Bianca B. Perren

Quantifying recent ecological changes in remote lakes of North America and greenland using sediment diatom assemblages.

Background Although arctic lakes have responded sensitively to 20th-century climate change, it remains uncertain how these ecological transformations compare with alpine and montane-boreal counterparts over the same interval. Furthermore, it is unclear to what degree other forcings, including atmospheric deposition of anthropogenic reactive nitrogen (Nr), have participated in recent regime shifts. Diatom-based paleolimnological syntheses offer an effective tool for retrospective assessments of past and ongoing changes in remote lake ecosystems. Methodology/Principal Findings We synthesized 52 dated sediment diatom records from lakes in western North America and west Greenland, spanning broad latitudinal and altitudinal gradients, and representing alpine (n = 15), arctic (n = 20), and forested boreal-montane (n = 17) ecosystems. Diatom compositional turnover (β-diversity) during the 20th century was estimated using Detrended Canonical Correspondence Analysis (DCCA) for each site and compared, for cores with sufficiently robust chronologies, to both the 19th century and the prior ∼250 years (Little Ice Age). For both arctic and alpine lakes, β-diversity during the 20th century is significantly greater than the previous 350 years, and increases with both latitude and altitude. Because no correlation is apparent between 20th-century diatom β-diversity and any single physical or limnological parameter (including lake and catchment area, maximum depth, pH, conductivity, [NO3 −], modeled Nr deposition, ambient summer and winter air temperatures, and modeled temperature trends 1948–2008), we used Principal Components Analysis (PCA) to summarize the amplitude of recent changes in relationship to lake pH, lake:catchment area ratio, modeled Nr deposition, and recent temperature trends. Conclusions/Significance 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.

Rapid Lacustrine Response to Recent High Arctic Warming: A Diatom Record from Sawtooth Lake, Ellesmere Island, Nunavut

Abstract Diatoms from Sawtooth Lake (79°20′N, 81°51′W) 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.

Twentieth-century warming revives the world's northernmost lake

Although recent ecological changes are widespread in Arctic lakes, it remains unclear whether they are more strongly associated with climate warming or the deposition of reactive nitrogen (Nr) from anthropogenic sources. We developed a 3500-yr paleolimnological record from the world’s northernmost lake to explore this question. Microfossils indicate that siliceous diatoms and chrysophytes were abundant initially, but disappeared 2400 yr ago in concert with Neoglacial cooling. Microfossils reappear in 20 th -century sediments and reach unprecedented concentrations in sediments deposited after ca. A.D. 1980, tracking increasing summer temperatures in the absence of evidence for atmospheric nutrient subsidies. These results indicate that current warming in northern Greenland is unprecedented in the context of the past 2400 yr, and that climate change alone is responsible for the marked biological changes observed.

The cultural eutrophication of Lac la Biche, Alberta, Canada: a paleoecological study

A multiproxy paleoecological investigation of Lac la Biche, a large boreal lake in northeastern Alberta, Canada, revealed that the lake was eutrophic before European settlement but has undergone additional cultural eutrophication in the past 30 to 50 years. Annual fluxes to sediments of phosphorus, nitrogen, carbon, and inorganic sediments have in- creased with time. A declining N-P ratio has increasingly favored nitrogen-fixing cyanobacteria. Increased deposition of microbial pigments and diatom frustules and a recent shift in diatom species also indicate increasing eutrophication. Bio- genic silica increased with time, but there is no evidence of a near-surface decline that would indicate silica limitation. Stable isotopes suggest that an increasing proportion of carbon deposited in sediments is of in-lake origin, indicating in- creased productivity. In the basin nearest the town of Lac La Biche, an increase in d 15 N followed the construction of the sewage treatment plant, but more recently, decreased d 15 N in both basins suggests that nitrogen fixation has become a more important source of nitrogen. Despite documented damage to the fishery of the lake, zooplankton fossils do not show evidence of a strong trophic cascade. The study illustrates the power of a multiproxy approach in obtaining reliable paleolimnological conclusions.

Biogeochemical evidence for hydrologic changes during the Holocene in a lake sediment record from southeast Greenland

Holocene paleoclimate records from Greenland help us understand the response of the Greenland Ice Sheet and regional oceanic and atmospheric circulation systems to natural climate variability in order to place recent changes in a longer-term perspective. Here biogeochemical analysis of a lake sediment core from southeast Greenland is used to define changes in moisture balance and runoff during the Holocene in a catchment near the margin of the Greenland Ice Sheet. A 1.6 m sediment core that spans the last 8.8 ka was recovered from Flower Valley Lake on Ammassalik Island. Magnetic susceptibility, diatoms, bulk biogeochemical properties (TOC, C/N, δ13Corg), and lipid biomarkers (n-alkanes; C16–C31) reveal changes in clastic sedimentation and the relative input of terrestrial- and aquatic-derived organic matter. Hydrogen isotope values (δD) of mid- (n-C25) and long-chain (n-C29, n-C31) n-alkanes allow reconstruction of δD of precipitation and summertime evaporation of lake water. Following a period of early lake ontogeny and landscape stabilization after deglaciation from 8.4 to 7.0 ka, the mid Holocene, 8.4–4.1 ka, is characterized by greater evaporative enrichment of the lake water as indicated by δD records. After 4.1 ka, there is a decrease in evaporative enrichment of the lake water. There is also an abrupt transition to more variable sedimentation marked by sharp increases in magnetic susceptibility, C/N, δ13Corg, and the concentration of long-chain n-alkanes, showing periodic delivery of terrestrial organic matter and clastic sediment to the lake. Higher insolation during the mid Holocene resulted in a warmer and drier climate with longer ice-free periods in the summer and enhanced evaporation of lake water. The reduction in insolation and colder temperatures during the late Holocene caused a reduction in evaporation of lake water over the last 4.1 ka and was accompanied by periodic increases in surface runoff, which correspond with intervals of cold Greenland Ice Sheet surface temperatures.

A paleoecological perspective on 1450 years of human impacts from a lake in southern Greenland

A multiproxy sedimentary record from Lake Igaliku in southern Greenland documents 1450 years of human impacts on the landscape. Diatoms, scaled chrysophytes, and C and N geochemistry show perturbations consistent with recent agricultural activities (post-ad 1980), superimposed upon long-term environmental variability. While the response to Norse agriculture (~ad 986–1450) is weak, the biological response to the last 30 years of modern sheep farming is marked, with drastic changes in diatom taxa, δ13C and δ15N isotopic ratios, and a sharp increase in scaled chrysophytes. Indeed, current conditions in the lake during the last 30 years are unprecedented in the context of the last 1450 years. The dominant driver for recent changes is likely an intensification of agricultural practices combined with warming summer temperatures. Warm temperatures and agricultural disturbance together during Norse Landnám did not lead to the marked changes seen in the modern lake environment over the last 30 years. The synergistic response between increased climate warming and agriculture will likely have unanticipated effects. These findings confirm the sensitivity of Arctic lakes to external anthropogenic forcing and are the first analyses of their kind for the effects of agriculture in Greenland.

The history and impacts of farming activities in south Greenland: an insight from lake deposits

Agriculture in southern Greenland has a two-phase history: with the Norse, who first settled and farmed the region between 985ad and circa 1450ad, and with the recent reintroduction of sheep farming (1920ad to the present). The agricultural sector in Greenland is expected to grow over the next century as anticipated climate warming extends the length of the growing season and increases productivity. This article presents a synthesis of results from a well-dated 1500-year lake sediment record from Lake Igaliku, south Greenland (61°00′N, 45°26′W, 15m asl) that demonstrates the relative impacts of modern and Norse agricultural activities. Pollen, non-pollen palynomorphs (NPPs), sediment mass accumulation rates, diatoms and stable isotopes of nitrogen provide a comprehensive history of both phases of agriculture and their associated impacts on the landscape and adjacent lake. The initial colonisation of southern Greenland is marked by a loss of tree birch pollen, a rise in weed taxa, and an increase in coprophilous fungi and sediment accumulation rate consistent with land-use changes. The biological and chemical proxies within the lake, however, show only slight changes in diatom taxa, and a rise in δ15N. After the Norse demise and during the Little Ice Age, most of the markers return to pre-settlement conditions. However, the continuation of non-indigenous plant taxa suggests that the landscape did not completely return to a pre-disturbance state. After 1988, the character of the lake changed markedly: mesotrophic diatoms and N isotopes all reveal major shifts consistent with a trophic shift, together with a sharp rise in sediment accumulation rate. The post-1988 lake environment, affected by modern farming development, is unprecedented within the context of the last 1500 years. These results demonstrate the potential of lake sediment studies paired with archaeological investigations to reveal the relationship between climate, environment and human societies.

Alder, Nitrogen, and lake ecology: Terrestrial-aquatic linkages in the postglacial history of lone spruce pond, southwestern Alaska

Diatoms, combined with a multiproxy study of lake sediments (organic matter, N, δ15N, δ13C, biogenic silica, grain size, Cladocera and chironomids, Alnus pollen) from Lone Spruce Pond, Alaska detail the late-glacial to Holocene history of the lake and its response to regional climate and landscape change over the last 14.5 cal ka BP. We show that the immigration of alder (Alnus viridis) in the early Holocene marks the rise of available reactive nitrogen (Nr) in the lake as well as the establishment of a primarily planktonic diatom community. The later establishment of diatom Discostella stelligera is coupled to a rise of sedimentary δ15N, indicating diminished competition for this nutrient. This terrestrial-aquatic linkage demonstrates how profoundly vegetation may affect soil geochemistry, lake development, and lake ecology over millennial timescales. Furthermore, the response of the diatom community to strengthened stratification and N levels in the past confirms the sensitivity of planktonic diatom communities to changing thermal and nutrient regimes. These past ecosystem dynamics serve as an analogue for the nature of threshold-type ecological responses to current climate change and atmospheric nitrogen (Nr) deposition, but also for the larger changes we should anticipate under future climate, pollution, and vegetation succession scenarios in high-latitude and high-elevation regions.

Holocene dynamics of the Southern Hemisphere westerly winds and possible links to CO2 outgassing

The Southern Hemisphere westerly winds (SHW) play an important role in regulating the capacity of the Southern Ocean carbon sink. They modulate upwelling of carbon-rich deep water and, with sea ice, determine the ocean surface area available for air–sea gas exchange. Some models indicate that the current strengthening and poleward shift of these winds will weaken the carbon sink. If correct, centennial- to millennial-scale reconstructions of the SHW intensity should be linked with past changes in atmospheric CO2, temperature and sea ice. Here we present a 12,300-year reconstruction of wind strength based on three independent proxies that track inputs of sea-salt aerosols and minerogenic particles accumulating in lake sediments on sub-Antarctic Macquarie Island. Between about 12.1 thousand years ago (ka) and 11.2 ka, and since about 7 ka, the wind intensities were above their long-term mean and corresponded with increasing atmospheric CO2. Conversely, from about 11.2 to 7.2 ka, the wind intensities were below their long-term mean and corresponded with decreasing atmospheric CO2. These observations are consistent with model inferences of enhanced SHW contributing to the long-term outgassing of CO2 from the Southern Ocean.The strength of the Southern Hemisphere westerly winds varied throughout the Holocene, according to a reconstruction from lake sediments, with periods of stronger winds coincident with higher atmospheric CO2 levels.

Lake Sediments as an Archive of Land use and Environmental Change in the Eastern Settlement, Southwestern Greenland

Abstract Palaeoenvironmental studies from continental and marine sedimentary archives have been conducted over the last four decades in the archaeologically rich Norse Eastern Settlement in Greenland. Those investigations, briefly reviewed in this paper, have improved our knowledge of the history of the Norse colonization and its associated environmental changes. Although deep lakes are numerous, their deposits have been little used in the Norse context. Lakes that meet specific lake-catchment criteria, as outlined in this paper, can sequester optimal palaeoenvironmental records, which can be highly sensitive to both climate and/or human forcing. Here we present a first synthesis of results from a well-dated 2000-year lake-sediment record from Lake Igaliku, located in the center of the Eastern Settlement and close to the Norse site Garðar. A continuous, high-resolution sedimentary record from the deepest part of the lake provides an assessment of farming-related anthropogenic change in the landscape, as well as a quantitative comparison of the environmental impact of medieval colonization (AD 985—ca. AD 1450) with that of recent sheep farming (AD 1920—present). Pollen and non-pollen palynomorphs (NPPs) indicate similar magnitudes of land clearance marked mainly by a loss of tree-birch pollen, a rise in weed taxa, as well as an increase in coprophilous fungi linked to the introduction of grazing livestock. During the two phases of agriculture, soil erosion estimated by geochemical proxies and sediment-accumulation rate exceeds the natural or background erosion rate. Between AD 1010 to AD 1180, grazing activities accelerated soil erosion up to ≈8 mm century-1, twice the natural background rate. A decrease in the rate of erosion is recorded from ca. AD 1230, indicating a progressive decline of agro-pastoral activities well before the end of the Norse occupation of the Eastern Settlement. This decline could be related to possible climate instabilities and may also be indirect evidence for the shift towards a more marine-based diet shown by archaeological studies. Mechanization of agriculture in the 1980s caused unprecedented soil erosion up to ≈21 mm century-1, five times the pre-anthropogenic levels. Over the same period, diatom assemblages show that the lake has become steadily more mesotrophic, contrary to the near-stable trophic conditions of the preceding millennia. These results reinforce the potential of lake-sediment studies paired with archaeological investigations to understand the relationship between climate, environment, and human societies.