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1.
The Arcellacean (Thecamoebian) fauna was assessed in five Holocene sediment cores obtained from James and Granite lakes in the Temagami region of northeastern Ontario. In addition, palynological analysis was carried out on two of these cores, one each from James and Granite lakes. The first indication of postglacial colonization by plants was the appearance of rare Cupressaceae pollen, dated to 10,800 yr BP. Plant diversity began to increase by 10,770 yr BP when Pinus spp. and Larix migrated into the area. The first appearance of arcellaceans occurred after 9650 yr BP in assemblages dominated by Centropyxis constricta and opportunistic Centropyxis aculeata. High abundances of charophytes in the cores until 8800 yr BP indicated that macroalgae were proliferating at this time. This deposition is interpreted to have occurred during the draining of an ice-marginal lake following the retreat of the Laurentide Ice Sheet. Based on pollen analysis, warmer conditions associated with the Holocene Hypsithermal prevailed in the area from 6250 to 4115 yr BP. The stable, open Great Lakes – St. Lawrence type forest that developed here at the beginning of the Hypsithermal continues to prevail to the present. The periodic colonization of the lake by beavers (Castor canadensis) acted as a control on water-level and eutrophication through the Holocene. Evidence of eutrophication was indicated in the core samples by the abundance of high levels of the alga Pediastrum and the arcellacean Cucurbitella tricuspis. Eutrophication periodically developed when beavers dammed a site, causing the rate of flow in drainage streams to slow and stagnant conditions occurred. When the site became depleted of the nearby trees, which were preferred by beaver (Betula, Alnus and Populus), the dam would be abandoned, causing the water-level to drop. Stagnant conditions were reduced as flow levels increased, reducing eutrophication and resulting in recovering forest stands. In addition, the lowering water levels would result in encroachment of the forest along the lake shore. This cycle occurred many times in the history of this lake as indicated by fluctuations in the size of arcellacean populations.  相似文献   

2.
Swan Lake is a small kettle lake located on the Oak Ridges Moraine; a moraine that is recognized as an important source of ground water for the nearby and rapidly expanding Greater Toronto Area. A paleolimnological reconstruction using pollen and diatoms from the lake sediments showed significant changes in biological community composition through the last ∼400 years. Alterations in the diatom and pollen assemblages were most dramatic ca. A.D. 1850, correlating with the highest sediment flux in the lake between the period ca. A.D. 1850 and A.D. 1870. These changes were directly linked to regional deforestation and agricultural activities associated with European settlement. The pollen record from ca. A.D. 1850 to present day indicated that tree species (e.g. Pinus spp., Tsuga canadensis) were declining, while grass (Poaceae) and invasive species (e.g. Ambrosia) were increasing. Around A.D. 1850, the diatom flora changed from an assemblage dominated by large, benthic species (e.g. Sellaphora pupula, Pinnularia cf. maior, and Stauroneis phoenicenteron) to an assemblage characterized by smaller, tychoplanktonic (e.g. Fragilaria tenera, Staurosirella pinnata) and epiphytic (e.g. Achnanthidium minutissimum, Rossithidium linearis) taxa. This diatom community change supports the intermediate disturbance hypothesis which predicts a high level of diversity and richness following an intermediate to intense disturbance of short duration. Phosphorus concentrations in Swan Lake were inferred using a diatom-based regional calibration model, and the results indicated marked changes in lake water chemistry through time (from below detection limits before land clearance and settlement to 19.3 μg l−1 in the current sediments), which were concurrent with episodes of regional deforestation and land-use change. Although the sediment and biological records indicate that the lake ecology has stabilized over the last 30–50 years, paleolimnological records show that the water quality and biology of Swan Lake has changed dramatically and not returned to pre-settlement conditions. Swan Lake presents a detailed record of the impact created by deforestation and urban development with a population of <50 individuals per km2. Detailed paleolimnological studies like Swan Lake, in tandem with global human footprint studies, can create realistic estimates of land-use impacts at the global scale.  相似文献   

3.
A 1600 year paleoecological record of environmental change is developed from a small lake in the Nothofagus forest of southern Chile (45.5°S, 72°W). High resolution fossil pollen, charcoal, sedimentological, and chrysophycean stomatocyst analyses are used to investigate the impacts of natural and anthropogenic disturbances on terrestrial and lacustrine environments. Chronological control is based on a combination of 210Pb and 14C dating. Temporal resolution during the past 150–200 yr is ca. 8 yr/sample. The macroscopic charcoal record correlates very closely with historical and dendroecological records of 20th century anthropogenic burning in this region. The chrysophyte stratigraphy indicates that this burning had immediate impacts on the lake itself, while the pollen record provides evidence for a succession of vegetation changes lagging slightly behind the disturbance. These palynological changes are very similar to the pollen signal of European disturbance in northeastern North America. Pre-European shifts in chrysophyte assemblages may be due to the influence of earthquake activity on the lake, though there is no corroborative evidence in the pollen or charcoal records. This study demonstrates that high resolution paleoecological methods can be used to help bridge the temporal gap between traditional ecological and paleoecological studies of environmental change in the temperate forests of southern South America.  相似文献   

4.
Lake Nakaumi, southwest Japan, is an enclosed lagoon characterized by polyhaline and halocline conditions. Since the last century, its ecological state has been altered by eutrophication. We used a paleolimnological approach and studied multiple proxies, including chemical compounds, diatoms, foraminifera and molluscs, to infer the eutrophication history of the ecosystem. Eutrophication in Lake Nakaumi was associated with several factors, including increased nutrient loading, input of herbicides, and dike building since the 1920s. The ecological condition of this lake was divided into several stages that reflect the eutrophication process after the 1940s. A catastrophic “regime shift” from a clear state with aquatic vegetation to a turbid one with phytoplankton occurred in the early 1950s. Environmental degradation in the Honjo area, a part of Lake Nakaumi, was attributed primarily to physical changes caused by the construction of an enclosing dike. Eutrophication occurred almost simultaneously with the physical changes to the Honjo area in the 1970s. Until recently, no regime shift was observed in this area, though the core-top sediments show possible symptoms of incipient change.  相似文献   

5.
This study provides a high resolution multi-proxy record of the response of an aquatic ecosystem (Alexander Lake) to forest clearance in New Zealand in the late twentieth century (ca. 1950–2006 AD). New chironomid-based transfer functions for lake water total nitrogen (TN) concentration were applied to the Alexander Lake chironomid record. A test of the significance of reconstructions based on multiple model types indicates that a model with the highest r2 and lowest root mean squared error of prediction may not necessarily perform the best when applied to a particular site. The chironomid-based TN reconstruction and other proxies suggest a complex response by a stained water (dystrophic) lake in a forested catchment to deforestation. Minor perturbations and nutrient influx may favour increased phytoplankton production, but continued light attenuation by dissolved organic carbon and humic compounds prevents proliferation of submerged macrophytes. Complete mechanical forest clearance resulted in a short term pulse of nutrients and eutrophication. The long term effect of deforestation was to increase light penetration and favour the growth of submerged macrophytes. Continued eutrophication of Alexander Lake could be due to a contribution of bird-derived nutrients. Deforestation around Alexander Lake has created a perfect moulting site for Paradise Shelducks (Tadorna variegata Gmelin). The input of total phosphorus from T. variegata could be enough to trigger blooms of Microcystis that currently occur in the lake. Changes in bird behaviour in response to changes in vegetation should therefore be considered a possible result of past (including prehistoric) and future deforestation in New Zealand.  相似文献   

6.
Lake Uddelermeer (The Netherlands) is characterized by turbid conditions and annual blooms of toxic cyanobacteria, which are supposed to be the result of increased agricultural activity in the twentieth century AD. We applied a combination of classic palaeoecological proxies and novel geochemical proxies to the Holocene sediment record of Lake Uddelermeer (The Netherlands) in order to reconstruct the natural variability of the lake ecosystem and to identify the drivers of the change to the turbid conditions that currently characterize this lake. We show that the lake ecosystem was characterized by a mix of aquatic macrophytes and abundant phytoplankton between 11,500 and 6000 cal year BP. A transition to a lake ecosystem with clear-water conditions and relatively high abundances of ‘isoetids’ coincides with the first signs of human impact on the landscape around Lake Uddelermeer during the Early Neolithic (ca. 6000 cal year BP). An abrupt and dramatic ecosystem shift can be seen at ca. 1030 cal year BP when increases in the abundance of algal microfossils and concentrations of sedimentary pigments indicate a transition to a turbid phytoplankton-dominated state. Finally, a strong increase in concentrations of plant and faecal biomarkers is observed around 1950 AD. Canonical Correspondence Analysis suggests that reconstructed lake ecosystem changes are best explained by environmental drivers that show long-term gradual changes (sediment age, water depth). These combined results document the long-term anthropogenic impact on the ecosystem of Lake Uddelermeer and provide evidence for pre-Industrial Era signs of eutrophication.  相似文献   

7.
We studied high-resolution stratigraphy of phosphorus (P) forms in the Holocene sediments of large shallow Lake Peipsi (Estonia/Russia) in order to evaluate the lake ecosystem response to environmental changes and track the lake’s eutrophication history. We distinguished four main periods in the history of Lake Peipsi, each having likely different factors responsible for the distribution pattern of P fractions in the sediment record. We suggest that in the oldest period, from ca. 10.4 up to 7.3 cal ka BP, the sediment composition was mainly determined by rising water level, the second period dated 7.3–2.3 cal ka BP was governed mainly by stable hydrology and P loading, while the third period between 2.3 and 0 cal ka BP was primarily influenced by emerging anthropogenic impact. The sediments from the last period since 1950 are subject of ongoing diagenetic processes but still reflect rapid eutrophication of the lake. Comparison of the results with periods derived from other sediment proxies proved the usability of P fractions stratigraphy in reconstruction of the development of lakes.  相似文献   

8.
根据太白湖沉积柱中硅藻、孢粉、粒度、磁化率、元素含量等指标记录,冗余分析结果表明松孢粉的百分含量、沉积物磷浓度、频率磁化率及有机碳含量是能显著解释水环境变化的最小变量组合,解释的硅藻变率百分比达51.5％,仅比所有沉积指标共同解释的信息量少6.4%。主要根据该4指标的古环境意义,对该湖近200年来的营养演化驱动机制进行了探讨。太白湖近代富营养化过程表明:在温暖湿润的气候背景下,较高营养背景的湖泊极易在人类活动的驱动下发生富营养化,因此对湖泊资源必须进行合理的人为开发与利用。  相似文献   

9.
As many as 2500 interdune lakes lie within the Nebraska Sand Hills, a 50000 km stabilized sand sea. The few published data on cores from these lakes indicate they are typically underlain by less than two m of Holocene lacustrine sediments. However, three lakes in the southwestern Sand Hills, Swan, Blue, and Crescent, contain anomalously thick marsh (peat) and lacustrine (gyttja) sediments. Swan Lake basin contains as much as 8 m of peat, which was deposited between about 9000 and 3300 years ago. This peat is conformably overlain by as much as 10.5 m of gyttja. The sediment record in Blue lake, which is 3 km downgradient from Swan lake, dates back to only about 6000 years ago. Less than two m of peat, which was deposited from 6000 to 5000 years ago, is overlain by 12 m of gyttja deposited in the last 4300 years. Crescent Lake basin, one km downgradient from Blue Lake, has a similar sediment history except for a lack of known peat deposits. Recently, a 8-km long segment of a paleovalley was documented running beneath the three lakes and connecting to the head of Blue Creek Valley. Blockage of this paleovalley by dune sand during two arid intervals, one shortly before 10500 yr BP and one in the mid-Holocene, has resulted in a 25 m rise in the regional water table. This made possible the deposition of organic-rich sediment in all three lakes. Although these lakes, especially Swan, would seem ideal places to look for a nearly complete record of Holocene climatic fluctuations, the paleoclimatic record is confounded by the effect dune dams have on the water table. In Swan Lake, the abrupt conversion from marsh to lacustrine deposition 3300 years ago does not simply record the change to a wetter regional climate; it reflects the complex local hydrologic changes surrounding the emplacement and sealing of dune dams, as well as regional climate.  相似文献   

10.
Diatom and geochemical data from Crawford Lake, Ontario, have been used to document limnological responses to periods of cultural disturbance resulting from native Iroquoian occupation of the watershed (1268–1486 AD) and Euro-Canadian agriculture and deforestation (1867 AD–present). Here, we further develop the high-resolution nature of the Crawford Lake sediment record to examine the physical, chemical and biological aspects of limnological response to human disturbances in the lake catchment area with exceptional detail. We report detailed diatom abundance and flux data for individual taxa from Crawford Lake, and further describe the relationship between assemblage composition and environmental conditions using canonical correspondence analysis (CCA). Diatom assemblage data are used to calculate diatom inferred-total phosphorus (DI-TP) concentrations for the past ∼1,000 years. We also examine the diatom community response during and after periods of disturbance by Iroquoian and Euro-Canadian populations, and compare this response to existing geochemical proxies of lake production and new elemental geochemical indicators of catchment area erosion. In particular, we explore the differing limnological response to the two distinct periods of cultural eutrophication and examine the limnological processes that occurred during the period of␣low (or no) human activity (1487–1866 AD), when geochemical indicators of lake production recovered to pre-disturbance conditions, but diatom assemblages notably did not. Our results illustrate the highly susceptible nature of diatom communities to periods of anthropogenic disturbance, and emphasize that ecological indicators (such as diatom assemblages) should be included with other proxies (such as nutrient concentrations and physical characteristics) when assessing disturbance and recovery in lake systems.  相似文献   

11.
Shallow lakes are among the most threatened ecosystems in the world and many contemporary studies have demonstrated declines in biodiversity due to anthropogenic forcing. Mostly, however, these studies have not covered the full period of human-induced diversity change in lakes which is typically over decades-centuries. Here we provide two examples of palaeoecological studies focussed on reconstructing biodiversity changes in contrasting shallow lake environments that demonstrate the efficacy of the approach—a shallow UK lake and a suite of floodplain lakes (called billabongs) in the Murray-Darling basin, Australia. In the Murray-Darling billabongs, complex sedimentary processes operate, sediment chronologies are less certain and replication of sites is needed to confirm patterns. The combination of sediment records from 10 billabongs showed that diatom diversity changes pre- and post-European (>1850) disturbance were inconsistent; however, reductions in diversity were more common and appear to reflect reductions in macrophyte abundance. At Felbrigg Lake, a multi-proxy study with strong chronological control demonstrated divergent responses of macrophyte, diatom, cladoceran and chironomid richness and diversity to a century of eutrophication. Eutrophication of the site was qualitatively inferred from changes in the macrophyte community in turn reconstructed from plant macrofossils. Benthic cladocerans showed a consistent decline in richness through the record, reflecting the gradual reduction in their macrophyte associated habitat over the past century. Diatom richness and diversity responses were complex, with increases in diversity and richness linked to both increases and decreases in macrophyte species richness and abundance. Chironomid richness and diversity patterns were less consistently linked to eutrophication. The loss of the dominant zooplanktivore (perch) in the 1970s was reflected in the richness and diversity profiles for all groups. Our study reveals clear potential for using sediment cores to infer biodiversity change in shallow lakes and shallow lake regions. However, given the contrasting patterns of diversity change for the different biological groups both in Felbrigg Lake and between Felbrigg and the billabongs, caution is required when interpreting whole-ecosystem biodiversity changes (or the absence of change) based on single as opposed to multi-proxy studies.  相似文献   

12.
A 1.2 m sediment core from Lake Forsyth, Canterbury, New Zealand, records the development of the catchment/lake system over the last 7000 years, and its response to anthropogenic disturbance following European settlement c. 1840 AD. Pollen was used to reconstruct catchment vegetation history, while foraminifera, chironomids, Trichoptera, and the abundance of Pediastrum simplex colonies were used to infer past environmental conditions within the lake. The basal 30 cm of core records the transition of the Lake Forsyth Basin from a tidal embayment to a brackish coastal lake. Timing of closure of the lake mouth could not be accurately determined, but it appears that Lake Forsyth had stabilised as a slightly brackish, oligo-mesotrophic shallow lake by about 500 years BP. Major deforestation occurred on Banks Peninsula between 1860 AD and 1890 AD. This deforestation is marked by the rapid decline in the main canopy trees (Prumnopitys taxifolia (matai) and Podocarpus totara/hallii (totara/mountain totara), an increase in charcoal, and the appearance of grasses. At around 1895 AD, pine appears in the record while a willow (Salix spp.) appears somewhat later. Redundancy analysis (RDA) of the pollen and aquatic species data revealed a significant relationship between regional vegetation and the abundance of aquatic taxa, with the percentage if disturbance pollen explaining most (14.8%) of the constrained variation in the aquatic species data. Principle components analysis (PCA) of aquatic species data revealed that the most significant period of rapid biological change in the lakes history corresponded to the main period of human disturbance in the catchment. Deforestation led to increased sediment and nutrient input into the lake which was accompanied by a major reduction in salinity. These changes are inferred from the appearance and proliferation of freshwater algae (Pediastrum simplex), an increase in abundance and diversity of chironomids, and the abundance of cases and remains from the larvae of the caddisfly, Oecetis unicolor. Eutrophication accompanied by increasing salinity of the lake is inferred from a significant peak and then decline of P. simplex, and a reduction in the abundance and diversity of aquatic invertebrates. The artificial opening of the lake to the Pacific Ocean, which began in the late 1800s, is the likely cause of the recent increase in salinity. An increase in salinity may have also encouraged blooms of the halotolerant and hepatotoxic cyanobacteria Nodularia spumigena.  相似文献   

13.
Taihu Lake is the third largest freshwater lake in China and has been experiencing eutrophication problems for several decades. Diatoms in short sediment cores from three bays in northern Taihu Lake were studied in addition to 1-year of seasonal phytoplankton samples in order to evaluate the rate and magnitude of nutrient enrichment. The dominant species found in the phytoplankton samples appeared in high percentages in the surface sediment samples, suggesting that the latter faithfully record the modern diatom flora. The diatom preservation status varied among the three cores, while in all cores the preservation deteriorated with sediment depth. Due to the superior diatom preservation in the core from Mashan Bay, the fossil diatom record of this core and an established diatom total phosphorus (TP) transfer function were used to reconstruct the nutrient history of Taihu Lake. Diatom assemblages changed from Aulacoseira-dominated to other eutrophic planktonic species, such as Stephanodiscus minutulus, Cyclostephanos tholiformis, Cyclotella atomus, C. meneghiniana and S. hantzschii in ca. 1980. Diatom-inferred TP concentrations exhibited little change prior to 1980, with values around 50 μg/l. However, after 1980 TP concentrations increased significantly and remained in excess of 100 μg/l, reflecting eutrophication of Taihu Lake. Comparison with TP measurements in the water column from 1988 to 2004, as well as the analogue analysis among fossil and modern samples, demonstrates that the diatom-TP inference model can reliably hindcast past TP concentrations. Therefore, the baseline TP value of about 50 μg/l, can be used as a restoration target for Taihu Lake. However, due to the complexity of this very large, shallow aquatic ecosystem, caution should be exercised when employing the diatom record to track eutrophication. Further studies on the mechanism of diatom distribution, evolution and preservation are recommended for Taihu Lake.  相似文献   

14.
We studied the eutrophication history of a tropical shallow reservoir in the S?o Paulo metropolitan region, southeast Brazil. We analyzed grain size, geochemistry, diatom assemblages, and land-use records in a sediment core from the reservoir to infer its trophic state history during the last ~110?years (1894?C2005). Eighty diatom species were observed in the core and shifts in the relative abundances of planktonic and benthic taxa indicate major limnological changes associated with complex interactions between hydrologic factors and eutrophication. Discostella stelligera was associated with deforestation and water physical changes whereas Aulacoseira granulata, a species abundant throughout the core, was mostly associated with high flux conditions and erosion events, regardless of trophic state. Eutrophication was triggered by construction of the city zoo (1958) and installation of the S?o Paulo State Department of Agriculture (1975) within the Gar?as watershed, and increasing loads of untreated sewage from these institutions. The data suggest that deterioration in water quality began after ~1975 and markedly accelerated after ~1990. The reservoir has been hypereutrophic since 1999. Steady increases in geochemical proxies for trophic state, along with a decrease in C/N ratios, indicated higher nutrient concentrations and the prevalence of autochthonous production towards the core top. Appearance of Achnanthidium catenatum ~1993 highlighted the onset of a marked eutrophication phase. The subsequent dominance of Planothidium rostratum and Cyclotella meneghiniana suggested a sharp shift to a hypereutrophic state since 1999. Land-use history proved valuable for validating the chronology and interpreting anthropogenic impacts. Multi-proxy analysis of the sediment record provided an effective tool for tracking ecological shifts in the reservoir ecosystem. This study provides the first reconstruction of lake eutrophication history in Brazil and highlights the importance of hydrological/physical changes as drivers of diatom assemblage shifts in reservoirs, which may confound trophic state inferences based on shifts in the planktonic/benthic diatom ratio.  相似文献   

15.
东南湿润区是我国生态环境问题最为突出的区域之一,水体的富营养化是困扰该区域经济和社会发展的主要问题。尤其近些年来,在政府鼓励和市场推动双重作用下,丘陵山区的开发力度逐步加强,越来越多的原生态竹林被开发为经济型用地(如茶园)。但该土地利用方式的转变在带来经济效益的同时,也改变了下垫面土壤孔隙结构和土壤水文过程,从而极大地影响着营养盐随土壤水分的迁移和转化。在国家自然科学基金青年科学基金项目“太湖流域丘陵区坡面土壤水文过程物理机制及模拟研究”的资助下,在以下3个方面取得了重要进展：①不同土地利用坡面土壤水分时空变化与影响因素;②坡面水文过程与水量平衡;③坡面土壤水文过程影响机制。目前对太湖流域丘陵区土壤水文过程研究虽取得一些进展,但其影响机制仍不十分明确,有待进一步深入的探讨。上述成果的取得以及未来的持续探索,对于太湖水体富营养化与流域面源农业面源污染控制具有重要的环境意义,进而为推动我国流域生态文明建设提供理论支持。  相似文献   

16.
Closed-basin alkaline lakes record climate change particularly well because they generally contain a sedimentary record that is high in carbonate mineral content from which climate proxies can be determined. Various approaches are used to estimate paleo-lake level and volume (δ18O, dating of “shoreline” tufas, biotic proxies, etc.), yet all carry certain caveats that limit their usefulness. Ultimately, the relationship between the chemistry of the lake, the volume of the lake, and the response of the proxy will determine how well a proxy serves a paleolimnologic purpose. Here, we discuss the use of carbonate-associated sulfate (CAS), the sulfate contained within the lattice of carbonate minerals that precipitate in lake water, as a proxy for lake water chemistry and by extension, lake volume. Walker Lake, an alkaline closed-basin lake in western Nevada, has experienced a well-documented lake-level decline since 1880 and provides a test case for CAS as a lake-level proxy. By extracting the CAS from sedimentary carbonate and tufas that have been age dated, we can relate these values to lake sulfate content based on historical or other proxy data. We confirm that CAS tracks lake sulfate. Our study of sedimentary carbonates demonstrates that CAS is a linear function of lake sulfate through a range of 10–25 mM, which corresponds to a change in lake level of 30 m. As confirmation of the CAS technique, we analyzed a stromatolitic tufa dated using AMS 14C. The CAS trend in the stromatolite suggested that it grew during a lake-level decline, a result consistent with other proxy data. Finally, laboratory experiments were conducted that demonstrate CAS is monotonically correlated with sulfate concentration and that precipitation kinetics are not likely a major control on CAS in alkaline lakes, but that ionic strength of the solution exerts a strong control on CAS.  相似文献   

17.
石臼湖湿地水环境质量评价及富营养化状况研究   总被引：2，自引：0，他引：2  
评价了石臼湖水质现状及富营养化状况。结果表明,石臼湖的水质级别为Ⅲ级、Ⅳ级,污染程度为轻、中度污染;影响各水域水质的主要污染物为高锰酸盐指数、硫化物、总氮、总磷;水体富营养化较为严重,高锰酸盐指数、总磷严重超标。造成石臼湖水环境问题的主要原因为渔业与养殖、工业废水、生活污水等污染因素。  相似文献   

18.
Pollen and spores with resistant exines are preferentially preserved in soils, and during periods of soil erosion they can become incorporated into lake sediments. As a result, the contemporary vegetation may be poorly represented by the palynomorphs in the lake sediments because of the reworked component of inwashed pollen and spores. We record the proportion of palynomorphs with corroded exines in sediment cores from four lakes in the eastern North Island of New Zealand to document changing sources of palynomorphs over the last 2000 years. During this period, the catchments experienced major vegetation disturbances, both natural (from volcanism and fire) and anthropogenic including deforestation ca. 600 years ago, and the European conversion of fern-scrubland to pasture in the 19th century. Corroded palynomorphs are more abundant in inwashed sediments than authigenic sediments. Catchment soil disturbance was minor during the forested period, and characterised by small, inwashed, sediment pulses after storms, and a relatively low percentage of corroded palynomorphs. Although initial Maori forest clearance by fire led to a temporary increase in erosion in one lake catchment, rapid replacement of forest by a dense bracken fern cover helped to minimise soil erosion and reworking of palynomorphs in this period. European pastoralists replaced the bracken fern with shallow-rooted pasture grasses about 150 years ago. In erosion prone lake catchments, this led to a rapid increase of inwashed eroded soils and littoral sediments, and their component of resistant palynomorphs, reaching the lake sediments. As a result, the palynological records from these catchments during the European period are distorted by reworking. By contrast, over the same period, the palynological record from a lake with no inflowing streams and stable catchment soils more faithfully represented the contemporary vegetation cover. Exine corrosion has been used to help identify periods of reworking in the lake sediments and to allow for a correction of distortion caused by reworking.  相似文献   

19.
Playa lake systems tend to be overlooked archives of paleoenvironmental change due to the likelihood of a short and intermittent record of deposition. Groundwater-fed wetlands associated with these climate-sensitive playas, however, preserve changes in hydrologic budget and are thus valuable archives for semiarid regions. This study examines the paleoecological record of a groundwater-fed wetland from Lake Solai, Kenya. Biological proxies are used to reconstruct paleoenvironmental change and climate impacts over the past millennium. Dry conditions persisted between CE 1115 and 1490, followed by wetter conditions during the Little Ice Age. Near surface sediments indicate increasing anthropogenic impact through pastoralism.  相似文献   

20.
Fish introduction and eutrophication are important disturbances to aquatic ecosystems, especially to oligotrophic plateau lakes that are generally considered to be very vulnerable ecosystems. Planktivorous fish Neosalanx taihuensis were introduced to Lake Fuxian, an oligotrophic (TP 17 μg/l) deep (average depth 89.7 m) plateau lake in southwest China, in the middle of the 1980s. After the introduction, N. taihuensis became the dominant fish species, and the total fish yield increased about threefold. Although the lake is still oligotrophic, the trophic state of Lake Fuxian has started to shift with increasing nutrient supply (eutrophication) due to an increase in human activities in the drainage basin. This study investigated the effects of N. taihuensis introduction and eutrophication on the cladoceran community of Lake Fuxian by examining changes in cladoceran assemblages and abundance, as well as the morphological features of Bosmina microfossils in the lake sediment. Absolute abundance of total Bosmina increased substantially after the middle of the 1980s. In addition, dominance of Bosmina with straight antennules was replaced by Bosmina with hooked antennules. The morphological variables (length of carapace, antennule and mucro) of Bosmina all decreased when planktivorous fish N. taihuensis achieved relatively large numbers. Eutrophication was the most important process determining cladoceran abundance, while fish introduction played an important role in structuring the cladoceran community in this oligotrophic, deep plateau lake.  相似文献   

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