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1.
Banks Island (N.W.T.) has become a focal point for climate change studies in the Canadian Arctic. However, long-term climatic and environmental data are very sparse from this large island, as they are for the entire southwestern region of the Canadian Arctic Archipelago. In this paleolimnological study, diatom species assemblage shifts documented in cores collected from a pond and a lake on Banks Island were interpreted to represent a response to climate warming commencing in the nineteenth century. We found that, although the timing and overall nature of the species changes in the two cores were consistent, the signal was muted in the deeper site likely as a result of differences in ice cover extent and duration between lakes and ponds. A high-resolution study was also conducted from a second pond, at sub-decadal resolution, that only spanned the last ∼60 years. In the deeper lake site, Fragilaria construens and F. pinnata dominated the assemblages, similar to those noted in other high Arctic regions where lakes are characterized by extended ice cover. In contrast, Denticula kuetzingii dominated the shallower ponds and, in the case of the pond core representing the longer time period, this taxon increased in the post-1850 sediments, likely coincident with climate warming. In all cores, diatom assemblages became more diverse and Achnanthes species (particularly A. minutissima) increased from ∼1850 to the present, similar to changes documented in other Arctic regions. Beta diversity values calculated for the diatom species changes indicated that assemblage shifts in the Banks Island cores were of similar magnitude to those recorded in other Arctic regions with high species turnover, such as Ellesmere Island. A diatom-based Total Nitrogen (TN) transfer function previously developed for Banks Island was applied to the three 210Pb dated cores as an exploratory tool for inferring past changes in nitrogen concentrations. In both the lake and pond cores, diatom-inferred TN concentrations tended to increase in the more recent sediments, as may be expected with warming; however these trends were not very distinct.  相似文献   

2.
Ecology of testate amoebae (thecamoebians) in subtropical Florida lakes   总被引：1，自引：1，他引：0  
Fifty-seven surface sediment samples from 35 Florida lakes were collected to study testate amoebae. Seven genera, 17 species, and 28 strains were identified in the 46 sediment samples from 31 lakes that contained testate rhizopods. Seven species accounted for ≥90% of the individuals in all samples. Sediment total phosphorus (TPsed), organic matter (OM), and total carbon:total nitrogen ratio (TC:TN) were measured to assess the effect of these variables on thecamoebian assemblages. OM content was the only sediment variable that influenced presence/absence of thecamoebians. Samples with <5% OM contained no thecamoebians. Lakes with multiple surface sediment samples showed high Morisita–Horn similarity values (0.74–0.99), indicating that all sites at which samples were collected in a lake provided representative thecamoebian assemblages. No relationship was observed between thecamoebian diversity indices and sediment variables. Lake trophic state and pH were examined to explore potential water column influences on thecamoebian communities. Highest thecamoebian diversity indices were found in mesotrophic to eutrophic lakes with pH near 8.0. These results suggest that water column conditions have a greater influence on thecamoebian assemblages than do sediment variables. We used multivariate analysis to evaluate the relations between water quality variables and testate rhizopod assemblages. Canonical correspondence analysis (CCA) showed that alkalinity and pH are the water column variables that most influence the relative abundance of species. Thecamoebians thus hold promise as bioindicators of acidification in Florida lakes. Thecamoebian remains in lake sediment cores should be useful to infer past anthropogenic shifts in lake pH.  相似文献   

3.
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4.
Meretta Lake (Resolute Bay, Cornwallis Island, Nunavut, Canada) is a high arctic lake that received raw sewage for almost 50 years from the Canadian Department of Transport Base. The lake was sampled from 1968–72 during the International Biological Programme, as part of the Char Lake Project. As the number of users at the Transport Base declined throughout the 1990s, so too did the lake's nutrient levels, and Meretta Lake is now classified as oligotrophic. A previous diatom-based paleolimnological study revealed marked species assemblage shifts coincident with sewage inputs beginning in the late 1940s; however, because the core was taken at a time when nutrient levels were still relatively high (i.e., 1993), the diatom record did not yet track any signs of recovery. In this present study, we examined fossil diatom assemblages from a sediment core taken in 2001. Our results indicate a shift to the pre-impact diatom assemblages in the most recent sediments, indicating that the paleolimnological record is tracking the decreased nutrient inputs to this high arctic lake, and confirms that no significant lags exist in these largely ice-covered lakes.  相似文献   

5.
Until recently, major anthropogenic impacts on freshwater ecosystems were believed to be rare in North America prior to the period of European colonization. However, recent paleolimnological and archaeological data collected from the Canadian Arctic suggest that the whaling activities of Thule Inuit, who lived in small, nomadic communities, altered freshwater ecosystems centuries earlier. Using a comparative paleolimnological approach from two ponds situated adjacent to a former Thule winter settlement on south-eastern Bathurst Island (Nunavut, Arctic Canada), we record marked ecological changes in pond ecology due to eutrophication from the Thule’s activities. The geography of our study site provided an interesting and rare opportunity for a comparative paleolimnological study of long-term Thule impacts on polar limnology, because our two study ponds (only ~50 m apart) were nearly identical in size and in geological and climatic settings, but differed markedly in the magnitude of Thule influence. Here, we recorded striking changes in diatom species assemblages, spectrally-inferred primary production, and nutrient geochemistry, indicating eutrophication in a small pond draining 18 Thule whale houses. Input of marine-derived nutrients from sea mammal carcasses used by the Thule for both sustenance and the construction of winter settlements, as well as other anthropogenic activities, coincided with a notable increase in the eutrophic diatom taxon Stephanodiscus minutulus, whereas no comparable changes were recorded in the nearby control pond for the duration of the sedimentary record. Although the diatom changes recorded in the affected site persisted after the period of Thule occupation, the most recent sediments and water chemistry suggest that the pond has largely recovered to near pre-impact conditions.  相似文献   

6.
Stratigraphic changes in diatom assemblages from four small lakes on northeastern Ellesmere Island, high Arctic, Canada, provide a proxy lake-ice cover and paleoenvironmental record. Low absolute diatom abundances and a benthic Fragilaria (sensu lata) dominated assemblage during the postglacial (< 7.6 ka B.P.) to mid-Holocene record the moderating effects of locally retreating glaciers. Around 5.5 ka B.P. diatom concentrations begin to rise, reaching their highest levels (109 valves per g dry sediment) between 4.2 and 3 ka B.P., interpreted to be the warmest period in this region. Topoclimatic differences between lakes on Hazen Plateau and those lower in Lake Hazen Basin account for the initial decline in diatom abundances in the upper lakes after 3 ka B.P. This change is thought to reflect a lowering of the regional snowline, accordant with widely recognized Neoglacial advances on Ellesmere Island and Greenland. Lakes in lower Lake Hazen Basin maintained extensive summer ice free conditions until ~ 1.9 ka B.P., after which diatom abundances declined, suggesting prolonged summer lake-ice cover through the remainder of the recovered Holocene record. Differences between the records presented here and those from coastal areas of the Canadian high Arctic highlight the unique topoclimatic characteristics and continentality of the Lake Hazen region, and possible effects that local marine environments may have had on coastal records. Such differences serve to demonstrate the inherent geographic variability of paleoenvironmental records from the high Arctic.  相似文献   

7.
Chironomid, pollen, and rhizopod records from a permafrost sequence at Bol’shoy Lyakhovsky Island (New Siberian Archipelago) document the development of a thermokarst palaeo-lake and environmental conditions in the region during the last Interglacial (MIS 5e). Open Poaceae and Artemisia associations dominated vegetation at the beginning of the interglacial period. Rare shrub thickets (Salix, Betula nana, Alnus fruticosa) grew in more protected and wetter places as well. Saalian ice wedges started to melt during this time, resulting in the formation of an initial thermokarst water body. The high percentage of semi-aquatic chironomids suggests that a peatland-pool initially existed at the site. A distinct decrease in semi-aquatic chironomid taxa and an increase in lacustrine ones point to a gradual pooling of water in the basin, which could in turn induce thermokarst and create a permanent pond during the subsequent period. The highest relative abundance of Chironomus and Procladius reflects unfrozen water remaining under the ice throughout the ice-covered period during the later stage of palaeo-lake development. The chironomid record points to three successive stages during the history of the lake: (1) a peatland pool; (2) a pond (i.e., shallower than the maximum ice-cover thickness); and (3) a shallow lake (i.e., deeper than the maximum ice-cover thickness). The trend of palaeo-lake development indicates that intensive thermokarst processes occurred in the region during the last Interglacial. Shrub tundra communities with Alnus fruticosa and Betula nana dominated the vegetation during the interglacial optimum. The climate was moister and warmer than present. The results of this study suggest that quantitative chironomid-based temperature reconstructions from Arctic thermokarst ponds/lakes may be problematic due to other key environmental factors, such as prolonged periods of winter anoxia and local hydrological/geomorphological processes, controlling the chironomid assemblages.  相似文献   

8.
Holocene sediments in Lake Winnipeg are expressed in the lower Lake Agassiz sequence which is unconformably overlain by the Lake Winnipeg sequence. Nine sites, covering the North and South basins and the connecting Narrows, were selected for analysis of Holocene changes in thecamoebian faunae. Only the Lake Winnipeg sequence contains thecamoebians. This study indicates that biologic productivity and consequently the type of organic material in the sediments is the main control on thecamoebian taxa in Lake Winnipeg. Other factors controlling the distribution of thecamoebians are water chemistry and turbidity. Inorganic sediment geochemistry and water temperature do not appear to significantly influence the thecamoebian fauna of Lake Winnipeg. Variations in the abundance of key thecamoebian species along a north-south transect divide Lake Winnipeg into three distinct areas. The North Basin has remained relatively unchanged since the retreat of Lake Agassiz as indicated by the domination of Difflugia manicata throughout its history. This species appears to prefer Cyanophyta and diatoms as its food source. In the Narrows harsh conditions created by turbid waters and lack of algal food taxa result in Centropyxis aculeata replacing Difflugia manicata as the dominant species. In the South Basin three thecamoebian assemblages are recognized. Cucurbitella tricuspis, indicative of eutrophic conditions, dominates the most recent sediments of the South Basin. The underlying sediments are characterized by Difflugia globulus. In Lake Winnipeg this species is not a cold climate (arctic) indicator as suggested elsewhere but instead seems to prefer sediments containing green and yellow-green algal material. A Centropyxis-Arcella Assemblage occurs only at the base of the southernmost core where it is indicative of an early phase of hyposaline conditions as developed in shallow pools during the southward transgression of Lake Winnipeg. This study illustrates the usefulness of thecamoebians as paleolimnological indicators. Environmental changes are more significant in the restricted South Basin resulting in distinct thecamoebian assemblages. In contrast, the North Basin provided a stable environment throughout the late Holocene reflected in only subtle faunal changes.  相似文献   

9.
Arctic aquatic systems are considered to be especially sensitive to anthropogenic disturbance, which can have cascading effects on biological communities as aquatic food-web structure is altered. Bio-indicators that respond to major limnological changes can be used to detect and infer major environmental change, such as climate warming, with the use of paleolimnological techniques. A multi-proxy approach was used to quantify recent environmental changes at Baker Lake, Nunavut, Arctic Canada. Analyses of fossilized remains of chironomids and diatoms were conducted on a sediment core of 20 cm in length sampled at 0.5-cm intervals. A new surface sediment training set of subfossil chironomid assemblages from 65 lakes across the eastern Canadian Arctic generated a robust (r jack2 = 0.79) surface water paleotemperature transfer function. The transfer function was applied to stratigraphic intervals from the Baker Lake sediment core to generate a paleotemperature reconstruction of sub-decadal resolution. The surface water temperature reconstruction inferred a 2°C increase in mid-summer surface water temperature for Baker Lake over the last 60 years, which was corroborated by the local instrumental record spanning the period of 1950–2007 AD. The chironomid record shows a recent decline of several cold-water taxa and appearance of warm-water indicators. This shift in community structure began circa 1906 AD, and intensified after 1940 AD. The corresponding fossil diatom record showed an increase in small planktonic Cyclotella taxa over the past 60 years, intensifying in the last 5 years, which also suggests a warmer climate and longer ice-free periods. The shifts in the diatom assemblages began later than the shifts in the chironomid assemblages, and were of lower magnitude, reflecting differences in the mechanisms in which these two indicators respond to environmental change.  相似文献   

10.
Twenty high Arctic lakes and ponds were sampled for water chemistry and modern diatom assemblages in two distinct physiographic sectors of Sirmilik National Park, Nunavut, Canada. Sites on southwestern Bylot Island were warmer, more alkaline, less dilute, and had higher concentrations of nutrients, DOC and Chl-a (mesotrophic to oligo-mesotrophic), whereas sites on Qorbignaluk Headland on northern Baffin Island were deeper, very dilute, mostly oligotrophic and had lower pH. Diatom assemblages differed markedly between these two regions as a consequence of limnological differences between them. Paleolimnological records, spanning > 200 years and dated by 210Pb activity, were produced from each region to compare biological responses to recent warming inferred from glaciological studies on Bylot Island and regional syntheses for the Arctic. Diatom assemblages began to change around AD 1900 at both sites. At Qorbignaluk Headland, marked shifts in diatom community composition occurred during the twentieth century, with large increases in the abundance of planktonic diatoms. At Bylot Island, diatom community changes began around the same time, and involved modest decreases in planktonic diatoms and increases in inferred specific conductance, likely because of a decrease in the areal extent of the small lake as a response to warming. The study confirms that responses of freshwater ecosystems to climate warming vary depending on local physiographic factors.  相似文献   

11.
Chrysophyte scales have been used in several paleolimnological studies to track long-term environmental change, however little data exist for the many lakes in the Maritime provinces of eastern Canada. As part of a multi-disciplinary investigation of acidification and other environmental stressors in the Maritimes, chrysophytes scales were identified and enumerated from the sediments of 52 lakes from Nova Scotia and two lakes from New Brunswick. A total of 25 chrysophyte taxa were identified from the surface sediments, reflecting the modern-day chrysophyte assemblages. The dominant species included Mallamonas duerrschmidtiae and Mallomonas acaroides. Taxa of the genus Synura were present in some lakes, but mainly in the more southern sites. In general, the floras were less diverse than those recorded from similar studies in other temperate regions. This may be related to the fact that the calibration lake set contained only a relatively short limnological gradient, and the assemblages reflect the acidic to circumneutral conditions of these lakes. Synura petersenii, a taxon that has been linked to imparting taste and odor problems to lakes, and had been shown to increase in the recent sediments of many other Canadian lakes, was only rarely present. In contrast to other studies, scaled chrysophytes were very rare in the pre-industrial sediments, with substantial nineteenth century populations only present in four relatively deep (>19 m) lakes. Detailed stratigraphic analyses of eight sediment cores revealed that scaled-chrysophyte assemblages increased dramatically during the latter part of the twentieth century. Limnological changes associated with climate (e.g. increased thermal stratification due to a 1.5°C temperature increase since ~1850) may have influenced chrysophyte distributions in these lakes.  相似文献   

12.
Thermokarst lakes in the Siberian Arctic contain sediment archives that can be used for paleoenvironmental inference. Until now, however, there has been no study from the inner Lena River Delta with a focus on diatoms. The objective of this study was to investigate how the diatom community in a thermokarst lake responded to past limnogeological changes and what specific factors drove variations in the diatom assemblage. We analysed fossil diatom species, organic content, grain-size distribution and elemental composition in a sediment core retrieved in 2009 from a shallow thermokarst lake in the Arga Complex, western Lena River Delta. The core contains a 3,000-year record of sediment accumulation. Shifts in the predominantly benthic and epiphytic diatom species composition parallel changes in sediment characteristics. Paleoenvironmental and limnogeological development, inferred from multiple biological and sedimentological variables, are discussed in the context of four diatom zones, and indicate a strong relation between changes in the diatom assemblage and thermokarst processes. We conclude that limnogeological and thermokarst processes such as lake drainage, rather than direct climate forcing, were the main factors that altered the aquatic ecosystem by influencing, for example, habitat availability, hydrochemistry, and water level.  相似文献   

13.
We present a paleolimnological record spanning the Holocene from a small lake on Russell Island (Lake PW02), in the central Canadian Arctic Archipelago (74.07° N, 97.77° W, 182 m asl). Fragilarioid diatom types in the genera Pseudostaurosira, Staurosira and Staurosirella constitute >90% of valves in fossil samples. Using modern biogeographic data which specify the temperature optima of the Fragilarioid diatom taxa, we present new inferences about the timing of paleoclimatic changes in the central Arctic islands. The early Holocene was characterized by maximum values for sediment organic matter, and lower ratios of Staurosirella pinnata to Staurosira construens v. venter, suggesting warm summer air temperatures between about 9500−6500 cal year BP. Influxes of biogenic silica and diatom valves decreased following 4000 cal year BP, the sediment accumulation rate slowed and diatom taxa of the littoral zone diversified, suggesting cooler summers and more persistent lake ice. Variations in the species composition of the assemblages indicate paleoclimatic changes that are in broad agreement with other paleoenvironmental records from the Arctic including melt records from the Agassiz Ice Cap. Although autecological data remain incomplete for Fragilarioid taxa, our results indicate differences in these taxa in responses to paleoenvironmental change and underline the potential for the increased use of these taxa in paleoenvironmental reconstructions. The record from Lake PW02, as in other records from Arctic lakes with low algal diversity throughout the Holocene, shows a pronounced increase in diatom diversity since the 1920s, and diatom production since the 1970s far exceeds any recorded during the Holocene.  相似文献   

14.
Thecamoebians were examined from 71 surface sediment samples collected from 21 lakes and ponds in the Greater Toronto Area to (1) elucidate the controls on faunal distribution in modern lake environments; and (2) to consider the utility of thecamoebians in quantitative studies of water quality change. This area was chosen because it includes a high density of kettle and other lakes which are threatened by urban development and where water quality has deteriorated locally as a result of contaminant inputs, particularly nutrients. Fifty-eight samples yielded statistically significant thecamoebian populations. The most diverse faunas (highest Shannon Diversity Index values) were recorded in lakes beyond the limits of urban development, although the faunas of all lakes showed signs of sub-optimal conditions. The assemblages were divided into five clusters using Q-mode cluster analysis, supported by Detrended Correspondence Analysis. Canonical Correspondence Analysis (CCA) was used to examine species-environment relationships and to explain the observed clusterings. Twenty-four measured environmental variables were considered, including water property attributes (e.g., pH, conductivity, dissolved oxygen), substrate characteristics, sediment-based phosphorus (Olsen P) and 11 environmentally available metals. The thecamoebian assemblages showed a strong association with phosphorus, reflecting the eutrophic status of many of the lakes, and locally to elevated conductivity measurements, which appear to reflect road salt inputs associated with winter de-icing operations. Substrate characteristics, total organic carbon and metal contaminants (particularly Cu and Mg) also influenced the faunas of some samples. A series of partial CCAs show that of the measured variables, sedimentary phosphorus has the largest influence on assemblage distribution, explaining 6.98% (P < 0.002) of the total variance. A transfer function was developed for sedimentary phosphorus (Olsen P) using 58 samples from 15 of the studied lakes. The best performing model was based on weighted averaging with inverse deshrinking (WA Inv, rjack2 = 0.33, RMSEP = 102.65 ppm). This model was applied to a small modern thecamoebian dataset from a eutrophic lake in northern Ontario to predict phosphorus and performed satisfactorily. This preliminary study confirms that thecamoebians have considerable potential as quantitative water quality indicators in urbanising regions, particularly in areas influenced by nutrient inputs and road salts.  相似文献   

15.
Three lakes, informally named Lakes Inqua, Instaar and Mercer, from the Jackman Sound area in Southern Baffin Island were cored. The elevation of the three lakes are 36 m above high tide (aht), 10 m aht and 1 m aht. The cores from Lakes Inqua and Mercer penetrated the marine/lacustrine boundary; the core from Lake Instaar is estimated to have come within ca. 5 cm of this boundary. In addition, grab samples and water samples were retrieved from the lakes. The cores have been 14C dated (AMS). Emergence of the three lake basins from the sea occurred approximately 8650 BP (Lake Inqua), 8630 BP (Lake Instaar) and 7900 BP (Lake Mercer, averaged date). This suggests a very rapid rate (>5 m/100 yrs) of glacio-isostatic rebound for this area. Down-core analyses of diatom assemblages indicate that the paleolimnology in the three lakes was similar. The diatom analyses suggest a change in climate between approximately 8000 and 6000 BP, which is also reflected in the marine record from the same area, and the onset of the neoglacial period around 4500 to 3800 BP. A decrease in diatom-inferred pH is evident in the lakes' development and can probably be related to the general change of the watershed vegetation with time.  相似文献   

16.
Holocene paleolimnological records (diatoms, organic content, spectrally inferred sediment chlorophyll-a) from three West Greenland lakes (~67°N) situated along a transect from the outer coast to a nunatak at the periphery of the Greenland Ice Sheet are used to explore the nature of regional postglacial lake development and its relationship to Holocene climate evolution. The lakes were deglaciated asynchronously by approximately 4?ka (earliest on the coast) and thus their sediment records document different starting points of Holocene ontogeny, both temporally and paleoclimatically. Despite similar time-transgressive characteristics of the diatom stratigraphies, overarching climatic factors, principally effective moisture, and eolian inputs, govern individual lake development. The transition to Neoglaciation between 5.6 and 4?ka BP marks a shift toward a cooler, moister, windier climate from the aridity and higher temperatures of the mid-Holocene (8?C6?ka BP). A shift toward increased aridity, windiness, and eolian activity is documented in the interior lakes over the last 500?years. These lake records demonstrate the sensitivity of freshwater lakes in arid regions to changes in effective moisture and highlight the role of wind and eolian activity in Arctic lake environments.  相似文献   

17.
Subarctic and arctic lakes are the focus of many paleolimnological studies, as they are still among the least impacted lakes by humans. Hence they provide an excellent setting for studies on long-term climatic variability without the overriding effects of direct anthropogenic perturbation. On the other hand, these ecosystems are highly vulnerable to even moderate anthropogenic influence like long-distance airborne transport of nutrients and pollutants. The paleolimnological studies conducted in these areas usually include a multitude of different proxies, but so far only few have used the green algal group of Pediastrum Meyen. These algae, however, preserve well in sediments and can be identified to species level, which lends them potential as a paleo-proxy. In this study we analysed the present Pediastrum assemblages from surface-sediments of 16 subarctic lakes in Finnish Lapland as well as bottom samples from the same sediment cores, which are “spot” samples from the recent past. We found a total of 14 Pediastrum taxa, five of which occurred at moderate to high relative abundances. The majority of the taxa showed distinct relationships to environmental variables measured. Of these, pH and dissolved organic carbon (DOC) explained most of the variance in the distribution of Pediastrum and generally summarised the main environmental gradients in our data set well. Five of the studied lakes lacked Pediastrum taxa altogether, and Pediastrum occurred at low abundances in four additional lakes. All of these nine lakes have extremely low nutrient concentrations and generally lowest pH and DOC in the data set and were defined by barren catchment areas and scarce lake macrophyte growth. According to a top–bottom analysis of sediment cores, the Pediastrum assemblages of the study lakes have changed moderately, suggesting changed environmental conditions in the lakes. Although these changes appear to be climate-related, more studies are needed to confirm this hypothesis.  相似文献   

18.
Subfossil Cladocera were sampled and examined from the surface sediments of 35 thermokarst lakes along a temperature gradient crossing the tree line in the Anabar-river basin in northwestern Yakutia, northeastern Siberia. The lakes were distributed through three environmental zones: typical tundra, southern tundra and forest tundra. All lakes were situated within the continuous permafrost zone. Our investigation showed that the cladoceran communities in the lakes of the Anabar region are diverse and abundant, as reflected by taxonomic richness, and high diversity and evenness indices (H = 1.89 ± 0.51; I = 0.8 ± 0.18). CONISS cluster analysis indicated that the cladoceran communities in the three ecological zones (typical tundra, southern tundra and forest-tundra) differed in their taxonomic composition and structure. Differences in the cladoceran assemblages were related to limnological features and geographical position, vegetation type, climate and water chemistry. The constrained redundancy analysis indicated that TJuly, water depth and both sulphate (SO4 2?) and silica (Si4+) concentrations significantly (p ≤ 0.05) explained variance in the cladoceran assemblage. TJuly featured the highest percentage (17.4 %) of explained variance in the distribution of subfossil Cladocera. One of the most significant changes in the structure of the cladoceran communities in the investigated transect was the replacement of closely related species along the latitudinal and vegetation gradient. The results demonstrate the potential for a regional cladoceran-based temperature model for the Arctic regions of Russia, and for and Yakutia in particular.  相似文献   

19.
The Arctic has a disproportionately large response to changes in radiative forcing of climate, and arctic lacustrine ecosystems respond sensitively to these changes. The goal of this research is to generate high-resolution climate records for the past two millennia using multiple proxies in order to place 20th and 21st century climate and environmental change into a long-term context. We use a 14C- and 210Pb-dated surface core from Lake CF8 on northeastern Baffin Island, Arctic Canada to generate a high-resolution multiproxy reconstruction of climate and environmental change. Throughout the late Holocene, primary productivity in Lake CF8 was low, but increased almost 20-fold in the past 200 years. Insect (Chironomidae) assemblages also show dramatic changes since 1950 AD, with cold stenothermous chironomid taxa disappearing from the record altogether. These changes in productivity and chironomid assemblages are unprecedented in the past 5,000 years. The dramatic ecological shifts that occurred at Lake CF8 have also been observed elsewhere in the Arctic, and will likely continue at ever-increasing rates as anthropogenic inputs of green house gases continue to cause climate warming and enhanced lacustrine primary production.  相似文献   

20.
Diatoms are used widely for paleolimnological studies in lakes, but their use for studying the environmental history of reservoirs has not been tested extensively. Reservoirs have hydrodynamic characteristics intermediate between those of rivers and lakes. This study assessed the utility of diatom assemblages as recorders of long-term changes in hydrodynamics and spatial gradients in Liuxihe Reservoir, an impoundment in southern China. Four sediment cores were collected in the reservoir, from the riverine, transition and lacustrine zones. Each core was sectioned at 2-cm intervals to investigate the stratigraphic distribution of accumulated diatoms. Varve counting was used to develop a chronology for one of the cores. The unique characteristics of Liuxihe Reservoir, including its large size, great depth, long narrow morphology and strong thermal stratification for 10 months of the year, limit secondary sedimentation processes and preserve the varves, enabling development of an accurate chronology. Damming profoundly altered the physical environment of the former river, especially in the lacustrine zone, where the change is clearly illustrated by diatoms in the sediment. Diatom abundance increased as a consequence of nutrient enrichment after construction of the dam in 1958, but later decreased as the new impoundment stabilized. After damming, relative abundance of Cyclotella increased along with a simultaneous decrease of Navicula and Achnanthes in the lacustrine zone, most significantly in 1963, when Cyclotella replaced Navicula as the dominant genus. This switch was indicative of a general shift from a lotic to a lentic habitat. A longitudinal gradient was apparent in the patterns of sedimentation and diatom accumulation at different sites in the reservoir, with diatom abundance highest in the transition zone. In the long term, water discharge from the reservoir showed a weak, but significant negative correlation with diatom abundance in the lacustrine zone (r = −0.320, P = 0.03). In summary, diatom assemblages in the Liuxihe Reservoir sediments recorded past changes in hydrodynamics, suggesting that paleolimnological study of some impoundments is feasible.  相似文献   

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