Subarctic Environment Research Articles

Detection of permanent open water surfaces in central Siberia with ENVISAT ASAR wide swath data with special emphasis on the estimation of methane fluxes from tundra wetlands

Permanent water bodies not only store dissolved CO2 but are essential for the maintenance of wetlands in their proximity. From the viewpoint of greenhouse gas (GHG) accounting wetland functions comprise sequestration of carbon under anaerobic conditions and methane release. The investigated area in central Siberia covers boreal and sub-arctic environments. Small inundated basins are abundant on the sub-arctic Taymir lowlands but also in parts of severe boreal climate where permafrost ice content is high and feature important freshwater ecosystems. Satellite radar imagery (ENVISAT ScanSAR), acquired in summer 2003 and 2004, has been used to derive open water surfaces with 150 m resolution, covering an area of approximately 3 Mkm2. The open water surface maps were derived using a simple threshold-based classification method. The results were assessed with Russian forest inventory data, which includes detailed information about water bodies. The resulting classification has been further used to estimate the extent of tundra wetlands and to determine their importance for methane emissions. Tundra wetlands cover 7% (400 000 km2) of the study region and methane emissions from hydromorphic soils are estimated to be 45 000 t d−1 for the Taymir peninsula.

Ralph Erskine, (Skiing) Architect

In this paper I focus on Ralph Erskine's enduring image in architectural discourse as the Arctic Architect of Modernism. My interest lies with the relationship between portrayals of Erskine - both in textual accounts an images - and the way his sub-Arctic projects, especially his unrealised utopian projects for an ‘Ideal Town' north of the Arctic Circle, have been canonised in architectural discourse as exemplars of an architecture that is truly regional in character and, moreover, ideally suited to the unique cultural - especially with regard to indigenous populations - and environmental habitats of Arctic and sub-Arctic environments.

Oral and physiological paleohealth in cold adapted peoples: Northeast Asia, Hokkaido

This paper examines variables useful in reconstructing oral (caries, antemortem tooth loss, alveolar defects) and physiological (cribra orbitalia, linear enamel hypoplasia) well-being in two bioarchaeological assemblages from Hokkaido, Japan: Okhotsk (n = 37 individuals) and Jomon (n = 60). Findings are compared and contrasted with each other, with published series from Honshu Japan, and samples from climatically near-equivalent Alaska. It was found that more meaningful comparisons of Hokkaido paleohealth could be made with Alaskan material, rather than the more southerly Jomon. Results were ambiguous with respect to physiological well-being. Low levels of LEH in the cold-adapted samples suggest operating in arctic and subarctic environments with marine-based subsistence regimes is not physiologically expensive. However, the relatively high levels of cribra orbitalia in Hokkaido, relative to Alaska, suggest the picture is not straightforward: the reasons for elevated cribra orbitalia in Hokkaido are unclear. The subarctic and arctic samples formed three broadly similar groupings in terms of oral health profiles: (1) Aleuts and Eskimo; (2) Ipiutak and Tigara; (3) Hokkaido Jomon, Okhotsk, and Kodiak Island. Differences between these groupings could be explained with a combination of sample demographics and subsistence orientations. The extremely high frequency of caries in one sample, caribou hunting Ipiutak, may have been influenced by factors such as low levels of dietary magnesium and potentially cariogenic foodstuffs, such as preparations of caribou stomach contents. It was concluded that oral health profiles are potentially sensitive to differences in subsistence strategies among cold-adapted hunter-gatherers, although they lack predictive value.

An integrated framework of lake‐stream connectivity for a semi‐arid, subarctic environment

AbstractLake‐stream networks dominate the semi‐arid, subarctic Precambrian shield. Such a network consists of a number of lake basin elements linked to each other by surface channels. To investigate the processes causing seasonal severance of flow connection in the lake‐stream system, a chain of lakes in northern Canada was studied in 2004. Water balance shows that rapid and substantial runoff from the local basin slopes during the snowmelt period led to a rise of lake levels above their outlet elevations to generate outflow. Continued summer evaporation caused draw down of lake storage below the outflow thresholds, represented by the lake outlet elevations. Outflow ceased and the lakes became disconnected. Summer rainfall in a semi‐arid environment was insufficient to overcome storage deficit to re‐establish flow connectivity among all lakes. For the drainage system as a whole, streamflow interruption or continuity depends on the linkage of its lake‐stream sub‐units. The principle of fill and spill governs runoff generation and flow connection between the lake elements. This principle is applied to a conceptual model of flow along a chain of lakes, taking account of (1) antecedent storage in individual lakes, (2) their storage change calculated through water balance and (3) the thresholds to be exceeded for outflows to occur. Copyright © 2007 John Wiley & Sons, Ltd.

Consequences of Variation in Tree Architecture and Leaf Traits on Light Capture and Photosynthetic Nitrogen Use Efficiency in Mountain Birch

ABSTRACTIn the subarctic environment, shortage of nitrogen is common and may have immediate effects on tree survival via lowered photosynthetic capacity in cold periods. Yet, despite the critical role of nitrogen, the subarctic tree mountain birch [Betula pubescens ssp. czerepanovii (Orlova) Hämet-Ahti] shows remarkable variability in tree architecture and leaf traits, which affect its nitrogen use and its ability to capture light. It is possible that intraspecific variation in these traits exhibits alternative strategies for maintaining efficient nitrogen use, provided it results in equal efficiency despite variation in the underlying traits, but true differences between tree individuals may also exist.Computer simulations were used to investigate how daily photosynthetic nitrogen use efficiency (DPNUE) and the potential rate of photosynthesis (Pmax) of whole birch trees depend on tree architecture, the area or number of leaves per shoot, or nitrogen invested per leaf. The simulations showed that mountain birch has the potential to achieve an equal Pmax or DPNUE by adjusting variation in many traits, but the potential was not realized as a large amount of variation remained when leaf traits measured in the field were used to estimate DPNUE and Pmax . Trade-offs between Pmax , DPNUE, and other tree functions such as resistance to herbivores, growth, maintenance, or reproduction are likely causes for variation.

Interactions of temperature and photoperiod in the control of flowering of latitudinal and altitudinal populations of wild strawberry (Fragaria vesca)

Floral induction and development requirements of a range of latitudinal and altitudinal Norwegian populations of the wild strawberry Fragaria vesca L. have been studied in controlled environments. Rooted runner plants were exposed to a range of photoperiods and temperatures for 5 weeks for floral induction and then transferred to long day (LD) at 20°C for flower development. A pronounced interaction of temperature and photoperiod was shown in the control of flowering. At 9°C, flowers were initiated in both short day (SD) and LD conditions, at 15 and 18°C in SD only, whereas no initiation took place at 21°C regardless of daylength conditions. The critical photoperiod for SD floral induction was about 16 h and 14 h at 15 and 18°C, respectively, the induction being incomplete at 18°C. The optimal condition for floral induction was SD at 15°C. A minimum of 4 weeks of exposure to such optimal conditions was required. Although the populations varied significantly in their flowering performance, no clinal relationship was present between latitude of origin and critical photoperiod. Flower development of SD‐induced plants was only marginally advanced by LD conditions, while inflorescence elongation and runnering were strongly enhanced by LD at this stage. The main shift in these responses took place at photoperiods between 16 and 17 h. Unlike all other populations studied, a high‐latitude population from 70°N (‘Alta’) had an obligatory vernalization requirement. Although flowering and fruiting in its native Subarctic environment and after overwintering in the field in south Norway, this population did not flower in the laboratory in the absence of vernalization, even with 10 or 15 weeks of exposure to SD at 9°C. Flowering performance in the field likewise indicated a vernalization requirement of this high‐latitude population.

Prolonged Persistence of Fecally Excreted Ivermectin from Reindeer in a Sub-Arctic Environment

In December 2001 and 2002, feces from reindeer calves treated with ivermectin were distributed on plots established on two types of forested reindeer pasture in northern Finland. The ungrazed plots were on an enclosure that had been fenced to prevent reindeer access for the last 6 years. The grazed plots were on an area that had been heavily stocked by reindeer during the last 5 years. After enclosures had been established, reindeer and large wildlife were prevented from entering by a fence. Topsoil samples (feces, vegetation, and soil) were collected monthly during the summers of the following 2 years, over a period of from 25 to 95 weeks after deposition. The samples were analyzed for ivermectin using HPLC. Although ivermectin degradation rapidly took place during the first spring, considerable residual ivermectin could be measured throughout the sampling time, showing that ivermectin in feces on pasture may not be photodegraded as rapidly as previously believed. The results support the need for further environmental evaluation studies on the use of ivermectin to control reindeer parasites.

Temporal and spatial variability of the beginning and end of daily spring freeze/thaw cycles derived from scatterometer data

The variability of snowmelt dates is important for the terrestrial carbon balance in boreal and subarctic environments. Scatterometers such as the K u-Band QuikScat have been proven applicable for the detection of surface thaw. We present an improved method for the capture of thaw events based on significance of diurnal differences with respect to long term noise. For each 10 km × 10 km grid point two products are derived for the major thaw period: 1) the onset of thaw and 2) the end of daily freeze/thaw cycles at the surface. Both dates may be related to biogeochemical processes, especially carbon fluxes in boreal forests. The onset of the spring thaw period coincides with the first days of increased CO 2 fluxes above the late winter baseline. The end of daily freeze/thaw cycles corresponds to the switch from source to sink in evergreen boreal forest environments as illustrated by comparison with eddy-flux tower data and xylem sap flow records from other investigators. The approach is suitable for detecting freeze/thaw cycle periods in boreal forest and tundra biomes. The mean absolute difference in end of freeze/thaw cycling date within the central Siberian study area (3 Mio km 2 comprising tundra, boreal forest and steppe grassland) was 9 days for 2000 to 2004. Largest mean differences occurred in the southern taiga and all tundra regions, which were highest for spring 2000. The improved extraction method delivers more precise products from the viewpoint of carbon accounting in evergreen boreal forest environments. This widens the application potential of scatterometer data beyond the current status.

Spatial Heterogeneity and Hierarchical Feeding Habitat Selection by Reindeer

Reindeer, Rangifer tarandus, live in subarctic and alpine environments with spatially and temporally heterogeneous resource distribution. In this study, we used a hierarchical approach to test whether reindeer responded to spatial heterogeneity during the plant growing season (divided into three distinct periods) in a mountainous subarctic environment in northern Sweden. A reindeer herd in northern Sweden was surveyed using radio-telemetry (8 female reindeer) and the selection of feeding habitats by observing individuals/groups (135 observations) using laser range-finding binoculars. Reindeer selected feeding areas (evaluated at 5-km grid size), as well as feeding habitats (evaluated at 0.5- and 1-km grid size) during spring, in response to high terrain ruggedness and habitat heterogeneity. Reindeer switched during summer to select against terrain ruggedness and habitat heterogeneity at the level of feeding habitats, while preferring southward facing habitats. During autumn, a broader spectrum of feeding habitats was used. We conclude that reindeer seem to adopt a hierarchical strategy in agreement with general foraging theory, and are capable of responding to seasonal changes in resource distribution occurring across spatial scales. Furthermore, our results support the idea that spatial heterogeneity is an important factor to large-sized herbivores at high and intermediate levels of habitat selection. Conservation of large continuous and undeveloped landscapes is an important management goal, as they provide a wide range of habitats necessary for animals such as reindeer that use large territories.

Impacts of Holocene climatic variations on alluvial fan activity below snowpatches in subarctic Québec

In the Lake Guillaume-Delisle area of subarctic Québec, storm-generated alluvial fans have been active sporadically throughout the Holocene. In this study, we propose that the persistence of late-lying snowpatches in fan catchments during Holocene cold episodes promoted alluvial fan activity by lowering the precipitation threshold required to trigger a torrential event. This hypothesis was tested by characterizing the depositional processes responsible for alluvial fan formation below snowpatches, and by reconstructing the Holocene alluvial fan activity. Stratigraphic and sedimentary analyses conducted on seven alluvial fans revealed that they were deposited by torrential activity leading to waterlaid, transient, or hyperconcentrated deposition. The chronology of the storm-generated alluvial fans — based on 22 radiocarbon dates — indicates that torrential activity was enhanced during the cooler Late Holocene (i.e., after ca. 3500 cal. yr BP). Snowier winters and cooler summers were beneficial to nival activity, allowing the persistence of larger snowpatches throughout the summer and fall seasons. Rainfall-induced thaw of such snowpatches during rainstorm events is inferred to have contributed to alluvial fan activity by increasing water availability. Three peaks of alluvial activity occurred during the Late Holocene (2950–2750, 1900–1400, and 800–300 cal. yr BP) and are indicative of increased storminess resulting in higher fan activity. Increased fan activity during cooler episodes was concurrent with increased runoff activity in the immediate pronival area. This stresses the importance of nivation below snowpatches and pinpoints the role of nivation in enhancing geomorphological activity during period of cooler and more humid climate in subarctic environments.

Analysis of anaerobic BTX biodegradation in a subarctic aquifer using isotopes and benzylsuccinates

In situ biodegradation of benzene, toluene, and xylenes in a petroleum hydrocarbon contaminated aquifer near Fairbanks, Alaska was assessed using carbon and hydrogen compound specific isotope analysis (CSIA) of benzene and toluene and analysis of signature metabolites for toluene (benzylsuccinate) and xylenes (methylbenzylsuccinates). Carbon and hydrogen isotope ratios of benzene were between − 25.9‰ and − 26.8‰ for δ 13C and − 119‰ and − 136‰ for δ 2H, suggesting that biodegradation of benzene is unlikely at this site. However, biodegradation of both xylenes and toluene were documented in this subarctic aquifer. Biodegradation of xylenes was indicated by the presence of methylbenzylsuccinates with concentrations of 17–50 μg/L in three wells. Anaerobic toluene biodegradation was also indicated by benzylsuccinate concentrations of 10–49 μg/L in the three wells with the highest toluene concentrations (1500–5000 μg/L toluene). Since benzylsuccinate typically accounts for a very small fraction of the toluene present in groundwater (generally < 1 mol%), the signature metabolite approach works best at higher toluene concentrations when it is not constrained by detection limits. In wells with lower toluene concentrations (410–640 μg/L), carbon and hydrogen isotopic values were enriched by up to ∼2‰ for δ 13C and ∼70‰ for δ 2H. This evidence of isotopic fractionation verifies the effects of biodegradation in these low concentration wells where metabolites may already be below detection limits. The combined use of signature metabolite and CSIA data is particularly valuable given the challenge of verifying biodegradation in subarctic environments where degradation rates are typically much slower than in temperate environments.

Climate effects on formation of jellyfish and ctenophore blooms: a review

Much speculation and some evidence suggest that jellyfish and ctenophore populations have increased in recent decades. Unfortunately, few past records exist with which to compare current populations, and our knowledge of how environmental factors affect jellyfish population size is meagre. Human enterprise has wrought many changes in the ocean that are hypothesized to favour jellyfish, including eutrophication, reduction of fish stocks, and global warming. In addition to anthropogenic changes, natural climate cycles may affect jellyfish populations. Records of jellyfish and ctenophore abundance that appear to be related to indices of climate variations (temperature, salinity, North Atlantic Oscillation, North Pacific Decadal Oscillation, El Niño Southern Oscillation) are reviewed. In eleven species studied from subtropical, temperate and subarctic environments, warm temperatures were related to large population sizes; three scyphozoan species in the North Sea, and one mesopelagic hydromedusan were exceptions to that trend. One tropical scyphomedusan species was decimated by unusually warm, salty El Niño conditions in Palau. Because climate changes have complex ecosystem-level effects, the proximate causes of jellyfish increases are difficult to deduce. Therefore, the effects of temperature, salinity and prey on asexual production of new medusae from the benthic polyps of scyphomedusae and hydromedusae also are reviewed. Experiments on temperate species show greater and more rapid production of medusae at warmer temperatures. Salinity also had significant effects, and was especially important for estuarine species. Temperature and salinity affect asexual reproduction rates directly through metabolism, and indirectly through prey capture. Ocean warming may shift the distributions, expand the seasonal occurrence, and increase the abundances of temperate-boreal species. Populations living near their thermal maximum may suffer negative consequences of warming.

Effects of reindeer browsing on tundra willow and its associated insect herbivores

Summary Browsing by large mammals may strongly constrain the growth and reproduction of woody plants, and may alter the food quality and resource availability for herbivorous insects. The response of the plants may vary between different growth stages, and the preference of herbivores may be related to plant age. Understanding the effects of reindeer Rangifer tarandus browsing on the growth of woody forage plants is important in formulating guidelines for reindeer grazing management, especially in low productivity subarctic environments. We studied the effects of summer browsing by reindeer on the growth and reproduction of willow Salix phylicifolia and on the abundance of its insect herbivores, by studying plants inside and outside exclosures over a period of 6 years. The experiment was run in northern Finland and included 80 willow genets in an area near the timberline formed by mountain birch Betula pubescens. At the beginning of the experiment, half of the willows were cut at ground level to rejuvenate ramets. Reindeer feeding was more intense on rejuvenated willow compared with old willow, and the effects of browsing were more pronounced on rejuvenated plants. Reindeer browsing reduced the height of willow by c. 50%, shoot length by c. 30% and accelerated dieback of the shoots by c. 50%. Browsed willow produced fewer shoots, with fewer buds and floral catkins, than unbrowsed willow. Browsing also reduced the densities of the most common insect herbivores: leaf beetles of the genus Gonioctena (Coleoptera: Chrysomelidae) and gall‐inducing sawflies (Hymenoptera: Tenthredinidae) belonging to the genera Phyllocolpa, Eupontania and Euura. Synthesis and applications. We show that reindeer browsing in summer reduces biomass and diminishes reproductive success of willow; it also lowers the numbers of its associated insect herbivores. Our results suggest that this effect will be most evident in low‐productivity tundra heaths where alternative forage plants, such as relatively palatable and productive graminoids, are scarce. We advise that reindeer should be maintained below the present levels of 2–3 reindeer km−2 to sustain the long‐term persistence of important forage plants such as willow in these low productivity habitats.

Circadian organization of a subarctic rodent, the northern red-backed vole (Clethrionomys rutilus).

Arctic and subarctic environments are exposed to extreme light: dark (LD) regimes, including periods of constant light (LL) and constant dark (DD) and large daily changes in day length, but very little is known about circadian rhythms of mammals at high latitudes. The authors investigated the circadian rhythms of a subarctic population of northern red-backed voles (Clethrionomys rutilus). Both wild-caught and third-generation laboratory-bred animals showed predominantly nocturnal patterns of wheel running when exposed to a 16:8 LD cycle. In LL and DD conditions, animals displayed large phenotypic variation in circadian rhythms. Compared to wheel-running rhythms under a 16:8 LD cycle, the robustness of circadian activity rhythms decreased among all animals tested in LL and DD (i.e., decreased chi-squared periodogram waveform amplitude). A large segment of the population became noncircadian (60% in DD, 72% in LL) within 8 weeks of exposure to constant lighting conditions, of which the majority became ultradian, with a few individuals becoming arrhythmic, indicating highly labile circadian organization. Wild-caught and laboratory-bred animals that remained circadian in wheel running displayed free-running periods between 23.3 and 24.8 h. A phase-response curve to light pulses in DD showed significant phase delays at circadian times 12 and 15, indicating the capacity to entrain to rapidly changing day lengths at high latitudes. Whether this phenotypic variation in circadian organization, with circadian, ultradian, and arrhythmic wheel-running activity patterns in constant lighting conditions, is a novel adaptation to life in the arctic remains to be elucidated.

Measuring the Extent of Chemical Weathering in Subarctic Alpine Environments: Implications for Future Research

Chemical transformations associated with the weathering of bedrock to regolith, and ultimately to soils are poorly known despite the importance of these transformations in the generation of surface and groundwater solutes, the preparation of bedrock for erosion, the production of plant nutrients, and ultimately the formation of soil itself. This lack of knowledge is particularly apparent in polar and subpolar lands where traditionally chemical processes have been regarded as subordinate to mechanical weathering processes. This paper examines the nature of chemical changes associated with the transformation of bedrock to soil in three subarctic alpine locations by comparing weathering indices calculated from whole-rock chemical analyses of surface soil, regolith, rock fragments, and bedrock. The results of this largely post hoc integration of our previous reconnaissance studies suggests that as bedrock weathers to soil there is a loss of silicon, alkali earths, and alkalis relative to resistant chemical constituents, as might be expected from first principles. However, the loss is not systematic, as bedrock fragments derived from crustal rock represent the most resistant elements of a weathering/soil profile, and surface soils commonly experience eolian addition of chemical constituents. The results of this study are viewed as being “enticing” and suggest a potentially productive line of chemical weathering investigation in polar and subpolar environments that will throw light on patterns of release of chemical constituents in the initial stages of the production of residual soils.

Simulating vegetation cover dynamics with regards to long-term climatic variations in sub-arctic landscapes

Iceland is facing severe land degradation in many parts of the country. This study aims to increase the understanding of the complex interactions and interconnectivity between the critical factors that help maintain the land degradation processes in sub-arctic environments. A holistic approach in the form of a causal loop diagram (CLD) is applied for diagnosing the influencing factors. To further study the relationship between vegetation cover and its degradation, a dynamic model that uses a long-term temperature data as the main indicator function is constructed to simulate potential vegetation cover during the Holocene. The results depict an oscillating vegetation cover. Gradual degradation in potential vegetation cover begins ca. 3000 BP and accelerates greatly after ca. 2500 BP. From the time of the Norse settlement in the latter halve of the 9th century to present time, the simulated vegetation cover retreats ca. 25% in relation to climatic cooling.

Operation of a geophysical HTS SQUID system in sub-arctic environments

Transient ElectroMagnetic geophysical prospecting using SQUID sensors has demonstrated potential for improved target detection at late response times compared to conventional coil sensors. We have developed a three-axis, rf SQUID sensor system which has been extensively operated in sub-Arctic conditions by a geophysics contractor. Due to the harsh environmental and operating conditions, the system is designed to operate in sub-zero temperatures (as low as minus 40/spl deg/C) and is ruggedly packaged whilst still remaining quite portable. Auto-tuning of the rf electronics has been implemented by adjusting the rf SQUID control parameters via a microprocessor controller. After a small amount of training, regular field crews have operated two of these systems in the field continuously for periods of months at a time. An example, comparing SQUID B field data to coil dB/dt data, is presented in this paper.

Stable isotope profiles ofSerripes groenlandicus shells. I. Seasonal and interannual variations of Alaskan Coastal Water in the Bering and Chukchi Seas

Serripes groenlandicus shells were collected alive from the eastern regions of the Bering and Chukchi seas where Alaskan Coastal Water occupies dominantly. High-resolution stable isotope profiles of the shell carbonates exhibit distinct, apparently annual, cycles caused by seasonal variations of temperature, seawater isotopic composition and other factors. The shell σ18O profiles show a moderately high-amplitude seasonal temperature cycle with considerable interannual variability. Superimposed on the temperature-controlled cycles are periodic excursions which are associated with reduced salinity, and more negative seawater σ18O, related presumably to an increased freshwater flux to the Alaskan Coastal Current during the summer: Comparison between two oxygen isotope profiles supports the occurrence of seasonal and interannual variability of Alaskan Coastal Water in the eastern part of the Bering and Chukchi Shelf. The cycles in the σ13C profiles are interpreted as being controlled primarily by phytoplankton productivity and seasonal temperature. In addition, a trend of more negative σ13C values towards the later stage of growth may reflect physiological changes associated with sexual maturity, gametogenesis, and slower growth rate. The oxygen and carbon isotope profiles as a biorecorder of bivalve shells are substantiated in the subarctic environments, probing the seasonal and internanual variation of Alaskan Coastal Water.

Polychlorinated naphthalene levels, distribution, and biomagnification in a benthic food chain in the Baltic Sea.

The scientific literature contains little information regarding bioaccumulation and biomagnification of polychlorinated naphthalenes (PCNs) in food webs. Here we present new information on the food chain transfer of PCNs within a food chain in a subarctic environment PCNs (tetra- to hepta-chloro congeners) were measured in surface sediments and in a marine benthic food chain, comprising amphipods, isopods, and fourhorned sculpins. Samples were collected from five locations in the Gulf of Bothnia, northern Baltic Sea. PCN concentrations in the sediments were similar to background levels determined previously in sediments from the northern hemisphere. Measurement of the carbon content of the sediments allowed the calculation of biota to sediment accumulation factors (BSAFs). Tetra- and penta-CNs exhibited BSAF values greater than one, while BSAFs for the more chlorinated PCNs were less than one. This suggests more efficient assimilation, by amphipods, of the less chlorinated PCNs. A decrease in sigmaPCN concentrations from the lowest to the highest trophic level was demonstrated (amphipods: 10-69 ng/g lw, isopods: 3.9-16 ng/g lw; fourhorned sculpins: 0.54-1.5 ng/g lw). Biomagnification factors (BMFs) were calculated based on the concentrations of the congeners. These indicated that a few congeners biomagnified significantly: the highest BMFs (0.09-1.4) were found for 2,3,6,7-substituted congeners and those lacking adjacent hydrogen-substituted carbon atoms.

Energy Exchange During River Icing Formation in a Subarctic Environment, Yukon Territory

La formation de nappes de glace stratifiée est un phénomène hydrologique cou- rant en milieu subarctique. Il résulte de l'accumulation d'écoulements successifs pendant l'hiver. La taille et l'épaisseur des couches de glace sont déterminées par l'interaction entre les systèmes microclimatologique et hydrologique de l'eau de surface. On étudie ici les échanges d'énergie suivant différentes épaisseurs de nappes de glace. Dans le cas d'épaisses nappes d'écoulement, les couches de glace prennent plus de temps à geler entièrement en raison de la plus grande quantité de chaleur latente emmagasinée dans les volumes d'eau. Des températures de l'air plus douces vont davantage ralentir le processus. Dans de telles conditions, l'eau circulant entre le couvert de glace supérieur et la glace sous-jacente procure des quantités d'énergie appréciables. De 60 à 87 % de l'énergie est fournie par l'eau libre. Avec des températures progressivement plus froides, l'englaciation accélérée réduit le temps d'écoulement de l'eau, diminuant ainsi la quantité relative d'énergie fournie par l'eau. Dans ce cas, l'énergie est livrée par la chaleur latente libérée par le gel de l'eau contenue dans la couche de débordement. Dans certaines conditions, l'absortion du rayonnement solaire produit aussi une quantité considérable d'entrée d'énergie dans le régime thermique. Cette énergie est libérée par l'entremise de pertes appréciables de chaleur irradiante. Pendant la formation d'une nappe de glace, la chaleur latente est la moins importante, ne comptant que pour 6 à 17 % de la perte totale de chaleur.