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1.
Biogeomorphology adds the element “biological dynamics” (of populations or communities) to chemical and physical geomorphic factors and thus complicates the framework of geomorphic processes. Such biological complications of the animal-induced transport of solids in streams should be particularly important in crayfish, as crayfish affect this transport through their overall activity and intraspecific aggression levels, which could be modified by shelter availability or the establishment of dominance hierarchies among individuals not knowing each other. Using experimental streams, we tested these hypotheses by measuring how shelter availability or residential crayfish group invasion by unknown individuals affected the impact of the crayfish Orconectes limosus on the (i) transport of gravel at baseflow (during 12 experimental days); (ii) sediment surface characteristics (after 12 days); and (iii) critical shear stress causing incipient gravel motion during simulated floods (after 12 days). The two potentially important factors shelter availability or residential group invasion negligibly affected the crayfish impact on gravel sediments, suggesting that habitat unfamiliarity (a third potentially important factor affecting crayfish activity) should increase the crayfish-induced sediment transport. Because habitat unfamiliarity is associated with sporadic long-distance migrations of a few crayfish individuals, this third factor should play a minor role in real streams, where crayfish biomass should be a key factor in relations with crayfish effects on sediments. Therefore, we combined the results of this study with those of previous crayfish experiments to assess how crayfish biomass could serve in modelling the gravel transport. Crayfish biomass explained 47% of the variability in the baseflow gravel transport and, in combination with the coefficient of variation of the bed elevation and algal cover, 72% of the variability in the critical gravel shear stress. These results encourage more research on the topic, as an increasing number of eliminations of abiotic and biotic factors that could complicate the animal-induced sediment transport in streams would facilitate the use of biological variables (e.g., bioturbator biomass) in future modelling of the transport of solids.  相似文献   

2.
Radical grain size changes between two main units of a sedimentary megacycle in a foreland basin are commonly interpreted to result from changes in tectonic activity or climate in the adjacent mountain range. In central Nepal, the Cenozoic Siwalik molasse deposits exposed in the frontal Himalayan folds are characterized by such a radical grain size transition. Locally gravel deposits completely replace sands in vertical succession over approximately a hundred metres, the median grain size (D50) displaying a sharp increase by a factor of ca. 100. Such a rapid gravel‐sand transition (GST) is also observed in present‐day river channels about 8–20 km downstream from the outlet of the Siwalik Range. The passage from gravel‐bed channel reaches (proximal alluvial fans) to sand‐bed channel reaches (distal alluvial fans) occurs within a few kilometres on the Gangetic Plain in central Nepal, and the D50 ratio between the two types of channels equals ca. 100. We propose that the dramatic and remarkably similar increase in grain size observed in the Neogene Siwalik series and along modern rivers in the Gangetic foreland basin, results from a similar hydraulic process, i.e. a grain sorting process during the selective deposition of the sediment load. The sudden appearance of gravels in the upper Siwalik series would be related to the crossing of this sorting transition during progressive southward migration of the gravel front, in response to continuous Himalayan orogen construction. And as a consequence, the GST would be diachronous by nature. This study demonstrates that an abrupt change in grain size does not necessarily relate to a change in tectonic or climatic forcing, but can simply arise from internal adjustment of the piedmont rivers to the deposition and run out of coarse bedload. It illustrates, in addition, the genesis of quartz‐rich conglomerates in the Himalayan foreland through gravel selective deposition associated with differential weathering, abrasion processes and sediment recycling during thrust wedge advance and shortening of the foreland basin.  相似文献   

3.
The characteristics of foredunes created in a municipal management program on a developed barrier island are evaluated to identify how landforms used as protection structures can be natural in appearance and function yet compatible with human values. Shoreline management zones include a naturally evolving, undeveloped segment; a noneroding, developed segment; eroding and noneroding segments of an “improved beach” where dunes have been built by artificial nourishment; and a privately built, artificially nourished dune on the shoreline of an inlet.A disastrous storm in 1962 resulted in an aggressive program for building dunes using sand fences, vegetation plantings, purchase of undeveloped lots, and sediment backpassing to maintain beach widths and dune elevations. The present nourished and shaped foredune in the improved beach is higher, wider, and closer to the berm crest than the natural dune. Restricted inputs of aeolian sand keep the surface flat and poorly vegetated. A stable section of this engineered shore has a wider beach, and sand fences have created a higher foredune with greater topographic diversity. The cross shore zonation of vegetation here is more typical of natural dunes, but the environmental gradient is much narrower. The privately built dune is low, narrow, and located where it could not be created naturally. Foreshore and aeolian sediments in the undeveloped segment and the improved beach are similar in mean grain size (0.16–0.21 mm) and sorting (0.31–0.39φ), but sediment on the surface of the nourished dune is coarser (28.1% gravel) with a more poorly sorted sand fraction (1.30φ) representing lag elements on the deflation surface.Willingness to enhance beaches and dunes for protection has reduced insurance premiums and allowed the municipality to qualify for funds from the Federal Emergency Management Agency (FEMA) to replace lost sediment, thus placing an economic value on dunes. Success of the management program is attributed to: (i) timing property-purchase and dune-building programs to periods immediately after storms (causing residents to accept high dunes that restrict access or views); (ii) instituting a vigorous education program (reminding residents of hazards during nonstorm periods); (iii) maintaining control over local sediment supplies (to keep pace with erosion and create new shoreline environments); (iv) investing private and municipal economic resources in landforms (qualifying them for external funds for replacement); and (v) maintaining, augmenting, or simply tolerating biodiversity and natural processes (retaining a natural heritage).  相似文献   

4.
中国北方农牧交错带鄂尔多斯高原段土壤表层粒度特征   总被引：1，自引：0，他引：1  
土壤粒度是描述土壤性质的重要参数,研究农牧交错带土壤表层粒度对土壤质量和沙化程度评价具有科学意义。对不同沉积物类型及土地利用方式土壤表层(0~5 cm深度)粒度进行分析。结果表明:(1)沙黄土、覆沙黄土残积物、风化残积物中粉粒最多,其次为极细砂和细砂,这3个粒级含量之和表现为沙黄土(96.51%)>覆沙黄土残积物(88.29%)>风化残积物(77.58%);风成沙土壤表层以细砂(53.85%)和中砂(26.13%)为主。(2)平均粒径由小到大依次为沙黄土(4.78Φ)、覆沙黄土残积物(4.62Φ)、风化残积物(3.80Φ)、风成沙(2.46Φ);分选性由差到好依次为风化残积物、覆沙黄土残积物、沙黄土、风成沙;偏度呈现为正偏或极正偏;峰值表明风成沙(2.30)粒径分布最为集中。(3)同一地表沉积物中,砾石和极粗砂等粗颗粒在农田较多,粉粒在草地较多,极细砂在林地较多;风成沙中,粉粒在固定沙丘略多,细砂在半固定沙丘较多,砾石、极粗砂等粗颗粒仅在流动沙丘地中分布。(4)土壤颗粒分形维数表现为沙黄土(2.5242)>覆沙黄土残积物(2.4373)>风化残积物(2.3554)>风成沙(2.2815);地表沉积物类型不同,表层土壤分形维数与粒级含量相关性有着明显差异。  相似文献   

5.
Over the past two decades there has been a growing interest in the geomorphological mosaic along large floodplain rivers where channel dynamics are seen to drive habitat-patch creation and turnover and to contribute to high biological diversity. This has required a new perspective on fluvial geomorphology that focuses on biological scales of space and time. This study examines the spatial pattern of surface fine sediment accumulations along a reach of a large gravel-bed river, the Tagliamento River in NE Italy; an area with a moist Mediterranean climate and seasonal flow regime. The study investigates changes in sediment characteristics during the summer low-flow period between April and September. Focussing on five areas representing a gradient from open, bar-braided to wooded island-braided morphologies, the paper demonstrates the importance of riparian vegetation and aeolian–fluvial interactions.Significant contrasts in particle size distributions and organic content of freshly deposited sand and finer sediments were found between sampling areas, geomorphological settings, and sampling dates. In particular, wooded floodplain and established islands supported consistently finer sediment deposits than both open bar surfaces and the lee of pioneer islands, and in September significantly finer sediments were also found in deposits located in the lee of pioneer islands than on open bar surfaces. Overall, the September samples had a greater variability in particle size characteristics than those obtained from the same sites in April, with a general coarsening of the D5 (φ) (i.e., the coarse tail of the particle size distribution). Also in September, crusts of fine sediment (30 μm < D50 < 64 μm) had formed on the surface of some of the open bar and pioneer island deposits within the more open sampling areas along the study reach. These crusts possessed similar particle size characteristics to aeolian crusts found in more arid environments. They were significantly finer than April samples and September subcrust samples obtained from the same sites and had similar particle size characteristics to some samples taken from wooded floodplain, established island surfaces and the lee of pioneer islands that were not crusted.Local climatological and river level data confirm significant wind and rainfall events during a period of consistently low river levels between the April and September sampling periods. These support deflation, deposition and rain wash of finer sediment during the summer, with windblown sediments being deposited on bar surfaces and in the lee of pioneer islands where wood and young trees provide foci for accelerated sedimentation and island growth as well as on marginal floodplains and established islands. We conclude that along braided rivers in moist settings but with a distinct dry season, aeolian reworking of sediment deposits may have a more important role in driving habitat dynamics than previously considered.  相似文献   

6.
A palynological investigation was conducted on two cores with Holocene sediments collected from the northeastern littoral part of the border Lake Doirani in northern Greece. The radiocarbon dates indicated that the analyzed sediments accumulated during the last 5000 yrs. The pollen-stratigraphic record revealed the environmental changes in the catchment area, starting from a natural undisturbed landscape to one modified by increasing anthropogenic influences. The tree vegetation dominated by Quercus woods in the lowlands and byPinus, Abies, and Fagus at higher altitudes, lasted for the period 2900 - 830 cal. B.C. Subsequently it was replaced by xerothermic herb and tree vegetation as a result of intensive human activity - and farming and stock-breeding. The accumulation of sediments with more sand and gravel in historical time was the result of increased erosion.  相似文献   

7.
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   

8.
Frequent and serious aeolian disasters occur in the upper and middle reaches of the Yarlung Zangbo River, which runs through the high-elevation Tibet Plateau. Sediment geochemical characteristics can be used as a proxy to identify the sediment's provenance. To determine the provenance of aeolian sediments in the river's basin, we analysed major and trace element contents from surface samples and local clastic rocks throughout the basin. We found that the major and trace elements differed between the middle reaches, upper reaches and regions south of the river. Major element contents were similar in the upper and middle reaches, but trace elements differed. Al2O3, MgO and Na2O concentrations increased from the upper reaches to the lower reaches, and in the lower reaches, MgO and Na2O were enriched compared with the crustal average. The similarities between samples in the lower part of the upper reaches and those in the middle reaches indicated that sediment transported by fluvial systems from the upper reaches were first deposited in the wide valleys near Xigaze, where they formed a large area of shifting sand. These deposits were then transported by the wind to the river's middle reaches, where they formed a large area of shifting sand. When we compared aeolian sediment in the middle reaches with the local clastic rocks, they appeared to be unrelated. The difference between sediments south of the river and those in the middle reaches means that the southern sediments were not transported to the middle reaches. Therefore, the aeolian sediment in the middle reaches of the Yarlung Zangbo River mainly came from the lower part of the upper reaches, not from the local clastic rocks.  相似文献   

9.
Wind erosion has major impacts on dune growth, desertification, and architecture on sea coasts. The deflation threshold shear velocity is a crucial parameter in predicting erosion, and surface moisture greatly affects this threshold and thus sand stability. Wind tunnel studies have shown that reduced moisture contents decrease entrainment thresholds and increase wind erosion, but field and wind tunnel test data is lacking for tropical humid coastal areas. In this study, we investigated the influence of surface moisture contents (at 1 mm depth) on sand entrainment and erosion using tropical humid coastal sands from southern China. Shear velocities were deduced from velocity profiles above the sand. The threshold shear velocity increased linearly with increasing ln100M (M, gravimetric moisture content). The increase was steepest below a moisture content of 0.0124 (i.e., at M1.5, the moisture content in the sand at a matric potential of − 1.5 MPa). We compared several popular models that predict threshold shear velocity of moisture sediment, and found substantial differences between their predicted results. At a surface moisture content of 0.0124, the predicted increase in the wet threshold shear velocity compared with the dry threshold shear velocity ranged from 34% to 195%. The empirical model of Chepil and Selah simulated the data well for M < 0.0062 (i.e., 0.5M1.5), whereas Belly's empirical model simulated the data best for M > 0.0062. Wind erosion modulus increased with increasing effective wind velocity following a power function with a positive exponent at all moisture contents, but decreased with increasing surface moisture content following a power function with a negative exponent. When wind speed and moisture content varied simultaneously, wind erosion modulus was proportional to the 0.73 power of effective wind velocity, but inversely proportional to the 1.48 power of M. The increase in resistance to erosion at low moisture contents probably results from cohesive forces in the water films surrounding the sand particles. At a moisture content near M1.5, wind erosion ceases nearly for all wind velocities that we tested.  相似文献   

10.
通过对独库公路三岔河道班沟等处泥石流扇形地上不同形态的泥石流堆积、冰川堆积和岩屑坡的沉积砾石组构对比研究,发现在不同沉积相中,砾石ab面的组构特点互不相同,都随其不同的沉积动力和地形等沉积条件的差别而有别,所以利用砾石组构这一研究方法,不但可划分不同的沉积相,而且在第四纪地质与沉积学研究中具有重要的意义。  相似文献   

11.
1 Introduction The sand deserts in China are the product of long-term wind erosion from clastic sediments and remodified by wind in the arid environments (Wu, 1987; Gao et al., 1993). The origin of aeolian sand is an elementary issue in studying morpholog…  相似文献   

12.
Groundwater dominance has important effects on the hydrological and geomorphological characteristics of river systems. Low suspended sediment concentrations and high water clarity are expected because significant inputs of sediment-free spring water dilute the suspended sediment generated by storms. However, in many Mediterranean rivers, groundwater dominance is characterised by seasonal alternations of influent and effluent discharge involving significant variability on the sediment transport regimes. Such areas are often subject to soil and water conservation practices over the centuries that have reduced the sediment contribution from agricultural fields and favour subsurface flow to rivers. Moreover, urbanisation during the twentieth century has changed the catchment hydrology and altered basic river processes due to its ‘flashy’ regime. In this context, we monitored suspended sediment fluxes during a two-year period in the Na Borges River, a lowland agricultural catchment (319 km2) on the island of Mallorca (Balearic Islands). The suspended sediment concentration (SSC) was lower when the base flow index (i.e., relative proportion of baseflow compared to stormflow, BFI) was higher. Therefore, strong seasonal contrasts explain the high SSC coefficient of variation, which is clearly related to dilution effects associated with different groundwater and surface water seasonal interactions. A lack of correlation in the Q-SSC rating curves shows that factors other than discharge control sediment transport. As a result, at the event scale, multiple regressions illustrate that groundwater and surface water interactions are involved in the sedimentary response of flood events. In the winter, the stability of baseflow driven by groundwater contributions and agricultural and urban spills causes hydraulic variables (i.e., maximum discharge) to exert the most important control on events, whereas in the summer, it is necessary to accumulate important volumes of rainfall, creating a minimum of wet conditions in the catchment to activate hydrological pathways and deliver sediment to the drainage network. The BFI is also related to sediment delivery processes, as the loads are higher with lower BFI, corroborating the fact that most sediment movement is caused by stormflow and its related factors. Overall, suspended sediment yields were very low (i.e., < 1 t km− 2 yr− 1) at all measuring sites. Such values are the consequence of the limited sediment delivery attributable to soil conservation practices, low surface runoff coefficients and specific geomorphic features of groundwater-dominated rivers, such as low drainage density, low gradient, steep valley walls and flat valley floors.  相似文献   

13.
We have measured the distribution coefficient (Kd) of210Po and210Pb in laboratory systems and in natural freshwater systems. In the laboratory systems, an inverse relationship was observed between the particle concentration of sand or lake sediment, and the distribution coefficients of210Po and210Pb. The slope of the log-linearK d vs particle concentration relation is consistent with existingK d-particle concentration theories. These laboratory observations are consistent with similar measurements in two lakes. TheK d values of Po and Pb for the bottom sediment-pore water system with a high particle concentration were 10 to 100 times lower than those for dilute concentrations of particles suspended in the lake water. TheK d of210Pb in the sediments was >104 so that the diffusive transport of210Pb has only a small influence on the interpretation of210Pb concentration-depth profiles and the210Pb dating of these sediments.This is the second of a series of papers to be published by this journal following the 20th anniversary of the first application of210Pb dating of lake sediments. Dr P. G. Appleby is guest editing.  相似文献   

14.
Rapid urbanization around Palm Springs, California has encroached upon the sand habitats of the endangered Coachella Valley fringetoed lizard (Uma inornata). The Nature Conservancy and the Coachella Valley Preserve System are concerned about maintaining an adequate sediment supply to the dune environments that urban growth might interrupt. Determining the source of the existing aeolian sediments is a critical first step toward ensuring a perpetual sediment supply. Geochemical links between the aeolian sediment sources and the Coachella Valley Preserve dunes were discovered by mapping the aeolian sand distribution and analyzing trace elements using multivariate statistical techniques.  相似文献   

15.
Tao Tang   《Geomorphology》2003,49(3-4):231-254
Dissolution of extensive outcrops of limestone and dolostone in humid tropical and subtropical southern China produced numerous caves and residual hills that are referred as tower karst. This study identifies and relates the physical and chemical characteristics of the surface sediment with the limestone bedrock in Guilin to assess the influence of the limestone dissolution process on sediment composition.The results of this study indicated that (i) both limestone and dolostone of the region are very pure (99.5% and 98.5% of CaCO3 and MgCO3, respectively); (ii) the material composition of limestone and dolostone is different from that of soil and sediment of the region: constituents of surface sediments are highly related with the clastic sedimentary rocks, such as the mudstone, but show negative correlation with limestone and dolostone; (iii) the limestone formations are highly resistant to physical weathering and disintegration; their durability versus physical weathering and their high susceptibility to chemical dissolution account for why residual towers can form and persist; (iv) a dual-zone environmental structure exists vertically downward from the surface in Guilin: the zone of unconsolidated clastic sediments that is predominantly acidic, and the zone of karstified limestone that is predominantly basic. The evidence suggests that the environment and processes differ in these two zones. The chemical dissolution of limestone that formed tower karst of the region is not mainly responsible for the accumulation of clastic sediment on the surface.  相似文献   

16.
The downstream fining of fluvial sediments is a fundamental tenet of drainage systems and, for decades, has been the subject of considerable research. Most of this research has focused on variability in channel-bed material. Other sedimentological components such as channel bars and banks, however, represent distinctively different processes occurring at various flow magnitudes and durations and thus provide an opportunity to examine a more comprehensive set of controls on the larger fluvial system. This study analyses downstream patterns of sediment size and composition for channel-bed material, bars, and banks in the Llano River watershed (11,568 km2) in central Texas, USA.Fluvial deposits in the study area were characterized through field, laboratory, and statistical analyses and standard sedimentary indices (d16, d50, d84, sorting) were computed. Two hundred thirty-eight sediment samples were collected at 15 sites along the main-stem channel with sampling occurring at the low-flow channel (thalweg), lateral bars, banks, and overbank locations. Channel-bar deposits are characterized by a downstream reduction in particle size, but low-flow-channel deposits have a substantially weaker trend, a discrepancy possibly attributed to uniformity and continuity of hydraulic sorting mechanisms during moderate and high flows. Channel-bar deposits reveal an abrupt downstream reduction in gravel size in the upper watershed, which is attributed to an increase in drainage area. Further, an abrupt gravel-to-sand transition occurs immediately downstream of a distinct lithologic change from mostly carbonate rocks to igneous and metamorphic rocks. The downstream decrease in channel-bar particle size occurs despite an increasingly constricted alluvial valley, commonly associated with greater unit stream power and relatively coarse sediment. Contrasting with channel-bed material, particle size of channel banks increases downstream, which is attributed to the addition of sand-sized sediment from igneous and metamorphic rocks. The consideration of distinctive sedimentological components of a dynamic fluvial system represents a more comprehensive and nuanced study of the topic of downstream sediment trends than prior studies, which is important to a range of engineering, biological, and planning issues at the watershed scale.  相似文献   

17.
谢轲  尹辉 《中国沙漠》2021,41(6):111-119
沉积物的粒度特征对于分析区域沉积动力环境有着重要意义。本文以额济纳盆地为研究区,以戈壁、河床和沙丘3个主要地貌单元采集的表层沉积物样本的粒度测量结果为数据源,基于粒度参数分析和端元建模分析,研究了沉积物的粒度特征、端元构成和反映的沉积动力环境。结果表明：（1）额济纳盆地表层沉积物粒度曲线具有单峰和多峰分布,其中戈壁和沙丘沉积物组成上以砂质为主,河床沉积物组成上以砂质和粉砂质为主。（2）表层沉积物可分解出4个主要端元,端元1到端元4的主要物质依次为粗粉砂、细砂、中砂和细砂混合物以及粗砂和中砂混合物。（3）区域环境特点分析表明,表层沉积物端元1主要代表了以流水为主的动力过程,端元2主要代表了风力作用和局部地形环境复合的过程,端元3和端元4主要是风力主导下的多外营力复合以及局部环境共同作用的结果。  相似文献   

18.
The sidewall effects of a wind tunnel on aeolian sand transport were investigated experimentally. A wind tunnel was used to conduct the experiments with a given channel height of 120 cm and varying widths (B) of 40, 60, 80, 100 and 120 cm. Both vertical profiles of wind velocity and sand mass flux were measured at different locations across the test section. The results show that the wind velocity with saltation first increases and then decreases to a minimum, from the sidewall to the central line of the wind tunnel. The discrepancy among wind velocities at different locations of the transverse section decreases with decreasing tunnel width. The wind friction velocity across the wind tunnel floor, with the exception of the region closest to the sidewalls, does not deviate strongly in wide wind tunnels from that along the central line, whereas it does vary in narrow tunnels. The sand mass fluxes, with the exception of some near-bed regions, are larger along the central line of the wind tunnel than they are at the quarter width location from the sidewall. Unlikely previously reported results, the dimensionless sand transport rate, Qg / (ρu3) (where Q is the total sand transport rate, g is the gravitational acceleration constant, ρ is the air density, and u is the wind friction velocity), first decreases and then increases with the dimensionless friction velocity, u / ut (where ut is the threshold friction velocity). The above differences may be attributed to the sidewall effects of the wind tunnel. A dimensionless parameter, FB = u / (gB)1/2, is defined to reflect the sidewall effects on aeolian sand transport. The flows with FB of 0.33 or less may be free from the sidewall effects of the wind tunnel and can ensure accurate saltation tunnel simulation.  相似文献   

19.
The investigations on the organic carbon （OC） of core sediments were carried out in Chongming east tidal fiat （CM） during Scirpus mariqueter growing stage （from April to December 2004） in Yangtze Estuary. The Yangtze River annually transports a runoff discharge of 30,000 m^3/s, carrying about 480 million tons of sediments to the estuarine and coastal area, which formed a great OC pool. In the sampling spots, seven quadrats of 50 cm × 50 cm and five sediments cores of 20 cm deep （40 cm deep in December） were randomly established in order to collect vegetations and core sediments samples during the low tide each month except November. After pretreatment, the core sediments were sieved and their OC contents were measured according to the potassium dichromate method. The results show that the higher surface sediment OC content in summer comes from allochthonous terrigenous particle settlements on the Chongming east middle tidal fiat S. mariqueter zone. In autumn and winter, the decomposing of the defoliated S. mariqueter increases the surface sediments OC content. Settling velocity, sediment temperature and S. mariqueter growth are the main factors that can control the sediment carbon content. Summer is the “carbon losing” period of the tidal fiat sediments, while from September, it changes into the “carbon accumulating” period of sediment OC pool because of the decomposing of dead S. mariqueter community in the sediments. From this alternation of “carbon losing” period and “carbon accumulating” period, we conclude that carbon in the OC pool of the middle tidal fiat S. mariqueter zone sediments mainly comes from the atmospheric carbon rooted by S. mariqueter photosynthesis.  相似文献   

20.
The zooplankton community structure in lakes is highly influenced by size-selective predation by fish, with small zooplankton species dominating at high predation pressure. Remains of cladocerans are preserved in the sediment and may be used to trace historical changes in fish predation. We determined how contemporary data on planktivorous fish were related to the size of Daphnia ephippia (dorsal length) in the surface sediment (0-1 cm) of 52 mainly shallow lakes with contrasting densities of fish and nutrients (TP: 0.002-0.60 mg P l-1). Density of fish expressed as catch per unit effort, in terms of numbers in multiple mesh-sized gill nets (CPUEn), decreased significantly with increasing mean size of ephippia. The relationship was improved by adding TP as an independent variable, now explaining 90% of the variation in CPUEn on the full data set covering lakes in Denmark, Greenland and New Zealand, and 78% if only data on Danish lakes were used. CPUE by weight of planktivorous fish and mean weight of Daphnia in the pelagial during summer were also related to ephippial size. By including contemporary data on established relationships between the sizes of egg-bearing female Daphnia and ephippia, we inferred changes in the CPUEn, mean size of ephippia-bearing Daphnia and summer mean body weight of Daphnia from ephippial size in four lakes during the past 1-2 centuries. In a hypertrophic lake subject to periodic fish kills, Daphnia mean body weight was high and CPUEn was low compared with those in two eutrophic lakes, while CPUEn was low and Daphnia body weight was high in the least eutrophic, clearwater lake. Estimated CPUEn and Daphnia mean weight in the surface sediment of these four lakes corresponded well with contemporary data. Only small changes in ephippial size with time were observed in the clearwater lake and in one of the lakes that had suffered early eutrophication, while major changes occurred in the two other lakes that had been subjected to a major increase in nutrient input or fish kills. We conclude that Daphnia ephippia preserved in the surface sediments of lakes may be a useful and efficient method to quantify the present-day abundance of planktivorous fish and Daphnia mean size. The method is particularly valid in surveys aimed to give a general picture of the fish stock and the ecological state in a set of lakes in a region rather than a precise estimate for a single lake. Though some evidence is provided, more work is needed to evaluate whether the equations are valid for hind-casting in down-core palaeoecological studies.  相似文献   

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