Introduction

Arctic and subarctic regions are particularly sensitive to environmental changes, and especially those related to climatic warming (Serreze et al., 2000). Average surface air temperature has increased by ∼0.3°C per decade in the 20th century (Keyser et al., 2000), and increases are predicted to continue to about 4°C in subarctic Canadian regions over the next century (Houghton et al., 1996). Circumpolar treeline regions are especially important for studies of environmental and climatic change, as vegetation growth is closely associated with climatic factors such as temperature (Timoney et al., 1992). Recent increased concern over climatic warming emphasizes the importance of environmental studies in northern treeline regions.

The central Canadian arctic treeline region covers a large geographic area containing a multitude of lakes and ponds, which are further characterized by diverse catchment characteristics. The abundances and diversity of sites pose challenges for regional studies, as sufficiently large numbers of lakes must be studied in order to track changes on landscape scales. As climate change affects hydrological conditions (MacDonald et al., 2000), as well as the physical (e.g., thermocline depth, temperature) and chemical (e.g., ionic concentrations) properties of lakes (Rouse et al., 1997), changes in these limnological properties will also influence the composition of aquatic biota (e.g., diatoms: Douglas and Smol, 1999; Lotter et al., 1999). Moreover, lakes from different ecozones (e.g., boreal forest, arctic tundra) will likely respond differently to environmental changes.

In order to assess or characterize environmental changes across large regions, long-term water quality data or baseline conditions are required. Unfortunately, almost no long-term monitoring data exist in arctic and subarctic regions. Without an estimate of past environmental conditions, it is not possible to assess the magnitude and direction of recent environmental changes. Paleolimnological methods provide a means to reconstruct past environmental conditions (Smol, 2002).

Diatoms (Bacillariophyceae) are the most extensively used biological indicators in paleolimnological studies (Stoermer and Smol, 1999). They are abundant, ecologically diverse, and, due to the siliceous nature of their cell walls, diatom valves are often well preserved in lake sediments. Furthermore, diatoms exhibit a variety of life strategies, have specific ecological preferences, and their short life spans enable them to respond rapidly to environmental changes. Collectively, these characteristics make diatoms effective paleo-indicators of environmental change, and are especially useful in arctic and subarctic regions (Douglas and Smol, 1999; Lotter et al., 1999).

Paleolimnological techniques are rapidly being integrated into a variety of studies dealing with ecosystem change, and especially those related to lake management issues (Smol, 2002). One of the major drawbacks of detailed paleolimnological studies, however, is that they are often labor-intensive, and so only a few sites can typically be studied in detail (e.g., at close sediment intervals). For some types of environmental assessments, such detailed analyses, although desirable, are not feasible, and so paleolimnologists have developed sampling methods that allow for relatively quick assessments of environmental change using the so-called top-bottom approaches. Indicators (e.g., diatoms) present in the surficial sediments of the lakes (e.g., 0–0.5 cm, the so-called top sample) are assumed to represent present-day or recent conditions, whereas the diatoms preserved in sediments deposited from preindustrial times (e.g., pre-1850 for most regions of Canada; the so-called bottom sample) represent “natural” or “baseline” conditions (Dixit et al., 1999). Sediment samples have the advantage that they represent an integrated sample of diatoms from all habitats within the lake, as well as an integration of several years of sediment accumulation. This “top-bottom” approach allows researchers to investigate large numbers of lakes over broad geographic areas, thereby making regional environmental assessments possible. However, this technique also has some limitations. As comparisons are only made between two discrete time slices (i.e., top versus bottom), it is not possible to precisely pinpoint the timing of environmental changes. Rather, the goal of this type of study is to determine, on a regional scale, whether the present-day environmental conditions are similar to those during preindustrial times, and if not, what changes have occurred.

Although the “top-bottom” approach has been used in a variety of lake management studies (e.g., acid rain: Cumming et al., 1992; Smol et al., 1998; Korsman, 1999; eutrophication: Hall and Smol, 1996; Dixit et al., 1999), it has rarely been applied to arctic or subarctic regions (e.g., Korsman, 1999; Michelutti et al., 2000), and has yet to be used for a paleoclimatic assessment. In this study, we apply this “top-bottom” approach to make a regional assessment of environmental changes by comparing preindustrial to modern diatom assemblages from 50 lakes spanning the northern boreal forest treeline and the tundra ecozones in the central Canadian Subarctic. Our data show that the most prominent regional shifts in diatom assemblages were related to changes in aquatic habitats, most likely as a result of late-19th- and 20th-century climatic warming.

Site Description

The 50 study lakes were selected to cover a large geographic area in the central Canadian arctic treeline region (Fig. 1). The transect of lakes (all unnamed, and so referred to as TK-6 to TK-58 with the exception of Slipper Lake [SL]) spans a portion of Canada's circumpolar treeline from the northeastern side of Yellowknife (62°32.72′N, 114°07.13′W) to the northern proximity of the Thelon Game Sanctuary (66°41.32′N, 104°55.86′W) (Fig. 1, Table 1). The region is characterized by gradually changing vegetation and climate, including the boreal forest ecozone to the south, the forest-tundra transitional ecotone, and the arctic tundra ecozone to the north (Fig. 1). Vegetation is influenced by many factors, including climate, soils, the degree of permafrost development, the extent of snow accumulation, bedrock geology, and nutrient levels (Ritchie, 1993). The study area is underlain by Precambrian Shield bedrock consisting of gneiss, migmatite, granitoid intrusions, and metamorphosed volcanic and sedimentary rocks (Padgham and Fyson, 1992). Soils are poorly developed due to late deglaciation of the region and the presence of underlying permafrost. Permafrost influences soil conditions and is loosely associated with climatic factors such as air temperature, precipitation, and vegetation (Bonan, 1992). Consequently, the southern limit of continuous permafrost generally follows the position of arctic treeline, as it has restricting effects on vegetation (MacDonald and Gajewski, 1992).

Warmer seasonal temperatures and higher annual precipitation occur in the boreal forest sites to the south in comparison to the more northern sites in the arctic tundra. The boreal forest region near Yellowknife has an average annual precipitation of 267 mm (Canada, 1990), and mean July daytime and January nighttime temperatures of 22.5°C and −30°C, respectively (Canada, 1991), and lakes have an ice-free period of approximately 120 d (Wedel et al., 1990). In contrast, northern sites in the tundra have an ice-free period of only ∼90 d, an average annual precipitation of 200 mm, and mean July daytime and January nighttime temperatures of 12.5°C and −37.5°C, respectively (Canada, 1990; Canada, 1991).

The three ecozones (i.e., boreal forest, forest-tundra, arctic tundra; Fig. 1) were delineated using aerial photographs, topographical maps, field observations, and maps developed by satellite imagery (Canada, 1996). The zonations closely adhered to descriptions given by Ritchie (1993) and Timoney et al. (1993).

The boreal forest is primarily composed of a closed-crown, continuous forest with a lichen floor. Conifers, including Picea mariana (black spruce), P. glauca (white spruce), Abies balsamea (balsam fir), and Pinus banksiana (jackpine), represent the majority of the trees (Ritchie, 1993). Conifers also dominate the transition zone (e.g., Picea mariana, P. glauca, and Larix laricina [larch] [Sirois, 1992]), but several deciduous tree species (e.g., Populus balsamifera [balsam poplar], P. tremuloides [trembling aspen], and Betula papyrifera [paper birch]) are also present. The arctic tundra ecozone is characterized mainly by sedges, lichen-heath, and dwarf shrub vegetation (Ritchie, 1993). A more detailed site description is given in Rühland and Smol (2002).

Materials and Methods

Results

Discussion

Our 50 study lakes, which span a diverse array of environments from the treeline ecozone in the central Canadian Arctic, have clearly undergone major changes in diatom communities since preindustrial times, with a general shift toward a higher relative abundance of the planktonic C. stelligera complex and a decrease in the relative abundance of benthic Fragilaria species (Figs. 2, 3a and b, Table 2). These floristic changes likely occurred during a period of potential anthropogenic disturbance, beginning in the 19th century. Therefore, we explore several hypotheses that could potentially explain these changes, including (1) acidification, (2) nutrient increases, (3) chemical contaminants, and (4) climatic change.

One possible explanation for the recent diatom changes is acidification due to acid precipitation. It is widely accepted that acidification is an important stressor in some poorly buffered systems and that diatoms are excellent indicators of lakewater pH (Battarbee et al., 1999). However, in relatively isolated regions, such as in our study area, atmospheric transport and deposition of sulphuric and nitric acids are very low, even though their concentrations have increased since the mid-1900s (Yalcin and Wake, 2001). Furthermore, our study lakes are presently slightly alkaline, well buffered (mean pH of 7.2; Table 1). Moreover, as has been shown in many paleo-acidification studies, a characteristic feature of many acidified lakes is the loss of Cyclotella species (Battarbee et al., 1999), contrasting with the trends that we record in this study. The overall decrease in the abundance of acidophilic Fragilaria virescens var. exigua in the modern sediments (Fig. 3c) also conflicts with the hypothesis that acidification was responsible for these diatom changes. These conclusions are supported by our environmental reconstruction (Rühland, 2001), based on modern diatom distributions along a DIC gradient, that showed no consistent changes between modern and preindustrial DIC estimates.

Lakes in remote areas could possibly be affected by atmospheric deposits of nutrients from anthropogenic sources (Wolfe et al., 2001). However, based on our modern environmental data, together with our temporal diatom trends, it was clear that there was no relationship between abundances of our most common taxa (Fragilaria and Cyclotella species) and nutrients (total phosphorus [TP] and total nitrogen [TN]) in both the modern assemblages and in the amount of change between the modern and fossil assemblages (r² between 0.003 and 0.07). In fact, the modern distribution of Cyclotella species along the TP and TN gradients revealed that higher species abundances were recorded in lakes with low nutrients (i.e., oligotrophic). Furthermore, the majority of our lakes were ultraoligotrophic to oligotrophic (TP <10 µg L⁻¹). Collectively, these findings strongly suggest that the diatom changes we observed over the past two hundred years were not related to human-induced nutrient increases.

Recently, there has been concern over the impact that long-range transport of heavy metals and of persistent organic pollutants (POPs), such as organochlorines and polycyclic aromatic hydrocarbons, have on arctic environments (Muir et al., 1996; Doubleday, 1997; Betts-Piper, 2001; Rawn et al., 2001). Production of many of these compounds commenced in the early to mid-1900s and therefore may be a possible explanation for our observed diatom changes. Although paleolimnological studies examining the relationships between arctic diatoms and chrysophytes and contaminants are rare (e.g., Betts-Piper, 2001; Michelutti et al., in press; Paterson et al., in press), these studies have shown that POPs such as polychlorinated biphenyls (PCBs), even in high concentrations (Paterson et al., in press), have had no noticeable impact on arctic algal assemblages over the past ∼200 yr.

Our data make a strong case for dismissing acidification, nutrient enrichment and contaminants as possible causes for the diatom changes observed in this study. The remaining hypothesis that we examined to explain these floristic changes is climatic change. Climatic warming may affect diatom assemblage composition through a longer growing season and extended ice-free periods that affect lakewater characteristics, habitat availability, and, ultimately, aquatic biota (e.g., Smol, 1983, 1988; Psenner and Schmidt, 1992; Douglas and Smol, 1999, 2000; Ohlendorf et al., 2000; Catalan et al. 2002a, b; Koinig et al., 2002; Korhola et al., 2002; Lotter et al., 2002). For example, increased duration and stability of thermal stratification with climatic warming alters various lakewater properties, such as nutrient and light availability and pH (e.g., Sorvari and Korhola, 1998; Lotter and Bigler, 2000; Ohlendorf et al., 2000; Sorvari et al., 2002). With warming, these limnological changes would generally favor the growth of planktonic diatom species, such as the C. stelligera complex, in arctic environments (Smol, 1988; Battarbee et al., 2002; Sorvari et al., 2002).

The changes observed in our 50-lake top-bottom study are similar to the marked diatom changes reported in the High Arctic (Douglas et al., 1994), in Finnish Lapland (Sorvari and Korhola, 1998; Rautio et al., 2000; Sorvari et al., 2002), and in recent studies of several remote European alpine lakes (Lotter and Bigler, 2000; Catalan et al., 2002a,b; Koinig et al., 2002; Korhola et al., 2002; Lotter et al., 2002; see Battarbee et al., 2002, for overview). These studies have also suggested that the present-day distribution of planktonic Cyclotella stelligera diatoms appear to be induced by warmer conditions during the later part of the thermal stratification period (before fall turnover). In general, planktonic diatoms and air temperature were found to be related to the duration of ice cover, as first considered by Smol (1983, 1988). In agreement with our findings, these new studies have consistently found increases in planktonic Cyclotella taxa in the recent sediments of lakes that have been associated with a warming climate, and related limnological changes (e.g., extended and/or more stable thermal stratification, nutrient and light availability, etc.).

Sorvari and Korhola (1998) and Sorvari et al. (2002) recorded strikingly similar diatom assemblage changes in several Finnish Lapland lakes. They found consistent and synchronous increases in small planktonic Cyclotella species and decreases in benthic diatom taxa in the upper sediments of thermally stratified lakes. Again, these floristic changes were attributed to longer ice-free periods and increased stability of thermal stratification. Similarly, in our set of 50 lakes, deeper sites recorded stronger thermal stratification and also recorded the largest increases in Cyclotella species (Fig. 6). Cremer et al. (2001) have also found a diatom assemblage change from planktonic Cyclotella pseudostelligera to benthic Fragilaria taxa in an East Greenland lake. They too suggested that this floristic change was likely due to a change in the degree of ice cover related to climatic warming. In our study, the overall changes in benthic (Fragilaria spp.) to planktonic (Cyclotella spp.) diatoms (Fig. 3) agree with these other paleolimnological studies in that the potential mechanism for this floristic change may be attributed to climatically induced changes over the past ∼150 yr.

In sites dominated by benthic diatom taxa, Cameron et al. (2002) and Sorvari et al. (2002) recorded much smaller diatom changes over the last few hundred years. Sorvari et al. (2002) suggested that the absence of planktonic diatoms in their shallow, isothermal lakes elicited a more muted response to climate change, whereas Cameron et al. (2002) suggested the lack of change recorded in their lake may be due to its higher acidity. Similarly, in our study, shallow lakes generally did not record marked changes in species distributions.

A higher relative abundance of Fragilaria species in our bottom sediments can be interpreted as generally colder conditions, and related limnological changes (e.g., more extensive ice and snow cover). Several paleolimnological studies from High Arctic regions have recorded marked declines in these taxa in post-1850 sediments, as warming was believed to have occurred (Douglas et al., 1994; Wolfe, 2000; Smith, 2002). For example, Smol (1988) and Smith (2002) observed that small Fragilaria species were dominant in the Arctic during colder temperatures where environmental conditions, such as longer seasonal ice cover, would provide a more favorable environment for the survival of these benthic aquatic diatoms. These observations are consistent with our interpretations of temporal and regional trends in Fragilaria species observed in our study.

Further support for climate-related mechanisms influencing our species distributions were provided by Lotter and Bigler's (2000) detailed study of diatoms from a Swiss alpine lake, where benthic Fragilaria taxa were more abundant under ice cover and in sediment deposited during the Little Ice Age cooling interval. Conversely, planktonic diatom communities quickly sank to the lake bottom following ice cover as conditions became less favorable for their growth, confirming Smol's (1988) hypothesis linking diatom life strategies and lake ice cover in the High Arctic.

Detailed diatom analyses of a sediment core from Slipper Lake (SL) corroborate our landscape temporal trends recorded in the top-bottom study. Diatom assemblage composition had been relatively stable during the earlier history of the Slipper Lake core, with the clearest changes occurring during the 19th century (Fig. 7), approximately coinciding with post–“Little Ice Age warming” ca. A.D. 1850 (Overpeck et al., 1997; Bradley, 2000; Grove, 2001). The C. stelligera complex became a prominent component of the assemblage by the mid-1800s; this expansion predates the period of known nutrient and contaminant increases by about a century (ca. post-1950s). Another notable change occurred at approximately 13.5 cm (estimated early A.D. 1500s) when small, benthic Fragilaria taxa (dominant in the previous sediment levels) diminish to trace levels (i.e., <1% relative abundance) (Fig. 7). This clear decrease in alkaliphilic taxa (ca. A.D. 1500) again predates the impact of anthropogenic disturbances. These diatom changes are consistent with a change in climate and consequent changes in the aquatic habitat, supporting the changes observed in our 50-lake top-bottom study.

Summary and Conclusions

Increases in planktonic Cyclotella taxa since preindustrial (ca. A.D. 1850) times in lakes from the Canadian central arctic region are likely due to climatically induced limnological changes associated with a longer ice-free period and/or enhanced thermal stratification. These results have important implications for future climate scenarios, as continued warming will result in more dramatic changes in these sensitive arctic environments. These data suggest that other anthropogenically induced stressors (e.g., acid rain, nutrient enrichment, persistent organic pollutants) are likely not the primary mechanisms influencing these diatom species changes. However, the potential for synergistic effects may be heightened, as these lakes are already undergoing marked environmental changes. Our results show that the application of a paleolimnological lake-management tool (i.e., the “top-bottom” approach) can be effectively used in an arctic treeline environment to test hypotheses and to track environmental changes on a regional scale. In the absence of long-term lake monitoring data, this paleolimnological approach should be considered for other arctic regions.