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1.
This study addresses the effects of climate, land-use, and atmospheric nitrogen (N) deposition on the Fallen Leaf Lake watershed, Lake Tahoe Basin, through diatom and geochemical analyses of sediment cores. Four diatom zones are recognized from a core taken at Fallen Leaf Lake (FLL), a site moderately impacted by human activities: (1) Pre-Little Ice Age Zone (840–1385), (2) Little Ice Age Zone (1385–1810) characterized by Stephanodiscus alpinus and Aulacoseira subarctica, (3) Transitional Zone (1810–1950) of warming and anthropogenic influence with increased Lindavia rossii-ocellata group and Discostella stelligera and decreased Pseudostaurosira brevistriata, and (4) Anthropogenic Zone (1950–2010) characterized by a rapid increase of mesotrophic diatoms of the Fragilaria tenera-nanana group, Tabellaria flocculosa strain IIIP, and Nitzschia gracilis. The Transitional Zone increases in elemental Co, Zn, and Sn that may be attributed to an increase in coal burning and smelting activities in California and Nevada. Beginning around 1910 and accelerating in the 1940s, increased building, land-use, and recreation around FLL caused an increase in terrestrial sedimentary input. Down core proxies for atmospheric N deposition in the FLL watershed are, at best, weakly expressed and appear to be overshadowed by stronger signals. Lack of support for N deposition includes the asynchrony in the appearance of the N-sensitive diatom Asterionella formosa in FLL and a lower impact site at Gilmore Lake, and an uninformative δ15N record. Asterionella formosa is a dominant component in the FLL water column today, but has been present in similar abundances for at least the last 1200 years. Asterionella formosa is present in the water column at Gilmore Lake and absent from the sediment, indicating a very recent appearance. The data collected show that the FLL record is sensitive to climatic cooling during the Little Ice Age and to anthropogenic activities commencing in the 1800s that increased throughout the latter half of the twentieth century; however the effects of anthropogenic N deposition in these lakes could not be substantiated.  相似文献   

2.
Based on data on taxed-cropland area and on the number of households in historical documents, a probabilistic model of cropland distribution and a cropland area allocation model were designed and validated. Cropland areas for the years AD976, 997, 1066, and 1078 were estimated at the level of Lu (an administrative region of the Northern Song Dynasty). The results indicated that (1) the cropland area of the whole study region for AD976, 997, 1066, and 1078 was about 468.27 million mu (a Chinese unit of area, with1 mu=666.7m2), 495.53 million mu, 697.65 million mu, and 731.94 million mu, respectively. The fractional cropland area (FCA) increased from 10.7% to 16.8%, and the per capita cropland area decreased from 15.7 mu to 8.4 mu. (2) With regard to the cropland spatial pattern, the FCA of the southeast, north, and southwest regions of the Northern Song territory increased by 12.0%, 5.2%, and 1.2%, respectively. The FCA of some regions in the Yangtze River Plain increased to greater than 40%, and the FCA of the North China Plain increased to greater than 20%. However, the FCA of the southwest region (except for the Chengdu Plain) in the Northern Song territory was less than 6%. (3) There were 84.2% Lus whose absolute relative error was smaller than 20% in the mid Northern Song Dynasty. The validation results indicate that our models are reasonable and that the results of reconstruction are credible.  相似文献   

3.
The complex relationship between the Yangtze River and Poyang Lake controls the exchange of water and sediment between the two, and exerts effects on water resources, flooding, shipping, and the ecological environment. The theory of energy is applied in this paper to investigate the physical mechanisms that determine the nature of the contact between the Yangtze River and Poyang Lake and to establish an energy difference (Fe) index to quantify the interactions between the two systems. Data show that Fe values for this interaction have increased since the 1950s, indicating a weakening in the river effect while the lake effect has been enhanced. Enclosure of the Three Gorges Reservoir (TGR) has also significantly influenced the relationship between the river and the lake by further reducing the impacts of the Yangtze River. The river effect also increases slightly during the dry season, and decreases significantly at the end of the flooding period, while interactions between the two to some extent influence the development of droughts and floods within the lake area. Data show that when the flow of the five rivers within this area is significant and a blocking effect due to the Yangtze River is also clearly apparent, floods occur easily; in contrast, when the opposite is true and the flow of the five rivers is small, and the Yangtze River can accommodate the flow, droughts occur frequently. Construction and enclosure of the TGR also means that the lake area is prone to droughts during September and October.  相似文献   

4.
Due to methodological challenges there are only a few studies that focus on macrophyte dynamics in large lakes despite their notable role in a lake’s ecosystem functioning. This study investigates composition and productivity changes of the submerged vegetation of Lake Karakul, Pamir Mountains (Tajikistan), using sedimentary ancient DNA metabarcoding and elemental (C/N) and isotopic (δ13C, δ15N) measurements of Stuckenia cf. pamirica (Baagøe) Z. Kaplan (Potamogetonaceae) leaf remains. No Stuckenia cf. pamirica leaf remains were found for 28.7–26.1 cal ka BP, when both Potamogetonaceae and Chara (L.) DNA sequences were recorded, suggesting sparse submerged vegetation at the coring site. This agrees with the inference of a deep lake reached using geochemical proxies. From 26.1 to 17.5 cal ka BP a few macrophyte remains and high numbers of Potamogetonaceae sequences were recovered: lake level was probably low, as suggested by other studies on the lake. Another phase of increased numbers of Chara sequences and the absence of Stuckenia cf. pamirica leaf remains was found between 17.5 and 12.2 cal ka BP, which coincides with a lake-level transgression at Lake Karakul as indicated by paleo-shoreline investigations. Analyses of macrophyte remains reveal intermediate paleo-productivity from 6.9 cal ka BP and high paleo-productivity from 2.2 cal ka BP onwards. From comparisons with other studies, we suggest that lake-level changes are the main driver for the submerged vegetation composition and productivity at the coring site in Lake Karakul and underline our conclusions by depicting the present-day distribution of Stuckenia cf. pamirica and Chara within the lake.  相似文献   

5.
Preventing biodiversity loss is a key aim of modern conservation, and paleolimnology can inform conservation strategies for target species and habitats where other data are unavailable. Care must be taken to fully understand the possibilities and limits of such techniques, particularly where they concern single species. This study uses plant and seed distribution data to inform macrofossil reconstructions of the rare macrophyte Najas flexilis (Slender Naiad) in Scotland, UK. It answers three questions: (a) How does the location of N. flexilis seeds in the surface sediments relate to the distribution of N. flexilis plants? (b) How do the numbers of seeds in surface sediments correlate with % cover of N. flexilis plants across lakes with differing N. flexilis abundances? (c) What are the implications of these findings for paleolimnology? Percentage N. flexilis cover and number of N. flexilis seeds in surface sediments were recorded at ~100 sample points at each of three sites; one where the species was abundant, one where it was occasional and one where it was extinct. At all sites, N. flexilis seeds were present in surface sediments across the entire lake. No correlation between % cover N. flexilis and the number of seeds in surface sediments was found within individual sites. The distribution of seeds in these lakes appeared to be related to multiple environmental and ecological variables including latitude and longitude (proxies for water currents). This is attributed to the ability of seed-bearing N. flexilis plants to fragment and float large distances on water. Between sites, there was a significant difference in the mean seed counts, with higher mean seed counts corresponding to higher abundances of N. flexilis plants. It is concluded that N. flexilis is likely to be well represented in sediment cores taken from any location within a basin, but that care should be taken when inferring changes in N. flexilis abundance from changes in the numbers of seeds in sediment samples. This work demonstrates that the reproductive ecology (number of seeds produced and dispersal mechanisms) is an important factor to consider when attempting reconstructions of single aquatic plant populations from macrofossil records.  相似文献   

6.
This study represents a step towards developing seasonal climate inferences by using high-resolution modern data sets. The importance of seasonal climate changes is highlighted by the instrumental record of a meteorological station close to our study site (lac du Sommet in the Laurentian Mountains, Québec, Canada): Between 1966 and 2001, May temperatures increased significantly by 3.1°C (r = 0.41, n = 35, p < 0.01) but annual mean temperatures only by 0.6°C (r = 0.21, n = 35, p > 0.05). Comparison of this instrumental record with fossil diatom assemblages in a sediment core from lac du Sommet showed that axis one of a principal component analysis (PCA) of the fossil diatoms was best correlated with wind velocity in June (r = 0.62, n = 19, p < 0.005) and that past diatom production was significantly enhanced in periods with colder July temperatures (r =  ?0.77, n = 19, p < 0.0005) and higher wind velocity in June (r = 77, n = 19, p < 0.0005). The strong impact of the spring and summer conditions on overall diatom composition and productivity suggests that seasonal lake responses to climate are more important than annual mean temperatures. However, the seasonal dynamics of diatom communities are not well understood, and seasonality is rarely inferred effectively from lake sediment studies. Our research presents a pilot study to answer a twofold question: Is it possible to identify diatom communities which are typical for warmer or colder seasonal climate using sediment traps, and if it is, can this knowledge be used to infer seasonal climate conditions from fossil diatom assemblages? To address these questions, the seasonal dynamics of diatom communities and water chemistry were studied using sediment traps and water samples at biweekly intervals in four lakes distributed along an altitudinal gradient in the Laurentian Mountains from May through October 2002. Date of ice break-up was significantly related to the diatom assemblages taken in spring and uncorrelated to other significant environmental variables. Summer water temperature, circulation of the water column and pH explained a significant part of the biological variance in summer, and total nitrogen (TN) explained most of the biological variance in autumn. To infer these variables, weighted averaging partial least squares models were applied to the seasonal data sets. Inferred ice break-up dates were significantly correlated with number of days below 0°C in April (r = 0.52, n = 19, p < 0.025), inferred circulation of the water column was significantly related to measured wind velocity in June (r = 0.64, n = 19, p < 0.005), inferred summer water temperature and inferred pH was significantly related to measured July air temperature (r = 0.50, r =  ?53, n = 19, p < 0.025) and inferred TN autumn concentrations had an inverse relationship to August temperatures (r =  ?0.53, n = 19, p < 0.01). This comparison of the historical record with diatom-inferred seasonal climate signals, based on the comparison of fossil diatom assemblages with modern sediment trap data of high temporal resolution, provides a promising new approach for the reconstruction of seasonal climate aspects in paleolimnological studies.  相似文献   

7.
Ecological and biogeographical studies of Neotropical non-marine ostracods are rare, although such information is needed to develop reliable paleoecological and paleoclimatic reconstructions for the region. An extensive, yet little explored South American area of paleoclimatic interest, is the arid-semiarid ecotone (Arid Diagonal) that separates arid Patagonia from subtropical/tropical northern South America, and lies at the intersection of the Pacific and Atlantic atmospheric circulation systems. This study focused on the Laguna Llancanelo basin, Argentina, a Ramsar site located within the Arid Diagonal, and was designed to build a modern dataset using ostracods (diversity, spatial distribution, seasonality, habitat preferences) and water chemistry. Cluster and multivariate analysis of the data indicated that salinity is the most significant variable segregating two ostracod groups. Limnocythere aff. staplini is the only species that develops abundant populations in the saline ephemeral Laguna Llancanelo during almost all seasons, and is accompanied by scarce Cypridopsis vidua in summer. The latter species is abundant in freshwater lotic sites, where Ilyocypris ramirezi, Herpetocypris helenae, and Cyprididae indet. are also found in large numbers. Darwinula stevensoni, Penthesilenula incae, Heterocypris incongruens, Chlamydotheca arcuata, Chlamydotheca sp., Herpetocypris helenae, and Potamocypris smaragdina prefer freshwater lentic conditions (springs), with C. arcuata and Chlamydotheca sp. found only in the Carapacho warm spring, which has a year-round constant temperature of ~20 °C. Seasonal sampling was necessary because some taxa display a highly seasonal distribution. Species that were recorded have either subtropical or Patagonian affinities, although a few taxa are endemic or common to both regions. These data can serve as modern analogues for reconstructing the late Quaternary history of the area, and to investigate the extent and position of the arid/semiarid ecotone (Arid Diagonal) during past glacial/interglacial cycles.  相似文献   

8.
We examined long-term relationships between Cladocera and limnogeological and climate variables in a late Holocene lake sediment sequence to assess cladoceran responses to climate change and reconstruct past climate variations in southern Finland, near the Baltic Sea coast. Elemental composition, organic matter, and inferred water quality variables were used to constrain paleoenvironmental (until 4500 cal BP) conditions and human impact. Fossil Cladocera assemblages, ephippia, and body size trends were utilized to estimate mean July air temperature (T Jul) variability, open-water season length, and water-temperature regimes. Results revealed stable cladoceran communities, dominated by Eubosmina, until 500 cal BP, followed by major assemblage changes including increases in Chydorus cf. sphaericus, Alonella spp., and Bosmina longirostris. Pb and Cu concentrations were associated temporally with increasing human impact and warming climate during the past few centuries, and were the most significant factors in explaining cladoceran community changes, based on redundancy analysis. A Cladocera-based T Jul reconstruction estimated elevated temperatures for the end of the Holocene Thermal Maximum, a cooler period during ~3000–2000 cal BP, slightly increased temperatures during 1200–800 cal BP corresponding to the Medieval Climate Anomaly (MCA), and lower temperatures during the Little Ice Age (LIA), 800–200 cal BP. The reconstruction also suggests that significant climate warming took place during the twentieth century. Cladocerans mostly associated with warm periods included B. longirostris and Pleuroxus uncinatus, whereas Alona affinis, A. quadrangularis, and C. cf. sphaericus were associated with colder climate. Compared to a reference Chironomidae-inferred T Jul reconstruction from the same region, discrepancies were apparent in amplitude of temperature change, as the temperature variability in the cladoceran-based record was muted until ~200 cal BP. During the LIA, increased ephippia and body size agreed with the temperature reconstruction, but suggested a severely shorter open-water season and reduced water temperatures compared to the preceding late Holocene episodes, even though the cladoceran T Jul showed quite similar temperatures for these periods. Our results suggest that fossil cladoceran assemblages, ephippia, and morphological attributes respond sensitively to long-term climate fluctuations and this record reflects well the major climate events of the late Holocene and provides realistic paleoclimatic estimates for maritime southern Finland.  相似文献   

9.
Compared to larger lakes, ponds have rarely been the focus of palaeoecological studies. A common feature of ponds, especially those subject to eutrophication, is mass surface coverings of lemnoid Araceae (duckweed) which have severe implications for ecological processes in small waterbodies, in particular lowered oxygen content. To help understand the implications of duckweed dominance for the long-term ecology of ponds, and to determine the potential for palaeoecological studies in ponds more generally, we develop a new diatom-based Lemna-indicator metric. Recent studies of diatom host-plant relationships have shown significant associations between duckweed and the epiphytes Lemnicola hungarica and Sellaphora saugerresii (formally known as Sellaphora seminulum). To determine the potential of these species as palaeo-indicators of long-term duckweed dynamics in ponds, we investigated the diatom composition of surface sediment assemblages in sets of duckweed and non-duckweed-dominated ponds in Norfolk, eastern England. In addition, we undertook diatom analysis of two cores from a small farmland pond (Bodham Rail Pit) subject to a known duckweed dominance event (1999–2005). Both L. hungarica and S. saugerresii were significant predictors of past Lemna dominance in the surface sediments. Further, in the core study, both diatom species accurately and closely tracked the documented “on–off” duckweed cycle. Our study suggests huge potential for using ponds in palaeoecological studies and for diatom-based investigations of floating plant histories.  相似文献   

10.
In recent years, global reanalysis weather data has been widely used in hydrological modeling around the world, but the results of simulations vary greatly. To consider the applicability of Climate Forecast System Reanalysis (CFSR) data in the hydrologic simulation of watersheds, the Bahe River Basin was used as a case study. Two types of weather data (conventional weather data and CFSR weather data) were considered to establish a Soil and Water Assessment Tool (SWAT) model, which was used to simulate runoff from 2001 to 2012 in the basin at annual and monthly scales. The effect of both datasets on the simulation was assessed using regression analysis, Nash-Sutcliffe Efficiency (NSE), and Percent Bias (PBIAS). A CFSR weather data correction method was proposed. The main results were as follows. (1) The CFSR climate data was applicable for hydrologic simulation in the Bahe River Basin (R 2 of the simulated results above 0.50, NSE above 0.33, and |PBIAS| below 14.8. Although the quality of the CFSR weather data is not perfect, it achieved a satisfactory hydrological simulation after rainfall data correction. (2) The simulated streamflow using the CFSR data was higher than the observed streamflow, which was likely because the estimation of daily rainfall data by CFSR weather data resulted in more rainy days and stronger rainfall intensity than was actually observed. Therefore, the data simulated a higher base flow and flood peak discharge in terms of the water balance, except for some individual years. (3) The relation between the CFSR rainfall data (x) and the observed rainfall data (y) could be represented by a power exponent equation: y=1.4789x 0.8875 (R 2=0.98, P<0.001). There was a slight variation between the fitted equations for each station. The equation provides a theoretical basis for the correction of CFSR rainfall data.  相似文献   

11.
Rainfall erosivity is an important climatic factor for predicting soil loss. Through the application of high-resolution pluviograph data at 5 stations in Huangshan City, Anhui Province, China, we analyzed the performance of a modified Richardson model that incorporated the seasonal variations in parameters α and β. The results showed that (1) moderate to high seasonality was presented in the distribution of erosive rainfall, and the seasonality of rainfall erosivity was even stronger; (2) seasonal variations were demonstrated in both parameters α and β of the Richardson model; and (3) incorporating and coordinating the seasonality of parameters α and β greatly improved the predictions at the monthly scale. This newly modified model is therefore highly recommended when monthly rainfall erosivity is required, such as, in planning soil and water conservation practices and calculating the cover-management factor in the Universal Soil Loss Equation (USLE) and Revised Universal Soil Loss Equation (RUSLE).  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.
Fragilarioid diatom taxa are often deemed ubiquitous in shallow lake systems. Their presence has been described as contributing to statistical noise in paleolimnological studies of cold-temperate lakes. In shallow, warm-temperate lakes of Florida, long-term transitions from assemblages dominated by Aulacoseira spp. to fragilarioid taxa, particularly Pseudostaurosira brevistriata, Staurosira construens var. venter, and Staurosirella pinnata, often occur. Distinctly higher limnetic nutrient optima are demonstrated by these fragilarioid taxa than by planktonic Aulacoseira spp. Community successions occur during eutrophication, and progressive replacement of Aulacoseira spp. and other planktonic taxa by fragilarioid taxa is concurrent with and apparently related to the onset of cyanobacterial dominance. We examine successions from Aulacoseira-dominated to fragilarioid-dominated assemblages in sediment cores from subtropical Florida lakes that have undergone eutrophication. Diatom profiles are compared with sedimented pigments, nitrogen stable isotopes of organic matter, and with silica accumulation rates. These study lakes have little if any macrophyte presence. Their light-extinction depths are extremely shallow, yet diatom communities are dominated by bottom-dwelling rather than planktonic taxa. Frequent wind-generated mixing, sometimes to lake bottoms, is sufficient to sustain the light needs of benthic and tychoplanktonic taxa. We conclude that assemblage changes generally are not caused by reduced water depths, silica limitation, nor increased incipient stratification, but that cyanobacteria are responsible for reducing planktonic Aulacoseira in favor of fragilarioid taxa. Cyanobacteria blooms persist over a wide seasonal range because of warm climate and high limnetic nutrient concentrations in Florida lakes. Cyanobacteria progressively displace and outcompete Aulacoseira and other planktonic taxa as eutrophication proceeds. Reduced light availability, changes in mineral/nutrient availability, and other aspects of competitive exclusion, such as cyanobacterial allelotoxins, might contribute to observed changes. Climate warming is not likely to account for Aulacoseira reduction as in colder regions because it is less pronounced in this subtropical district. Lakes with low nutrient levels and less cyanobacteria still sustain large Aulacoseira populations, and decreases in limnetic nutrients sometimes lead to the return of planktonic Aulacoseira. Rather than simply representing statistical noise for paleolimnological reconstructions, shifts to certain fragilarioid taxa indicate when subtropical Florida lakes progressed to hypereutrophic conditions that were marked by cyanobacterial proliferation.  相似文献   

14.
Eutrophication, prompted by anthropogenic activities and climate change has led to multiple adverse effects in freshwater systems across the world. As instrumental measurements are typically short, lake sediment proxies of aquatic primary productivity (PP) are often used to extend the observational record of eutrophication back in time. Sedimentary pigments provide specific information on PP and major algal communities, but the records are often limited in the temporal resolution. Hyperspectral imaging (HSI) data, in contrast, provide very high seasonal (sub-varve-scale) resolution, but the pigment speciation is limited. Here, we explore a combined approach on varved sediments from the Ponte Tresa basin, southern Switzerland, taking the advantages of both methods (HSI and high performance liquid chromatography, HPLC) with the goal to reconstruct the recent eutrophication history at seasonal to interannual resolution. We propose a modified scheme for the calibration of HSI data (here: Relative Absorption Band Depth between 590 and 730 nm RABD590–730) and HPLC-inferred pigment concentrations (here: ‘green pigments’ {chlorophyll a and pheophytin a}) and present a calibration model (R2?=?0.82; RMSEP?~?12%). The calibration range covers >?98% of the spectral index values of all individual pixels (68 µm?×?68 µm) in the sediment core. This allows us to identify and quantify extreme pigment concentrations related to individual major algal blooms, to identify multiple algal blooms within one season, and to assess interannual variability of PP. Prior to the 1930s, ‘green pigment’ concentrations and fluxes (~?50 µg g?1;?~?2 µg cm?2a?1, chlorophyll a and pheophytin a) and interannual variability was very low. From the 1930s to 1964, chlorophyll a and pheophytin a increased by a factor of ~?4, and ββ-carotene appeared in substantial amounts (~?0.4 µg cm?2a?1). Interannual variability increased markedly and a first strong algal bloom with ‘green pigment’ concentrations as high as 700 µg g?1 is observed in 1958. Peak eutrophication (~?12 µg cm?2a?1 chlorophyll a and pheophytin a) and very high interannual variability with extreme algal blooms (‘green pigment’ concentrations up to 1400 µg g?1) is observed until ca. 1990, when eutrophication decreases slightly. Maximum PP values after 2009 are likely the result of internal nutrient cycling related to repeated deep mixing of the lake.  相似文献   

15.
Changes in macrophyte communities have occurred over the past decades in many oligotrophic softwater lakes with low carbon availability. Slow-growing isoetid species have been replaced by faster-growing elodeid species. Commonly, these changes are explained by anthropogenic nutrient enrichment or acidification of the lake water. Here we present a multi-proxy study in which we analysed plant macrofossils, pollen and spores, as well as sedimentological data from several cores taken from a SW Norwegian softwater lake. Our results indicate that the elodeid macrophyte Callitriche hamulata first appeared in this lake in the 1970s. Proliferation of C. hamulata occurred in the 1990s, replacing the hitherto dominant submerged Isoëtes macrophyte vegetation. Independent lines of evidence, such as diatom-inferred TP and pH reconstructions, showed no change during the past 200 years, therefore ruling out both acidification and phosphorus enrichment of the lake as possible causes for the observed change in the macrophyte community. Alternatively, expansion of Callitriche at the expense of Isoëtes may have been related to increased aquatic carbon availability, although nitrogen enrichment may also have been important.  相似文献   

16.
Plant macrofossils and pollen were analyzed from sediment cores to identify long-term changes in the aquatic plant community of Jones Creek, Thousand Islands National Park, Ontario, Canada. Six sediment cores were recovered from Jones Creek in February 2014. One complete core and five top/bottom samples were analyzed for plant macrofossil abundance and diversity. Sediment analysis and 210Pb dating confirmed a productive wetland throughout the core, dating back beyond 1883 AD. Jones Creek is currently dominated by thick stands of cattails, particularly the hybrid white cattail (Typha x glauca Godr). The relative abundance of Typha pollen began to increase in the late nineteenth and early twentieth century, reaching a relative abundance of nearly 40% in the modern day surface sediment. Common macrofossils recovered from the sediment record included seeds of Carex, Schoenoplectus, Najas, and Eleocharis. There is evidence that community composition, as recorded by the macrofossil record, has shifted in Jones Creek in response to human activities. In particular there has been a reduction in sedge species between historical and present day conditions as the wetland shifted from a sedge dominated wet-meadow wetland to a cattail dominated system. The results of our study indicate that future restoration efforts should be directed towards reintroducing native sedge species that were present prior to major changes in land-use that occurred in the St. Lawrence region throughout the nineteenth and twentieth centuries.  相似文献   

17.
To evaluate the comparability of neo- and paleo-limnology, we made year-to-year as well as seasonal comparisons of contemporary zooplankton data and cladoceran remains in thick (9–42 mm) annual laminations in sediment of Lake Vesijärvi, southern Finland. We calculated the expected annual exuviae production of nine planktonic taxa in the water column using contemporary zooplankton records, and compared the value to the observed net accumulation of their remains in deep sediment for 7 years. Although all of the species studied occur commonly in the lake pelagic zone, deposition of remains differed significantly among taxa. The observed accumulation of three Bosmina species and Chydorus sphaericus was similar to or exceeded expected values, suggesting good preservation of their remains as well as focusing of sediment into the deepest part of the lake. The accumulation of Limnosida and Leptodora remains exceeded expected values several fold, suggesting under-representation of these species in pelagic plankton samples, as well as efficient transportation and spatial averaging of their remains in the sediment. Daphnia, Diaphanosoma, and Ceriodaphnia were clearly under-represented in the sediment due to poor preservation of their remains and use of a 50-μm sieve in sediment processing. Thus, sieving should be taken into account as a possible source of bias. Correlation between relative abundances of all species in the sediment and in the plankton was weak (r s = 0.54, P < 0.001), but for well-preserved species, the correlation was strong (r s = 0.91, P < 0.001). Inter-annual variation in the deposition of remains suggested that resuspension and sediment focusing may vary between years, thus making it difficult to interpret absolute abundances, even though the deepest part of the basin shows clear varve formation and seasonality is well recorded. Detailed study of the uppermost lamination showed the seasonal succession of a cladoceran community within an annual varve as well as differences in the seasonality and intensity of ephippia production among the species. We recommend that results of cladoceran analyses be expressed in several ways (relative abundances, per unit dry weight, per unit organic matter, and as net accumulation values) before drawing final conclusions, as each approach may reveal a different aspect of the deposition process. In addition, sedimentation differences between epilimnetic and hypolimnetic species should be considered in stratified lakes.  相似文献   

18.
In North America, land use practices of the last two centuries have strongly influenced aquatic communities and freshwater quality, but the impacts of prehistoric land use on freshwater resources remain poorly documented. Here we investigate the influence of prehistoric and historical land use on Horseshoe Lake, Illinois, USA, an oxbow lake in a floodplain of the Mississippi River that is adjacent to Cahokia, the largest prehistoric indigenous population center north of Mexico. Diatom assemblages from Horseshoe Lake’s sedimentary record track shifts in aquatic environmental conditions over the last ca. 1600 years. During the period of prehistoric population growth and agricultural intensification associated with Cahokia’s emergence (ca. 600–1200 CE), the relative abundance of Aulacoseira granulata—a planktonic diatom associated with shallow eutrophic lakes—increased. Following the abandonment of Cahokia in the 14th century CE, the diatom flora of the lake shifted from planktonic Aulacoseira taxa to the epiphytic taxa Cocconeis and Gomphonema. This shift in diatom assemblages is consistent with a reduction of nutrient inputs to the lake and/or reduced fishing pressure as prehistoric populations abandoned the area. Following the intensification of historic settlement after 1800 CE, diatom assemblages shift to epipelic species of small Staurosira and Fragilaria, indicating a reduction in aquatic macrophytes and increased turbidity. Our results document prehistoric indigenous impacts on a freshwater system beginning nearly 1000 years before European colonization of the Americas and demonstrate the antiquity of human impacts on freshwater resources in North America.  相似文献   

19.
太湖平原区河网结构变化对调蓄能力的影响   总被引：4，自引：2，他引：2  
城镇化快速发展导致河网水系萎缩,连通性受阻,河网调蓄能力降低,加大了区域洪灾风险和洪涝损失.基于1960s,1980s,2010s的1:5万地形图水系及三个控制站的水位数据,选取河网密度(Rd),水面率(WP),盒维数(D),水位Hurst指数(H),分析了近50年来太湖流域武澄锡虞地区的水系演变与河网调蓄能力变化;并借助代表性单元流域概念,以水位Hurst指数来表征河网调蓄能力,从河网调蓄的物理机理出发,探讨了河网调蓄与河网结构参数的关系.结果表明:① 受快速城镇化影响,1960s-2010s武澄锡虞地区河网密度,水面率及盒维数分别下降了18.87%,24.59%,7.60%,且1980s至2010s各水系指标的衰减率均高于1960s-1980s;② 1960-2010年,研究区年均及汛期的水位Hurst指数呈下降趋势,其所反映的河网调蓄能力与该地区河网水系各指标变化同步,这表明城镇化引起的河流水系衰减导致河网调蓄能力发生明显下降,非汛期水位Hurst指数先增加后降低,表现出一定的不确定性;③ 基于REW概念与水位Hurst指数值,初步构建了河网结构参数与调蓄能力的关系式,并对关系式进行了验证,基本可行,可在一定程度上揭示城镇化下水系变化对河网调蓄能力的影响,从而为评价平原水网区河网调蓄能力提供了一种相对简便的方法.  相似文献   

20.
We reconstructed the paleohydrologic and climatic history of the Lake Neor region, NW Iran, from the end of the late glacial to the middle Holocene (15,500–7500 cal yr BP). Subfossil chironomid and pollen assemblages in a sediment core from a peatland located south of Lake Neor enabled identification of four main hydrologic phases. The period 15,500–12,700 cal yr BP was characterized by a relatively dry climate with an open landscape, suggested by the abundance of Irano-Turanian steppe plants (e.g. Amaranthaceae, Artemisia and Cousinia). Dominance of several shallow-water and semi-terrestrial chironomid taxa (e.g. Pseudosmittia, Smittia/Parasmittia and Paraphaenocladius/Parametriocnemus) during this period is indicative of lower water tables in the wetland. Between 12,700 and 11,300 cal yr BP, chironomid taxa indicate higher wetland water tables, as suggested by the presence of Zavrelia, Chironomus anthracinus/plumosus-type and Micropsectra, which are inhabitants of open-water, lacustrine areas. The open-steppe vegetation remained dominant in the watershed during this time. Increasing wetland moisture could be explained by: (1) cool summers that reduced the evaporation rate; and/or (2) a decrease in duration of the summer dry season. The period 11,300–8700 cal yr BP was characterized by lower wetland moisture, contemporaneous with a delay in the expansion of deciduous forest, suggesting persistent dry climate conditions throughout the beginning of the Holocene, which may have been related to the intensified seasonality of precipitation. Around 8700 cal yr BP, higher wetland water levels, inferred from chironomids, occurred simultaneously with the onset of regional deciduous forest expansion, probably caused by a shortening of the summer dry period. We concluded that chironomids are appropriate paleoecological proxies to investigate global and local hydrologic variability in the Middle East.  相似文献   

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