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1.
In recent decades, softwater lakes across Canada have experienced a wide array of anthropogenic influences, with acidification and climate warming of particular concern. Here, we compare modern and pre-industrial sedimentary diatom assemblages from 36 softwater lakes located on the Canadian Shield in south-central Ontario to determine whether lake acidification or reduced calcium availability was the main stressor responsible for recent declines in Ca-sensitive cladoceran taxa. Regional surveys of south-central Ontario water chemistry have identified the pH recovery of many formerly acidified lakes, and our fossil diatom-inferred pH analyses indicate that modern lakewater pH in the 36 study lakes is similar to (or higher than) pre-industrial levels, with diatom assemblages from both time periods dominated by taxa with similar pH preferences. In addition, modern diatom assemblages compared to pre-industrial assemblages contained higher relative abundances of planktonic diatom taxa (e.g. Asterionella formosa and the Discostella stelligera complex) and lower relative abundances of heavily silicified diatoms (e.g. Aulacoseira taxa) and benthic fragilarioid taxa. These taxonomic shifts are consistent with warming-induced changes in lake properties including a longer ice-free period, decreased wind speed and/or increased thermal stability. We conclude that recent changes observed within the cladoceran assemblages of these lakes are not a response to acidification, but are likely a consequence of Ca declines. In addition, our data suggest that regional climate warming is now responsible for the diatom changes observed in this region.  相似文献   

2.
We compared cladoceran assemblages in modern and pre-industrial sediments from cores taken in 48 softwater lakes located in four main regions of Nova Scotia (Canada) to evaluate the impacts of acidification and other recent environmental stressors. Lakes in Kejimkujik National Park showed significant increases in Holopedium relative abundances and significant decreases in Alona and other chydorids since pre-industrial times, which appear to be related to declines in pH and calcium (Ca) concentrations caused by acidic deposition. Lakes in Bridgewater also showed a significant decrease in Alona, as well as a significant increase in macrothricid (Acantholeberis and Ophryoxus) taxa that cannot be explained by declines in pH, although declines in [Ca] have been recorded. Lakes in Yarmouth did not show any significant regional changes in major cladoceran species groups. Still, pre-industrial assemblages in these lakes significantly differed from modern assemblages, with assemblage changes being lake-specific and likely related to interactions between local and regional stressors acting on individual lakes. Finally, lakes in Cape Breton Highlands National Park, located on the taiga plateau, have received historically lower levels of sulphate deposition relative to other regions in the province, and recorded a significant decrease in Alona similar to Bridgewater lakes. The province-wide decrease in Alona across different acid deposition rates suggests that limnological changes related to climate warming may be responsible. Overall, this study shows that acidification history is an important predictor of cladoceran assemblage changes since pre-industrial times in Nova Scotia lakes, although multiple stressors result in complex Cladocera responses in some regions.  相似文献   

3.
Cladocera as indicators of trophic state in Irish lakes   总被引：1，自引：0，他引：1  
We examined the impact of lake trophic state on the taxonomic and functional structure of cladoceran communities and the role of nutrient loading in structuring both cladoceran and diatom communities. Surface sediment assemblages from 33 Irish lakes were analysed along a gradient of total phosphorus concentration (TP; 4.0–142.3 μg l−1), using a variety of statistical approaches including ordination, calibration and variance partitioning. Ordination showed that the taxonomic structure of the cladoceran community displayed the strongest response to changes in lake trophic state, among 17 measured environmental variables. Trophic state variables chlorophyll-a and TP explained about 20% of the variance in both cladoceran and diatom assemblages from a set of 31 lakes. Procrustes analysis also showed significant concordance in the structure of cladoceran and diatom communities (P < 0.001). Thus, lake trophic state affects the taxonomic structure of both primary and secondary producers in our study lakes. We also found a significant decrease in relative abundance of taxa associated with both macrophytes and sediments, or sediments only, along the TP gradient (r = −0.49, P = 0.006, n = 30), as well as an increase in the proportion of the planktonic group (r = 0.43, P = 0.017, n = 30). This suggests that cladoceran community structure may also be shaped by lake trophic state indirectly, by affecting habitat properties. We found no relationship between lake trophic state and the relative abundance of each of three cladoceran groups that display different body size. We compared community structure between bottom and top sediment samples in cores from six Irish lakes. Results revealed similar trajectories of nutrient enrichment over time, as well as a strong shift in cladoceran functional structure in most systems. This study confirms that Cladocera remains in lake sediments are reliable indicators of lake trophic state. This study also highlights the fact that taxonomic and functional structure should both be considered to account for the multiple factors that shape cladoceran communities.  相似文献   

4.
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   

5.
We present isotope, cladoceran and diatom results from investigations of Eemian sediments of the palaeolake at Ruszkówek, central Poland. Our analyses of the 15-m-thick sediments indicate that sandy silts occur on the bottom, followed by calcareous gyttja, interbedded with lake marl. The upper part of the sequence contains peat and peaty sands. Values of δ18O change from −9.4‰ to −3.3‰ and δ13C values oscillate between −3.2‰ and +7.0‰. Nine isotope zones (Is) were defined and characterized using stable isotope analysis of carbonates. Fifteen species of subfossil Cladocera were found and six faunal zones were distinguished (Cladocera zones). One hundred and twenty-three diatom taxa, representing 31 genera were identified, enabling us to discern six Diatom Assemblage Zones. The isotope, cladoceran, and diatom data correlate well with pollen data that define seven phases of evolution of the palaeolake at Ruszkówek. The palaeolake began during the final phase of the Wartanian (Late Saalian Glaciation). During the early Eemian, the palaeolake reached its maximum depth. During the Early Vistulian glaciation, the palaeolake declined. Changes in the cladoceran and diatom communities indicate initial oligotrophic conditions in the lake, then an increase to mesotrophy, and finally eutrophic conditions.  相似文献   

6.
Upper Saalian (Illinoian) glaciolacustrine deposits in central Poland, preserved in a tectonic graben, were exposed in an opencast lignite mine and investigated using sedimentological and micro-paleontological methods. The extraglacial lake sediments provide the first records of late Saalian cladoceran communities in central Europe, recovered from glaciolacustrine deposits. Sedimentation was dominated by a supply of clastics that fluctuated with the seasons, forming rhythmites. In addition to seasonal cyclicity, sedimentary and environmental conditions changed every several years to decades, with periods of increased inflow to the lake delivering sandy material, and periods of almost stagnant water dominated by suspension settling. The sediments contain Cladocera assemblages that indicate the lake was initially deep, oligotrophic, and filled with moderately cold water. Changes in Cladocera community composition and abundance were perhaps responses to climate seasonality. Zones without Cladocera were associated with seasons of higher inflow and sediment supply, and directly or indirectly, with tectonic activity in the graben. Earthquakes, documented by the presence of seismites, caused not only deformation of unconsolidated lake-bottom sediments, but possibly also changes in habitat characteristics. Combined sedimentological and biological data were used to infer the lake’s history and show that deposits of glaciolacustrine lakes can be used as indicators of past ecological and climate changes.  相似文献   

7.
Chrysophycean stomatocysts were identified and enumerated from the surface sediments of 60 lakes located on the Interior Plateau of British Columbia. The lakes span a salinity gradient from freshwater to hypersaline (0.0–92.4 g L–1), with the majority being fresh to hyposaline. One hundred and ten stomatocyst morphotypes, almost all of which were previously described, were identified from the lake sediments. The first axis of direct gradient analysis, which was highly significant, was essentially a salinity axis (i.e. [Ca], [Mg], [K], [Na], [SO4], [DIC], and [Cl]). Most cysts were found to have fairly broad tolerances, with the narrowest tolerances occurring among morphotypes with the lowest salinity optima. Weighted-averaging regression and calibration techniques were used to develop an inference model to measure the relationship between measured average lakewater salinity and stomatocyst inferred salinity (apparent r2=0.80). Simple weighted-averaging produced a model with a lower bootstrapped RMSE of prediction than weighted-averaging with tolerance downweighting. These data indicate that chrysophyte stomatocysts are useful quantitative indicators of past lakewater salinity (in the freshwater to hyposaline range) in B.C. lakes, and can be used to strengthen the interpretations from diatom-inference models already developed from the same region.  相似文献   

8.
Surface sediment diatoms from 35 soft-water lakes in southern Quebec were studied to examine the relationship with lakewater pH. The lakes ranged in pH from 5.25 to 7.66. The species composition and the relative abundance of diatoms in the study lakes was found to be closely related to pH and/or factors closely associated with pH. Predictive models were developed to infer lakewater pH using simple linear regression equations of index alpha, index B, and multiple regressions using pH preference categories. Among the predictive models examined here, the multiple regression technique provided the highest correlation coefficient (r2=0.88) and the lowest standard error (±0.26 of a pH unit) in computing diatom-inferred pH. This model appears to be the most appropriate to reconstruct lake pH histories in Quebec region.  相似文献   

9.
The surface sediment diatom and chrysophyte assemblages from 33 Sudbury lakes were added to our published 72 lake data set to expand and refine the diatom and chrysophyte-based inference models that we had earlier developed for this region. Our calibration data set now includes 105 lakes, representing gradients for multiple environmental variables (e.g., lakewater pH, metals, and transparency). The revised models are based on the weighted averaging calibration and regression approach and include bootstrap error estimates. The pH model was the strongest (r2 boot = 0.75, RMSE boot = 0.50). The chrysophyte-inferred pH model (r2 boot = 0.79, RMSE boot = 0.48) that we developed was as robust as the diatom pH model. Diatom and chrysophyte inferred pH models were then applied to top (surface sediments representing current conditions) and bottom (generally from > 30 cm deep representing pre-industrial conditions) sediment diatom and chrysophyte assemblages of 19 Killarney area lakes near Sudbury. The top and bottom inferred pH results were compared to early-1970s measured pH data. These data suggest that, although many of the poorly buffered Killarney lakes had experienced acidification, marked pH recovery has occurred in many lakes within the last 25 years. Despite the stunning pH recovery, the present-day diatom and chrysophyte assemblages are significantly different from assemblages present during pre-industrial times. Our results suggest that biological recovery may require more time than chemical recovery. It is also likely that these lakes may never recover biologically because other anthropogenic stressors (e.g., climate warming and increased exposure to UV-B radiation) may now have greater influence on biological communities in Killarney/Sudbury area lakes than acidification.  相似文献   

10.
In order to assess how best to manage impacted lake systems, one needs to understand the trophic functioning of the lake system and the recent states through which the lake may have transitioned. Lakes in the middle and lower reaches of the Yangtze have been heavily impacted over recent decades. In order to understand recent changes in functional status, we examined sediment cores covering the last 120?years from two lakes in the same catchment with differing status: one algal-dominated (Taibai Lake) and the other macrophyte-dominated (Longgan Lake). Chironomid head capsules were identified from both sites and an expanded chironomid-total phosphorus (TP) transfer function (21 sites were added to the 30-lake model previously developed by Zhang et al. 2006) was used to assess the lakes?? response to recent anthropogenic change. Quantitative chironomid-inferred TP (CI-TP) reconstructions showed that Taibai Lake experienced clear changes in trophic status since the 1860s. Before the 1950s, the CI-TP concentration was relatively stable around 50?C80???g?L?1, while it reached to 80?C130???g?L?1 in the latter period. CI-TP for Longgan Lake, however, showed a relative decline from the range of 50?C75???g?L?1 since the 1880s to 30?C40???g?L?1 in recent years, accompanied by strong evidence from the chironomids for increased macrophyte biomass as TP levels declined. Both reconstructions agreed with diatom inferences of TP from the same lakes. The stark difference between these two sites is thought to reflect a function of macrophyte development, with Taibai Lake losing its plants through increased nutrient levels and internal recycling, whereas Longgan Lake, which is much bigger in area and hence potentially more resilient to change, was able to develop macrophyte communities over the same time period. The positive feedbacks associated with abundant macrophytes retained the clear water state of Longgan Lake, but a further increase in nutrients might lead to decrease in resilience of the relatively stable macrophyte state and loss of benthic pathways of primary production, which would push the lake towards eutrophication. Unless nutrient inputs to Longgan are controlled, Longgan Lake might lose macrophyte communities and follow a developmental pathway similar to that observed in Taibai Lake.  相似文献   

11.
Sediment cores were used to estimate in-lake alkalinity generation resulting from sulfate reduction relative to inferred changes in lakewater pH and trophic status over the last century in three Connecticut lakes. Despite being situated in geological settings with crystalline bedrock and thin, poorly buffered soils, and being impacted with high rates of acidic precipitation, none of the study lakes have declined in inferred pH based on scaled chrysophyte and diatom remains. In fact, the pH of one of the lakes, Coventry Lake, has significantly increased over the last century. Over the last 44 to 69 years the amount of sulfur stored in the sediments from each lake increased from ~two to three times resulting in mean rates of alkalinity generation ranging from 78 to 145 meq m–2 yr–1, significantly higher than the 45 to 48 meq m–2 yr–1 of hydrogen ions falling directly on the lake surfaces. In-lake alkalinity generation resulting from sulfate reduction has been sufficient to neutralize all of the acid falling directly onto the lake surfaces, as well as between 9% and 25% of the acid deposited onto the surrounding watersheds. Despite the increased importance of in-lake alkalinity generation, our findings support the hypothesis that significant amounts of alkalinity are also being generated in the catchments of the study lakes. The bulk of the increases in stored sulfur in all three lakes were as Fe sulfides and not in the form of organic sulfur, suggesting that the increases were the result of dissimilatory bacterial reduction of sulfate. As a result of the large increases in storage of Fe sulfides the ratio of total iron to chromium reducible sulfur (Fe:CRS) has declined in all cores over time. Despite the overall decline in Fe:CRS in recent sediments, values are still largely above 3 in more recent sediments of two of the lakes. However, values of Fe:CRS have dropped below 1 in surface sediments of Uncas Lake, suggesting that in-lake loading of phosphorus may be responsible for a recent shift in the algal flora towards a slightly more eutrophic condition.  相似文献   

12.
The stable carbon isotope composition, expressed as δ13C values, of chitinous resting stages of planktivorous invertebrates can provide information on past changes in carbon cycling in lakes. For example, the δ13C values of cladoceran ephippia and bryozoan statoblasts have been used to estimate the past contribution of methane-derived carbon to lake food webs and variations in the δ13C value of planktonic algae. Limited information, however, is available concerning seasonal variations in δ13C values of these organisms and their resting stages. We measured the seasonal variation in δ13C values of Daphnia (Branchiopoda: Cladocera: Daphniidae) and their floating ephippia over a 2-year period in small, dimictic Lake Gerzensee, Switzerland. Floating ephippia of Ceriodaphnia (Branchiopoda: Cladocera: Daphniidae) and statoblasts of Plumatella (Phylactolaemata: Plumatellida: Plumatellidae) were analysed during parts of this period. Furthermore, δ13C values of remains from all three organism groups were analysed in a 62-cm-long sediment core. Throughout the year, Daphnia δ13C values tracked the δ13C values of particulate organic matter (POM), but were more negative than POM, indicating that Daphnia also utilize a relatively 13C-depleted carbon source. Daphnia ephippia δ13C values did not show any pronounced seasonal variation, suggesting that they are produced batch-wise in autumn and/or spring and float for several months. In contrast, δ13C values of Ceriodaphnia ephippia and Plumatella statoblasts followed variations in δ13CPOM values, Ceriodaphnia values being the most negative of the resting stages. Average cladoceran ephippia δ13C values in the flotsam agreed well with ephippia values from Gerzensee surface sediments. In contrast, average Plumatella statoblast δ13C values from the flotsam were 4‰ more negative than in the surface sediments. In the sediment core, δ13C values of the two cladocerans remained low (mean ?39.0 and ?41.9‰) throughout the record. In contrast, Plumatella had distinctly less negative δ13C values (mean ?32.0‰). Our results indicate that in Gerzensee, Daphnia and Ceriodaphnia strongly relied on a 13C-depleted food source throughout the past 150 years, most likely methane-oxidising bacteria, whereas this food source was not a major contribution to the diet of bryozoans.  相似文献   

13.
Remains of Cladocera were examined in short sediment cores from three Adirondack lakes with mean pHs below 5 and a fourth with a mean pH of 6.5. These cores were collected as part of the Paleoecological Investigation of Recent Lake Acidification (PIRLA I) project. Historical and paleolimnological evidence suggests that pH has decreased in each of the acid lakes in recent decades. In all of the study cores, the greatest changes in net accumulation rates, assemblage composition, and species richness occurred in recently deposited sediments. The similar timing of events in all lakes suggests that a regional disturbance was responsible. In the three acid lakes, there was a strong association of changes in cladoceran assemblages and diatom, chrysophyte, and geochemical evidence of acidification. The occurrence of recent changes in non-acid Windfall Pond indicates that other factors may also have affected Cladocera in the study lakes.This is the fifteenth of a series of papers to be published by this journal which is a contribution of the Paleoecological Investigation of Recent Lake Acidification (PIRLA) project. Drs. D. F. Charles and D. R. Whitehead are guest editors for this series.  相似文献   

14.
Aquatic macrophytes play a key role in providing habitat, refuge and food for a range of biota in shallow lakes. However, many shallow lakes have experienced declines in macrophyte vegetation in recent decades, principally due to eutrophication. As changes in macrophyte composition and abundance can affect overall ecological structure and function of a lake, an assessment of the timing and nature of such changes is crucial to our understanding of the wider lake ecosystem. In the typical absence of historical plant records, the macro-remains of macrophytes preserved in lake sediments can be used to assess long-term changes in aquatic vegetation. We generated recent (150–200 years) plant macrofossil records for six English lakes subject to conservation protection to define past macrophyte communities, assess trajectories of ecological change and consider the implications of our findings for conservation targets and strategies. The data for all six lakes reveal a diverse submerged macrophyte community, with charophytes as a key component, in the early part of the sedimentary records. The stratigraphies indicate considerable change to the aquatic vegetation over the last two centuries with a general shift towards species more typically associated with eutrophic conditions. A common feature is the decline in abundance of low-growing charophytes and an increase in tall canopy-forming angiosperms such as fine-leaved Potamogeton species, Zannichellia palustris and Callitriche species. We hypothesise, based on findings from long-term datasets and palaeoecological records from enriched shallow lakes where plants are now absent, that the observed shifts provide a warning to managers that the lakes are on a pathway to complete macrophyte loss such that nutrient load reduction is urgently needed. It is the sound understanding of present-day plant ecology that affords such reliable interpretation of the fossil data which, in turn, provide valuable context for current conservation decisions.  相似文献   

15.
Present climate warming strongly affects limnological and ecological properties of lakes and may cause regime shifts that alter structure and function in the water bodies. Such effects are especially pronounced in climatologically extreme areas, e.g. at high altitudes. We examined a sediment core from Lake Oberer Landschitzsee, Austrian Alps, which spans the period from the Little Ice Age (LIA) to present. We investigated whether post-LIA climate warming altered aquatic invertebrate communities and limnological status in this sensitive high Alpine lake. Fossil Cladocera (Crustacea) and Chironomidae (Diptera) and organic matter in the core were analyzed. Chironomids were used to assess the lake??s benthic quality (i.e. oxygen availability). An instrumental Alpine temperature record was used to assess whether changes in the biotic assemblages correspond to post-LIA temperature trends. The planktonic and macro- and microbenthic invertebrate communities exhibit almost complete and simultaneous species turnover after the LIA, from about AD 1850 onward, when Sergentia coracina-type replaced oxyphilous Micropsectra contracta-type as the dominant macrobenthic taxon, and phytophilous Acroperus harpae outcompeted Alona affinis and Alona quadrangularis in the microbenthos. These directional community shifts corresponded with a period of reduced benthic quality, higher sediment organic content, and progressive climate warming, superimposed on Alpine land-use changes, until the early twentieth century. Detected changes suggest increased productivity and lower benthic oxygen availability. Faunal shifts were even more pronounced during the late twentieth century, simultaneous with enhanced warming. A new planktonic Cladocera species, Bosmina longirostris, typically absent from high Alpine lakes, colonized the lake and gradually became dominant toward the core top. Results show that post-LIA climate warming, coupled with increasing benthic and planktonic production, substantially altered the limnological and ecological status of this remote Alpine lake. Observed faunal turnovers provide evidence that temperature-driven ecological thresholds, whether associated directly or indirectly with greater human activity, have been crossed. Species abundances and distributions have changed in response to post-LIA and late twentieth century climate warming.  相似文献   

16.
McNearney Lake is an acidic (pH=4.4) lake in the Upper Peninsula of Michigan with low acid neutralizing capacity (ANC=-38 eq L-1) and high SO inf4 sup2- and aluminium concentrations. Oligotrophy is indicated by high Secchi transparency and by low chlorophyll a, total phosphorus, and total nitrogen concentrations. The lake water is currently acidic because base cations are supplied to the lake water at a low rate and because SO inf4 sup2- from atmospheric deposition was not appreciably retained by the lake sediments or watershed and was present in the water column.This interdisciplinary paleolimnological study indicates that McNearney Lake is naturally acidic and has been so since at least 4000 years B.P., as determined from inferred-pH techniques based on contemporary diatom-pH relationships. Predicted pH values ranged from 4.7 to 5.0 over the 4000-year stratigraphy. Considerable shifts in species composition and abundance were observed in diatom stratigraphy, but present-day distributions indicate that all abundant taxa most frequently occur under acidic conditions, suggesting that factors other than pH are responsible for the shifts. The diatom-inferred pH technique as applied to McNearney Lake has too large an uncertainly and is not sensitive enough to determine the subtle recent changes in lakewater pH expected from changes in atmospheric deposition because: (1) McNearney Lake has the lowest pH in the contemporary diatom data set in the region and confidence intervals for pH predictions increase at the extremes of regressions; (2) other factors in addition to pH may be responsible for the diatom species distribution in the lake and in the entire northern Great Lakes region; (3) McNearney Lake has a well-buffered pH as a consequence of its low pH and high aluminium concentrations and is not expected to exhibit a large pH change as a result of changes in atmospheric deposition; and (4) atmospheric deposition in the region is modest and would not cause a pH shift large enough to be discernable in McNearney Lake.Elevated atmospheric deposition is indicated in recent sediments by Pb, V, and polycyclic aromatic hydrocarbon accumulation rates and to a lesser extent by those of Cu and Zn; however, these accumulation rates are substantially lower than those observed for acidified lakes in the northeastern United States. Although atmospheric loadings of materials associated with fossil fuel combustion have recently increased to McNearney Lake and apparently are continuing, the present study of the diatom subfossil record does not indicate a distinct, recent acidification (pH decrease).Order of the first two authors is alphabetical  相似文献   

17.
Wetlands and lakes in the Tanana Valley, Alaska, have provided important resources for prehistoric humans who inhabited this region. We examine an ~11,200?cal?yr BP record of environmental and paleolimnological changes from Quartz Lake in the middle Tanana Valley. Our data are also presented in the context of recent archaeological findings in the lake??s general vicinity that have 18 associated AMS 14C dates. We analyzed the stable-carbon and nitrogen isotope composition of total organic matter from the core, coupled with oxygen and carbon isotope analyses of Pisidiidae shells (fingernail clams), in addition to chironomid assemblage changes. Lacustrine sediments began to accumulate at ~11,200?cal?yr BP. Initially, autochthonous production was low and allochthonous organic input was negligible between 11,000 and 10,500?cal?yr BP, and were associated with relatively cool conditions at Quartz Lake at ~10,700?cal?yr BP. After 10,500?cal?yr BP, autochthonous production was higher coincident with a shift to chironomid assemblages dominated by taxa associated with warmer summer climates. A decrease in ??13C values of total organic carbon (TOC) and organic content of the sediment between 9,000 and 4,000?cal?yr BP may indicate declining autochthonous primary production. This period ended with an abrupt (~7???) decrease in the ??18O values from Pisidiidae shells at ~3,000?cal?yr BP, which we hypothesize represented an episodic connection (flood) of the lake with flow from the nearby (~6?km) Tanana River. Our findings coincide with evidence for major flooding at other locations connected to the Tanana River and further afield in Alaska. From ~3,000?cal?yr BP Quartz Lake subsequently appeared to become a relatively closed system, as indicated by the ??18OPisidiidae and ??13CPisidiidae data that are positively correlated and generally higher, which also correlates with a shift to moderately higher abundances of littoral chironomids. The cause of the transition to closed-basin conditions may have been geomorphic rather than climatic. This evidence of a progressively stronger evaporative influence on the lake??s closed hydrology after ~3,000?cal?yr BP is consistent with our modern ??18O and ??D water data from Quartz Lake that plot along a regional evaporative line we base on isotopic measurements from other local lakes and rivers.  相似文献   

18.
The remains of diatoms, cladocerans, and midges are usually the most abundant of freshwater organisms and to now have been most useful in interpreting past conditions in a lake. Each taxocene consists of two separate communities, one in the warm littoral zone and the other offshore. Remains of inshore organisms are moved offshore by wind-generated currents, the amount of transport varying with individual characteristics of the lakes. Nowhere do remains of the two communities become completely integrated numerically, although the remains of the littoral chydorid Cladocera become integrated by species before they are incorporated into the sediments. The taxa of the planktonic Eubosmina and of the offshore midges correspond to levels of productivity in present-day lakes, and hence changes in the fossil record are commonly regarded as indicating eutrophication over time. The deepwater midges respond to the concentration of dissolved oxygen in deep water, which may be controlled more by a decrease in volume of deep water through accumulation of sediments than by any real increase in edaphic productivity. While such changes are going on offshore during the Holocene, the littoral communities of cladocerans and midges are scarcely changing at all, suggesting a different response of the inshore from the offshore communities to longterm changes resulting from increasing productively or from other functions. Thus, considering these different responses of the two communities of organisms and their incomplete mixing, the remains of littoral and offshore taxa recovered from an offshore core of sediments must be tabulated separately and interpreted separately. For any studies involving accumulation rates, there must be an understanding of the integration of inshore and offshore remains, its variation over the lake bottom, and how it may have varied with marked fluctuations in water level.This is the third of a series of papers to be published by this journal that was presented in the paleolimnology sessions organized by R. B. Davis and H. Löffler for the XIIth Congress of the International Union for Quaternary Research (INQUA), which took place in Ottawa, Canada in August 1987. Drs. Davis and Löffler are serving as guest editors of this series.  相似文献   

19.
An increase in the frequency and intensity of marine storm surges is a predicted consequence of climate warming, and therefore it is important to better understand the biological responses to such events in coastal regions. In late September 1999, a major storm surge resulted in a saltwater intrusion event over a large area of the Mackenzie Delta (NT, Canada) front, causing rapid salinization of lakes on the alluvial plain. Due to a lack of long-term ecological monitoring data in the region, the impacts that the saltwater intrusion event had on the biota of affected lakes were unknown. We used high-resolution paleolimnological approaches to reconstruct past assemblage changes in Cladocera from impacted Lake DZO-29 (unofficial name) in order to determine how different cladoceran species responded to a major increase in lake salinity following the 1999 storm surge. Camptocercus were extirpated from the lake following the saltwater intrusion and have not recovered. We also observed an initial decrease in Alona relative abundance following the marine flooding, likely reflecting a loss of A. quadrangularis, A. barbulata, and A. costata from the lake. A. circumfimbriata, Chydorus biovatus, C. brevilabris, and Bosmina spp. were abundant both before and after the saltwater intrusion, and Paralona pigra was present following the storm surge, but not prior to it. The most notable shift in Cladocera in the recent sedimentary record, however, occurred much earlier, with an increase in pelagic Bosmina taxa and a subsequent decrease in the benthic/littoral taxa Chydorus and Camptocercus, an assemblage shift that is consistent with a response to climate warming in this region, and strongly correlated to other changes in the lake inferred to be as a result of regional warming.  相似文献   

20.
The hydroseral development of a former small lake in Southern Finland was studied by means of subfossil cladoceran remains, diatoms, pollen, plant macrofossils and other sediment data. The diatom analysis shows the lake to have become markedly acid during the Early Holocene. This is reflected in the cladoceran communities in the form of a fall in the proportions of Bosmina longirostris etc. and a reciprocal rise in Bosmina (Eubosmina) longispina, an increase in the proportions of the chydorid species indicative of acidity, and the appearance of certain new morphotypes. The concentrations of both planktonic and littoral species and the numbers of such species increase with acidification. Advancement in the hydroseral succession is reflected in the disappearance of benthic species from the chydorid communities and a pronounced increase in exclusively phytophile species. Sedimentation and the drop in water levels are seen to have led to a spread of helophytes and floating-leaved plants over the water body around 6500–7000 B.P., and a vegetation-filled swamp was created at the site. The zooplankton was practically exterminated, but the concentrations of littoral cladocerans reached their peak at this point. The basin became overgrown completely at the beginning of the Subboreal chronozone (approx. 4600–4800 B.P.), simultaneously with the low-water phase observed in many lakes. It became covered with a Sphagnum stand, and this in turn led to complete destruction of the cladoceran communities. The planktonic/littoral ratio among the Cladocera closely reflected the relation between open water and the macrophyte zone as a function of time. Climatically induced rises and falls in water level are shown to have played a significant role in promoting the advancement of the hydroseral succession.  相似文献   

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