首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
We present a palaeolimnological study encompassing five centuries of trophic-state change of the dike-breach lake De Waay located on the Rhine-Meuse delta (the Netherlands). Diatom-inferred total phosphorus (TP) concentrations indicate hypertrophic epilimnetic conditions (>300 μg l−1 TP) since the formation of the lake in the fifteenth century until the end of the eighteenth century. Cladocera data support the reconstructed trophic state and indicate turbid conditions in lake De Waay during this period. High inferred TP concentrations as well as the amount of Ti in the sediment reflect numerous flooding events. From the nineteenth century onwards reconstructed TP concentrations decreased to 40–150 μg l−1 due to improvements in sewage and dike systems that considerably diminished direct river flooding and seepage-derived nutrients. As a consequence, the increased stability of littoral habitats led to an increased diversity of the Cladocera assemblages. The most significant decrease in TP concentrations to ~40 μg l−1 occurred between about 1900 and 1930. This mesotrophic phase was a consequence of the isolation of the lake from catchment drainage and the introduction of a highly elaborate flood control during this period. However, since the mid twentieth century a eutrophication trend is preserved in the record, likely related to increased agricultural activity in the vicinity of the lake. Our results emphasize that land-use and trophic-state history must be taken into account when evaluating the ecological status of lakes for water management and protection actions, especially for lakes in landscapes that are strongly modified by human action.  相似文献   

2.
Lake Naivasha, Kenya, is one of a number of freshwater lakes in the East African Rift System. Since the beginning of the twentieth century, it has experienced greater anthropogenic influence as a result of increasingly intensive farming of coffee, tea, flowers, and other horticultural crops within its catchment. The water-level history of Lake Naivasha over the past 200 years was derived from a combination of instrumental records and sediment data. In this study, we analysed diatoms in a lake sediment core to infer past lacustrine conductivity and total phosphorus concentrations. We also measured total nitrogen and carbon concentrations in the sediments. Core chronology was established by 210Pb dating and covered a ~186-year history of natural (climatic) and human-induced environmental changes. Three stratigraphic zones in the core were identified using diatom assemblages. There was a change from littoral/epiphytic diatoms such as Gomphonema gracile and Cymbella muelleri, which occurred during a prolonged dry period from ca. 1820 to 1896 AD, through a transition period, to the present planktonic Aulacoseira sp. that favors nutrient-rich waters. This marked change in the diatom assemblage was caused by climate change, and later a strong anthropogenic overprint on the lake system. Increases in sediment accumulation rates since 1928, from 0.01 to 0.08 g cm−2 year−1 correlate with an increase in diatom-inferred total phosphorus concentrations since the beginning of the twentieth century. The increase in phosphorus accumulation suggests increasing eutrophication of freshwater Lake Naivasha. This study identified two major periods in the lake’s history: (1) the period from 1820 to 1950 AD, during which the lake was affected mainly by natural climate variations, and (2) the period since 1950, during which the effects of anthropogenic activity overprinted those of natural climate variation.  相似文献   

3.
Evaluation of land-use effects on coastal and marine ecosystems requires better understanding of the role of rivers in regulating mass transport from terrestrial to oceanic environments. Here we take advantage of the presence of a riverine lake to use paleoecological techniques to quantify impacts of logging, European-style agriculture, urbanization and continued terrestrial disturbance on mass transport and water quality in the northern drainage of the Mississippi River. Two 2-m sediment-cores recovered in 1999 from Lake St. Croix, a natural impoundment of the St. Croix River, were dated using 210Pb and 137Cs, and analyzed for historical changes (c. 1840–present) in sediment magnetic susceptibility, inorganic and organic matter content, biogenic silica, fossil pigments, and diatom microfossils. Inorganic sediment accumulation increased threefold between the mid-1800s and present, whereas clear signs of eutrophication were only evident after the mid-twentieth century when biogenic silica accumulation increased sixfold, diatom accumulation rates increased 20- to 50- fold, and the diatom community shifted from predominantly benthic species to assemblages composed mainly of planktonic taxa. Similarly, fossil pigment concentrations increased during the 1960s, and diatom-inferred total phosphorus (DI-TP) increased from ~30 μg TP l−1 c. 1910 to ~60 μg l−1 since 1990, similar to historical records since 1980. Together, these patterns demonstrate that initial land clearance did not result in substantive declines in water quality or nutrient mass transport, instead, substantial degradation of downstream environments was restricted to the latter half of the twentieth century. This is one of eight papers dedicated to the “Recent Environmental History of the Upper Mississippi River” published in this special issue of the Journal of Paleolimnology. D. R. Engstrom served as guest editor of the special issue.  相似文献   

4.
A knowledge of pre-disturbance conditions is important for setting realistic restoration targets for lakes. For European waters this is now a requirement of the European Council Water Framework Directive where ecological status must be assessed based on the degree to which present day conditions deviate from reference conditions. Here, we employ palaeolimnological techniques, principally inferences of total phosphorus from diatom assemblages (DI-TP) and classification of diatom composition data from the time slice in sediment cores dated to ~1850 AD, to define chemical and ecological reference conditions, respectively, for a range of UK lake types. The DI-TP results from 169 sites indicate that reference TP values for low alkalinity lakes are typically <10 μg L−1 and in many cases <5 μg L−1, whilst those for medium and high alkalinity lakes are in the range 10–30 and 20–40 μg L−1, respectively. Within the latter two alkalinity types, the deeper waters (>3 m mean depth) generally had lower reference TP concentrations than the shallow sites. A small group of shallow marl lakes had concentrations of ~30 μg L−1. Cluster analysis of diatom composition data from 106 lakes where the key pressure of interest was eutrophication identified three clusters, each associated with particular lake types, suggesting that the typology has ecological relevance, although poor cross matching of the diatom groups and the lake typology at type boundaries highlights the value of a site-specific approach to defining reference conditions. Finally the floristic difference between the reference and present day (surface sample) diatom assemblages of each site was estimated using the squared chord distance dissimilarity coefficient. Only 25 of the 106 lakes experienced insignificant change and the findings indicate that eutrophication has impacted all lake types with >50% of sites exhibiting significant floristic change. The study illustrates the role of the sediment record in determining both chemical and ecological reference conditions, and assessing deviation from the latter. Whilst restoration targets may require modification in the future to account for climate induced alterations, the long temporal perspective offered by palaeolimnology ensures that such changes are assessed against a sound baseline.  相似文献   

5.
We propose a palaeolimnological method for inferring past total phosphorus (TP) concentrations in lake water from spectrophotometrically-measured sedimentary pigments, particularly total carotenoids (TC). Our approach is based on a highly significant statistical correlation (P < 0.0001) between pigment concentrations (total carotenoids) in the surface sediment of 28 Italian lakes (subalpine, large, deep, shallow, volcanic) and TP concentrations measured in these lakes at overturn when the core was collected. A transfer function was developed from this “training” set, and used to estimate past TP concentrations from pigment concentrations in sediment cores. The results generally agreed with TP values as measured by long-term water quality monitoring programs. Contrasting results were obtained by a comparison with diatom-inferred TP. While the diatom model showed a tendency to overestimate TP values higher than 100 μg l−1, the pigment model correctly estimated TP in lakes when TP was <100 μg l−1, but not when lakes were rich in macrophytes. In fact, lakes with extensive populations of aquatic submersed macrophytes and epiphytes are outliers in terms of the TC versus TP relationship. The root mean square error of prediction of the pigment model is lower than those derived from certain diatom—based inference models. The predicted and residual values are not related to the estimated values and their average is not statistically different from zero. Errors were estimated via a ‘leave-one-out’ re-sampling technique. The proposed method permits rapid and relatively inexpensive determination of reference trophic conditions.  相似文献   

6.
We present a sedimentary geochemical record of human perturbation in the watershed and related changes in trace metals (Cr, Ni, Zn and Pb), phosphorus and abundant rock-forming elements (Al, Fe, K, Mg, etc.) in Chaohu Lake over the past 500 years, a period spanning historical agricultural expansion and modern economic development. The record exhibited a stable terrestrial detrital input to Chaohu Lake before ca. 1540 AD, a period with less human perturbation of the watershed, which resulted in low and constant values of trace metals and phosphorus (TP) concentrations as well as Al, Fe, K, Mg, fine silt (<16 μm), the chemical index of alteration (CIA) and K/Na ratio. Two periods of successive marked increases in Al, Fe, K, Mg, fine silt, chemical index of alteration (CIA) and K/Na ratio occurred after ca. 1540 AD and 1950 AD. The former period apparently resulted from enhanced well-weathered topsoil erosion in the watershed related to the expansion of arable land, and the latter resulted from further enhancement of human perturbation in the watershed during the modern period. Concentrations of trace metals and TP were positively correlated with that of the rock-forming elements and fine silt. Trace metal pollution was limited during 1540–1950 AD, according to the low enrichment factors (EFs = 0.8–1.1), when an increase in trace metal concentrations was also linked to changes in detrital input. In addition to detrital regulation, pollution also contributed to an increase in TP concentrations (average EF 1.4) during 1540–1950 AD and the average accumulation rate of anthropogenic phosphorus was 87.3 mg m−2 a−1. Anthropogenic phosphorus increased further and Pb and Zn pollution also occurred after 1950 AD, reaching maximum values after 1980 AD, when the average accumulation rates of anthropogenic Zn, Pb and phosphorus (mainly in the form of NaOH-P) were 242.2, 43.3 and 811.8 mg m−2 a−1, respectively. The increase in phosphorus pollution in recent decades is probably from the domestic sewage sources of Hefei City and non-point sources related to agricultural utilization of commercial fertilizer, whereas Zn and Pb pollution is probably derived mainly from industrial sources of Hefei City, as deduced from their spatial variations in the sediments of the river mouths.  相似文献   

7.
Trace metals and polycyclic aromatic hydrocarbons (PAH) were analyzed in sediment cores from three central Alberta lakes to determine the contributions of local coal-fired power plants to contaminant loadings. In Wabamun Lake, with four power plants built since 1950 within a 35-km radius, sediment concentrations of mercury, copper, lead, arsenic and selenium have increased by 1.2- to 4-fold. Trace metal enrichments were less pronounced in Lac Ste. Anne and Pigeon Lake, situated 20 km north and 70 km south of Wabamun Lake, respectively. Total Hg flux to Wabamun Lake sediments (21–32μg m−2 yr−1) has increased 6-fold since 1950, compared to 2- and 1.5-fold increases in Lac Ste. Anne and Pigeon Lake, respectively, since circa 1900. Total PAH flux to surface sediments was 730–1100μg m−2 yr−1 in Wabamun Lake, 290–420μg m−2 yr−1 in Lac Ste. Anne, and 140–240μg m−2 yr−1 in Pigeon Lake. Without adoption of pollution-abatement technology that compensates for increases in generating capacity, continued expansion of coal-burning industry in Alberta will result in increased contaminant deposition, primarily from local sources.  相似文献   

8.
A 72-lake diatom training set was developed for the Irish Ecoregion to examine the response of surface sediment diatom assemblages to measured environmental variables. A variety of multivariate data analyses was used to investigate environmental and biological data structure and their inter-relationships. Of the variables used in determining a typology for lakes in the Irish Ecoregion, alkalinity was the only one found to have a significant effect on diatom assemblages. A total of 602 diatom taxa were identified, with 233 recorded at three or more sites with abundances ≥1%. Generally diatom data displayed a high degree of heterogeneity at the species level and non-linear ecological responses. Both pH and total phosphorus (TP) (in the ranges of 5.1–8.5 and 4.0–142.3 μg l−1 respectively) were shown to be the most significant variables in determining the surface sediment diatom assemblages. The calibration models for pH and TP were developed using the weighted averaging (WA) method; data manipulation showed strong influences on model performances. The optima WA models based on 70 lakes produced a jack-knifed coefficient of determination (r 2 jack) of 0.89 with a root mean squared error (RMSEP) of 0.32 for pH and r 2 jack of 0.74 and RMSEP of 0.21 (log10 μg l−1) for TP. Both models showed strong performances in comparison with existing models for Ireland and elsewhere. Application of the pH and TP transfer functions developed here will enable the generation of quantitative water quality data from the expanding number of palaeolimnological records available for the Irish Ecoregion, and thus facilitate the use of palaeolimnological approaches in the reconstruction of past lake water quality, ecological assessment and restoration.  相似文献   

9.
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   

10.
Biomonitoring past salinity changes in an athalassic subarctic lake   总被引：1，自引：0，他引：1  
A short sediment core was taken from a small saline lake located on an intermontane plateau in the central Yukon Territory, Canada. In July 1990, chemical analyses indicated that, although the lake was shallow (Zmax=1.1 m), it was also chemically stratified, with hyposaline (9.9 to 10.0 g L−1) surface waters and slightly mesosaline (22.0 g L−1) deeper waters. The surface water was dominated by Na+ and HCO 3 − . To our knowledge, this is the northernmost athalassic saline lake yet recorded. Quantification of algal (diatom, chrysophyte, and pigment) and invertebrate (chironomid, ceratopogonid, andChaoborus) fossils at four stratigraphic levels indicated that the lake sediments preserved numerous biological indicators that could be used to infer recent lake development. Many of the taxa are found in other athalassic salt lakes. The most striking stratigraphic change was a remarkable drop in the species richness of diatoms and invertebrates in the recent sediments, which parallels the elimination of species characteristic of less saline conditions. Halophilous taxa dominate the most recent sediments, indicating the development of more saline conditions. At the same time, a significant shift in chrysophyte cyst composition was observed. Fossil carotenoids and chlorophylls indicated a decrease in total algal abundance in recent sediments, as green and blue-green algae replaced diatoms and chrysophytes. Together, these paleolimnological data suggest a recent shift to drier conditions or increased evaporation in the central Yukon Territory.  相似文献   

11.
In order to assess the recent anthropogenic environmental changes in Lake Kitaura, central Japan, changes during the past few centuries were reconstructed from results of radiometric and tephrochlonological age determination, magnetic susceptibility measurements, total organic carbon analyses, total nitrogen analyses and fossil diatom analyses on a sediment core from the lake. A total of six major and sub-zones are recognized according to the diatom fossil assemblages, and we discuss aquatic environmental change in Lake Kitaura mainly based on these diatom assemblage change. Zone Ia and Zone Ib (older than AD 1707) are marine to brackish. In Zone IIa (AD␣1707–AD 1836), most of the brackish diatoms disappeared, and were replaced by freshwater species indicating a decrease in salinity. We interpret the salinity decrease in Zone I–IIa as a sea-level fall during the Little Ice Age. The salinity of the lake decreased to near freshwater conditions in Zone IIb (AD 1836–AD 1970), which could arise from alteration in River Tone or development of a sandspit in the mouth of River Tone in addition to sea-level change. In Zone IIIa (AD 1970–AD 1987), the diatom assemblage indicates a freshwater environment, and sedimentation rates increase rapidly. These changes reflect sedimentary environment change and an ecosystem transition due to the construction of the tide gate. In Zone IIIb (AD 1987–AD 2002), the diatom flux (valves cm−2 y−1) increased and species composition changed. The changes in Zone IIIb show a good agreement with limnological monitoring data gathered from the lake. These paleolimnological data suggest that the recent human-induced changes of the aquatic environment of the lake after the 1970s exceed rates during the period concerned in this study.  相似文献   

12.
We analyzed 210Pb-dated sediment cores from four relatively shallow lakes (zmax < 10 m) in the Upper Ocklawaha River Basin, Florida, USA to compare primary producer community structure before and after anthropogenic impacts. We measured physical and chemical sediment variables including density, organic matter (OM), water-soluble phosphorus, polyphosphate (Poly-P), total P (TP), total carbon to total nitrogen mass ratios of OM (TC:TN), biogenic silica (diatoms, sponge spicules), total amorphous silica, and stable carbon and nitrogen isotope ratios of bulk OM. Principal component analysis showed that diatom biogenic silica, TC:TN, Poly-P and TP displayed discernible stratigraphic changes associated with the shift in the primary producer community. We applied k-means cluster analysis to these variables to identify macrophyte-derived, transitional, and phytoplankton-derived sediments. Our approach provides an objective method for identifying sediment sources that may be applied to shallow lakes in other regions. The four study lakes shifted from a macrophyte-dominated state to a transitional state before major anthropogenic disturbances, and became phytoplankton-dominated after ~1950.  相似文献   

13.
Surface sediments from 68 small lakes in the Alps and 9 well-dated sediment core samples that cover a gradient of total phosphorus (TP) concentrations of 6 to 520 g TP l-1 were studied for diatom, chrysophyte cyst, cladocera, and chironomid assemblages. Inference models for mean circulation log10 TP were developed for diatoms, chironomids, and benthic cladocera using weighted-averaging partial least squares. After screening for outliers, the final transfer functions have coefficients of determination (r2, as assessed by cross-validation, of 0.79 (diatoms), 0.68 (chironomids), and 0.49 (benthic cladocera). Planktonic cladocera and chrysophytes show very weak relationships to TP and no TP inference models were developed for these biota. Diatoms showed the best relationship with TP, whereas the other biota all have large secondary gradients, suggesting that variables other than TP have a strong influence on their composition and abundance. Comparison with other diatom – TP inference models shows that our model has high predictive power and a low root mean squared error of prediction, as assessed by cross-validation.  相似文献   

14.
We used elemental carbon, nitrogen, phosphorus and hydrogen ratios (C/N, N/P and H/C) with total organic carbon (TOC) and total phosphorus (TP) as well as stable carbon and nitrogen isotopes (δ13C and δ15N) to investigate the source and depositional conditions of organic matter in sediments from Zeekoevlei, the largest freshwater lake in South Africa. Typical C/N (10–12), H/C ratios (≥1.7) and δ13Corganic values (−22 to −19‰) together with the increase in TOC concentration indicate elevated primary productivity in lower middle (18–22 cm) and top (0–8 cm) sections of the sediment cores. Seepage of nutrients from a nearby waste water treatment plant, rapid urbanization and heavily fertilized farming in the catchments are responsible for the increased productivity. Consistent with this, measured δ15Norganic values (∼11‰) indicate increased raw sewage input towards the top-section of the core. Although cyanobacterial blooms are evident from the low δ15N values (∼3‰) in mid-section of the core, they did not outnumber the phytoplankton population. Low N/P ratio (∼0) and high TP (100–2,200 mg l−1) support cyanobacterial growth under N limited condition, and insignificant input of macrophytes towards the organic matter pool. Dredging in 1983, caused sub-aerial exposure of the suspended and surface sediments, and affected organic matter preservation in the upper mid-section (12–14 cm) of the core.  相似文献   

15.
Diatom-based transfer functions for inferring epilimnetic total phosphorus (TP) have been developed from a data set of 33 southeastern Australian water storages. Regular institutional monitoring of these sites has allowed comparison of models developed from TP data covering different time periods. A model based on mean annual TP performs better than models derived from winter maximum TP, spring minimum TP or TP nearest the time of diatom sampling. A mean annual TP model (WA-PLS 2 component) has a jack-knifed diatom-inferred versus measured TP correlation coefficient (r 2 jack) of 0.69 and a root-mean-square-error of prediction (RMSEP) of 0.246 log10g TP l–1, while alternative models have RMSEP > 0.27. Deletion of two samples with uncharacteristic species composition and environmental conditions improved performance of the mean annual TP model (r 2 jack= 0.74; RMSEP = 0.233 log10g TP l–1). Comparison with other published diatom-TP calibration models indicates that this model performs relatively well, with possible contributing factors including the extensive characterisation of TP (with an average 15 determinations making up the annual mean) and the dominance of planktonic diatoms in most sites. Downcore application of the model will allow the reconstruction of reservoir nutrient histories since commissioning, and thus provide a basis for understanding and management of reservoirs.  相似文献   

16.
Environmental change in Lake Taihu and its catchment since the early to middle part of the twentieth century has left a clear geochemical record in the lake sediments. The human activities in the lake and its catchment responsible for the change include agriculture, fishery, urbanisation, sewage and industrial waster disposal. Sediment cores were collected from Meilian Bay of northern Lake Taihu to investigate the record of anthropogenic impacts on the lake’s ecosystem and to assess its natural, pre-eutrophication baseline state. Two marked stratigraphic sediment units were identified on the basis of total phosphorus concentration (TP), pigments, total organic carbon (TOC)/total nitrogen (TN), δ13C and δ15N corresponding to stages in the lake history dominated by phytoplankton, and by aquatic macrophytes. Results show that as TP loading increased from the early 1950s the lake produced sediments with increasing amounts of organic matter derived from phytoplankton. In the early 1950s, the first evidence for eutrophication at the Meilian Bay site is recorded by an increase in C/N values and in sediment accumulation rate, but there is little change in phosphorus concentrations, pigments, δ13C and δ15N at this time. After 1990 a more rapid increase in trophic status took place indicated by increased levels of phosphorus, pigments, δ15N and by decreased δ13C and TOC/TN values in the lake sediments. The first increase in trophic status of the early 1950s results mainly from agricultural development in the catchment. In contrast, the acceleration from ca. 1990 originates from the recent development of fisheries and the urbanisation and industrialisation of the catchment.  相似文献   

17.
The lead pollution history, based on the accumulation rate of total Pb and ratio of stable isotopes (206Pb/207Pb), was studied in the annually laminated sediment of a small lake in Finland (62°20′ N; 25°41′ E). The sediment chronology based on varve counting provided a unique opportunity to explore and date signals of Pb emissions, including the ancient metallurgical activities of the Roman Empire at the beginning of the Current Era. Changes in the ratio of stable isotopes gave a pronounced signal of the atmospheric Pb fallout in AD 32–392, although this was not distinguishable in the accumulation rate of total Pb, as it was observed in previous work. Calculated accumulations of the ancient pollution Pb were low, the highest values being 0.2–0.3 mg m2 a−1 in AD 144–392, corresponding 14–21% of the accumulation of total Pb. The accumulation of pollution Pb collapsed in the fifth century and remained at or close to the background level up to the eleventh century. After this, the accumulation rate of pollution Pb began to increase and reached 1.2 mg m2 a−1 in AD 1420–1439, corresponding to 44% of the total Pb accumulation. During five centuries, from AD 1420–1895, the average accumulation of pollution Pb was 2.6 mg m2 a−1, the variation being from 0.8 to 4.8 mg m2 a−1. The accumulation of Pb started to increase exponentially in the early twentieth century, and the highest accumulations of pollution Pb (11–22 mg m2 a−1, corresponding 50–76% of the annual accumulation of total Pb) were dated to AD 1926–1985. The banning of the use of leaded fuel has led to a pronounced decrease in the accumulation of pollution Pb since the 1980s, and the present accumulation rate represents the level that prevailed 80–120 years ago.  相似文献   

18.
Sediment accumulation rate (SAR) is an important physical parameter in all lakes and increases have been observed in many over the last c.100 years. This has been ascribed to changes in land-use and land-management causing accelerated catchment soil erosion and an increase in autochthonous organic matter production. The EU Water Framework Directive requires that assessment of biological, hydromorphological and chemical elements of water quality should be based on the degree to which present day conditions deviate from those expected in the absence of significant anthropogenic influence, termed reference conditions. Currently however, the reference condition for sediment accumulation rate for lakes of different types is undefined. To improve our understanding of the controls on SARs we compiled SAR and lake typology data for 207 European lakes derived from 210Pb dated cores to assess how rates have changed through time (in 25 year classes) both overall and for lakes of different types. Seventy-one percent of these sediment cores showed surface SARs higher than “basal” (mainly nineteenth century) rates, 11% showed no change while 18% showed a decline. Lakes were then classified into lake-types using four variables: alkalinity (3 classes), altitude (3 classes), maximum depth (2 classes) and lake area (2 classes). This generated a possible 36 lake classes of which 25 were represented in the dataset. Nine lake-types contained >10 lakes. Little change in SAR occurred prior to 1900 and most increases occurred in more recent periods, in particular 1950–1975 and post-1975. This indicates a general acceleration in SAR in European lakes during the second half of the twentieth century. Reference SARs were estimated for six lake-types with the highest number of sites. European mountain lakes had the lowest reference SAR (0.005 ± 0.003 g cm−2 yr−1) while lowland, high alkalinity sites had the highest (0.03–0.04 g cm−2 yr−1). SARs for other lake-types ranged between 0.012 and 0.024 g cm−2 yr−1. Using the mountain lake-type as an example, the 1850 reference SAR appears to show good agreement with available data for lakes beyond Europe indicating these values may be more broadly applicable. Contemporary SARs in lakes of all classes showed exceedence over their defined reference SAR. This may be partly due to diagenetic processes. Greatest exceedences were found in shallow, low altitude lakes and these are considered to be the ones under the greatest threat from continued elevation of SAR. It is considered that climate change may play a progressively more important role in driving SAR in the future.  相似文献   

19.
Rapid urbanization and increased tourism around Nainital Lake in the Kumaun Himalayan region in north India has raised concerns about sediment and water pollution. Lead-210 dated sediment cores from the lake represent ~95 years of accumulation and yield a mean sedimentation rate of ~4.7 mm year−1. Total organic carbon (TOC), percent N and S and their atomic C/N and C/S ratios, stable isotopes (δ13C, δ15N, and δ34S), and specific biomarkers (n-alkanes and pigments) were measured in the core. Organic matter is primarily derived from in-lake algal production and TOC flux varies from 1.0 to 3.5 g m−2 year−1. Sediments are anoxic (Eh −328 to −187 mV) and have low (0.10–0.30 g m−2 year−1) N, but high (0.37–1.0 g m−2 year−1) S flux. Shifts in δ13C, δ15N, and δ34S suggest in-lake microbial processes dominated by denitrification and sulfate reduction. The sediments are dominated by short-chain hydrocarbons with low Carbon Preference Index values. The pigments indicate a gradual shift to cyanobacterial domination of the phytoplankton community in recent years. Despite an increase in external input of nutrients, the trophic state of the lake has remained largely unchanged, and the perceived human-induced impacts are limited.  相似文献   

20.
Marine eutrophication of estuaries and coastal waters is considered to be a significant problem worldwide. In the semi-enclosed Baltic Sea, where the nutrient load has strongly increased from its natural level, this has led to marked changes in the coastal ecosystems. A key to successful management of coastal waters is reliable scientific evidence of their past state. The palaeolimnological record of subfossil diatoms was used to study the rate and magnitude of eutrophication over the last ca. 200 years in two urban and three rural sites. The urban sites showed marked increases in the percentage abundance of planktonic diatoms (from <50 to ca. 90% and from <5 to ca. 70%) and diatom-inferred total dissolved nitrogen (from <800 to ca. 3000 μg l−1 and from <400 to ca. 800 μg l−1), and a decrease in species richness starting in the 19th – early 20th century with increased urbanisation. At both sites a clear recovery was observed after the cessation of waste water loading by the mid 1980s. The present planktonic diatom assemblages of these embayments, however, show no change back to the pre-disturbance diverse benthic communities. In contrast, the changes observed in the rural sites were only moderate and occurred later starting in the 1940s. No marked increases in diatom-inferred total dissolved nitrogen were seen, however, all sites showed an increase in small planktonic taxa (from ca. 1–6% to 8–36%) indicating increased nutrient enrichment and turbidity. These small floristic changes could be seen as an early warning signal despite little change in the inferred nutrient concentrations. The results have implications for the European Water Framework Directive, which requires European surface waters to be of good ecological status, defined both by biological and chemical quality elements.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号