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1.
 Arid karst landscapes that have been degraded by human activities provide a challenge for rehabilitation and an opportunity to test ideas about the stability and resilience of limestone ecosystems. The Nullarbor Plain is the largest arid karst area in Australia (220 000 km2) and is divided into extensive closed karstic depressions separated by low rocky ridges, while the dominant vegetation is chenopod shrubland. Since European settlement there has been considerable change in the vegetation, with significant reduction in shrub and grass cover over large areas of the plain. These changes are related to a state and transition model of vegetation dynamics which incorporates climatic variability, fire history and grazing pressure from sheep, kangaroos and rabbits. A partial sediment budget using 137Cs inventories reveals local and regional patterns of soil redistribution within this arid karst landscape. Rehabilitation of eroded soil in pastoral lands has been accomplished at several sites but is labour intensive and vulnerable to climatic fluctuations. Given the low stock numbers, limited number of people involved, and poor economic returns, it would be sensible to make pastoral activities on the Nullarbor secondary to conservation priorities. This would necessitate a change in land ethic to stewardship, with emphasis on rehabilitation and control of feral animals. Management of increased numbers of visitors to the caves and karst also requires that resource inventories and management plans for each area be drawn up and used. Received: 1 June 1995 · Accepted: 4 December 1995  相似文献   

2.
Environmental change in NW Iberia between 7000 and 500 cal BC   总被引：1，自引：0，他引：1  
We review research done on environmental changes in northwest (NW) Iberia spanning from the beginning to the late Holocene (7000–500 cal. BC). The type of archives (peat bogs, lake sediments, colluvium, soils, etc.) and proxies (pollen, element concentrations, isotopes, etc.) that were used to reconstruct changes on climate, soils, vegetation and atmospheric metal pollution are briefly described. Then we synthesize what the records suggest about the ecological history of NW Iberia. We identified four main phases: 7000–5000, 5000–3000, 3000–1500 and 1500-500 cal. BC. Each phase is determined by a set of environmental conditions, a combination of changes in climate, vegetation, soils and human impact. Human activities seem to have been involved in landscape changes in NW Iberia since at least 5000 cal. BC, with an increasing degree of anthropisation through time, which accelerated by 1500 cal. BC. The interaction between human activities and natural changes expressed as modifications in the vegetation cover, the elimination of the soil resources in many areas and its concentration in more localized, control-demanding sectors, as well as a progressive acidification and pollution of continental ecosystems. To a great extent, the present landscape in NW Iberia is the end product of these complex interactions, a cultural landscape.  相似文献   

3.
The relief of high limestone plateaus of the Transcaspian region (Ustyurt, Steppe Mangyshlak, and Krasnovodsk) is leveled and primarily tectonic: ridges and depressions (tens and hundreds of kilometers in length and tens, rarely a few hundred meters in amplitude) are composed of gentle (10–40) positive and negative deformations of the Neogene cover. The plateaus are bounded by ledges (chinoks). Against the background of the gentle hilly topography, one can observe smaller but sufficiently long (tens of kilometers) groups of arc-shaped parallel limestone ridges (2–8 m high) that crosscut large morphological irregularities. All topographic forms on the plateau resulted from a prolonged (many millions years) weathering and arid denudation. Therefore, insignificant differences of stability appeared in limestones armoring the plateau. Analysis of the composition and structure of limestones in linear arc-shaped ridges of ledges and the plateau surface showed that the limestones represent an extremely shallow-water facies of shelf sediments. The arc-shaped limestone ridges on the plateau are exposed, partially modified, accumulative, and biogenic-accumulative forms of bottom relief. Similar accumulative forms coupled with biogenic buildups are known in nontropic carbonate shelves of recent seas. The finding of coastal-marine forms on the limestone plateau surface suggests a shallow-water sedimentation regime in Neogene basins persisting over short time spans. Study of such forms is of interest for the refinement of water depths, wave–wind regime, and currents in Neogene basins, as well as for prospecting for oil–gas fields and placer accumulations.  相似文献   

4.
Western European loess sequences of the last glaciation (100,000–15,000 years BP) exhibit strong, cyclic variations of the sedimentation rate, which are coeval to the Greenland stadial/interstadial cycles and the Heinrich events. These North-Atlantic rapid climate changes appear, thus, as a potential cause for the sedimentation variations, via changes in dust intensity cycle. Here we make a first step in testing this hypothesis, by modelling the impact of the North-Atlantic abrupt climate variations on dust emission. Our dust emission calculations use meteorological fields generated by the LMDZ atmospheric general circulation model at a resolution down to 60 km over Western Europe. Three numerical experiments are run, representing a Greenland stadial, an interstadial and a Heinrich event. Orbital parameters and ice-sheet configuration correspond to conditions from Marine Isotope Stage 3 (60,000–25,000 years BP), a period characterized by strong millennial-scale climate variability. The only differences we impose in the boundary conditions regard the North-Atlantic surface temperature and sea-ice cover in the latitudinal band 30°–63°N. The changes in wind, precipitation, soil moisture and snow cover from one simulated state to another result in small differences in dust emission intensity. In contrast, when the inhibition of the aeolian erosion by vegetation is taken into account, the dust fluxes for the cold climate states (Greenland stadial and Heinrich event) become generally more than twice higher than those for the relatively warmer Greenland interstadial, in agreement with the loess data. These results support the hypothesis that the North-Atlantic millennial-scale variability is imprinted in Western European loess profiles, and point to vegetation changes as the main factor responsible for millennial-scale sedimentation variations. An analysis for the English Channel and southern North Sea areas, major potential dust sources, shows that the seasonality of dust emission is not controlled by the wind speed, as in modern large deserts, but by the surface conditions. Consequently, the dusty season lasts from late winter to early summer, with maximum activity in April–May, and is shifted towards summer when the climate is colder.  相似文献   

5.
The current regulatory requirement for cover soils in landfills and surface impoundments is that the soils attain, upon compaction, a very low hydraulic conductivity of 10–7 cm/s or less. Although the influence of the interaction between waste chemicals and clay soil on waste migration has been extensively studied, attempts to incorporate as design components the effects of sulfidic (sulfide-bearing) clays on the integrity of clay caps have largely been ignored. These influences may include increasing the permeability of the cover to percolating moisture, enhancing erosion of clay covers, and killing of vegetation on downslopes of the cover. Consequently, it is suggested that clay cap designers test the acid-generating capabilities of potential clay cap materials before exploiting these earth formations. This can be done by incubating a sample of the candidate capping material (with pH > 3.5) under moist aerobic conditions (field capacity) at room temperature. The soil will be said to contain sulfidic materials if it shows a drop in pH (1 1 by weight in water) of 0.5 or more units to a pH value of 4.0 or less within eight weeks. Decisions should then be made as to whether the soil should be avoided or used with amendments to the cap design.Although some of the authors of this article are employees of the United States Enviornmental Agency, the paper has not been subjected to Agency review and no official endorsement should be inferred.  相似文献   

6.
Atle Nesje   《Quaternary Science Reviews》2009,28(21-22):2119-2136
During the early Holocene abrupt, decadal to centennial-scale climate variations caused significant glacier variations in Norway. Increased freshwater inflow to the North Atlantic and Arctic Oceans has been suggested as one of the most likely mechanisms to explain the abrupt and significant Lateglacial and early Holocene climatic events in NW Europe. The largest early Holocene glacier readvances occurred 11,200, 10,500, 10,100, 9700, 9200 and 8400–8000 cal. yr BP. The studied Norwegian glaciers apparently melted away at least once during the early/mid-Holocene. The period with the most contracted glaciers in Scandinavia was between 6600 and 6000 cal. yr BP. Subsequent to 6000 cal. yr BP the glaciers started to advance and the most extensive glaciers existed at about 5600, 4400, 3300, 2300, 1600 cal. yr BP, and during the ‘Little Ice Age’. Times with overall less glacier activity were apparently around 5000, 4000, 3000, 2000, and 1200 cal. yr BP. It has been proposed that several glacier advances occurred in Scandinavia (including northern Sweden) at 8500–7900, 7400–7200, 6300–6100, 5900–5800, 5600–5300, 5100–4800, 4600–4200, 3400–3200, 3000–2800, 2700–2000, 1900–1600, 1200–1000, and 700–200 cal. yr BP. Glaciers in northern Sweden probably reached their greatest ‘Little Ice Age’ extent between the 17th and the beginning of the 18th centuries. Evidence for early Holocene glacier advances in northern Scandinavia, however, has been questioned by more recent, multi-disciplinary studies. The early to mid-Holocene glacier episodes in northern Sweden may therefore be questioned.Most Norwegian glaciers attained their maximum ‘Little Ice Age’ extent during the mid-18th century. Cumulative glacier length variations in southern Norway, based on marginal moraines dated by lichenometry and historic evidence, show an overall retreat from the mid-18th century until the 1930s–40s. Subsequently, most Norwegian glaciers retreated significantly. Maritime outlet glaciers with short frontal time lags (<10–15 years) started to advance in the mid-1950s, whereas long outlet glaciers with longer frontal time lags (>15–20 years) continued their retreat to the 1970s and 1980s. However, maritime glaciers started to advance as a response to higher winter accumulation during the first part of the 1990s. After 2000 several of the observed glaciers have retreated remarkably fast (annual frontal retreat > 100 m) mainly due to high summer temperatures. The general glacier retreat during the early Holocene and the Neoglacial advances after 6000 cal. yr BP are in line with orbital forcing, due to the decrease of Northern Hemisphere summer solar insolation and the increase in winter insolation. In addition, regional weather modes, such as the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO), play a significant role with respect to decadal and multi-decadal climate variability.  相似文献   

7.
The Late Paleolithic sites existed in cold and dry climat (Ustinovka I, 10–25 ky ago; Ust-Ulma, 19, 360 ± 65 BP) with warmer stages (Suvorovo IV, 15,300 ± 140 BP) and on the Pleistocene/Holocene transition, near 10,000 BP (Suvorovo III). The Final Paleolithic sites existed in Late Glacial time, 10,000–12,000 BP (Gorbatka III, lower level) and in Boreal period, 8,000–9,000 BP (Ilistaya I, 7, 840 ± 60 BP; Timofeevka I). The Developed Neolithic sites existed during the warm and humid Middle-Late Atlantic period, 6,000–8,000 BP; the Late Neolithic sites — 3,000–5,000 BP. The human impact in the Paleolithic-Neolithic was restricted to burning and trampling the vegetation near the sites; the bases of paleoeconomy were hunting, fishing, gathering. The presence of pastoral anthropogenic indicators in pollen spectra of the Neolithic sites may reflect the beginning of cattle breeding.The Bronze Age (end of the 4th-2nd millenia BC) provides the first reliable evidence of cultivated plants (foxtail and Japanese millet) and domestic animals (pig, dog). In the paleoeconomy agriculture and cattle breeding appeared; the first stage of human impact increased (4,000–4,500 BP, ie 2550–3230 cal. BC) is reflected in pollen spectra from cultural layers. From the Early Iron Age to the Middle Ages (end of 2nd millenium BC-13th century AD) agriculture and cattle breeding were the base of paleoeconomy. From the 8th to the 10th centures AD, due to the development of ploughing (arable), the human impact intensified.Paper was presented on the 27th International Geographical Congress, Washington, DC, USA, August 9–14, 1992 (section Human Induced Environmental Change, the Ancient Past)  相似文献   

8.
The Voltýov gold deposit in Central Bohemia is situated in the contact zone of granitic rocks of Variscan age and the Upper Proterozoic and Lower Paleozoic metasediments of the Krásná Hora Metamorphic Islet — the relict of an original sedimentary cover. Goldbearing quartz mineralization of vein and stockwork types is developed in rocks of both geological units, its form depending upon lithology. While ore shoots in metasediments and orthogneiss form generally regular bodies several tens of meters thick, those developed in granodiorite are rather irregular, without any indication of the tectonic control of the mineralization. The mineralized zone peters out extremely rapidly downwards, with a marked decrease both in extent and in gold contents. The gold-bearing mineralization is formed by two generations of gold, both of high fineness (over 900) and very fine-grained (5–20 m). The gold is accompanied by accessory arsenopyrite, maldonite and other Bi-Te minerals, scheelite, and exceptionally chalcopyrite, pyrite, and sulphosalts. Gold contents are 2–3 g/t on average. In addition, late quartz-carbonate mineralization, with U and Se minerals and remobilized gold, is also present. Detailed soil prospecting (25 × 50 m grid) of the deposit detected a large and constrasting gold halo with gold contents exceeding 2 ppm (–200 mesh fraction), accompanied by higher concentrations of characteristic pathfinders — above all As and Bi. The halo extent exceeds the area of the ore outcrop by several times. The origin of gold mineralization in the deposit is explained by remobilization of gold from orthogneiss by the intrusion of the Central Bohemian Pluton.  相似文献   

9.
The extent of desertification on Saudi Arabia   总被引：2，自引：0，他引：2  
Desertification is the process that turns productive deserts into non-productive deserts as a result of poor land-management. Desertification reduces the ability of land to support life, affecting wild species, domestic animals, agricultural crops and humans. The reduction in plant cover that accompanies desertification leads to accelerated soil erosion by wind and water. South Africa is losing approximately 300–400 million tons of topsoil every year. As vegetation cover and soil layer are reduced, rain fall impact and run-off increases. This paper discusses the extent of desertification, its potential threat to sustained irrigated agriculture and possible measures adopted to control ongoing desertification processes to minimize the loss of agricultural productivity in an arid country such as the Kingdom of Saudi Arabia.  相似文献   

10.
This paper studies the CO2 distribution of soil atmosphere in the Shilin National Park. The measurement sites were chosen according to different topographic features and different vegetations. Seven measurement sites on 3 cross sections were chosen to pass through 3 karstic depressions or on the slopes of depressions. All measurement results show soils with pH values lower than 7.0 (from 5.4 to 6.6). There are 2 cases for the pH values of soil in different topographic features: the pH values of 2 profiles on the ridges or upper slopes of depressions are lower than those in the depressions; and the pH values of 2 soil profiles on the slopes of depressions are higher than those in the depressions. Most samples show relatively low humidity and CO2 contents on the ridges or slopes of depressions compared with soil profiles in the depressions. High CO2 contents occur at depths from -40 to -80 cm and high and dense grassland shows high CO2 contents in the soil atmosphere. Grass roots may grow and are distribu  相似文献   

11.
Carbon biogeochemistry of the Betsiboka estuary (north-western Madagascar)   总被引：1，自引：0，他引：1  
Madagascar’s largest estuary (Betsiboka) was sampled along the salinity gradient during the dry season to document the distribution and sources of particulate and dissolved organic carbon (POC, DOC) as well as dissolved inorganic carbon (DIC). The Betsiboka was characterized by a relatively high suspended matter load, and in line with this, low DOC/POC ratios (0.4–2.5). The partial pressure of CO2 (pCO2) was generally above atmospheric equilibrium (270–1530 ppm), but relatively low in comparison to other tropical and subtropical estuaries, resulting in low average CO2 emission to the atmosphere (9.1 ± 14.2 mmol m−2 d−1). Despite the fact that C4 vegetation is reported to cover >80% of the catchment area, stable isotope data on DOC and POC suggest that C4 derived material comprises only 30% of both pools in the freshwater zone, increasing to 60–70% and 50–60%, respectively, in the oligohaline zone due to additional lateral inputs. Sediments from intertidal mangroves in the estuary showed low organic carbon concentrations (<1%) and δ13C values (average −19.8‰) consistent with important inputs of riverine imported C4 material. This contribution was reflected in δ13C signatures of bacterial phospholipid derived fatty acids (i + a15:0), suggesting the potential importance of terrestrial organic matter sources for mineralization and secondary production in coastal ecosystems.  相似文献   

12.
It has been shown that large-scale weather patterns in both the tropical South Pacific (El Niño-Southern Oscillation, or ENSO, events) and the North Pacific (Pacific-North American, or PNA, patterns) have strong teleconnection effects on the air, ice, and ocean environments of the Bering Sea. This signal apparently comes via the atmosphere and not the ocean. The connection between variability of the Bering Sea and the ENSO and PNA appears to be the winter position of the Aleutian Low. Interannual variability in air temperatures, ice cover, and surface winds in the Bering Sea generally are in phase with each other, whereas sea-surface temperatures (SST) tend to lag these variables by 1–3 months. These Bering Sea time-series are significantly correlated with the Southern Oscillation Index (SOI) time-series (an indicator of ENSO events) when the Bering sea data are lagged behind the SOI for up to 18 months. The correlations suggest that warming in the Bering Sea follows negative anomalies in the SOI (i.e., El Niño events). Cooling in the Bering Sea tends to follow positive anomalies (i.e., precursors of El Niños) in the SOI. Maximal correlations for the PNA also lag the SOI by a mouth or two.Analyses of variance indicate that the SOI can explain 30–40% of the variability in the Bering Sea. Stepwise multiple regressions can explain up to 54% of the variation in air temperatures, up to 39% of the variation in sea ice cover, and up to 46% of the variation in SST in the Bering Sea. PNA and SOI were significant variables only in the equation for air temperatures, indicating a close relationship between them and the atmosphere in the Bering Sea and suggesting that energy is transmitted to the water and ice via the atmosphere. The three variables airtemps, ice, and SST were significant each time they were used as independent variables, indicating a rapid and strong feedback relationship among them.Three ENSO events have occurred since the mid-1970s, but none have been typical. There have been either two positive SOI anomalies preceding an El Niño or there have been none preceding an El Niño. When there has been a positive anomaly, ice cover has been above normal, but neither a positive anomaly nor above-normal ice has occurred in the past two ENSO events. An ice retreat has occurred any time there has been an ENSO event, except in the case of the great El Niño of 1982–1983; the anomalous position of the Aleutian Low at that time explains the lack of response of the ice. Finally, one ice retreat occurred that was unrelated to an ENSO event, but was related to a PNA event.  相似文献   

13.
The seismic ground motion hazard is mapped in the Sikkim Himalaya with local and regional site conditions incorporated through geographic information system. A strong motion network in Sikkim comprising of 9 digital accelerographs recorded more than 100 events during 1998–2002, of which 41 events are selected with signal-to-noise ratio 3 for the estimation of site response (SR), peak ground acceleration (PGA) and predominant frequency (PF) at all stations. With these and inputs from IRS-1C LISS III digital data, topo-sheets, geographical boundary of the State of Sikkim, surface geological maps, soil taxonomy map in 1:50,000 scale and seismic refraction profiles, the seismological and geological thematic maps, namely, SR, PGA, PF, lithology, soil class, %slope, drainage, and landslide layers are generated. The geological themes are united to form the basic site condition coverage of the region. The seismological themes are assigned normalized weights and feature ranks following a pair-wise comparison hierarchical approach and later integrated to evolve the seismic hazard map. When geological and seismological layers are integrated together through GIS, microzonation map is prepared. The overall site response, PGA and predominant frequency show an increasing trend in the NW–SE direction peaking at Singtam in the lesser Himalaya. As Main Boundary Thrust (MBT) is approached, the attribute value increases further. A quasi-probabilistic seismic hazard index has been proposed based on site response, peak ground acceleration and predominant frequency. Six seismic hazard zones are marked with percent probability <22%, 22–37%, 37–52%, 52–67%, 67–82%, >82% at 3 Hz and <20%, 20–34%, 34–48%, 48–61%, 61–75%, >75% at 9 Hz. In the microzonation vector layer of integrated seismological and geological themes also six major zones are mapped, with percent probability <15%, 15–31%, 31–47%, 47–63%, 63–78%, >78% at low frequency end. The maximum risk is attached to the probability greater than 78% in the Singtam and its adjoining area. These maps are generally better spatial representation of seismic hazard including site-specific analysis.  相似文献   

14.
Soil samples were collected along a north–south transect extending from Manitoba, Canada, to the US–Mexico border near El Paso, Texas in 2004 (104 samples), a group of sites within New Orleans, Louisiana following Hurricane Katrina in 2005 (19 samples), and a Gulf Coast transect extending from Sulphur, Louisiana, to DeFuniak Springs, Florida, in 2007 (38 samples). Samples were collected from the top 40 cm of soil and were screened for the presence of total Bacillus species and Bacillus anthracis (anthrax), specifically using multiplex-polymerase chain reaction (PCR). Using an assay with a sensitivity of 170 equivalent colony-forming units (CFU) g−1 field moist soil, the prevalence rate of Bacillus sp./B. anthracis in the north–south transect and the 2005 New Orleans post-Katrina sample set were 20/5% and 26/26%, respectively. Prevalence in the 2007 Gulf Coast sample set using an assay with a sensitivity of 4 CFU g−1 of soil was 63/0%. Individual transect-set data indicate a positive relation between occurrences of species and soil moisture or soil constituents (i.e., Zn and Cu content). The 2005 New Orleans post-Katrina data indicated that B. anthracis is readily detectable in Gulf Coast soils following flood events. The data also indicated that occurrence, as it relates to soil chemistry, may be confounded by flood-induced dissemination of germinated cells and the mixing of soil constituents for short temporal periods following an event.  相似文献   

15.
Pollen and δ13CTOM data obtained from two contrasting lake sequences (Lakes Kamalété and Nguène), located 200 km apart in the lowland rainforest of Gabon, provide complementary local and regional 1500-yr records of high resolution (15–30 yr) vegetation change. A combination of aquatic, semi-aquatic and terrestrial pollen showed in both records that the tropical rainforest increased during periods of high rainfall and decreased during drought intervals. The strong fluctuations of water balance at decadal scale during the “Medieval Warm Period” ( 1100–800 cal yr BP) coincided with a noticeable increase in shade-intolerant taxa, indicating recurring rainforest canopy disturbance. The δ13CTOM signal showed high-amplitude variations in both records, which positively correlates with the rainforest dynamics and local vegetation changes. The similar trends in both the pollen and the δ13CTOM signals between these sites demonstrate the regional broadly synchronous timing of shifting hydrological conditions. The largely positive co-variation between strong fluctuations of hydrological conditions and changes in rainforest structure and composition indicate that regional climatic change is probably the driving force for major rainforest dynamics in Gabon. Any significant anthropogenic impact on vegetation has not been clearly identified, and this issue still needs to be resolved independently by obtaining detailed archeological records across the interval 1400–800 BP, which currently seem to be extremely rare or not easily available.  相似文献   

16.
Inactive parabolic dunes are present in southeastern Maryland, USA, along the east bank of the Potomac River. More elongate and finer-grained eolian deposits and paha-like ridges characterize the Potomac River–Patuxent River upland and the west side of Chesapeake Bay. These ridges are streamlined erosional features, veneered with eolian sediment and interspersed with dunes in the low-relief headwaters of Potomac- and Patuxent-river tributaries. Axis data for the dunes and ridges indicate formation by WNW–NW winds. Optically stimulated luminescence and radiocarbon age data suggest dune formation from  33–15 ka, agreeing with the 30–13 ka ages Denny, C.S., Owens, J.P., Sirkin, L., Rubin, M., 1979. The Parsonburg Sand in the central Delmarva Peninsula, Maryland and Delaware. U.S. Geol. Surv. Prof. Pap. 1067-B, 16 pp. suggested for eolian deposits east of Chesapeake Bay. Age range and paleowind direction(s) for eolian features in the Bay region approximate those for late Wisconsin loess in the North American midcontinent. Formation of midcontinent loess and Bay-region eolian features was coeval with rapid growth of the Laurentide Ice Sheet and strong cooling episodes (δ18O minima) evident in Greenland ice cores. Age and paleowind-direction coincidence, for eolian features in the midcontinent and Bay region, indicates strong mid-latitude WNW–NW winds for several hundred kilometers south of the Laurentide glacial terminus that were oblique to previously simulated anticyclonic winds for the last glacial maximum.  相似文献   

17.
New chemical analyses and a review of published data show that there is a compositional diversity between volcanics of basaltic composition found in the M.A.R. rift valley, M.A.R. transform faults and aseismic ridges. The basaltic rocks from the M.A.R. transform faults are less mafic (depleted in olivine content) than those from the M.A.R. rift valley. The transform fault basalts have a higher range of TiO2 content (1–4%), of Fe2O3+FeO content (8–14%) and a lower range of Cr content (50–500 ppm) and Ni content (50–300 ppm). The volcanics from aseismic ridges around the world are considered to be the more felsic types of the two provinces. They have a higher range of variation for their TiO2 and Fe2O3+FeO (1.6–5%; 9–15%; respectively) and a lower range of variability for their Cr and Ni (<250 and 100 ppm respectively) than both the M.A.R. rift valley and transform fault volcanics.It is suggested that transform faults have, by faulting, exposed more fractionated types of basaltic rocks (may be as intrusives) than the rift valley volcanics. While aseismic ridges have undergone a greater degree of differentiation than both transform faults and rift valley volcanics.Contribution n 475 du Département Scientifique, Centre Océanologique de Bretagne Contribution No. 3803 of Woods Hole Oceanographic Institution (USA)  相似文献   

18.
The Deepwater Horizon oil spill was the largest marine oil spill in US waters to date and one of the largest worldwide. Impacts of this spill on salt marsh vegetation have been well documented, although impacts on marsh macroinvertebrates have received less attention. To examine impacts of the oil spill on an important marsh invertebrate and ecosystem engineer, we conducted a meta-analysis on fiddler crabs (Uca spp.) using published sources and newly available Natural Resources Damage Assessment (NRDA) and Gulf of Mexico Research Initiative (GoMRI) data. Fiddler crabs influence marsh ecosystem structure and function through their burrowing and feeding activities and are key prey for a number of marsh and estuarine predators. We tested the hypothesis that the spill affected fiddler crab burrow density (crab abundance), burrow diameter (crab size), and crab species composition. Averaged across multiple studies, sites, and years, our synthesis revealed a negative effect of oiling on all three metrics. Burrow densities were reduced by 39 % in oiled sites, with impacts and incomplete recovery observed over 2010–2014. Burrow diameters were reduced from 2010 to 2011, but appeared to have recovered by 2012. Fiddler crab species composition was altered through at least 2013 and only returned to reference conditions where marsh vegetation recovered, via restoration planting in one case. Given the spatial and temporal extent of data analyzed, this synthesis provides compelling evidence that the Deepwater Horizon spill suppressed populations of fiddler crabs in oiled marshes, likely affecting other ecosystem attributes, including marsh productivity, marsh soil characteristics, and associated predators.  相似文献   

19.
Chesapeake Bay tidal wetlands are experiencing a broad-scale, aggressive invasion by the non-native, clonal grass Phragmites australis. The grass is often managed with herbicides in efforts to restore native plant communities and wildlife habitat. Management efforts, however, can act as a disturbance, resulting in increased light availability, potentially fostering reinvasion from soil seedbanks. If native vegetation establishes quickly from seedbanks, the site should have greater resiliency against invasion, while disturbed sites where native plants do not rapidly establish may be rapidly colonized by P. australis. We surveyed the soil seedbank of three vegetation cover types in five Chesapeake Bay subestuaries: areas where P. australis had been removed, where P. australis was left intact, and with native, reference vegetation. We determined the total germination, the proportion of the seedbank that was attributable to invasive species, the richness, the functional diversity, and the overall composition of the seedbanks in each of the cover types (i.e., plots). After 2 years of herbicide treatment in the P. australis removal plots, vegetation cover type impacted the total germination or the proportion of invasive species in the seedbank. In contrast, we also found that seedbank functional composition in tidal brackish wetlands was not influenced by vegetation cover type in most cases. Instead, plots within a subestuary had similar seedbank functional composition across the years and were composed of diverse functional groups. Based on these findings, we conclude that plant community recovery following P. australis removal is not seed-limited, and any lack of native vegetation recruitment is likely the result of yet-to-be-determined abiotic factors. These diverse seedbanks could lead to resilient wetland communities that could resist invasions. However, due to the prevalence of undesirable species in the seedbank, passive revegetation following invasive plant removal may speed up their re-establishment. The need for active revegetation will need to be assessed on a case-by-case basis to ensure restoration goals are achieved.  相似文献   

20.
E. N. Kairu 《GeoJournal》1993,29(4):351-358
This paper discusses the results of an experiment whereby all the major radiation and energy fluxes were measured/estimated for a mature tea canopy at Kericho, Kenya. The fluxes include incoming short-wave radiation, terrestrial radiation, latent heat flux and the sensible heat fluxes between the canopy and the air and that into and out of the soil. The pertinent data were simultaneously collected by use of 21X microloggers. Global radiation varied from 40 Wm–2 to 1160 Wm–2 depending on sky conditions. High values were observed early in the morning and late in the afternoon. Net radiation varied between — 100 Wm–2 at night and 600 Wm–2during the daytime. The largest proportion of net radiation was consumed by the latent heat flux. Advection was found to be minimal.  相似文献   

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