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The ecological responses of remote lakes to post-industrial environmental changes are complex. However, two regions reveal concentrations of sites with elevated 20th-century diatom β-diversity: the Arctic where temperatures are increasing most rapidly, and mid-latitude alpine lakes impacted by high Nr deposition rates. We predict that remote lakes will continue to shift towards new ecological states in the Anthropocene, particularly in regions where these two forcings begin to intersect geographically.
There is mounting evidence that recent ecological and biogeochemical changes have occurred in remote lakes, defined here as those lacking any immediate, catchment-scale, anthropogenic influences. The implication is that these ecosystems can no longer be considered pristine, largely because of their high sensitivity to climate change ,  which, in some regions, is compounded by significant inputs of reactive anthropogenic nitrogen (hereafter Nr; comprising all biologically-, photochemically- and radiatively- active nitrogenous compounds in the atmosphere and biosphere ) delivered by atmospheric deposition –. Part of the sensitivity of arctic and alpine lakes is attributable to limnological characteristics including dilute water chemistry, low primary production, and high flushing rates associated with nival hydrological regimes. The paucity of long-term climatic and environmental monitoring data in remote regions can be alleviated by the use of proxy data from high-resolution sedimentary records. Geochemical signatures and biological remains are continuously archived in the sediments accumulating at the bottoms of lakes. Diatoms (Bacillariophyceae) are unicellular aquatic photoautotrophs that respond rapidly to changes in water chemistry mediated by environmental change. Their siliceous cell walls are often preserved in lake sediments with sufficient fidelity to allow taxonomic identifications and ecological inferences, thus producing an archive of limnological history. In an evolving literature, sediment diatom records have been applied successfully to a range of global environmental issues including lake acidification , eutrophication , and climate change , . Many of these data augment the evidence that the planet has entered the Anthropocene , , the era of human dominance over key biogeochemical cycles, with direct climatic and ecological repercussions.
During the 20th century, changes in diatom assemblages from both arctic and alpine lakes have accelerated relative to the previous 350 years. Elevated 20th century β-diversity is associated primarily with climate warming and Nr deposition, with strong regional variations in the degree of influence attributable to either forcing factor. Given future scenarios for both climate change ,  and Nr deposition , the diatom β-diversity trends are unlikely to be reversed, and in our view will only become exacerbated as the 21stcentury progresses. Our study contributes further proof that distinctive biological fingerprints exist for the Anthropocene , while extending this notion to include the microbiota of remote lakes. As with organisms such as birds  and higher plants , future trajectories of lake diatom communities include states for which no prior analogs exist. Although the full range of ecological implications remain poorly understood, changes at the base of food webs necessarily entail consequences for higher trophic levels, while modifying the biogeochemical cycling of major elements including, but not limited to, carbon, nitrogen, phosphorus and silicon. Increases of primary production and organic matter sedimentation may also influence the recruitment of metals, both natural and anthropogenic, to sediments. The prognosis for truly unperturbed lake ecosystems, if indeed any still remain, is that they are highly susceptible to marked biological reorganizations. Lakes that have already entered new biological regimes will continue to change as humankind tightens its grip on both the global climate system and key biogeochemical cycles.
Materials and Methods
Site Selection, Core Chronology, and Diatom Analysis
Fifty-two diatom stratigraphies were compiled from lakes in western and northern North America and west Greenland (Fig. 1), spanning latitudes from 37.67°N to 79.33°N, and altitudes from 12 m asl to 3546 m asl (Table 1). The range of lake-water pH is 5.9–8.4, representing a large environmental gradient that is captured by diatom assemblages ranging, accordingly, from acidophilous to circumneutral to alkaliphilous. Sites can be categorized into arctic (>60°N; n = 20), alpine (above altitudinal tree-line; n = 15), and temperate montane-boreal lakes (<60°N and forested; n = 17) (Table 1). Despite being restricted to a single cordillera (the Rocky Mountains), the alpine lake population spans 15° of latitude, from southern Colorado to Alberta. The arctic lakes include sites from the continental Northwest Territories, much of the Canadian archipelago, northern Quebec, and west Greenland, together spanning 18° of latitude and 61° of longitude. To close the geographical gap (Fig. 1), and for meaningful comparisons with arctic and alpine sites, we also included a number of low elevation lakes in forested catchments of western Canada (Alberta, central British Columbia, and Vancouver Island). None of the lakes in our analysis is affected by direct point-source anthropogenic activities (e.g. shoreline development, effluent discharge, acidification). Most of the sites have been previously published upon in the context of local and regional studies (Table 1).