediment transfer into the lake (Anderson et al., 2012). Around 5000 cal a BP at AT4, the input of allochthonous sediment altered the thermal, light, and nutrient status of the lake, leading to high abundances (<50%) of planktonic D. stelligera complex (Fig. 3). Conversely, at AT1 the Neoglacial was marked by a shift to an acidophilus and benthic-dominated assemblage (e.g., P. brevistriata, P. biceps; Fig. 2). This shift in diatom assemblages during the Neoglacial reflects a reduction in the hydrological transfer of sediments and DOC from the catchment (Anderson et al., 2012), resulting in clearer, oligotrophic waters. These contrasting responses to Neoglacial cooling highlight the importance of catchment geomorphology in determining lake and catchment filtering which produces variable ecological trends. The latter point about filtering is reflected in the differences in timing of significant changes in the diatom assemblages observed in the lakes (Fig. 9). 6. Conclusion e spatial and temporal variability in ecological change This study has highlighted the spatial and temporal complexity of lake ecological development and ontogeny. Regardless of age and climatic setting the study lakes demonstrate comparable long-term patterns of ecological development (as diatom DCA/CA axis 1 scores), interpreted as high alkalinity in the early stages followed by a progressive oligotrophication and declining alkalinity from the mid Holocene onwards (~4500 cal a BP). These long-term trends are driven by changes in lake water chemistry and nutrient availability, which are dictated by time-dependent ontogeny processes (Fritz and Anderson, 2013). The similarities in the DCA/CA axis 1 scores of the four study lakes and four additional lakes (SS49, SS16 and

A.C. Law et al. / Quaternary Science Reviews 126 (2015) 1e16

SS32; Perren et al. 2012; and SS2; Anderson et al., 2008) from south-western Greenland reflect the transition of diatom assemblages in relation to changes in water chemistry and nutrients (ontogeny). Taken together, these results indicate that long-term ontogeny is independent of climate in this region. In contrast, primary production maxima occurred during the mid-Holocene thermal maximum (8000 e 5000 cal a BP) at all lakes, indicating that climate forcing may induce regionally coherent patterns of inlake production. These results suggests that the controls on lake ontogeny in the low Arctic are complex and differ to those in the high Arctic where, due to the simplicity of lake catchments (e.g. limited vegetation and soil cover), lake ontogeny is directly controlled by climate (Smol, 1983; Wolfe et al., 2000; Wolfe, 2002). The lakes also demonstrate individual histories and despite overall assemblage change in response to declining alkalinity and nutrients, lake and catchment characteristics (e.g., geomorphology, lake bathymetry) and climate variability have, at some lakes, interacted with ontogeny processes to cause short-term excursions in the direction and pace of ecological change. For example, the setting of the inland lakes complicated their long-term ontogeny trends. These lakes are more ecologically sensitive to climatemediated changes than the coastal lakes because they are strongly influenced by variations in the precipitation: evaporation ratio (McGowan et al., 2003; Anderson and Leng, 2004) and, furthermore, can be affected by aeolian deposition. In contrast, neighbouring coastal lakes AT1 and AT4 responded in contrasting ways ecologically to Neoglacial cooling due to differences in geomorphology and soil and vegetation development. This study has illustrated the importance of establishing regional ontogeny trends as a template for interpreting ecological proxies in order to differentiate between natural long-term ecological change and short-term climate mediated responses in lakes. It is also essential to consider how lake setting (both climatologically and in terms of characteristics such as geology, geomorphology, soils and vegetation) and lake-catchment interactions causes temporal variability in lake sediment records in the same geographic region. Without this knowledge, ecological “baselines” cannot be defined and records cannot be used accurately to infer how future climate change may modify lake ecology and lake catchments. Acknowledgements This research was funded by a Loughborough University Development Trust Studentship awarded to ACL. Financial support for the fieldwork was provided by a Royal Geographical Society Postgraduate Award, a QRA New Workers' Research Award to ACL and a University of Nottingham New Lecturer's Fund award to SMcG. Dating was funded by NERC Awards to N.J. Anderson and S.McGowan (Allocation Numbers: 1403.0409, 1242.1007, and 1403.0409), thanks to Charlotte Bryant. Thanks also to Jesper Olsen (Queens University, Belfast) for creating the age-depth models, Teresa Needham and Graham Morris for help in the pigment laboratory and Theo Law and Nick Wallerstein for assistance in the field. We also thank Keely Mills for providing comments on an earlier version of the paper. References Aebly, F.A., Fritz, S.C., 2009. Palaeohydrology of kangerlussuaq (Sondre stromfjord), West Greenland during the last ~8000 years. Holocene 19, 91e104. Anderson, N.J., Bennike, O., Christofferson, K., Jeppesen, E., Markager, S., Miller, G., Renberg, I., 1999. Limnological and palaeolimnological studies of lakes in southwestern Greenland. Geol. Greenl. Surv. Bull. 183, 68e74. Anderson, N.J., Brodersen, K.P., Ryves, D.B., Mcgowan, S., Johansson, L.S., Jeppesen, E., Leng, M.J., 2008. Climate versus in-lake processes as controls on

15

the development of community structure in a low-Arctic lake (South-West Greenland). Ecosystems 11, 307e324. Anderson, N.J., Harriman, R., Ryves, D.B., Patrick, S.T., 2001. Dominant factors controlling variability in the ionic composition of West Greenland Lakes. Arct. Antarct. Alp. Res. 33, 418e425. Anderson, N.J., Leng, M.J., 2004. Increased aridity during the early Holocene in West Greenland inferred from stable isotopes in laminated-lake sediments. Quat. Sci. Rev. 23, 841e849. Anderson, N.J., Liversidge, A.C., Mcgowan, S., Jones, M.D., 2012. Lake and catchment response to Holocene environmental change: spatial variability along a climate gradient in southwest Greenland. J. Paleolimnol. 48 (1), 409e222. Anderson, N.J., Stedmon, C.A., 2007. The effect of evapoconcentration on dissolved organic carbon concentration and quality in lakes of SW Greenland. Freshw. Biol. 52, 280e289. Antoniades, D., Douglas, M.S.V., Smol, J.P., 2005. Quantitative estimates of recent environmental changes in the Canadian High Arctic inferred from diatoms in lake and pond sediments. J. Paleolimnol. 33, 349e360. Bennike, O., 2000. Palaeoecological studies of Holocene lake sediments from west Greenland. Palaeogeogr. Palaeoclimatol. Palaeoecol. 155, 285e304. Bennike, O., Anderson, N.J., Mcgowan, S., 2010. Holocene palaeoecology of southwest Greenland inferred from macrofossils in sediments of an oligosaline lake. J. Paleolimnol. 43 (4), 787e798. Battarbee, R.W., 1991. Recent paleolimnology and diatom-based environmental reconstruction. In: Shane, L.C.K., Cushing, E.J. (Eds.), Quaternary Landscapes. University of Minnesota Press, Minneapolis, pp. 129e174. Battarbee, R.W., 2000. Palaeolimnological approaches to climate change, with special regard to the biological record. Quat. Sci. Rev. 19, 107e124. Bigler, C., Grahn, E., Larocque, I., Jeziorski, A., Hall, R., 2003. Holocene environmental change at Lake Njulla (999 m asl), northern Sweden: a comparison with four small nearby lakes along an altitudinal gradient. J. Paleolimnol. 29 (1), 13e29. €cher, T.W., 1949. Climate, soil and lakes in continental West Greenland in relation Bo to plant life. Medd. Grønl. 147, 1e63. Boyle, J.F., 2007. Loss of apatite caused irreversible early-Holocene lake acidification. Holocene 7 (4), 543e547. Bouchard, G., Gajewski, K., Hamilton, P.B., 2004. Freshwater diatom biogeography in the Canadian Arctic Archipelago. J. Biogeo 31, 1955e1973. Boyle, J.F., Chiverrell, R., Plater, A., Thrasher, I., Bradshaw, E., Birks, H., Birks, J., 2013. Soil mineral depletion drives early Holocene lake acidification. Geology 41 (4), 415e418. Bradshaw, E.G., Jones, V.J., Birks, H.J.B., Birks, H.H., 2000. Diatom responses to lateglacial and early Holocene environmental changes at Kråkenes, western Norway. J. Paleolimnol. 23, 21e34. Carpenter, S.R., Leavitt, P.R., Elser, J.J., Elser, M.M., 1988. Chlorophyll budgets: response to food web manipulation. Biogeochem 6, 79e90. Chen, N., Bianchi, T.S., Mckee, B.A., Bland, J.M., 2001. Historical trends of hypoxia on Louisiana shelf: application of pigments as biomarkers. Org. Geochem 32, 543e561. Cremer, H., Melles, M., Wagner, B., 2001a. Holocene climate changes reflected in a diatom succession from Basaltsø, East Greenland. Can. J. Bot. 79, 649e656. Cremer, H., Wagner, B., 2004. Planktonic diatom communities in high Arctic lakes (Store Koldewey, Northeast Greenland). Can. J. Bot. 82 (12), 1744e1757. Cremer, H., Wagner, B., Melles, M., Hubberten, H.W., 2001b. The postglacial environmental development of Raffles SØ, East Greenland: inferences from a 10,000 year diatom record. J. Paleolimnol. 26, 67e87. Crocker, R.L., Major, J., 1955. Soil development in relation to vegetation and surface age at Glacier Bay, Alaska. J. Ecol. 43, 427e448. Dahl-Jensen, D., Mosegaard, K., Gundestrup, N., Clow, G.D., Johnsen, S.J., Hansen, A.W., Balling, N., 1998. Past temperatures directly from the Greenland ice sheet. Science 282, 268e248. Douglas, M.S.V., Smol, J.P., 1994. limnology of high Arctic ponds (Cape Herschel, Ellesmere Island, nwt). Arch. Hydrobiol. 131, 401e434. Eisner, W.R., Tornqvist, T., Koster, E.A., Bennike, O., Vanleeuwen, J.F.N., 1995. Paleoecological studies of a Holocene lacustrine record from the Kangerlussuaq (Søndre Strømfjord) region of West Greenland. Quat. Res. 43 (1), 55e66. Engstrom, D.R., Fritz, S.C., Almendinger, J.E., Juggins, S., 2000. Chemical and biological trends during lake evolution in recently deglaciated terrain. Nature 408, 161e166. Finkelstein, S.A., Bunbury, J., Gajewski, K., Wolfe, A.P., Adams, J.K., Devlin, J.E., 2014. Evaluating diatom-derived Holocene pH reconstructions for Arctic lakes using an expanded 171-lake training set. J. Quat. Sci. 29 (3), 249e260. Fredskild, B., 1973. Studies in the vegetational history of Greenland: Palaeobotanical investigations of some holocene lake and bog deposits. Medd. Grønl. 198, 1e245. Fredskild, B., 1983. The Holocene vegetational development of the Godthabsfjord area, West Greenland. Medd. Grønl. 10, 1e28. Fredskild, B., 1984. Holocene palaeo-winds and climatic changes in West Greenland as indicated by long-distance transported and local pollen in lake sediments. In: € MOrner, N.A., Karlen, W. (Eds.), Climatic Changes on a Yearly to Millennial Basis. Reidel Publishing, Dordrecht, pp. 163e171. Fritz, S.C., Anderson, N.J., 2013. The relative influences of climate and catchment processes on Holocene lake development in deglaciated areas. J. Paleolimnol. 49 (3), 349e362. Fritz, S.C., Engstrom, D.R., Juggins, S., 2004. Patterns of early lake evolution in boreal landscapes: a comparison of stratigraphic inferences with a modern chronosequence in Glacier Bay, Alaska. Holocene 14, 828e840.

16

A.C. Law et al. / Quaternary Science Reviews 126 (2015) 1e16

Foged, N., 1953. Diatoms from West Greenland. Medd. Grønl. 147, 1e86. Foged, N., 1955. Diatoms from Peary Land, North Greenland. Medd. Grønl. 128, 1e90. Foged, N., 1958. The diatoms in the basalt area and adjoining areas of Archean rock in West Greenland. Medd. Grønl. 156, 1e145. Foged, N., 1972. The diatoms from four postglacial deposits in Greenland. Medd. Grønl. 194, 1e66. Foged, N., 1977. The diatoms in four postglacial deposits at Godthåbsfjord, West Greenland. Medd. Grønl. 199, 1e64. Hasholt, B., Søgaard, H., 1978. Et forsøg på en klimatisk-hydrologisk regionsinddeling af Holsteinborgs Kommune (Sisimiut). Geogr. Tidss 77, 72e92. Haworth, E.Y., 1976. Two late-glacial (Late Devensian) diatom assemblage profiles from northern Scotland. New Phytol. 77, 227e256. Heggen, M.P., Birks, H.H., Anderson, N.J., 2010. Long-term ecosystem dynamics of a small lake and its catchment in west Greenland. Holocene 20, 1207e1222. Hogan, E.J., Mcgowan, S., Anderson, N.J., 2014. Nutrient limitation of periphyton growth in Arctic lakes in south-west Greenland. Polar Biol. 37 (9), 1331e1342. Jensen, S.M., Hansen, H., Secher, K., Steenfelt, A., Schjoth, F., Rasmussen, T.M., 2002. Kimberlites and other ultramafic alkaline rocks in the Sisimiut-Kangerlussuaq region, southern West Greenland. Geol. Greenl. Surv. Bull. 191, 57e66. Joynt, E.H., Wolfe, A.P., 2001. Paleoenvironmental inference models from sediment diatom assemblages in Baffin Island lakes (Nunavut, Canada) and reconstruction of summer water temperature. Can. J. Fish. Aquat. Sci. 58, 1222e1243. Krammer, K., Lange-Bertalot, H., 1986-1991. Bacillariophyceae. Süsswasserflora von Mitteleuropa, 2/1e2/4. G. Fischer Verlag, Stuttgart. Kaufman, D.S., Ager, T.A., Anderson, N.J., Anderson, P.M., Andrews, J.T., Bartlein, P.J., Brubaker, L.B., Coats, L.L., Cwynar, L.C., Duvall, M.L., Dyke, A.S., Edwards, M.E., Eisner, W.R., Gajewski, K., Geirsdottir, A., Hu, F.S., Jennings, A.E., Kaplan, M.R., Kerwin, M.N., Lozhkin, A.V., Macdonald, G.M., Miller, G.H., Mock, C.J., Oswald, W.W., Otto-Bliesner, B.L., Porinchu, D.F., Ruhland, K., Smol, J.P., Steig, E.J., Wolfe, B.B., 2004. Holocene thermal maximum in the western Arctic (0e180 degrees W). Quat. Sci. Rev. 23, 529e560. Leavitt, P.R., Bunting, L., Fritz, S.C., Anderson, N.J., Baker, P.A., Catalan, J., Conley, D.J., Hobbs, W., Jeppesen, E., Korhola, A., Mcgowan, S., Ruhland, K., Rusak, J., Simpson, G., Werne, J., 2009. Paleolimnological evidence of the effects on lake ecosystems of energy and mass transfer from climate and humans. Limnol. Oceanogr. 54, 2330e2348. Leavitt, P.R., Carpenter, S.R., 1989. Effects of sediment mixing and benthic algal production on fossil pigment stratigraphies. J. Paleolimnol. 2, 147e158. Leavitt, P.R., Hodgeson, D.A., 2001. Sedimentary Pigments. Kluwer Academic Publishers, Dordrecht, the Netherlands. Leng, M.J., Anderson, N.J., 2003. Isotopic variation in modern lake waters from western Greenland. Holocene 13, 605e611. Leng, M.J., Wagner, B., Anderson, N.J., Bennike, O., Woodley, E., Kemp, S.J., 2012. Deglaciation and catchment ontogeny in coastal south-west Greenland: implications for terrestrial and aquatic carbon cycling. J. Quat. Sci. 27 (6), 575e584. Mantoura, R.F.C., Llewellyn, C.A., 1983. Anal. Chim. Acta 151, 297e314. Mcgowan, S., Barker, P., Haworth, E.Y., Leavitt, P.R., Maberly, S.C., Pates, J., 2012. Humans and climate as drivers of algal community change in Windermere since 1850. Freshwat. Biol. 57, 260e277. Mcgowan, S., Ryves, D.B., Anderson, N.J., 2003. Holocene records of effective precipitation in West Greenland. Holocene 13, 239e249. Michelutti, N., Wolfe, A.P., Briner, J.P., Miller, G.H., 2007. Climatically controlled chemical and biological development in Arctic lakes, 2005e2012 J. Geophys. Res. Biogeo 112, G3. Pearsall, W.H., 1921. The development of vegetation in the English Lakes, considered in relation to the general evolution of glacial lakes and rock basins. Philos. T. Roy. Soc. B 92, 259e284. Perren, B., Anderson, N.J., Douglas, M., Fritz, S., 2012. The influence of temperature, moisture, and eolian activity on Holocene lake development in West Greenland. J. Paleolimnol. 48, 223e239. Perren, B., Douglas, M., Anderson, N.J., 2009. Diatoms reveal complex spatial and temporal patterns of recent limnological change in West Greenland. J. Paleolimnol. 42 (2), 233e247.

Pienitz, R., Smol, J.P., Birks, H.J.B., 1995. Assessment of freshwater diatoms as quantitative indicators of past climatic change in the Yukon and Northwest Territories, Canada. J. Paleolimnol. 13 (1), 21e49. Renberg, I., 1990a. A procedure for preparing large sets of diatom slides from sediment cores. J. Paleolimnol. 4, 87e90. Renberg, I., 1990b. A 12600 years perspective of the acidification of Lilla Oresjtin, southwest Sweden. Philos. T. Roy. Soc. B 327, 357e361. Renberg, I., 1991. The HON-Kajak sediment corer. J. Paleolimnol. 6 (2), 167e170. Reuss, N.S., Anderson, N.J., Fritz, S.C., Simpson, G.L., 2013. Responses of microbial phototrophs to late-Holocene environmental forcing of lakes in south-west Greenland. Freshw. Biol. 58, 690e704. Round, F.E., 1957. The late-glacial diatom succession in the Kentmere Valley deposit. 1. Introduction, methods and flora. New Phytol. 56, 98e126. Rouse, W.R., Douglas, M.S.V., Hecky, R.E., Hershey, A.E., Kling, G.W., Lesack, L., Marsh, P., Mcdonald, M., Nicholson, B.J., Roulet, N.T., Smol, J.P., 1997. Effects of climate change on the freshwaters of Arctic and subarctic North America. Hydrol. Process 11, 873e902. Ryves, D.B., Juggins, S., Fritz, S.C., Battarbee, R.W., 2001. Experimental diatom dissolution and the quantification of microfossil preservation in sediments. Palaeogeogr. Palaeoclimatol. Palaeoecol. 172, 99e113. Ryves, D.B., Mcgowan, S., Anderson, N.J., 2002. Development and evaluation of a diatom-conductivity model from lakes in West Greenland. Freshw. Biol. 47, 995e1014. Ryves, D.B., Battarbee, R.W., Juggins, S., Fritz, S.C., Anderson, N.J., 2006. Physical and chemical predictors of diatom dissolution in freshwater and saline lake sediments in North America and West Greenland. Limnol. Oceanogr 51, 1355e1368. Saros, J.E., Anderson, N.J., 2014. The ecology of the planktonic diatom Cyclotella and its implications for global environmental change studies. Biol. Revs 90 (2), 522e541. Saros, J.E., Stone, J.R., Pederson, G.T., Siemmons, K.E.H., Spanbauer, T., Schliep, A., Cahl, D., Williamson, C.E., Engstrom, D.E., 2012. Climate-induced changes in lake ecosystem structure inferred from coupled neo- and paleo-ecological approaches. Ecology 93, 2155e2164. Saros, J.E., Strock, K.E., Mccue, J., Hogan, E., Anderson, N.J., 2013. Response of Cyclotella species to nutrients and incubation depth in Arctic lakes. J. Plankton Res. 36 (2), 450e460. Smol, J.P., 1983. Paleophycology of a high Arctic lake near Cape Herschel, Ellesmere Island. Can. J. Bot. 61, 2195e2204. Van Tatentove, F.G.M., Van Der Meer, J.J.M., Koster, R.D., 1996. Implications for deglaciation chronology from new AMS age determinations in central West Greenland. Quat. Res. 45, 245e253.  ter Braak, C.J.F., Smilauer, P., 1997. Canoco for Windows, Version 4.56. Biometrics e Plant Research International, Wageningen, The Netherlands. Wagner, B., Bennike, O., 2012. Chronology of the last deglaciation and Holocene environmental changes in the Sisimiut area, SW Greenland based on lacustrine records. Boreas 41 (3), 481e493. € m, J., Korhola, A., Blom, T., 1997. Diatoms as quantitative indicators of pH Weckstro and water temperature in subarctic Fennoscandian lakes. Hydrobiol. 347 (1e3), 171e184. Whitehead, D.R., Charles, D.F., Jackson, S.T., Smol, J.P., Engstrom, D.R., 1989. The developmental history of Adirondack (N.Y.) lakes. J. Paleolimnol. 2, 185e206. Wilson, C.R., Michelutti, N., Cooke, C.A., Briner, J.P., Wolfe, A.P., Smol, J.P., 2012. Arctic lake ontogeny across multiple interglaciations. Quat. Sci. Rev. 31, 112e126. Willemse, N.W., Koster, E.A., Hoogakker, B., Van Tatenhove, F.G.M., 2003. A continuous record of Holocene aeolian activity in West Greenland. Quat. Res. 59, 322e334. Willemse, N.W., Tornqvist, T.E., 1999. Holocene century-scale temperature variability from West Greenland lake records. Geology 27, 580e584. Wolfe, A.P., 2002. Climate modulates the acidity of Arctic lakes on millennial time scales. Geology 30, 215e218. Wolfe, A.P., Frechette, B., Richard, P.J.H., Miller, G.H., Forman, S.L., 2000. Paleoecology of a > 90,000-year lacustrine sequence from Fog Lake, Baffin Island, Arctic Canada. Quat. Sci. Rev. 19, 1677e1699.