Reinhard Pienitz

Rapid response of treeline vegetation and lakes to past climate warming

FUTURE greenhouse warming is expected to be particularly pronounced in boreal regions1, and consequent changes in vegetation in these regions may in turn affect global climate2–4. It is therefore important to establish how boreal ecosystems might respond to rapid changes in climate. Here we present palaeoecological evidence for changes in terrestrial vegetation and lake characteristics during an episode of climate warming that occurred between 5,000 and 4,000 years ago at the boreal treeline in central Canada. The initial transformation — from tundra to forest-tundra on land, which coincided with increases in lake productivity, pH and ratio of inflow to evaporation — took only 150 years, which is roughly equivalent to the time period often used in modelling the response of boreal forests to climate warming5,6. The timing of the treeline advance did not coincide with the maximum in high-latitude summer insolation predicted by Milankovitch theory7, suggesting that northern Canada experienced regionally asynchronous middle-to-late Holocene shifts in the summer position of the Arctic front. Such Holocene climate events may provide a better analogue for the impact of future global change on northern ecosystems than the transition from glacial to nonglacial conditions.

Assessment of freshwater diatoms as quantitative indicators of past climatic change in the Yukon and Northwest Territories, Canada

We identified, enumerated, and interpreted the diatom assemblages preserved in the surface sediments of 59 lakes located between Whitehorse in the Yukon and Tuktoyaktuk in the Northwest Territories (Canada). The lakes are distributed along a latitudinal gradient that includes several ecoclimatic zones. It also spans large gradients in limnological variables. Thus, the study lakes are ideal for environmental calibration of modern diatom assemblages. Canonical correspondence analysis, with forward selection and Monte Carlo permutation tests, showed that maximum lake depth and summer surface-water temperature were the two environmental variables that accounted for most of the variance in the diatom data. The concentrations of sodium and calcium were also important explanatory variables. Using weighted-averaging regression and calibration techniques, we developed a predictive statistical model to infer lake surface-water temperature, and we evaluated the feasibility of using diatoms as paleoclimate proxies. This model may be used to derive paleotemperature inferences from fossil diatom assemblages at appropriate sites in the western Canadian Arctic.

Effect of climate change relative to ozone depletion on UV exposure in subarctic lakes

The effect of stratospheric ozone depletion on increases in ambient levels of solar ultraviolet (UV) radiation in high-latitude regions has raised concerns about the response of northern ecosystems to environmental change. The concentration of coloured dissolved organic material, which is derived from terrestrial vegetation and acts as a screen for ultraviolet radiation, is low in high-latitude lakes. The underwater light environment in these lakes is therefore likely to be sensitive to small variations in the supply of this material, in addition to the effects of ozone depletion. Here we use fossil diatom assemblages in combination with bio-optical models to estimate the magnitude of past variations in the underwater light regime of a lake at the boreal tree line. We find large shifts in underwater UV-B, UV-A and photosynthetically available radiation associated with changes in the input of coloured dissolved organic material into subarctic lakes during the Holocene. The inferred changes in biological exposure to UV radiation were at least two orders of magnitude greater than those associated with moderate (30%) ozone depletion. Our findings indicate that freshwater ecosystems at present located across vegetation gradients will experience significant shifts in underwater spectral irradiance through the effects of climate change on catchment vegetation and the export of coloured dissolved organic material.

Long-term environmental change in Arctic and Antarctic lakes

1. Paleolimnological research in polar regions: An introduction. Reinhard Pienitz, Marianne S.V. Douglas and John P. Smol 2. Geochronology of high latitude lake sediments. Alexander P. Wolfe, Gifford H. Miller, Carrie A. Olsen, Steven L. Forman, Peter T. Doran and Sofia U. Holmgren 3. Physical and chemical properties and proxies of high latitude lake sediments. Scott F. Lamoureux and Robert Gilbert 4. Palynology of North American arctic lakes. Konrad Gajewski and Glen M. MacDonald 5. Algal indicators of environmental change in arctic and antarctic lakes and ponds. Marianne S.V. Douglas, Paul B. Hamilton, Reinhard Pienitz and John P. Smol 6. Aquatic invertebrates and high latitude paleolimnology. Ole Bennike, Klaus P. Brodersen, Erik Jeppesen and Ian R. Walker 7. Use of water isotope tracers in high latitude hydrology and paleohydrology. Thomas W.D. Edwards, Brent B. Wolfe, John J. Gibson and Dan Hammarlund 8. Lake sediments as records of arctic and antarctic pollution. Derek C.G. Muir and Neil L. Rose 9. Paleolimnology of the middle and high Canadian Arctic. Alexander P. Wolfe and I. Rod Smith 10. Paleolimnology of the North American Subarctic. Bruce P. Finney, Kathleen Ruhland, John P. Smol and Marie-Andree Fallu 11. Holocene paleolimnology of Greenland and the North Atlantic islands (north of 60 N). N. John Anderson, David B. Ryves, Marianne Grauert and Suzanne McGowan 12. Paleolimnological research from northern Russian Eurasia. Glen M. MacDonald, Thomas W.D. Edwards, Bruce Gervais, Tamsin E. Laing, Michael F.J. Pisaric, David F. Porinchu, Jeffrey A. Snyder, Nadia Solovieva, Pavel Tarasov and Brent B. Wolfe 13. Paleolimnological studies in arctic Fennoscandia and the Kola Peninsula (Russia).Atte Korhola and Jan Weckstrom 14. Paleolimnological studies from the Antarctic and subantarctic islands. Dominic A. Hodgson, Peter T. Doran, Donna Roberts and Andrew McMinn 15. Paleolimnology of extreme cold terrestrial and extraterrestrial environments. Peter T. Doran, John C. Priscu, W. Berry Lyons, Ross D. Powell, Dale T. Andersen and Robert J. Poreda 16. Epilogue: Paleolimnological research from arctic and antarctic regions. Reinhard Pienitz, Marianne S.V. Douglas and John P. Smol

Diatom assemblages and their relationship to environmental variables in lakes from the boreal forest-tundra ecotone near Yellowknife, Northwest Territories, Canada

The relationship between diatom (Bacillariophyceae) taxa preserved in surface lake sediments and measured limnological and environmental variables in 22 lakes near Yellowknife (N.W.T.) was explored using multivariate statistical methods. The study sites are distributed along a latitudinal gradient that includes a strong vegetational gradient of boreal forests in the south to arctic tundra conditions in the north. Canonical correspondence analysis (CCA) revealed that lakewater concentrations of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) each accounted for independent and statistically significant proportions of variation in the distribution of diatom taxa. Weighted-averaging (WA) models were developed to infer DIC and DOC from the relative abundances of the 76 most common diatom taxa. These models can now be used to infer past DIC and DOC concentrations from diatom assemblages preserved in sediment cores of lakes in the Yellowknife area, which may provide quantitative estimates of changes in lakewater chemistry related to past vegetational shifts at treeline.

Ice Shelf Microbial Ecosystems in the High Arctic and Implications for Life on Snowball Earth

Abstract The Ward Hunt Ice Shelf (83°N, 74°W) is the largest remaining section of thick (>10 m) landfast sea ice along the northern coastline of Ellesmere Island, Canada. Extensive meltwater lakes and streams occur on the surface of the ice and are colonized by photosynthetic microbial mat communities. This High Arctic cryo-ecosystem is similar in several of its physical, biological and geochemical features to the McMurdo Ice Shelf in Antarctica. The ice-mats in both polar regions are dominated by filamentous cyanobacteria but also contain diatoms, chlorophytes, flagellates, ciliates, nematodes, tardigrades and rotifers. The luxuriant Ward Hunt consortia also contain high concentrations (107–108 cm–2) of viruses and heterotrophic bacteria. During periods of extensive ice cover, such as glaciations during the Proterozoic, cryotolerant mats of the type now found in these polar ice shelf ecosystems would have provided refugia for the survival, growth and evolution of a variety of organisms, including multicellular eukaryotes.

Multi-proxy Holocene palaeoclimatic record from a saline lake in the Canadian Subarctic

Multi-proxy palaeolimnological analyses of a postglacial sedimentary sequence at a centennial-scale resolution from an athalassic saline lake in the Yukon were conducted to infer patterns of Holocene climatic change in the Canadian Subarctic, using sediment mineralogy and biostratigraphy (diatoms, pigments). Diatominferred quantitative estimates of palaeosalinity were obtained by use of transfer functions developed from a calibration set of 219 lakes from western North America. The sediment mineralogy and fossil pigments at the base of the core indicated a moderately deep non-stratified lake dominated by clastic influx, probably in a basin fed by glacial meltwater. The early-Holocene history (c. 11 000–8100 14C yr BP) was characterized by a relatively deep mesosaline lake with diatom-inferred salinities approximating 20 g L-1. The occurrence of both aragonite and dolomite, as well as elevated concentrations of chlorophyll and carotenoid pigments, support the interpretation of deepwater anoxia and possibly strong chemical stratification. High concentrations of the chemically stable b-carotene suggest that total algal abundance was particularly high during the early Holocene, when planktonic Cyclotella cf. choctawhatcheeana and Chaetoceros muelleri were the most common diatom taxa. Relatively fresh (2–15 g L-1) eutrophic conditions prevailed during the mid-Holocene period (c. 8000– 2000 yr BP), with four periods of alternating fresh and saline conditions. The diatom-inferred salinity profile reveals significant fluctuations within these cycles, but overall they indicate humid climatic conditions compared to today. Algal abundance is inferred to have declined three-fold relative to the early Holocene, particularly in the case of eukaryotic algae (e.g., diatoms, cryptophytes, chlorophytes). The recent history of the lake (about 2000 years BP until the present day) was marked by important changes in ionic composition (e.g., occurrence of gypsum and Mg-carbonates) and hydrologic conditions. The lakewater composition during the last two millennia was characterized by hypersaline Mg-SO4 brines. The palaeolimnological evidence from most proxies indicates a trend towards drier conditions during the past 2000 years. The various indicators reveal a complex history of frequent and rapid shifts in palaeosalinity and lake palaeoproductivity during the Holocene, and the effects of the Younger Dryas and ‘Little Ice Age’ episodes may be recorded in the palaeoclimate proxy data. The palaeoclimatic interpretation emerging from this high-latitude lake corroborates existing broad trends based on palynological studies in this region but provides evidence for more dynamic climatic change during the mid- and late Holocene.

Past and Future Changes in Arctic Lake and River Ice

Paleolimnological evidence from some Arctic lakes suggests that longer ice-free seasons have been experienced since the beginning of the nineteenth century. It has been inferred from some additional records that many Arctic lakes may have crossed an important ecological threshold as a result of recent warming. In the instrumental record, long-term trends exhibit increasingly later freeze-ups and earlier break-ups, closely corresponding to increasing air temperature trends, but with greater sensitivity at the more temperate latitudes. Broad spatial patterns in these trends are also related to major atmospheric circulation patterns. Future projections of lake ice indicate increasingly later freeze-ups and earlier break-ups, decreasing ice thickness, and changes in cover composition, particularly white-ice. For rivers, projected future decreases in south to north air-temperature gradients suggest that the severity of ice-jam flooding may be reduced but this could be mitigated by changes in the magnitude of spring snowmelt.

Diatomées lacustres de Jamésie-Hudsonie (Québec) et modèle de reconstitution des concentrations de carbone organique dissous

AbstractLes diatomees des eaux douces sont de plus en plus utilisees pour reconstituer les environnements du passe. Cette recherche a ete realisee dans le but d’evaluer ce potentiel dans une region ou peu d’informations sur les diatomees etaient disponibles. L’etude a permis d’identifier un ensemble de 516 especes de diatomees preservees dans les sediments de surface d’un transect latitudinal de 59 lacs du Nord-Ouest quebecois. A l’aide d’analyses statistiques multivariees, il a ete possible de determiner quelles sont les variables environnementales (parmi 49) qui exercent le plus d’influence sur la composition des communautes de diatomees. Ce sont les concentrations en magnesium, en sodium, en carbone organique dissous (COD) et en silice, ainsi que la profondeur des lacs qui semblent etre les variables qui influencent le plus la composition des communautes de diatomees dans cette region. Un modele de reconstitution du COD a ete cree. Ce modele pourra etre utilise dans des etudes paleolimnologiques afin d...

Climate Change and Mercury Accumulation in Canadian High and Subarctic Lakes

Mercury (Hg) profiles were compared to profiles of climate indicators including microfossil remains and algal-derived or S2 carbon (C) in dated sediment cores from 14 lakes spanning latitudinal and longitudinal gradients across the Canadian high and subarctic. Hg fluxes increased postindustrialization (post-∼1850) in 11 of these lakes (postindustrialization Hg fluxes (ΔHgF(F)) = 2-24 μg m(-2) y(-1)). Correction of HgF(F) for catchment contributions demonstrated that Hg deposition originating from catchment-independent factors, such as atmospheric deposition, increased since industrialization in all 14 lakes. Several of these lakes also showed postindustrial shifts in algal assemblages consistent with climate-induced changes. Eleven lakes showed post-1850s increases in S2F(F), suggesting that lake primary productivity has recently increased in the majority of our sites (ΔS2F(F) = 0.1-4 g m(-2) y(-1)). Other studies have interpreted significant relationships between Hg:S2 concentrations in Arctic sediment as support for the algal scavenging hypothesis, which postulates that Hg fluxes to Arctic sediments are largely driven by S2. However, in six of our lakes we observed no Hg:S2 relationship, and in one lake a significant negative Hg:S2 relationship was observed due to increased Hg and decreased S2 C deposition during the postindustrialization period. In six of the seven lakes where a significant positive Hg:S2 relationship was observed, algal assemblages either did not change through time or the timing of the shifts did not correspond to changes in Hg deposition. Our results demonstrate that, although Arctic lakes are experiencing a myriad of changes, including increased Hg and S2 deposition or changing algal assemblages, increased lake primary productivity does not appear to be driving changes in Hg fluxes to sediments.