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Using paleolimnology to track Holocene climate fluctuations and aquatic ontogeny in poorly buffered High Arctic lakes
Abstract
Fossil diatom assemblages, and spectrally-inferred dissolved organic carbon (DOC) and sedimentary chlorophyll-a (SedChla) were analysed on lake sediment cores from two poorly buffered lakes on Pim Island (High Arctic Canada) to assess their responses to Holocene climate changes and to document lake ontogeny. Following deglaciation, diatom assemblages were dominated by small benthic Fragilaria sensu lato taxa. During the mid-Holocene, there was an abrupt shift to more circumneutral and slightly acidophilous taxa dominated by Achnanthes and Navicula taxa. In the most recent sediments, we recorded an increase in the planktonic taxon Cyclotella radiosa. This shift of the last century is the most ecologically unique in the Holocene record and is indicative of longer ice-free summers consistent with modern climate warming. Inferred DOC and SedChla track some of the main Holocene climatic trends documented in the region, including the Holocene Thermal Maximum and Neoglacial period; however, changes in lakewater DOC did not likely drive any of the recorded shifts in diatom assemblages. Compared to nearby well-buffered sites, our poorly buffered lakes recorded a more dynamic diatom response to Holocene environmental change. The decreasing trend in diatom-inferred pH is likely due to changes in the acid neutralizing capacity (ANC) driven by the release of alkalinizing base cations from the easily weathered glacial deposits in the early Holocene and later by climate-driven pH dynamics and within-lake dissolved inorganic carbon (DIC) dynamics. The diatom com-munity composition in our study lakes is different and undergoes greater changes than in nearby well-buffered lakes suggesting that softwater lakes in the high Arctic may respond most sensitively to climate and other environmental stressors.
... West Lake is a poorly-buffered lake on Pim Island (Canadian High Arctic), and although the sediment record shows little directional change in the species assemblage (it has yet to cross the habitat thresholds related to an elongated ice free season), there is substantial variation in the high-resolution record due to climate-driven pH shifts. This is consistent with the Holocene record from this same lake [44], which showed a highly variable response reflecting climatedriven pH shifts in the diatom record. ...
... The Holocene diatom record at West Lake was dynamic in tracking climate-driven pH changes but, importantly, we did not find increases in epiphytic taxa reflective of the emergence of novel moss habitats in West Lake. The disagreement between our shorter sediment record and the Holocene record [44] (but which is not relevant to our study) was the timing of the increase in Cyclotella radiosa. In our core the increase occurred at about 1800 AD,~50 years earlier than in the Holocene record [44]. ...
... The disagreement between our shorter sediment record and the Holocene record [44] (but which is not relevant to our study) was the timing of the increase in Cyclotella radiosa. In our core the increase occurred at about 1800 AD,~50 years earlier than in the Holocene record [44]. We only have four pre-1800 AD intervals (after which C. radiosa appears in moderate abundances), so the discrepancy is relatively difficult to interpret with much certainty. ...
Gajewski offers a formal comment on Griffiths et al. (2017), a paper that explored how microclimates and their varying ice cover regimes on lakes and ponds in Arctic regions modified the diatom assemblage responses to recent warming. One of Gajewski's main criticisms is that the microclimate classification scheme used in Griffiths et al. (2017) is merely anecdotal; a claim which ignores the value of observational evidence and misunderstands the frequency that each site was visited or surveyed. We clarify that the study sites were visited multiple times via recurrent aerial surveys and ground observations dating back to the 1970s, which supports our microclimate classification scheme. Many of Gajewski's claims regarding climate, catchment characteristics, and ice melting properties from field locations he has not visited were refuted by veteran Arctic scientists with long-term field experience in these regions. In addition, Gajewski makes several criticisms concerning radioisotopic dating , core chronology, sediment mixing, diagenesis, and preservation of bioindicators that relate more to general paleolimnological assumptions than to conclusions reached by Grif-fiths et al. (2017). Research from the 1980s and 1990s, when scientific consensus on these issues was first reached, readily show that the methodologies and data interpretation of Grif-fiths et al. (2017) are sound. We appreciate the opportunity to expound on the finer details of the Griffiths et al. (2017) paper, work based on field research by the study's co-authors spanning almost three decades, with additional observations from colleagues dating back to the 1970s. We address Gajewski's criticisms with relevant literature, expert statements, and a few clarifying comments.
... West Lake is a poorly-buffered lake on Pim Island (Canadian High Arctic), and although the sediment record shows little directional change in the species assemblage (it has yet to cross the habitat thresholds related to an elongated ice free season), there is substantial variation in the high-resolution record due to climate-driven pH shifts. This is consistent with the Holocene record from this same lake [44], which showed a highly variable response reflecting climatedriven pH shifts in the diatom record. ...
... The Holocene diatom record at West Lake was dynamic in tracking climate-driven pH changes but, importantly, we did not find increases in epiphytic taxa reflective of the emergence of novel moss habitats in West Lake. The disagreement between our shorter sediment record and the Holocene record [44] (but which is not relevant to our study) was the timing of the increase in Cyclotella radiosa. In our core the increase occurred at about 1800 AD,~50 years earlier than in the Holocene record [44]. ...
... The disagreement between our shorter sediment record and the Holocene record [44] (but which is not relevant to our study) was the timing of the increase in Cyclotella radiosa. In our core the increase occurred at about 1800 AD,~50 years earlier than in the Holocene record [44]. We only have four pre-1800 AD intervals (after which C. radiosa appears in moderate abundances), so the discrepancy is relatively difficult to interpret with much certainty. ...
Gajewski offers a formal comment on Griffiths et al . (2017), a paper that explored how microclimates and their varying ice cover regimes on lakes and ponds in Arctic regions modified the diatom assemblage responses to recent warming. One of Gajewski’s main criticisms is that the microclimate classification scheme used in Griffiths et al . (2017) is merely anecdotal; a claim which ignores the value of observational evidence and misunderstands the frequency that each site was visited or surveyed. We clarify that the study sites were visited multiple times via recurrent aerial surveys and ground observations dating back to the 1970s, which supports our microclimate classification scheme. Many of Gajewski’s claims regarding climate, catchment characteristics, and ice melting properties from field locations he has not visited were refuted by veteran Arctic scientists with long-term field experience in these regions. In addition, Gajewski makes several criticisms concerning radioisotopic dating, core chronology, sediment mixing, diagenesis, and preservation of bioindicators that relate more to general paleolimnological assumptions than to conclusions reached by Griffiths et al . (2017). Research from the 1980s and 1990s, when scientific consensus on these issues was first reached, readily show that the methodologies and data interpretation of Griffiths et al . (2017) are sound. We appreciate the opportunity to expound on the finer details of the Griffiths et al . (2017) paper, work based on field research by the study’s co-authors spanning almost three decades, with additional observations from colleagues dating back to the 1970s. We address Gajewski’s criticisms with relevant literature, expert statements, and a few clarifying comments.
... They provide several references in support of their assertion that these had the longest ice-free season, however these references do not provide data that this area has a longer or shorter ice-cover season than the regions of the other classes. 34] present no environmental data and they mention that the dense associations of vascular plants are associated with streams from the glaciers. They further say that during the Little Ice Age (CE1450-1850), uplands were covered by more extensive snow, suggesting the area was colder than today. ...
... Griffiths et al. [1] interpret the diatoms in the uppermost sediments of West Lake as showing no change in association with recent climate changes. However, a Holocene diatom record is available for this site [34], and Griffiths et al. [1] completely change the interpretation of [34]. West Lake did show large changes in the diatoms in the past, for example, a nearly complete replacement of an early Holocene flora with another. ...
... Griffiths et al. [1] interpret the diatoms in the uppermost sediments of West Lake as showing no change in association with recent climate changes. However, a Holocene diatom record is available for this site [34], and Griffiths et al. [1] completely change the interpretation of [34]. West Lake did show large changes in the diatoms in the past, for example, a nearly complete replacement of an early Holocene flora with another. ...
Griffiths et al. (2017) analyzed several ponds and lakes from the Cape Herschel region of Ellesmere Island in order to “… explicitly examine the role of ice cover as the dominant driver of diatom assemblage change …”. I reanalyze their data and suggest that their classification scheme, that they propose is due to differences in ice cover seasonality (“warm”, “cool”, “cold”, and “oasis”), is confounded with other morphological and chemical variables that better explain the differences between the groups. The “cold” sites are the deepest (lakes) and differ from the small, shallow ponds that occasionally dry, which would therefore have different diatom assemblages and histories. The “oasis” sites are nutrient enriched and probably have more stable water supplies, thereby enabling an aquatic flora providing habitats for diatoms. A key part of their interpretation is that “warm” sites have responded more rapidly to recent climate change than “cold” or “cool” sites, but their chronologies do not allow for such a conclusion. There is no clear difference between “cool” and “warm” sites, and problems in dating the sequences means inferences about their histories are not supported by data. Their results, which are restricted to the past century, are contradicted by a Holocene sequence from the region.
... Rapid ecological reorganizations, which have been attributed to recent climate warming mediated through changing ice-cover dynamics, are presently occurring in lakes throughout the Northern Hemisphere (Smol et al. 2005;Smol and Douglas 2007;Rühland et al. 2008;Rouillard et al. 2012;Table 1, no. 4). Although these regime shifts are manifested at several trophic levels (Smol et al. 2005), the shifts have been described mainly from changes in the species composition of diatom communities and are characterized by increases in small planktonic diatoms, such as Cyclotella species, and a compensatory decrease in small periphytic (often Fragilaria) species. ...
... Lakewater pH has been proposed as an important driver of diatom assemblage changes in poorly buffered Arctic and Alpine lakes, with warm intervals resulting in higher pH values due to enhanced loss of CO 2 to the atmosphere and greater utilization of limnetic CO 2 by algal photosynthesis, compared with cooler periods with longer ice-cover (Michelutti et al. 2007;Rouillard et al. 2012;Wilson et al. 2012;Fritz and Anderson 2013). Two poorly buffered lakes in the high Arctic of Canada show regime shifts, with a shift in diatom dominance from small benthic Fragilaria taxa to a more diverse but less productive assemblage of small benthic and periphytic species, including slightly acidophilous Achnanthes and Navicula taxa (Rouillard et al. 2012; Table 1, no. 2). ...
... Lakewater pH has been proposed as an important driver of diatom assemblage changes in poorly buffered Arctic and Alpine lakes, with warm intervals resulting in higher pH values due to enhanced loss of CO 2 to the atmosphere and greater utilization of limnetic CO 2 by algal photosynthesis, compared with cooler periods with longer ice-cover (Michelutti et al. 2007;Rouillard et al. 2012;Wilson et al. 2012;Fritz and Anderson 2013). Two poorly buffered lakes in the high Arctic of Canada show regime shifts, with a shift in diatom dominance from small benthic Fragilaria taxa to a more diverse but less productive assemblage of small benthic and periphytic species, including slightly acidophilous Achnanthes and Navicula taxa (Rouillard et al. 2012; Table 1, no. 2). The dynamics behind the shifts are connected to the neoglacial cooling caused by decreasing summer insolation from the mid-to late Holocene, and the associated lengthening of ice cover and decreased lakewater pH (Rouillard et al. 2012). ...
Regime shifts in lake ecosystems can occur in response to both abrupt and continuous climate change, and the imprints they leave in paleolimnological records allow us to investigate and better understand patterns and processes governing ecological changes on geological time scales. This synthesis investigates paleolimnological records that display apparent regime shifts and characterizes the shifts as either smooth, threshold-like or bistable. The main drivers behind the shifts are also explored: direct climate influence on lakes, climate influence mediated through the catchment, lake ontogenetic processes and/or anthropogenic forcing. This framework helps to elucidate the relationship between driver and regime shift dynamics and the type of imprint that the associated regime shifts leaves in sediment records. Our analysis of the limited sites available (22 sites) show that smooth regime shifts are characterized with forcing and response variables acting on similar time scales, whereas regime shifts that demonstrate a threshold like response or a hysteresis response occur on shorter time scales than changes in drivers. The temporal resolution of the record, a common concern in paleo records, limits identification of the timing and rate of the regime shifts. When detected, past regime shifts offer rich opportunities to understand ecosystem responses to climate and other changes and to evaluate the mean state and natural variability of lake ecosystems on time scales of decades to millennia. There are a number of remaining challenges in understanding regime shifts and ecosystem dynamics in a paleolimnological perspective including lack of an appropriate temporal resolution and ecosystem feedback mechanisms. Combining paleoecology with contemporary studies can help clarify the scale of regime shifts and to distinguish patterns in ecosystem changes from natural variability.
... A problem in working with palaeolimnological records in general, and when investigating regime shifts in particular, is low temporal resolution. In some of the literature in our synthesis, the regime shift occurs between two samples, but given a resolution of 150 or even 300-400 years (Rouillard et al. 2012) per sample, it is impossible to establish whether the shift has taken place over 100, 10 or even one year. However, in all of the examples the forcing is acting on even longer time scales (Fig. 4), allowing identification of the general dynamics behind the shifts although the specifics are hidden in the time integration inherent to sediment records. ...
... Ice cover responds in a non-linear fashion to small changes in temperature (Weyhenmeyer et al. 2004), thus relatively subtle temperature changes can cause dramatic changes in lake state. Rapid ecological reorganizations driven by recent climate warming, mediated through changing ice-cover dynamics, are presently occurring in lakes throughout the Northern Hemisphere Smol and Douglas 2007a;Rühland et al. 2008;Holmgren et al. 2010;Rouillard et al. 2012;Table 1, no. 4). Although these regime shifts are manifested at several trophic levels , the shifts have been described mainly from changes in the species composition of diatom communities and are characterized by increases in small planktonic diatoms, such as Cyclotella species, and a compensatory decrease in small periphytic (often Fragilaria) species. ...
... Lakewater pH has been proposed as an important driver of diatom assemblage changes in poorly buffered Arctic and Alpine lakes, with warm intervals resulting in higher pH values due to enhanced loss of CO 2 to the atmosphere and greater utilization of limnetic CO 2 by algal photosynthesis, compared with cooler periods with longer ice-cover (Michelutti et al. 2007;Rouillard et al. 2012;Wilson et al. 2012;. Two poorly buffered lakes in the high Arctic of Canada show regime shifts, with a shift in diatom dominance from small benthic Fragilaria taxa to a more diverse but less productive assemblage of small benthic and periphytic species, including slightly acidophilous Achnanthes and Navicula taxa (Rouillard et al. 2012; Table 1, no. 2). ...
Regime shifts in lake ecosystems can occur in response to both abrupt and continuous climate change, and the imprints they leave in palaeolimnological records allow us to investigate and better understand patterns and processes governing ecological changes on geological time scales. This thesis aims at investigating palaeolimnological records of regime shifts in lakes during the Holocene to explore how lake ecosystems responded to climate changes and anthropogenic activities and to identify thresholds or tipping points that produced regime shifts. The thesis includes case studies of two Swedish lakes and a synthesis of recently published palaeolimnological records displaying regime shifts. In the first case study a detailed diatom record from Lake Kälksjön, west-central Sweden, was investigated for periods of abrupt ecological change associated with the 8.2 ka cooling event. Using change-point analysis we found two regime shifts in the diatom record: one in response to an abrupt erosion event at c. 8040 cal. yr BP, and another caused by climate warming following the 8.2 ka event. The study demonstrates that not only can regime shifts be detected in sediment records, they can also be quantified and statistically tested for, provided that the sampling resolution is high and the chronological control sufficiently precise. The second case study is focused on recent regime shifts between clear-water and turbid states in Lake Krankesjön in southern Sweden. We combined palaeolimnological records and limnological monitoring data, concentrating on the documented collapse of the clear-water state in 1975 and the subsequent recovery in the late 1980s, in order to increase our understanding of changing ecological patterns and processes in shallow lakes. We found that the shift from clear to turbid conditions was abrupt, while the recovery of the clear-water state was more gradual, and the complex and non-linear reactions of the lake to shifting conditions emphasizes the importance of careful lake and catchment management if a stabilization of the clear-water state is desired. In addition to these studies, the theoretical concept of regime shifts is expanded upon in our synthesis of palaeolimnological records with regime shifts, where we characterize the shifts as either smooth, threshold-like or bistable. The examples are also placed within a conceptual model of potential physical processes that produce regime shifts in lakes and the main drivers behind the shifts are identified: direct climate influence on lakes, climate influence mediated through the catchment, lake ontogenetic processes and/or anthropogenic forcing. This framework helps to elucidate the relationship between driver and regime shift dynamics and the type of imprint that the associated regime shifts leave in sediment records. When detected, past regime shifts and palaeolimnological records offer rich opportunities to increase the understanding of ecosystem responses to climate changes and to evaluate the mean state and natural variability of lake ecosystems on timescales of decades to millennia. Palaeolimnological archives provide a perspective on whether the pre-disturbance "natural" state that may be the target of restoration efforts is actually natural, or if it is an anomaly in lake history, and if this target is at all possible to reach or if it, due to shifting baselines and lake ontogeny, is no longer a realistic restoration endpoint. Furthermore, I argue that linking the timescales of contemporary ecology with palaeoecology/limnology may provide long-term records of lake history and opportunities to further disentangle the role of different forcings on lake ecosystems. An improved understanding of lake ecosystem dynamics and regime shifts in lakes through the cooperation of limnologist/ecologists and palaeolimnologists may better prepare us to face future climate change and to fully understand and perhaps mitigate the effects of global climate change on lake ecosystems.
... In high arctic lakes, which are often small and shallow and therefore closely connected to terrestrial and atmospheric processes, temporal fluctuations of lakes' bio-optical properties and organic carbon amounts and sources are probably mediated through periglacial catchment processes and ice cover through direct climatic forcing (cf. Rouillard et al. 2012) and, in some cases, superimposed on seabird impact that enhances allochthonous carbon transport (cf. Keatley et al. 2009;Luoto et al. 2014a, b). ...
... Pienitz & Vincent 2000). In addition to diatoms, new spectroscopic methods including visible-near-infrared and Fouriertransform infrared techniques and their calibration in space have been introduced to infer past lake-water organic carbon concentrations from lake sediments (Rosén & Persson 2006;Rouillard et al. 2011Rouillard et al. , 2012. Additionally, a recently introduced method utilizing spectroscopic UV-absorbance of cladoceran sedimentary remains has been used as an indirect proxy for palaeo-optical lake-water properties, including DOC (Nevalainen & Rautio 2014). ...
... In fact, in a nearby shallow freshwater pond (Kvalrosslaguna) in the Murchisonfjorden, this scenario occurred during the 20th century (Luoto et al. 2014a). In common with the Einstaken record, Rouillard et al. (2012) described high DOC concentrations in a freshwater lake in high arctic Canada during the early Holocene and they associated this DOC maximum with lush catchment vegetation and aquatic bryophytes. However, reduced sedimentary TOC and OM in Einstaken do not support this, suggesting that lake productivity did not increase (Fig. 3). ...
Long-term fluctuations in lake-water optical properties were examined using a Holocene sediment sequence and multi-proxy palaeolimnological approach in Lake Einstaken, Nordaustlandet, Svalbard. UV-absorbance of sedimentary cladoceran remains provided information on underwater UV exposure and changes in lake-catchment coupling processes were inferred from sediment geochemistry. In addition, aquatic community succession was used as an indicator for lake-water bio-optical properties and a Holocene record of sun activity (sunspots) was utilized to evaluate long-term solar forcing. The results indicated that the UV-absorbance of cladoceran remains was highest (i.e. maximum UV-induced pigmentation) for a short period during the early Holocene and for several millennia during the mid-Holocene. Sun activity was high during these time intervals, probably impacting the UV intensities, but it is probable that the amount of UV-attenuating compounds (e.g. dissolved organic carbon (DOC)) also significantly affected the underwater UV environment and were low during high UV exposure. Benthic autotrophic communities also responded to the millennial changes in lake-water optical properties. UV-resistant Nostoc cyanobacterial colonies were established during the mid-Holocene, indicative of high underwater UV intensities, and Fontinalis mosses thrived during the early Holocene, indicating a highly transparent water column. The results further suggested that underwater UV exposure decreased during the late Holocene, which is probably attributable to increased DOC and decreased solar forcing. Owing to the location of Lake Einstaken and its catchment in the periglacial barren landscape of the polar desert, the fluctuations of bio-optical lake-water properties were apparently forced by postglacial environmental processes and Holocene climate development. These factors controlled sea shoreline proximity, water discharge, ice-cover duration and littoral-benthic primary production and further affected the underwater UV environment. Although the role of solar forcing cannot be underestimated, the current record emphasizes the role of climate-mediated lake-catchment interactions in impacting bio-optical properties and UV exposure of high arctic aquatic systems.
... Lakewater DOC reconstructions from the high Arctic are rare. Rouillard et al. (2012) used reflectance spectroscopy on two high Arctic lake sediment cores to infer Holocene length DOC trends. They showed that lakewater DOC, inferred from the spectral signatures of sediments, tracked the main Holocene climatic trends including the Holocene Thermal Maximum and Neoglacial cooling. ...
... They showed that lakewater DOC, inferred from the spectral signatures of sediments, tracked the main Holocene climatic trends including the Holocene Thermal Maximum and Neoglacial cooling. The spectral approach used by Rouillard et al. (2011Rouillard et al. ( , 2012 has not yet received rigorous confirmation, but appears to show promise in reconstructing past trends in DOC levels. The diatom-based DOC model developed here can be used in conjunction with newly developed spectral approaches (Rouillard et al. 2011), allowing for two independent methods of reconstructing trends in DOC. ...
... Unfortunately, analog issues hinder the application of our DOC model to the Ellesmere Island ELLESMERE ISLAND DIATOMS (Arctic Canada) diatom stratigraphies in Rouillard et al. (2012), especially in the most recent sediments. However, some comparisons can still be made at a very basic level. ...
Arctic oases are regions of atypical warmth and relatively high biological production and diversity. They are small in area (2) and uncommon in occurrence, yet they are relatively well studied due to the abundance of plant and animal life contained within them. A notable exception is the lack of research on freshwater ecosystems within polar oases. Here, we aim to increase our understanding of freshwater diatom ecology in polar oases. Diatoms were identified and enumerated from modern sediments collected in 23 lakes and ponds contained within the Lake Hazen oasis on Ellesmere Island, and compared with diatom assemblages from 29 sites located outside of the oasis across the northern portion of the island. There were significant differences in water chemistry variables between oasis and northern sites, with oasis sites having higher conductivity and greater concentrations of nutrients and related variables such as dissolved organic carbon (DOC). Taxa across all sites were typical of those recorded in Arctic freshwaters, with species from the genera Achnanthes sensu lato, Fragilaria sensu lato, and Nitzschia dominating the assemblages. A correspondence analysis (CA) ordination showed that oasis sites generally plotted separately from the northern sites, although the sites also appear to plot separately based on whether they were lakes or ponds. Canonical correspondence analysis (CCA) identified specific conductivity, DOC, and SiO2 as explaining significant (P 0.05) and additional amounts of variation in the diatom data set. The most robust diatom‐based inference model was generated for DOC, which will provide useful reconstructions on long‐term changes in paleo‐optics of high Arctic lakes.
... While the majority of the sites studied in the High Arctic have shown some form of response to climate change (e.g.,123), not all freshwater ecosystems respond in a similar manner [4]. For example, paleolimnological research on Arctic lakes has shown that poorly buffered waterbodies are particularly susceptible to climate-driven pH changes over Holocene timescales (e.g.,5678). As long-term monitoring data are not available in most Arctic regions, paleolimnological proxies, such as diatoms (e.g., [9]), present us with a means to reconstruct past limnological and climatic shifts, thereby providing information about the natural variability of these ecosystems prior to the modern instrumental record. ...
... The addition of low alkalinity sites to the previously documented lakes and ponds from the eastern portion of Baffin Island [11, 12] have allowed us to create a diatom-based transfer function for reconstructing lakewater pH, which was otherwise not possible. This diatom-based pH model can now be applied downcore in poorly buffered lakes that will provide a sensitive record of past climate-driven pH dynamics (e.g.,5678). ...
... The lack of extensive carbonate-bearing bedrock, described by Kerr [10], has resulted in a weakened buffering capacity of the sites on western Bathurst Island. This reduction in CaCO 3 input may render these sites more susceptible to climate-driven pH changes similar to those observed in other poorly buffered High Arctic regions (e.g.,5678). Specific conductivity measurements from the western study sites (mean ¼ 97.7 mS/cm) were much lower than those recorded in the east by Lim et al. (2001b; mean ¼ 150.8 mS/cm). ...
We examined the influence of catchment geology, specifically differences in buffering capacity, on the limnological characteristics and surface sediment diatom assemblages from lakes and ponds from Bathurst Island, High Arctic Canada. Differences in buffering capacity exist on Bathurst Island due to a geological gradient that spans from carbonate-bearing limestone in the east, to more stable quartz sandstone, siltstone, and shale in the west. We collected physical and chemical limnological data, as well as surface sediment diatom assemblages from nine ponds on the poorly buffered western portion of the island and combined these observations with a previously published dataset of 31 lakes and ponds, from the well-buffered eastern region. The addition of these nine ponds expanded the pH gradient of the existing Bathurst Island dataset (pH 8.0–8.6) to pH 6.8–8.6. A regional, weighted average diatom-inferred pH model was developed and showed strength similar to other Arctic calibration sets (, root-mean-squared-error of prediction (RMSEP) = 0.298). Given the links between climate and pH shifts in the High Arctic, the ability to reconstruct pH should be a valuable tool for future paleolimnological studies.
... The diatom profile from West Lake shows no directional assemblage change, although it is more variable as compared to the other "cold" records ( Fig 6C) with fluctuations between two epipelic species. The dominant species and high variability are consistent with the late Holocene assemblages from a 10,000 year diatom record of West Lake [69] suggesting the shifts in our short record are generally not outside the range of variability of the past 4000 years. ...
... Diadesmis gallica [51]) and epipelic (e.g. Eucocconeis leptostriata, Amphora inariensis) species characterizes the pre-shift assemblages, assemblages that are typical for poorly buffered Arctic sites [69,81,82]}. While the "cold" lakes have similar DIC concentrations to the poorly-buffered "cool" sites, these deeper lakes have had, until recently, persistent ice pans in summer. ...
Recent climate change has been especially pronounced in the High Arctic, however, the responses of aquatic biota, such as diatoms, can be modified by site-specific environmental characteristics. To assess if climate-mediated ice cover changes affect the diatom response to climate, we used paleolimnological techniques to examine shifts in diatom assemblages from ten High Arctic lakes and ponds from Ellesmere Island and nearby Pim Island (Nunavut, Canada). The sites were divided a priori into four groups (“warm”, “cool”, “cold”, and “oasis”) based on local elevation and microclimatic differences that result in differing lengths of the ice-free season, as well as about three decades of personal observations. We characterized the species changes as a shift from Condition 1 (i.e. a generally low diversity, predominantly epipelic and epilithic diatom assemblage) to Condition 2 (i.e. a typically more diverse and ecologically complex assemblage with an increasing proportion of epiphytic species). This shift from Condition 1 to Condition 2 was a consistent pattern recorded across the sites that experienced a change in ice cover with warming. The “warm” sites are amongst the first to lose their ice covers in summer and recorded the earliest and highest magnitude changes. The “cool” sites also exhibited a shift from Condition 1 to Condition 2, but, as predicted, the timing of the response lagged the “warm” sites. Meanwhile some of the “cold” sites, which until recently still retained an ice raft in summer, only exhibited this shift in the upper-most sediments. The warmer “oasis” ponds likely supported aquatic vegetation throughout their records. Consequently, the diatoms of the “oasis” sites were characterized as high-diversity, Condition 2 assemblages throughout the record. Our results support the hypothesis that the length of the ice-free season is the principal driver of diatom assemblage responses to climate in the High Arctic, largely driven by the establishment of new aquatic habitats, resulting in increased diversity and the emergence of novel growth forms and epiphytic species.
... They are dominated by small, colonial fragilarioid diatom taxa (Staurosira venter, Staurosirella pinnata, and Stauroforma exiguiformis) that typically occupy benthic habitats and are generally associated with early post-glacial paleoenvironments where glacial runoff supplies some mineral nutrients to sustain diatom communities in recently established ultra-oligotrophic lakes. These taxa have been widely reported in those conditions in early post-glacial sediments of Arctic lakes for the Holocene (Smol 1983;Cremer et al. 2001;Finkelstein and Gajewski 2007;Rouillard et al. 2012) and for earlier interglacials (Wilson et al. 2012). At Lake RS29, this early post-glacial assemblage was quickly replaced by a more unusual assemblage dominated by Eunotia praerupta, Neidium affine, Pinnularia nodosa, and Tabellaria flocculosa. ...
... Taken as a whole, the diatom assemblages in Zone 1 suggest some supply of base cations from the watershed to the lake and a warm and wet enough climate to support the development of the littoral zone vegetation, providing periphytic diatom habitats and releasing DOC to the lake. Further, given the warmer climate at this time (see below), the thicker active layer may have increased particulate and DOC fluxes from the watershed to lakes, influencing diatom assemblages (Fritz and Anderson 2013); high DOC concentrations have been inferred spectroscopically from other High Arctic lakes during early Holocene thermal maxima (Rouillard et al. 2012). Indications of significant dissolution of diatom valves further characterized diatom Zone 1. ...
This study presents a diatom-based analysis of the post-glacial Holocene environmental history at Lake RS29 on Somerset Island in the Canadian High Arctic. Earliest post-glacial diatom assemblages (10 200–10 000 cal yr BP) consisted mainly of small, benthic fragilarioid taxa. Poor diatom preservation in the early Holocene (~10 000–6200 cal yr BP) is associated with warm conditions, as determined by pollen data from the same core and other paleoclimate estimates from the region. Analysis of this and other sites from across the Canadian Arctic suggest that zones of poor diatom preservation or diatom absence in lake sediment records may be associated with warm conditions. After 6200 cal yr BP, acidophilic assemblages consisting of Aulacoseira spp. and a suite of periphytic taxa indicate acidification since the mid-Holocene. During this time period, cooling causing changes in lake ice phenology was likely a major driver of the reconstructed mid-Holocene pH decline. Watershed processes, including reduced fluxe...
... Decreasing sulfur deposition rates allow catchment soils to recover from acidification, where the resulting decrease in acidity and/or ionic strength of the soil solution may increase the solubility of soil organic matter (OM) and consequently OC leaching (4,16,17). Some studies have linked the OC rise to recent changes in land-use practices, such as the drainage and burning of peatlands (18,19), or to changes in land cover (20). Other proposed explanations are the increase in atmospheric carbon dioxide levels (21) and the elevated atmospheric deposition of nitrogen (22,23), both of which support increased plant productivity. ...
... The models developed by Rosén (6) and Cunningham et al. (7) have been applied with promising results in other studies aiming at, for example, reconstructing past mire, permafrost, and treeline dynamics (6,(10)(11)(12); assessing factors driving changes in lake-water chemistry (13)(14)(15); and tracking ecosystem responses to climate variations (16,17). Rouillard et al. (18,19) used the same methodology to develop a comparable model for Arctic Canada, demonstrating that the approach to use VNIR spectroscopy to infer past lake-water OC concentrations is not restricted to Scandinavian lakes and can also be used in regions with different environmental settings. ...
Significance
Monitoring programs have recorded increases in organic carbon concentrations in northern lakes, which have important implications for water quality and ecosystem functioning. Current hypotheses interpret this trend in light of recent environmental changes such as acidification and climate but do not include an examination of long-term changes and their causes. We inferred past trends from sediment archives across central Sweden, allowing us to assess recent changes on a millennial scale. Our data demonstrate that a long-term decline beginning already in the 15th century preceded the recent organic carbon increase. This was a response to spatially extensive human–landscape interactions that included forest grazing and mire exploitation, which were common across Europe and altered carbon cycling between terrestrial and aquatic ecosystems.
... The general catchment maturation accompanied by decreases of glacial ice-sheet influence led to initiation of new ecological niches and thus diversification of e.g. epiphytic species (Wilson et al., 2012;Rouillard et al., 2012). ...
This study is based on multiproxy data gained from a 14C-dated 6.5 m long sediment core and a 210Pb-dated 23 cm short core retrieved from Lake Rauchuagytgyn in Chukotka, Arctic Russia. The main objectives are to reconstruct the environmental history and ecological development of the lake during the last 29k years and to investigate the main drivers behind bioproduction shifts. The methods comprise age-modeling and accumulation rate estimation, light-microscope diatom species analysis (74 samples), organic carbon, nitrogen, and mercury analysis. Diatoms have appeared in the lake since 21.8 cal ka BP and are dominated by planktonic Lindavia ocellata and L. cyclopuncta. Around the Pleistocene-Holocene boundary, other taxa including planktonic Aulacoseira and benthic fragilarioid (Staurosira) and achnanthoid species increase in their abundance. There is strong correlation between variations of diatom valve accumulation rates (DAR, mean 176.1 109 valves m2 a1), organic carbon accumulation rates (OCAR, mean 4.6 g m-2 a-1), and mercury accumulation rates (HgAR, mean 63.4 µg m-2 a-1). We discuss the environmental forcings behind shifts in diatom species and found responses of key-taxa to the cold glacial period, postglacial warming, Younger Dryas, and the Holocene Thermal Maximum. The short core data likely suggest recent change of the diatom community at 1907 CE related to human-induced environmental change. Significant correlation between DAR and OCAR in the Holocene interglacial indicates within-lake bioproduction as the main source of carbon deposited in the lake sediment. During both glacial and interglacial episodes HgAR is mainly bound to organic matter in the lake associated to biochemical substrate conditions. There were only ambiguous signs of increased HgAR during the industrialization period. We conclude that pristine Arctic lake systems can serve as CO2 and Hg sinks during warming climate driven by insolation-enhanced within-lake primary productivity. Maintaining intact natural lake ecosystems should therefore be of interest to future environmental policy.
... Both major assemblage shifts occurred prior to the acceleration of glacial inputs in 2007 CE, when catchment glaciers shifted from net mass gain to net mass loss [13]. This has been evident since the first cursory palaeolimnological study of Lake Hazen [15] An extensive body of Arctic palaeolimnological studies, performed on lakes in glacier-free catchments, have documented diatom compositional shifts similar to those recorded in Lake Hazen, including several from Ellesmere Island [27][28][29][30]. These studies ascribed the assemblage shifts to decreased duration of snow and ice cover and longer growing seasons associated with warming [18,[31][32][33][34]. Specifically, the initial rise in benthic fragilarioids occurs in response to the amelioration of diatom growing conditions with longer ice-free conditions in littoral environments. ...
Lake Hazen, the High Arctic's largest lake, has received an approximately 10-fold increase in glacial meltwater since its catchment glaciers shifted from net mass gain to net mass loss in 2007 common era (CE), concurrent with recent warming. Increased glacial meltwater can alter the ecological functioning of recipient aquatic ecosystems via changes to nutrient budgets, turbidity and thermal regimes. Here, we examine a rare set of five high-resolution sediment cores collected in Lake Hazen between 1990 and 2017 CE to investigate the influence of increased glacial meltwater versus alterations to lake ice phenology on ecological change. Subfossil diatom assemblages in all cores show two major shifts over the past approximately 200 years including: (i) a proliferation of pioneering, benthic taxa at approximately 1900 CE from previously depauperate populations; and (ii) a rise in planktonic taxa beginning at approximately 1980 CE to present-day dominance. The topmost intervals from each sequentially collected core provide exact dates and demonstrate that diatom regime shifts occurred decades prior to accelerated glacial inputs. These data show that diatom assemblages in Lake Hazen are responding primarily to intrinsic lake factors linked to decreasing duration of lake ice and snow cover rather than to limnological impacts associated with increased glacial runoff.
... Species from the genus Aulacoseira require a sufficient amount of silicon for normal development, which becomes available only under a good mixing of the water column (Ruhland and . The species Pseudostaurosira brevistriata and Staurosirella pinnata became dominant after 1870, as they were the first to inhabit the carbonate-rich waters of the recently thawed areas (Puusepp et al., 2010;Rouillard et al., 2012). The dominance of the Staurosirella pinnata also reflects an increase in paleotemperatures and climate warming (Weckstrom et al., 1997;Kumke et al., 2004). ...
We studied diatom assemblages of the proglacial arctic lake Bolshoy Kharbey (Bolshezemelskaya tundra, Russian Arctic) from a short sediments core covering the last ca. 200 years. In total, 121 taxa from 2 classes, 5 orders, 18 families, and 50 genera were identified. The diatom flora included species with mainly cosmopolitan distribution. The assemblages were dominated by alkaliphilic benthic diatoms preferring standing — flowing waters, indifferent to salinity and moderate temperature conditions. The main changes in diatom assemblages took place in ca. 1870 and 1980. After 1870, which can be attributed to the end of the Little Ice Age, the species richness rose, especially of planktonic centric and small penate diatoms. This rise of diatom diversity took place most probably in response to climate warming, related to it prolongation of the growing season and period of open water. A negative trend in the evenness of the diatom assemblages during the last decades can be seen as an early-warning signal indicating a decrease of stability of the lake ecosystem.
... Our results showed limited 20 th century coherence with climate variables. The increasing temperature trends we observed using data from multiple California stations were consistent with increasing temperature trends, especially post-1940, observed by Bonfils et al. (2008) Warmer temperatures have been shown to increase ANC due to increased weathering rates and productivity and thus could be contributing to ANC recovery (Rouillard et al. 2012). However, the linear temperature trend did not explain the complete decreasingincreasing ANC pattern and the PCA indicated correlation with different principal components ( Figure 2.6). ...
... According to literature data, the abundance of these two genera is determined by low concentrations of dissolved calcium, magnesium, carbonate, and hydrocarbonate ions and low pH values (Van Dam et al., 1994;Potapova et al., 2003;Stenina, 2008). In arctic water bodies with low mineralization rate, pH becomes the most important factor that affects the development of diatom communities (Rouillard et al., 2012). ...
This article discusses the results of a taxonomic and ecological investigation into diatoms from polygonal ponds and quaternary permafrost deposits of Bol'shoy Lyakhovsky Island (New Siberian Archipelago) and the reconstruction of climatic changes on the island during Late Pleistocene/Holocene transition using fossil diatom assemblages from the permafrost deposits. The taxonomic list of diatoms includes 159 dia-tom species. The main ecological factors that determine the distribution of diatoms in the investigated data set are mean air temperature in July, рН, conductivity, water depth, and the concentration of Si 4+ and Al 3+. An increase in water depth and stable lacustrine conditions in the Lateglacial–Holocene in the ancient thermokarst lake relate to Lateglacial warming before 11860 ± 160 years BP and during the early Holocene between 11210 ± 160 and 7095 ± 60 years BP.
... According to previous investigation in Arctic freshwaters recent factors influencing diatom species composition include lake-water pH, conductivity, nutrients, dissolved organic carbon, water temperature, and habitat availability Rühland et al., 2008;Biskaborn et al., 2012;Rouillard et al., 2012). Eunotia taxa are commonly acidophilous species found at low Ca 2+ and Mg 2+ concentrations and have lower optima for HCO 3 + CO 3 (Van Dam et al., 1994;Potapova and Charles, 2003). ...
... Виды Pseudostaurosira brevistriata, Staurosirella pinnata становятся доминантами , поскольку отмечаются первыми в богатых карбонатами водах недавно оттаявшей местности. Их доминирование также отражает условия щелочной среды, обусловленные большим притоком основных катионов c недавно оттаявших водосборов (Rouillard et al., 2012; Puusepp et al., 2010). Доминирование вида Staurosirella pinnata, по мнению ряда исследователей, также отражает увеличение палеотемператур и потепление климата (Weckstrom et al., 1997; Kumke еt al., 2004; Holmgren et al., 2010). ...
Arctic lakes, which are not affected by direct human impact, are excellent objects for paleoclimatic reconstructions. Diatoms can be a source of information about the changes that occurred in lakes. The recent Holocene sediments from deep glacial arctic Lake Bolshoy Kharbei of Bolshezemelskaya tundra were studied for diatoms. In total, 122 taxa of 2 classes, 5 orders, 18 families, 45 genus of cosmopolitan forms with a predominance of alkaliphilic benthic species of diatoms preferring a standing – flowing waters, indifferent to salinity and moderate temperature conditions were revealed. There is a trend in forming diatom flora of Holocene sediments of Lake Bolshoy Kharbei manifested in changes in the composition of dominants due to climate warming. As a response to increase of paleotemperature, the lengthening of the growing season and period of open water in the lake, the rise of the water level and the rate of water exchange took place, which contributed to the development of planktonic centric and small pennate diatoms with a consequent increasing of their valves share in the sediments.
... Along with the other diatoms , monoraphid diatoms can be used to infer environmental conditions at differing spatial and temporal scales. They are particularly useful as a diverse group of indicator organisms in multivariate environmental gradient analysis , including salinity (Bradbury, 1987;Wilson et al., 1994;Cumming et al., 1995;Laird et al., 1998;Michelutti et al., 2013), nutrients (Bradbury, 1975;Christie and Smol, 1993;Fritz et al., 1991;Reavie et al., 1995;Dixit et al., 1992;Siver, 1999;Potapova, 2003;Potapova and Carlisle, 2011), lake acidity (Koppen, 1978;Charles, 1985;Cumming et al., 1992;Kingston et al., 1992), dissolved organic carbon (Kingston and Birks, 1990;Rouillard et al., 2012), toxic metals , temperature (Kingston et al., 1983;Pienitz and Smol, 1992), and lake depth (Yang and Duthie, 1995;Brugam et al., 1998;Kingsbury et al., 2012). Multivariate community analysis of diatoms is used to understand paleo and modern environments (ter Braak and Juggins, 1993;Fritz et al., 1991;Vyverman et al., 2007). ...
This monophyletic group of diatoms has the evolutionary innovation of a raphe system, which has afforded its members the ability to move longer and faster, relative to their body size. Species within the 105 genera treated here occupy a wide range of habitats, including living adnate on substrates, being motile in benthic environments, living attached or in mucilaginous tubes, and even creating colonies to live in the plankton. We treat the following major groups of raphid diatoms: Eunotiales (6 genera), Achnanthales (11 genera), Naviculales (72 genera), Bacillariales (9 genera), Rhopalodiales (2 genera), and Surirellales (5 genera). For each group, we present scanning electron microscope images to illustrate valve morphology. For genera in each group, we provide original light microscope images, descriptions, and comments on ecological preferences.
... For example, a warmer climate and longer open-water period can result in a shift towards a more species-rich assemblage as new diatom habitats become available and, in deeper lakes, can also result in a change in diatom life strategy from mostly benthic to more planktonic assemblages with increased thermal stability and reduced mixing strength (Smol et al., 2005;Rühland et al., 2008). Diatoms have also been successfully used to track Arctic Holocene tree-line migration through diatom-inferred changes in dissolved organic carbon (DOC) (Rouillard et al., 2011(Rouillard et al., , 2012. For example, warmer temperatures during the Holocene thermal maximum followed by a return to cooler conditions during the Neoglacial, led to the advancement (and then retreat) of needle-leaf trees onto (and from) the catchments of tundra lakes for the first time, thereby increas-ing (and then decreasing) DOC delivery to the lake (Pienitz et al., 1999). ...
We present a systematic compilation of previously published Holocene proxy
climate records from the Arctic. We identified 170 sites from north of
58° N latitude where proxy time series extend back at least to
6 cal ka (all ages in this article are in calendar years before present – BP),
are resolved at submillennial scale (at least one value every
400 ± 200 years) and have age models constrained by at least one age
every 3000 years. In addition to conventional metadata for each proxy record
(location, proxy type, reference), we include two novel parameters that add
functionality to the database. First, "climate interpretation" is a series
of fields that logically describe the specific climate variable(s)
represented by the proxy record. It encodes the proxy–climate relation
reported by authors of the original studies into a structured format to
facilitate comparison with climate model outputs. Second,
"geochronology accuracy score" (chron score) is a numerical rating that
reflects the overall accuracy of 14C-based age models from lake and
marine sediments. Chron scores were calculated using the original
author-reported 14C ages, which are included in this database. The
database contains 320 records (some sites include multiple records) from six
regions covering the circumpolar Arctic: Fennoscandia is the most densely
sampled region (31% of the records), whereas only five records from
the Russian Arctic met the criteria for inclusion. The database contains
proxy records from lake sediment (60%), marine sediment (32%),
glacier ice (5%), and other sources. Most (61%) reflect
temperature (mainly summer warmth) and are primarily based on pollen,
chironomid, or diatom assemblages. Many (15%) reflect some aspect of
hydroclimate as inferred from changes in stable isotopes, pollen and diatom
assemblages, humification index in peat, and changes in equilibrium-line
altitude of glaciers. This comprehensive database can be used in future
studies to investigate the spatio-temporal pattern of Arctic Holocene
climate changes and their causes. The Arctic Holocene data set is available
from NOAA Paleoclimatology.
This study is based on multiproxy data gained from a 14C-dated 6.5 m long sediment core and a 210Pb-dated 23 cm short core retrieved from Lake Rauchuagytgyn in Chukotka, Arctic Russia. Our main objectives are to reconstruct the environmental history and ecological development of the lake during the last 29 kyr and to investigate the main drivers behind bioproduction shifts. The methods comprise age-modeling, accumulation rate estimation, and light microscope diatom species analysis of 74 samples, as well as organic carbon, nitrogen, and mercury analysis. Diatoms have appeared in the lake since 21.8 ka cal BP and are dominated by planktonic Lindavia ocellata and L. cyclopuncta. Around the Pleistocene–Holocene boundary, other taxa including planktonic Aulacoseira, benthic fragilarioid (Staurosira), and achnanthoid species increase in their abundance. There is strong correlation between variations of diatom valve accumulation rates (DARs; mean 176.1×109 valves m2 a1), organic carbon accumulation rates (OCARs; mean 4.6 g m−2 a−1), and mercury accumulation rates (HgARs; mean 63.4 µg m−2 a−1). We discuss the environmental forcings behind shifts in diatom species and find moderate responses of key taxa to the cold glacial period, postglacial warming, the Younger Dryas, and the Holocene Thermal Maximum. The short-core data likely suggest recent change of the diatom community at the beginning of the 20th century related to human-induced warming but only little evidence of atmospheric deposition of contaminants. Significant correlation between DAR and OCAR in the Holocene interglacial indicates within-lake bioproduction represents bulk organic carbon deposited in the lake sediment. During both glacial and interglacial episodes HgAR is mainly bound to organic matter in the lake associated with biochemical substrate conditions. There were only ambiguous signs of increased HgAR during the industrialization period. We conclude that if increased short-term emissions are neglected, pristine Arctic lake systems can potentially serve as long-term CO2 and Hg sinks during warm climate episodes driven by insolation-enhanced within-lake primary productivity. Maintaining intact natural lake ecosystems should therefore be of interest to future environmental policy.
The eastern North American Arctic has a complex 5000-year prehistory, during which many human population movements occurred over great distances. Archaeologists have interpreted these movements as resulting from many factors, however the effects of climate change are often hypothesized as primary drivers that can “push” human groups to leave some regions, or “pull” them to move to others. Here, we examine climate change at the regional scale over the Common Era using Arctic paleoclimate data derived from a wide suite of biological proxies and geochemical tracers. We consider available statistical composites of high resolution (sub-decadal) paleo-temperature reconstructions for the Arctic region, as well as local-scale reconstructions at century or sub-century scale resolution in three focal regions of archaeological significance relevant to population movements: Victoria Island, Foxe Basin/Baffin Island, and the High Arctic (Ellesmere Island/Northwestern Greenland). We emphasize the two most widespread, though variable, climate change events characteristic of this period: the Medieval Climatic Anomaly and Little Ice Age in the second millennium AD; we also evaluate the evidence for the Roman Warm Period and Dark Ages Cold Period in the first millennium AD, as context for later events. We integrate information on the timing and magnitude of these events across the Canadian Arctic Archipelago, and assess the degree to which they coincide with current understanding of major population movements, with particular emphasis on three migration episodes. First, the expansion of Late Dorset Paleo-Inuit to the Central and High Arctic beginning in the late first millennium AD is plausibly linked to warming temperatures of the MCA. Second, the migration of Thule Inuit from Alaska to the Eastern Arctic beginning ca AD 1250 is not linked to warmer temperatures as previously hypothesized, and is therefore more likely related to social factors in Alaska. Third, the abandonment of northern regions and new settlement of southern regions by Inuit in the mid-second millennium AD is likely linked to a combination of cooling climate and increasing availability of European trade goods. Together, these three case studies indicate that linkages between climate change and migration are complex, variable, and mediated through social and economic factors.
Changing lake-water total organic carbon (TOC) concentrations are of concern for lake management because of corresponding effects on aquatic ecosystem functioning, drinking water resources and carbon cycling between land and sea. Understanding the importance of human activities on TOC changes requires knowledge of past concentrations; however, water-monitoring data are typically only available for the past few decades, if at all. Here, we present a universal model to infer past lake-water TOC concentrations in northern lakes across Europe and North America that uses visible-near-infrared (VNIR) spectroscopy on lake sediments. In the orthogonal partial least squares model, VNIR spectra of surface-sediment samples are calibrated against corresponding surface-water TOC concentrations (0.5–41 mg L⁻¹) from 345 Arctic to northern temperate lakes in Canada, Greenland, Sweden and Finland. Internal model-cross-validation resulted in a R² of 0.57 and a prediction error of 4.4 mg TOC L⁻¹. First applications to lakes in southern Ontario and Scotland, which are outside of the model’s geographic range, show the model accurately captures monitoring trends, and suggests that TOC dynamics during the 20th century at these sites were primarily driven by changes in atmospheric deposition. Our results demonstrate that the lake-water TOC model has multi-regional applications and is not biased by post-depositional diagenesis, allowing the identification of past TOC variations in northern lakes of Europe and North America over timescales of decades to millennia.
Freshwater diatoms are more and more frequently used in paleoenvironmental studies. The purpose of this research was to assess their potential as bioindicators in a region where little information exists about diatoms. A total of 516 diatom species were identified in the surface sediments of 59 lakes located along a latitudinal gradient in northern Quebec. The use of multivariate analysis allowed to determine the environmental variables (among 49) which had the greatest influence on diatom community composition. The concentrations of magnesium, sodium, dissolved organic carbon (DOC) and silica, as well as water depth seemed to be the most influential variables. An inference model for DOC has been developed which is useful for paleolimnological reconstructions of past DOC concentrations, as well as other variables that are correlated with DOC, such as catchment inputs, water temperature, water transparency and UV penetration.
The effects of climatic variables on lake-water total organic carbon (TOC) concentrations and benthic and pelagic primary producers during the past 45 yr were assessed using the sediment records of two subarctic lakes, one with mires and one without mires connected to the lake. The lake with a mire showed large and synchronous changes in the planktonic to benthic (P: B) ratio of diatoms and concentrations of TOC inferred from near-infrared spectroscopy. During periods of warm temperatures, high precipitation, and long ice-free conditions, we inferred high TOC in the lake, and the diatom community was dominated by planktonic species. The stable carbon isotopic (δ 13C) values of sediment organic matter were negatively correlated with inferred TOC concentration and P: B ratio. We suggest that the changes in TOC and P: B ratio were a result of changing climate, permafrost degradation, and related changes in the catchment. Terrestrial organic matter, by its strong effect on the penetration of light through the lake water, possibly affected the habitats available for benthic photosynthesis and thus the δ 13C of the sediment organic matter. The large changes in recent times may also be because of unusually long ice-free periods, warmer temperatures, and other associated limnological changes. The lake with no mire next to the lake showed only minor changes in lake-water TOC during the same period and P: B ratio remained almost constant until the past 5 yr, when the P: B ratio increased rapidly. The observed changes in P: B ratio within this lake may be because of complex interactions of several climate-related variables. © 2009, by the American Society of Limnology and Oceanography, Inc.
Diatoms from Sawtooth Lake (798209N, 818519W) on the Fosheim Peninsula in Central Ellesmere Island, Canada were analyzed to assess the temporal extent and magnitude of climatic change in the High Arctic during the late Holocene. Diatom results from the sediment cores show an absence of diatoms throughout the last ;2.5 ka (4.6 m) until the 1920s. However, ca. 1926 (5.3-cm depth), a rapid colonization of diatoms in the lake occurred. Within the uppermost section of the core (;1920 to ;1997), the diatom flora shift from a small Fragilaria-dominated assemblage to a more diverse assemblage that is dominated by large planktonic taxa (e.g., Cyclotella bodanica) and large raphid benthic species. The postglacial nature of this assemblage suggests a decrease in ice cover and a concomitant increase in light and nutrient availability for diatom growth over the last ;75 yr. Of particular significance is this absence of diatoms prior to the ;1920s, which indicates that environmental conditions of the last ;75 yr are unlike any of the previous ;2500 yr.
High-resolution diatom stratigraphies from two lakes on eastern Baffin Island (Nunavut, Canadian Arctic) are used to reconstruct lake-water pH over the past 5 k.y. Despite contrasting geomorphic histories and markedly different diatom floras, the inferred pH of both lakes declined in concert with Neoglacial cooling. This result confirms that climate exerts a first-order influence on the acidity of these poorly buffered oligotrophic lakes, through links between lake ice cover, primary productivity, and dissolved inorganic carbon dynamics. Diatom-based pH inferences from dilute lakes can therefore assist paleoclimatic interpretations from lake sediments.
Small benthic fragilarioid diatoms (Bacillariophyceae) are dominant taxa in circumpolar Arctic lakes and ponds. These small forms are difficult to identify under light microscope (LM) because of valve size, the limited number of visible characters, and considerable morphological variability within populations. For this reason, ultrastructural characters visible only under a scanning electron microscope (SEM) such as areolae and spine structure are usually recommended for species- and even genus-level identification. This is, however, often impractical. In this study we show that selected characters of Staurosirella pinnata (Ehrenberg) Williams & Round and Staurosira venter (Ehrenberg) Cleve & Möller measurable under LM correlate with diagnostic characters that can only be evaluated under SEM. We measured 32 morphological characters from SEM observations of 510 valves representing Staurosirella pinnata sensu Hustedt, Staurosira venter sensu Hustedt, and two morphologically distinct forms of S. venter sensu lato from lakes in Canada, Fennoscandia, and Siberia. Specimens from the circumpolar Arctic region identified as Staurosirella pinnata can be distinguished from Staurosira venter sensu lato using valve width and areola length (at margin), both visible under LM. Specimens with areolae lengths (at margin) larger than 0.4 μm are Staurosirella pinnata. For specimens with areolae lengths (at margin) smaller than 0.4 μm, those with valves wider than 2.9 μm are Staurosira venter sensu lato, whereas those with valves smaller than 2.9 μm are Staurosirella pinnata. Application of these numerical criteria allows for correct identification of 94% of specimens to either Staurosira venter (including morphotypes) or Staurosirella pinnata.
Four ice cores from the top of the Agassiz Ice Cap and down a flow line have been variously analysed for 8 (18O), ECM (solid conductivity) and ice-melt layer stratigraphy. Stratigraphic correlation of volcanic horizons is used to date the last 8000 years of the cores. The timescales at the Wisconsin/ Holocene transition are pinned at the new GRIP ice-core date. Both 8 and summer-melt records from cores A84 and A87 imply summer temperatures have decreased from 8000 BP to the present by about 2.0°C. Differences in the various 8 series are explained in terms of local drift noise, excessive summer melt and ice flow originating from higher up the local dome where winter snow is scoured away. The present accumulation pattern along the flow line allows one to explain the smoothed differences in the δ records from 8000 BP to the present, but the massive summer melting between 8000 BP and the transition seems to have significantly altered the site and possibly introduced discontinuities. The massive summer melting in the early Holocene alters the volcanic acid (ECM) record in all the cores.
Diatoms from the surface sediments of 61 lakes on Baffin Island, Nunavut, were identified, enumerated, and interpreted quantitatively. The samples span a latitudinal transect from 62 to 74°N, reflecting climatic and vegetational gradients that range from low Arctic to transitional mid-Arctic to high Arctic. While the sampled lakes encompass both predominantly maritime and continental climatic regimes, sites have been deliberately restricted to basins in Precambrian crystalline terrain to mitigate the edaphic consequences of alkaline sedimentary bedrock. Canonical correspondence analysis, using forward selection and Monte Carlo permutation tests, identified pH, conductivity, summer lake water temperature, and mean annual air temperature as significant environmental controls over diatom assemblages. Using weighted-averaging regression and calibration, predictive models for these parameters have been developed. When applied to down-core assemblages, the summer lake water temperature model provides realistic reconstructions when compared with other paleoenvironmental records. Over the past 5000 years, the amplitude of reconstructed summer lake water temperature is on the order of 4.0°C, expressed primarily as progressive Neoglacial cooling culminating in the Little Ice Age. Diatom-inferred summer water temperatures have increased by 2.0°C in the past 150 years, again in agreement with independent paleoclimatic reconstructions.
Reflectance spectroscopy has made it possible to rapidly and nondestructively assess the chlorophyll content of plants and natural waters. However, to date this approach has not been applied to chlorophyll and chlorophyll deriv- atives preserved in lake sediments. Here, we explore the relationships between visible-near-infrared spectral properties of lake sediments and measured pigment concentrations for lakes that have been exposed recently to anthropogenic ni- trogen deposition. Down-core decreases in pigment concentrations and changes in reflectance properties effectively chronicle increases in whole-lake primary production since 1950. Specifically, reflectance spectra of sediments from four alpine lakes in Rocky Mountain National Park (Colorado Front Range, USA) preserve salient troughs near 675 nm that covary in magnitude with concentrations of chlorophyll a and associated pheopigments. The area of the trough in reflectance between 600 and 760 nm best explains the sum of total chlorophyll a and its derivatives (r2 = 0.82, n = 23, P < 0.01). This result suggests that chlorophyll a preserved in lake sediments can be remotely sensed using a simple index derived from reflectance spectroscopy, thus providing a new paleolimnological strategy for rapid exploratory as- sessments of changing lake trophic status.
The assumption that within-lake, deep-water sedimentary diatom assemblages are relatively uniform and that a single core is sufficient to depict lake ontogeny was tested for a small tarn on the southwestern Cumberland Peninsula of Baffin Island, Northwest Territories. Diatom transport and deposition were evaluated through analyses of periphytic, planktonic, and epipelic habitats. Diatom stratigraphies of four cores were used to test whether or not trends are comparable in different regions of the lake and throughout the Holocene. Among 12 surface-sediment stations, diatom distributions were alternately highly equitable or variable. Valves of evenly distributed genera (Aulacoseira and Achnanthes) are mixed in the water column prior to deposition. This is supported by plankton tow and periphyton samples, which were respectively dominated by Aulacoseira distans (and varieties) and Achnanthes altaica. Conversely, frequencies of several benthic taxa (e.g., Pinnularia biceps, species of Eunotia) varied up to 30% between stations, in patterns unrelated to water depth, and reflecting habitat specificity and minimal transport prior to burial. Of the four cores (38.0–95.5 cm), analysis of the two longest revealed three distinct zones: (i) a zone dominated by species of Fragilaria (> 9000 BP); (ii) a zone containing benthic acidophilic diatoms indicating natural acidification (9000–7000 BP); and (iii) a zone characterized by numerous species of Aulacoseira ranging from the mid to late Holocene. Clear differentiation of the lower two zones was impossible in the shorter cores, and radiocarbon dates suggest that sediment reworking truncated the earliest records of organic sedimentation at these sites. Correspondence analysis facilitated comparisons of the diatom stratigraphies and enabled the evaluation of core reproducibility. Central cores preserve the most useful paleolimnological records in this environment. Keywords: diatoms, paleolimnology, Arctic Canada, Baffin Island.
The spatio-temporal pattern of peak Holocene warmth (Holocene thermal maximum, HTM) is traced over 140 sites across the Western Hemisphere of the Arctic (0–180°W; north of ∼60°N). Paleoclimate inferences based on a wide variety of proxy indicators provide clear evidence for warmer-than-present conditions at 120 of these sites. At the 16 terrestrial sites where quantitative estimates have been obtained, local HTM temperatures (primarily summer estimates) were on average 1.6±0.8°C higher than present (approximate average of the 20th century), but the warming was time-transgressive across the western Arctic. As the precession-driven summer insolation anomaly peaked 12–10 ka (thousands of calendar years ago), warming was concentrated in northwest North America, while cool conditions lingered in the northeast. Alaska and northwest Canada experienced the HTM between ca 11 and 9 ka, about 4000 yr prior to the HTM in northeast Canada. The delayed warming in Quebec and Labrador was linked to the residual Laurentide Ice Sheet, which chilled the region through its impact on surface energy balance and ocean circulation. The lingering ice also attests to the inherent asymmetry of atmospheric and oceanic circulation that predisposes the region to glaciation and modulates the pattern of climatic change. The spatial asymmetry of warming during the HTM resembles the pattern of warming observed in the Arctic over the last several decades. Although the two warmings are described at different temporal scales, and the HTM was additionally affected by the residual Laurentide ice, the similarities suggest there might be a preferred mode of variability in the atmospheric circulation that generates a recurrent pattern of warming under positive radiative forcing. Unlike the HTM, however, future warming will not be counterbalanced by the cooling effect of a residual North American ice sheet.
The distribution patterns of surface sediment diatom assemblages from 49 lakes were used to explore the relationship between limnological variables and diatom assemblages as well as to assess an ecological classification system as a tool for the management and conservation of Icelandic freshwaters. Lakes were limnologically diverse ranging from deep, oligotrophic, ionically dilute lakes, to shallow lakes with a wide range of nutrient and ionic contents. Physical conditions (depth, surface area, surface water temperature) and nutrient and ion concentrations differed significantly among ecological lake categories (i.e., plateau, spring-fed, direct-runoff, valley, glacial, and coastal lakes) (ANOVA, p < 0.05). Diatom assemblages were taxonomically diverse (329 taxa) with strong representation of planktonic, benthic and periphytic forms. Small benthic Fragilaria sensu lato (19 species and varieties) were the most abundant with combined abundances > 20% in all but 4 of the lakes, most likely due to the generally cold lake water conditions in this subarctic region. Variation in diatom distributions was best explained by the combination of mean depth (influencing littoral versus planktonic habitats), surface water temperature, specific conductivity, alkalinity, total organic carbon, total nitrogen and SiO2 in a canonical correspondence analysis (CCA). However, these variables did not explain distribution patterns amongst small benthic Fragilaria taxa, although some weak relationships between some taxa and these variables were evident. Distinct diatom assemblages and limnological properties among ecological lake categories support the classification of Icelandic freshwaters based on major topographic, geological and hydrological characteristics. More detailed inclusion of lake depth along with lake basin form, as well as more refined categories of lake water origin and topographical positioning to better approximate regional climatic conditions, may improve the ecological classification of Icelandic freshwaters for conservation and management practices.
Two diatom-based transfer functions for the reconstruction of past levels of dissolved organic carbon (DOC) and water colour were applied to fossil diatom species assemblages from a coastal isolation basin on the eastern shore of Hudson Bay. Diatom stratigraphic changes and the diatom-inferred patterns of limnological change following the retreat of Holocene marine waters of the Tyrrell Sea revealed a highly variable Holocene lake trajectory associated with successional shifts in lake catchment vegetation and soil development. The main trends observed in the Holocene history of Lake Kachishayoot are: (1) a progressive loss of alkalinity over time; and (2) abrupt increases in DOC and water colour that coincide with the arrival of spruce (Picea mariana) in the catchment. Reconstructed DOC allowed the estimation of past depths of ultraviolet (UV) penetration in the water-column of Lake Kachishayoot. Past variations in biological UV exposure were inferred using optical models based on DOC concentrations and the response curves for DNA damage and UV photoinhibition of photosynthesis. The palaeoecological analyses revealed large changes in the underwater photobiological environment over the course of the Holocene period, from extremely high UV exposure after the initial formation of the lake and its isolation from the sea, to an order-of-magnitude lower biological UV exposure under the present conditions of catchment vegetation.
1] A new application of reflectance spectroscopy enables inferences of lake sediment chlorophyll a concentrations and hence of historical trends in lacustrine primary production. In a survey of six arctic lakes on Baffin Island (Nunavut, Canada), pronounced increases of spectrally-inferred chlorophyll a concentrations are consistently expressed in sediments deposited during the 20th century. Climate warming appears to be increasing both aquatic chlorophyll a production and its seques-tration to sediments, as these lakes enter new biological regimes that are largely unique in the context of the late Holocene. Citation:
1] We investigated the factors controlling lake evolution in Arctic ecosystems using a multiproxy paleolimnological approach on a small lake on Baffin Island, Arctic Canada. Lakewater pH was inferred from fossil diatom assemblages, whereas primary production was assessed from sediment concentrations of diatom valves and spectrally inferred chlorophyll a. Our reconstructed limnological variables registered synchronous changes and showed a close coupling to Holocene climatic fluctuations, as inferred by numerous independent paleoclimate proxies. Without exception, our highest pH and production values occurred during warm intervals, and vice-versa. A return towards paleolimnological conditions of the warm early Holocene has occurred since the midtwentieth century, corresponding to climate warming following the Little Ice Age. Maximum recent values of our reconstructed parameters are either directly comparable to, or in some cases exceed, values attained during the Holocene Thermal Maximum, 8000–10,000 years ago. Our data suggest that climate has a first-order influence on primary production and the regulation of in-lake DIC dynamics (and hence on lakewater pH) through its modulation of lake ice cover. We conclude that direct forcing by climate is more important than catchment processes in controlling the chemical and biological development of ice-dominated Arctic lake ecosystems, at the scale of the Holocene.
Les diatomées des eaux douces sont de plus en plus utilisées pour reconstituer les environnements du passé. Cette recherche a été réalisée dans le but d’évaluer ce potentiel dans une région où peu d’informations sur les diatomées étaient disponibles. L’étude a permis d’identifier un ensemble de 516 espèces de diatomées préservées dans les sédiments de surface d’un transect latitudinal de 59 lacs du Nord-Ouest québécois. À l’aide d’analyses statistiques multivariées, il a été possible de déterminer quelles sont les variables environnementales (parmi 49) qui exercent le plus d’influence sur la composition des communautés de diatomées. Ce sont les concentrations en magnésium, en sodium, en carbone organique dissous (COD) et en silice, ainsi que la profondeur des lacs qui semblent être les variables qui influencent le plus la composition des communautés de diatomées dans cette région. Un modèle de reconstitution du COD a été créé. Ce modèle pourra être utilisé dans des études paléolimnologiques afin de déterminer les concentrations en COD du passé, ainsi que d’autres variables qui y sont reliées, telles que les apports du bassin versant, la température, la transparence de l’eau et la pénétration des rayons UV.
Diatom-based paleolimnological studies are being increasingly used to track long-term environmental change in arctic regions.
Little is known, however, about the direction and nature of such environmental changes in the western Canadian high Arctic.
In this study, shifts in diatom assemblages preserved in a 210Pb-dated sediment core collected from a small pond on Melville Island, N.W.T., were interpreted to record marked environmental
changes that had taken place since the early 20th century. For most of the history of the pond recorded in this core, the
diatom assemblage remained relatively stable and was dominated by Fragilaria capucina. A major shift in species composition began in the early-20th century, with a sharp decline in F. capucina and a concurrent increase in Achnanthes minutissima. In the last 20years, further changes in the diatom assemblage occurred, with a notable increase in the Nitzschia perminuta complex. The assemblage shifts recorded at this site appear to be consistent with environmental changes triggered by recent
climatic warming.
Diatoms were examined in three lacustrine sediment records from Alert, northern Ellesmere Island, and from Isachsen, Ellef Ringnes Island. Diatom assemblages changed markedly since the mid-19th century following relatively stable community composition that spanned centuries to millennia. Three different assemblages, primarily composed of Fragilaria pinnata, Diadesmis spp., or Pinnularia spp., dominated the pre-1850 period at the three sites, but were replaced with different, more diverse assemblages in recent sediments. These species shifts occurred in the mid- to late-19th century in the Isachsen sites, and in the mid- to late-20th century in our Alert site. This difference in timing appears to be a result of the different sensitivities of lakes and ponds to environmental change, rather than of site-specific chemical properties. Reconstructions of pH using diatom inference models indicated increases from 0.5 to 0.8pH units at these sites over this period of assemblage change. The diatom-inferred pH record from Alert showed agreement with measured climate data from Alert over the last 30years. These marked community changes suggest that these sensitive high arctic sites have recently crossed important ecological thresholds due to environmental change, most likely related to recent warming.
Two sediment cores of 70 and 252cm length were recovered from Hjort Sø, a small lake on Store Koldewey, Northeast Greenland,
and studied with a multidisciplinary approach in order to reconstruct the local environmental history and to test the relevance
of proxies for paleoenvironmental information. The basal sediments from the longer core are dominated by clastic matter, which
was likely deposited during deglaciation of the lake basin. These clastic sediments are overlain by gyttja, which is also
present throughout the shorter core. AMS radiocarbon dating was conducted on plant macrofossils of 11 samples from the gyttja
in both cores. A reliable chronology was established for both cores, which dated the onset of organic accumulation at 9,500cal.
year BP. The Holocene temperature development, with an early to mid Holocene thermal maximum, is best reflected in the grain-size
composition. Nutrient availability was apparently low during the early Holocene and led to low productivity in the lake and
its vicinity. From ca. 7,000cal. year BP, productivity in the lake increased significantly, probably induced by external
nutrient input from goose excrements. From this time, micro- and macro-fossil remains reflect relatively well the climate
history of East Greenland, with a cooling during the middle Holocene, the medieval warming, and the Little Ice Age. The amount
of organic matter in the sequence seems to be more affected by lake ice cover or by nutrient supply from the catchment than
by temperature changes. The record from Hjort Sø thus reveals the difficulties in interpreting sedimentary records from high
arctic regions.
We live in a constantly changing environment, yet tracking ecological change is often very difficult. Long-term monitoring data are frequently lacking and are especially sparse from Arctic ecosystems, where logistical difficulties limit most monitoring programs. Fortunately, lake and pond sediments contain important archives of past limnological communities that can be used to reconstruct environmental change. Here, we summarize some of the pale-olimnological studies that have documented recent climate warming in Arctic lakes and ponds. Several hypotheses have been evaluated to determine if warming, resulting in changes in ice cover and related variables (eg increased habitat availability), was the factor most strongly influencing recent diatom and other biotic changes. Striking and often unprecedented community changes were evident in post-1850 sediments, and could be linked to ecological shifts consistent with warming. Because future temperature increases are predicted to be greatly amplified in polar regions, the ecological integrity of these sensitive ecosystems will be further imperiled.
A 115 cm-long core was raised from an irregularly shaped lake informally named 'Kap Inglefield Si,f, 1.5 km southeast of Kap Inglefield, North-West Greenland, at an elevation of 150 m. A 49 cm-thick organic sequence is underlain by banded sediment (sand alternating with clay/silt), deposited in an ice-dammed lake. Today the lake is held up at the outlet by bouldery till containing marine shells. The presence of till containing shells with low D/L ratios for aspartic acid at several inland sites, together with well preserved glacial sculpture along the coast (mirrowing features on nearby Ellesmere Island) and over 80 m of Holocene emergence, provides convincing evidence that an ice stream, draining from coalesced Greenland and Innuitian Ice Sheets over Kane Basin, filled Smith Sound at the last (late-Wisconsinan) glacial maximum. Four 14C age determinations have been made on Core 80-18, and the age of the basal organic sediment is 7210 ± 130 conventional radiocarbon years. The minerogenic sediment in Core 80-18, rich in rebedded pollen, is lacking in contemporary algae and zoological remains, whereas their number varies throughout the organic part of the core, giving some indication of climate and trophic fluctuations. In the organic sequence only 20 diatom species were recorded, and of these only five taxa were ever present at greater than trace levels. The diatom assemblage was dominated by small Fragilaria species, typical of early postglacial environments from most glaciated lakes, regardless of location or geologic substrate. Their continued abundance throughout the history of this lake reflects the extreme climate of the region. These harsh conditions are also reflected in the fossil Cladocera (Crustacea). Fragments of mosses occurred throughout the organic sequence; Calliergon giganteum dominated at most levels between 5 and 40 cm. A pollen- and microfossil diagram can be divided into five zones. From the bottom upward they are: A. Poaceae pioneer plant zone; B. Salix arctica - Polypodiaceae zone with many pioneer plants; C. Salix arctica zone with high pollen influx; D. Cyperaceae zone with Dryas, Oxyria and Salix; and E. Saxifraga oppositifolia - Papaver zone with many pioneer (fell-field) plants. In this uppermost zone up to 250,000 Pediastrum/ml indicate a slow sedimentation rate, as do the age determinations, probably caused by the development of a nearly permanent ice cover over the central part of the lake. Detailed analysis of the top 7 .6 cm of sediment in a second core revealed drastic changes in the chlorophycean (green algae) fossils. Scenedesmus is dominant below 4.7 cm, whereas Pediastrum is dominant above 4.6 cm. An AMS date of 4050 ± 110 conventional radiocarbon years pinpoints the time at which the cover of lake ice became more permanent.
Brings the total known vascular flora of the Hayes Sound Region to 117 species. Of 22 species new to the region, Carex amblyorhyncha and C. scirpoidea are reported for the first time from Ellesmere Island. The current climate is the principal factor contributing to the diversity of the flora.-from Authors
25 radiocarbon age determinations on marine molluscs, basal organic pond sediments, charred remains in archeological sites, and a variety of other materials have allowed the construction of an emergence curve for Cape Herschel, east-central Ellesmere Island (78°35′N, 74°40′W). Only a narrow fringe of land is present between the Prince of Wales Icefield and Smith Sound, yet emergence of the order of 135 m has taken place during the last 8500-8700 radiocarbon years. The spectacular and fresh-appearing glacial sculpture along both sides of Smith Sound, coupled with the rapid emergence in Holocene time and the fact that the oldest dates on marine shells at the fiord heads to the west are 3000-4000 yrs younger than those at Cape Herschel, provides convincing evidence that an ice stream filled Smith Sound (>500m deep) during the Late Wisconsinan glacial maximum. -from Author
This overview summarizes limnological and paleolimnological studies carried out at 36 reference ponds as part of the Cape Herschel Project on Ellesmere Island. Marked relief, proximity to the sea, and drainage-basin composition cause pronounced interpond differences in microclimate, water chemistry, and vegetation. The ponds freeze completely for 9 to 10 months of the year; in summer, water temperatures fluctuate diurnally. Ponds are shallow, clear, oligotrophic, and, with one exception, alkaline. Conductivity values fluctuate seasonally. Such High Arctic ponds may be especially sensitive monitors of future environmental change. Over 130 diatom taxa from 28 genera were identified. Benthic diatoms were abundant and exhibited varying degrees of microhabitat specificity; planktonic taxa were absent. Other biological indicators, such as chrysophytes and invertebrates, were also studied. Paleolimnological data indicate that diatom assemblages were relatively stable over the last few millennia, then underwent unparalleled changes beginning in the nineteenth century. The environmental factors causing these changes may be related to recent climatic warming.
The calibration curves and tables given in this issue of RADIOCARBON form a data base ideally suited for a computerized operation. The program listed below converts a radiocarbon age and its age error o s (one standard deviation) into calibrated ages (intercepts with the calibration curve), and ranges of calibrated ages that correspond to the age error. The standard deviation o C in the calibration curve is taken into account using (see Stuiver and Pearson, this issue, for details).
Introduction High Arctic environments continue to receive increased attention from the scientific community, policy makers, and the public at large because polar regions are considered to be especially sensitive to the effects of global climatic and other environmental changes (Rouse et al., 1997; ACIA, 2004; IPCC, 2007). Polar lakes and ponds, and the biota they contain, are important sentinels of environmental changes (Pienitz et al., 2004; Schindler & Smol, 2006) and have thus been the focus of many research programs (Vincent & Laybourn-Parry, 2008). There is considerable potential for using living and fossil diatom assemblages to track environmental trends in High Arctic regions (Smol & Douglas, 1996; Douglas et al., 2004a). A growing number of studies have examined the taxonomy, ecology, and paleoecology of High Arctic diatoms, as lakes and ponds are dominant features of most Arctic landscapes. Given the diversity and vastness of these regions, many exciting research opportunities exist. For example, about 18% (by area) of Canada’s surface waters are situated north of 60 °N (Statistics Canada, 1987), and Sheath (1986) estimated that tundra ponds cover approximately 2% of the Earth’s surface. The heightened interest in High Arctic environments, coupled with an increased accessibility of these remote regions (e.g. by helicopter), has resulted in a recent surge of interest in Arctic diatom research. Whilst some proxy techniques, such as palynology and dendroecology, have limited applicability in some High Arctic regions due to the paucity of higher plants (Gajewski et al., 1995), paleolimnological approaches using diatoms have become especially important for studies of long-term global environmental change.
Diatom analyses were conducted on a 105.5 cm sediment core from a small upland lake on southwestern Fosheim Peninsula, Ellesmere Island. The core represents approximately 6500 years of continuous deposition. Diatom floras were consistently dominated by Fragilaria sp. Three diatom zones can be identified that broadly reflect regional paleoclimatic conditions inferred independently. Diatom concentrations were high in the oldest sediments and assemblages were dominated by Fragilaria construens var. venter. Subsequently, after ca. 4000 BP, diatom concentrations decreased, frequencies of the F. pinnata complex expanded at the expense of F. construens var. venter, and several additional benthic taxa became increasingly common. The greatest diatom changes of the entire record took place in the upper 3 cm, when diatom concentrations and diversity rose dramatically. The diatom stratigraphy indicates significant differences between the character of the two warm paleoclimatic periods encompassed by the record, the relatively wet early to middle Holocene and the arid twentieth century.
As a result of a comprehensive assessment of the climate of the Canadian Arctic Islands and adjacent waters, five climatic regions were identified. The regional boundaries were delineated by an analysis of the influence of the major climatic controls while further regional subdivisions were arrived at through consideration of the fields of the standard observed meteorological elements. Short discussions of the climatic characteristics of each sub-region are given and tables outlining values of selected climatic elements are presented. A brief discussion of climatic change across the entire area is included.
We live in a constantly changing environment, yet tracking ecological change is often very difficult. Long-term monitoring data are frequently lacking and are especially sparse from Arctic ecosystems, where logistical difficulties limit most monitoring programs. Fortunately, lake and pond sediments contain important archives of past limnological communities that can be used to reconstruct environmental change. Here, we summarize some of the pale-olimnological studies that have documented recent climate warming in Arctic lakes and ponds. Several hypotheses have been evaluated to determine if warming, resulting in changes in ice cover and related variables (eg increased habitat availability), was the factor most strongly influencing recent diatom and other biotic changes. Striking and often unprecedented community changes were evident in post-1850 sediments, and could be linked to ecological shifts consistent with warming. Because future temperature increases are predicted to be greatly amplified in polar regions, the ecological integrity of these sensitive ecosystems will be further imperiled.
Numerous palcolimnological studies of Arctic lakes and ponds have shown marked shifts in both algal and invertebrate taxa within the past similar to 150 years that are consistent with recent climatic warming. However, the magnitude and timing of changes are often non-uniform, with large, deep lakes frequently exhibiting muted assemblage shifts relative to smaller ponds. The hypothesis that duration and extent of ice cover exerts an overriding influence on habitat availability for biota has been commonly invoked to explain these differences, and many studies indicate that changes in ice cover are important drivers of recent biological changes. However, a detailed paleolimnological comparison of two lakes from the same region that have similar water chemistry but different ice cover regimes has not yet been attempted. Here we examine the influence of prolonged ice cover on the rate, magnitude, and direction of fossil diatom species shifts over time in two remarkably similar and adjacent Ellesmere Island lakes that mainly differ in their periods of ice cover. These two lakes exhibit strikingly different paleolimnological diatom profiles, despite their physical proximity, similar depths, and nearly identical water chemistry. In the lake characterized by prolonged ice cover, we find little evidence of diatom-inferred environmental change over its recent history, while diatom assemblages have undergone dramatic changes in the lake with the shorter duration of ice cover. This study supports the general hypothesis that changes in ice cover are a principle determinant of shifting diatom assemblages in High Arctic lakes.
A sediment core from Lake BC01 (75°10.945′N, 111°55.181′W, 225 m asl) on south-central Melville Island, NWT, Canada, provides the first continuous postglacial environmental record for the region. Fossil pollen results indicate that the postglacial landscape was dominated by Poaceae and Salix, typical of a High Arctic plant community, whereas the Arctic herb Oxyria underwent a gradual increase during the late Holocene. Pollen-based climate reconstructions suggests the presence of a cold and dry period ~12,000 cal yr BP, possibly representing the Younger Dryas, followed by warmer and wetter conditions from 11,000 to 5000 cal yr BP, likely reflective of the Holocene Thermal Maximum. The climate then underwent a gradual cooling and drying from 5000 cal yr BP to the present, suggesting a late Holocene neoglacial cooling. Diatom preservation was poor prior to 5000 cal yr BP, when conditions were warmest, suggesting that diatom dissolution may in part be climatically controlled. Diatom concentrations were highest ~4500 cal yr BP but then decreased substantially by 3500 cal yr BP and remained low before recovering slightly in the 20th century. An abrupt warming occurred during the past 70 yr at the site, although the magnitude of this warming did not exceed that of the early Holocene.
Diatom assemblage changes over the Holocene were examined from a 14C-dated sediment core retrieved from Lake TK-2, a small low Arctic lake located ca. 200km north of the forest–tundra ecotone in mainland Nunavut, Canada. Notable changes in the diatom assemblages were recorded throughout the core, suggesting that the Holocene epoch in this region has been environmentally and climatically dynamic. The earliest diatom assemblages (ca. 9000cal yr BP) were dominated by taxa that are atypical of post-glacial assemblages commonly recorded throughout the Arctic, and may suggest that early Holocene conditions at Lake TK-2 were relatively warmer. A shift to dominance by small, benthic, opportunistic Fragilaria taxa followed (after ca. 8550cal. yr BP), more typical of Arctic assemblages during initial lake ontogeny, suggesting the onset of cooler, more alkaline conditions. An abrupt and short-lived marked decrease in diatoms between ca. 8550 and 8500cal. yr BP, with corresponding changes in physical and chemical indicators (e.g., sedimentation rate, siliciclastic content, % organic matter content), provides potential evidence for the 8.2k cooling anomaly, an event rarely recognized from other paleolimnological studies in the Canadian north. Following ca. 7000cal. yr BP, a substantial shift occurred to a more complex and diverse diatom assemblage that now included more acidophilic taxa. This compositional change is likely indicative of a natural, long-term loss of alkalinity in the lake, and marks the onset of the Holocene Thermal Maximum, consistent with the timing of this warm period for this region. The relatively stable diatom assemblage composition during the Neoglacial period was punctuated by fluctuations in key species potentially correlative with the so-called Medieval Warm Period and Little Ice Age. In the most recent sediments, the post-industrial expansion of the small, planktonic Cyclotella stelligera complex, and a concurrent decline in the heavily-silicified Aulacoseira lirata complex, are similar to shifts that are increasingly being recognized as geographically widespread diatom responses to recent climate warming. The Lake TK-2 diatom record provides important insights into the Holocene environmental history of this understudied region of the Canadian Arctic. Furthermore, it is one of the few Arctic lakes in which the 8.2k cold event is possibly expressed.
Twenty-five radiocarbon age determinations on marine molluscs, basal organic pond sediments, charred remains in archeological sites, and a variety of other materials have allowed the construction of an emergence curve for Cape Herschel, east-central Ellesmere Island (78°35′N, 74°40′W). Only a narrow fringe of land is present between the Prince of Wales Icefield and Smith Sound, yet emergence of the order of 135 m has taken place during the last 8500–8700 radiocarbon years. The highest in situ shells were collected at an elevation of 107.5 m, and ages of 8470 ± 100 BP (GSC-3314) and 8230 ± 70 BP (TO-230) were obtained on this material.The spectacular and fresh-appearing glacial sculpture along both sides of Smith Sound, coupled with the rapid emergence in Holocene time and the fact that the oldest dates on marine shells at the fiord heads to the west are 3000–4000 years younger than those at Cape Herschel, provides convincing evidence that an ice stream filled Smith Sound (> 500 m deep) during the Late Wisconsinan glacial maximum. The Smith Sound Ice Stream drained southward from the Greenland Ice Sheet and the Innuitian Ice Sheet, which were confluent over Kane Basin, and it overrode the top of Pim Island (550 m asl). Massive melt-off of ice must have been occurring at the transition from Pleistocene to Holocene time, and this melting continued until the mid-Holocene, when all investigated outlet glaciers were behind their present positions.
Fifty-five diatom taxa, representing mostly acidophilous and benthic species (e.g., Pinnularia biceps, Nitzschia bryophila, Navicula variostriata), were identified in a 56 cm long core from a small, deep lake (78°29.5′ N, 76°44.3′ W) in innermost Baird Inlet, Ellesmere Island, Arctic Canada. The size and composition of past algal populations appears to have been controlled by fluctuations in the extent of icefree conditions in the littoral zone. The development of planktonic diatoms may have been precluded by extended ice cover in the pelagic region, although planktonic chrysophytes (exclusive of the Synuraceae) were abundant throughout the lake's 9000 year history. Crysophycean success in this harsh environment is attributed to their versatile nutritional strategies, their motility, and their ability to form a good resting stage. Four fossil zones were delineated. Following the onset of organic sedimentation (~ 9000 years before present (BP)), the diatom flora was dominated by small Fragilaria species (zone 1), comparable to early postglacial communities from temperate lakes. A contemporary analog to this early assemblage was recorded in the surficial sediments of Proteus Lake (78°41.7′ N, 74°23.0′ W), on Pim Island, suggesting that these diatoms reflect slightly alkaline conditions and extended ice cover. As the climate warmed, the assemblage shifted to one dominated by Nitzschia bryophila and Pinnularia biceps (zone 2), followed by an increase in Navicula variostriata (zone 3), indicating that ice cover was at a minimum and that bryophytes were abundant. Climatic deterioration (~ 4000 years BP) coincided with a shift to a diverse assemblage of shallow water diatoms and a marked decline in chrysophyte and diatom populations (zone 4). These data suggest that, in arctic regions, diatoms may provide indirect information of past climates, since temperature is closely associated with habitat availability.
The freshwater diatom flora from arctic and subarctic regions of North America remains poorly known. The aim of this investigation is to improve our knowledge of diatoms in lakes from eastern subarctic Canada, and to provide a stronger foundation for the use of diatoms in limnological and palaeolimnological studies. To this end, we analysed the modern diatom assemblages in surficial sediments from 123 lakes in northern Québec and Labrador. The two study transects in Jamésie-Hudsonie (data set including 59 lakes and 38 environmental variables) and in Québec-Labrador (data set consisting of 64 lakes and 29 environmental variables) extend over a vast area from the boreal forest in the south to arctic tundra conditions in the north. Of a total of 516 diatom taxa in the Jamésie-Hudsonie data set, 218 species were used for the development of a transfer function for the reconstruction of dissolved organic carbon (DOC). In the Québec.Labrador data set, two inference models were developed for the reconstruction of water colour and alkalinity based on 128 of 303 diatom taxa. The majority of taxa belonging to this surprisingly species-rich boreal-subarctic diatom flora are illustrated in photographic plates, accompanied by a short description of the distribution of each taxon.
A 56 cm long core covering ca. 9,000 years was obtained from a small, unnamed lake (78°29.5'N, 4 76°46.8'W) at 295 m elevation, innermost Baird Inlet, Ellesmere Island. The vegetation is high arctic, but locally relatively rich, with areas of Cassiope tetragona–Empetrum–Vaccinium uliginosum heath on the steep slopes above the lake. Cores were taken from the basin centre through ca. 15 m of water. Basal sandy-silt is overlain by 51 cm of laminated silty-gyttja and fine detritus gyttja grading upwards to loose algal gyttja. Three radiocarbon dates indicate rates of sedimentation from 0.07 to 0.04 mm per year. Pollen concentration varies from 1,500 to 2,000 per cm3 in the lower part of the core to 100 to 200 in the upper part; calculated pollen influx is between less than one and 14 grains per cm2 per year. Four local pollen assemblage zones reflect an early pioneer phase (grass-sedge-Oxyria) in the lowermost organic sediment, followed by the appearance and spread of Salix some 8,000 years ago and then, some 7,000 years ago, by a rise in Ericales. A period of vegetational stability with dwarf-shrub heaths follows, lasting for over 3,000 years. The topmost zone shows some increase in indicators of bare ground and fell-field vegetation (Saxifraga, Ranunculus, Papaver, Caryophyllaceae, Dryas) and decline in pollen influx, hence deterioration of local conditions during the last 3,500 to 4,000 years.
We document hydrological and phytoplankton characteristics of nine lakes and two ponds on Store Koldewey, a culturally undisturbed island off Northeast Greenland. The limnological survey included the recording of temperature, conductivity, oxygen concentration and saturation, pH, ionic composition, transparency, and the diatom phytoplankton community. In summer 2003, the lakes were cold, monomictic, thermally unstratified, alkaline and likely oligotrophic water bodies. Diatom phytoplankton was present in six lakes and consisted of four dominant species (Aulacoseira tethera, Cyclotella pseudostelligera, C. rossii, and Fragilaria tenera). The concentration of planktonic diatoms varied distinctly between the lakes. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
A synthesis of over 200 diatom-based paleolimnological records from nonacidified/nonenriched lakes reveals remarkably similar taxon-specific shifts across the Northern Hemisphere since the 19th century. Our data indicate that these diatom shifts occurred in conjunction with changes in freshwater habitat structure and quality, which, in turn, we link to hemispheric warming trends. Significant increases in the relative abundances of planktonic Cyclotella taxa (P<0.01) were concurrent with sharp declines in both heavily silicified Aulacoseira taxa (P<0.01) and benthic Fragilaria taxa (P<0.01). We demonstrate that this trend is not limited to Arctic and alpine environments, but that lakes at temperate latitudes are now showing similar ecological changes. As expected, the onset of biological responses to warming occurred significantly earlier (P<0.05) in climatically sensitive Arctic regions (median age=ad 1870) compared with temperate regions (median age=ad 1970). In a detailed paleolimnological case study, we report strong relationships (P<0.005) between sedimentary diatom data from Whitefish Bay, Lake of the Woods (Ontario, Canada), and long-term changes in air temperature and ice-out records. Other potential environmental factors, such as atmospheric nitrogen deposition, could not explain our observations. These data provide clear evidence that unparalleled warming over the last few decades resulted in substantial increases in the length of the ice-free period that, similar to 19th century changes in high-latitude lakes, likely triggered a reorganization of diatom community composition. We show that many nonacidified, nutrient-poor, freshwater ecosystems throughout the Northern Hemisphere have crossed important climatically induced ecological thresholds. These findings are worrisome, as the ecological changes that we report at both mid- and high-latitude sites have occurred with increases in mean annual air temperature that are less than half of what is projected for these regions over the next half century.
Sediment cores from six small lake basins in the Canadian high Arctic reveal a gravel-rich (≤30% by weight) to gravel-poor (≥2%) diamict facies underlying massive, post-glacial, clayey silt. Ten other lakes contain a second diamict facies within what are interpreted to be glaciolacustrine sedimentary assemblages. The sedimentology, clast fabrics and fossil remains (diatoms, ostracodes and chironomid head capsules) within both diamict facies suggest that these deposits are not tills. Clast fabrics yielded low S1 (0·41–0·57) and high S3 (0·09–0·22) eigenvalues, placing them within the range of ice-rafted diamictons and glacigenic sediment flows. The high percentage of clast dip angles >45° (15–61%), random clast azimuth and lower diamict contacts conformable to underlying current-bedded sediment favours an origin as a rain-out or settling deposit. Samples of the matrix and scrapings of clasts from the diamicts revealed a diatom assemblage dominated by littoral and planktonic forms, such as are found in the littoral regions of the lakes today. This contrasts sharply with the assemblages within the overlying clayey silt, in which benthic forms predominate. Clasts are thus interpreted to have been rafted from the littoral areas of the lake. The process proposed to explain this is rafting by the lake ice cover in a glacial-marginal environment. Early season meltwater, impounded along the lateral margin of retreating cold-based glaciers, would buoyantly lift the lake ice cover and any adfrozen lake sediment. Higher lake levels and increased areal extent of seasonal freeze-on between the lake ice cover and the lake bed would allow the redeposition of littoral sediments to the benthic regions through greater lateral shifting of the ice cover as it broke up. Incision by meltwater streams into the lateral glacial margins would later isolate the lake, allowing seasonal warming of lake water, enough to support the growth and maturation of the ostracode and chironomid species found as fossils within the diamicts.
A Holocene sediment sequence from Lake Seukokjaure, a subarctic lake at tree-line in northern Sweden, was analyzed to assess
major changes in the structure and functioning of the aquatic ecosystem in response to climate change and tree-line dynamics.
The compiled multi-proxy data, including sedimentary pigments, diatoms, chironomids, pollen, biogenic silica (BSi), carbon
(C), nitrogen (N) elemental and stable-isotope records, and total lake-water organic carbon (TOC) concentration inferred from
near-infrared spectroscopy (NIRS), suggest that the Holocene development of Lake Seukokjaure was closely coupled to changes
in terrestrial vegetation with associated soil development of the catchment, input of allochthonous organic carbon, and changes
in the light regime of the lake. A relatively productive state just after deglaciation around 9700 to 7800calyears BP was
followed by a slight long-term decrease in primary production. The onset of the local tree-line retreat around 3200calyears
BP was accompanied by more diverse and altered chironomid and diatom assemblages and indications of destabilized soils in
the catchment by an increase in variability and absolute values of δ13C. An abrupt drop in the C/N ratio around 1750calyears BP was coupled to changes in the internal lake structure, in combination
with changes in light and nutrient conditions, resulting in a shift in the phototrophic community from diatom dominance to
increased influence of chlorophytes, likely dominated by an aquatic moss community. Thus, this study emphasizes the importance
of indirect effects of climate change on tree-line lake ecosystems and complex interactions of in-lake processes during the
Holocene.
Keywordspaleoecology-aquatic ecosystems-multi-proxy-pigments-stable isotopes-NIRS-microfossils-northern tree-line-light-Holocene