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1.
Large differences in δ 2H of primary producers between aquatic and terrestrial ecosystems are used to identify subsidies, discriminate organic matter sources, and reduce uncertainty in food web studies. Previous investigations of hydrogen isotope ratios suggest there may be predictable differences between the δ 2H of water and organic matter for different types of primary producers. We define the difference in the net isotopic discrimination between water and bulk organic matter (om) as: ΔH = (δ 2Hom ? δ 2Hwater) ÷ (1 + δ 2Hwater ÷ 1,000). We summarized ΔH values from published literature and we measured the δ 2H of water and primary producers in order to compare ΔH among aquatic and terrestrial primary producers. Measurements were made from three water body types (lake, river, coastal lagoon) and their associated watersheds. Although we predicted a large and equivalent net isotopic discrimination for aquatic primary producers, we found considerable variability among groups of aquatic producers. Macroalgae, benthic microalgae, and phytoplankton had more negative ΔH values (i.e. greater isotopic discrimination) than both aquatic macrophytes and terrestrial vegetation. The more positive δ 2Hom and hence lower ΔH of terrestrial vegetation was expected due to relative increases in the heavier isotope, deuterium, during transpiration. However, the more positive values of δ 2Hom and relatively low ΔH in aquatic macrophytes, even submerged species, was unexpected. Marine macroalgae had high variability in δ 2Hom as a group, but low variability within distinct species. Variability among types of primary producers in δ 2Hom and in ΔH should be assessed when hydrogen is used in isotopic studies of food webs.  相似文献   

2.
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3.
Natural floodplains are spatially heterogeneous and dynamic ecosystems but at the same time, a highly endangered landscape feature due to climate change and human impacts such as water storage, flood control and hydropower production. Flow is considered a master variable that shapes channel morphology and the heterogeneity, distribution, and turnover of floodplain habitats. Despite their highly dynamic nature, the relative abundance of different habitat elements (islands, gravel bars) in natural floodplains seems to remain relatively constant over ecological periods and is referred to as the shifting mosaic steady state concept. In this conceptual context, we analysed spatiotemporal changes in relative habitat abundance and channel complexity of an alpine floodplain from its near natural state in 1940 before water abstraction and levee construction until 2007 using historical aerial images. Within the first decades of impairment, the relative abundance of floodplain habitats that depend on flood and flow pulses such as parafluvial channels and islands shifted toward a greater abundance of terrestrial forest and grassland habitats. After 1986, the duration and frequencies of high‐precipitation events (>60 mm 24 h–1) triggering major, channel‐reworking floods increased substantially and caused a restructuring of the floodplain and decrease in the abundance of more terrestrial habitat types. These results are contrary to expectations of the shifting mosaic steady state concept yet suggest its potential application as an indicator of landscape transformation and human impacts on floodplain ecosystems. Last, the results raise the applied question as to whether an increased frequency of high flow events induced by climate change can contribute to floodplain restoration. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

4.
Chemicals, especially silica, have been suspected to cause genetic alterations in pottery industry workers. The present study aims to analyze the frequency of chromosomal aberrations (CA), micronucleus (MN) and DNA damage (comet assay) in the peripheral blood lymphocytes and the immunological alterations workers chronically exposed to silica and in control subjects. In our study, 50 silica exposed workers and 35 control subjects were recruited and silica level was measured by respirable dust and respirable quartz concentrations of badge dosimeter. The serum immunoglobulins (IgM, IgG, and IgA) of all the subjects were measured by using ELISA method. The individuals exposed to silica have a significant increase in the frequency of CA, MN and the total DNA damage (p < 0.05). Immunoglobulin elevation on silica exposed workers was statistically significant (p < 0.05) on comparison with their respective controls. Investigation of the smoking and alcohol habitats coupled with silica exposure in exposed and control subjects represents alcohol consumption and smoking as additional risk factors and must be avoided. Multiple linear regression analysis obtained for CA, MN and comet assay confirm these tests as biomarkers for silica exposed pottery workers. Some confounding factors also showed significant influence on exposed subjects. These results indicate the mutagenic risk in the working environment has a high probability of association with the silica dust exposure in pottery industries. Nevertheless, the present study will create awareness and public concern not only among the silica exposed workers but also to the welfare of their progeny.  相似文献   

5.
The present work is a continuation and improvement of the method suggested in Pisarenko et al. (Pure Appl Geophys 165:1–42, 2008) for the statistical estimation of the tail of the distribution of earthquake sizes. The chief innovation is to combine the two main limit theorems of Extreme Value Theory (EVT) that allow us to derive the distribution of T-maxima (maximum magnitude occurring in sequential time intervals of duration T) for arbitrary T. This distribution enables one to derive any desired statistical characteristic of the future T-maximum. We propose a method for the estimation of the unknown parameters involved in the two limit theorems corresponding to the Generalized Extreme Value distribution (GEV) and to the Generalized Pareto Distribution (GPD). We establish the direct relations between the parameters of these distributions, which permit to evaluate the distribution of the T-maxima for arbitrary T. The duality between the GEV and GPD provides a new way to check the consistency of the estimation of the tail characteristics of the distribution of earthquake magnitudes for earthquake occurring over an arbitrary time interval. We develop several procedures and check points to decrease the scatter of the estimates and to verify their consistency. We test our full procedure on the global Harvard catalog (1977–2006) and on the Fennoscandia catalog (1900–2005). For the global catalog, we obtain the following estimates: \( \hat{M}_{{\rm max} } \)  = 9.53 ± 0.52 and \( \hat{Q}_{10} (0.97) \)  = 9.21 ± 0.20. For Fennoscandia, we obtain \( \hat{M}_{{\rm max} } \)  = 5.76 ± 0.165 and \( \hat{Q}_{10} (0.97) \)  = 5.44 ± 0.073. The estimates of all related parameters for the GEV and GPD, including the most important form parameter, are also provided. We demonstrate again the absence of robustness of the generally accepted parameter characterizing the tail of the magnitude-frequency law, the maximum possible magnitude M max, and study the more stable parameter Q T (q), defined as the q-quantile of the distribution of T-maxima on a future interval of duration T.  相似文献   

6.
Ecosystem metabolism is an important measure of wetland restoration efficiency, and serves to indicate if the system is capable of processing energetic resources. Despite its value, ecosystem metabolism has rarely been included in monitoring programs. In this study, we aimed to achieve the following objectives: (i) compare net ecosystem production (NEP) rates of constructed vs. natural wetlands; (ii) identify the highest NEP rate habitats; and (iii) define the main environmental factors regulating NEP in different wetland types. Pelagic and benthic NEP rates and physicochemical features were measured in three natural and five constructed wetlands in the middle Ebro River floodplain (NE Spain). Statistical analyses showed pelagic NEP rates peaked in natural wetlands, which produced up to 187.5 mg C m?3 h?1 compared to lower rates in constructed wetlands (up to 46.2 mg C m?3 h?1). Pelagic NEP responded positively to temperature, total dissolved solids, and nutrients. Benthic NEP rates were 3 to 30-fold greater than pelagic in natural (up to 994.9 mg C m?3 h?1) and constructed (up to 1,551.5 mg C m?3 h?1) wetlands, and were heavily influenced by habitat type, with NEP peaking in areas dominated by submerged vegetation and fine organic sediment. Rapid recovery in aquatic communities (i.e. macroinvertebrate diversity) has been previously reported for the studied wetlands; however, our study suggests a slower recovery of functional processes (i.e. pelagic NEP) in constructed habitats. We therefore strongly advocate the inclusion of ecosystem function in the design and evaluation of restoration projects to optimise long-term wetland ecosystem sustainability.  相似文献   

7.
This study shows the assessment of radiation hazard parameters due to terrestrial radionuclides in the soil around artisanal gold mining for addressing the issue of natural radioactivity in mining areas. Hence, the levels 238U, 232Th, 40K and 226Ra in soil (using gamma spectrometry), 222Rn in soil and 222Rn in air were determined. Radiation hazard parameters were then computed. These include absorbed dose D, annual effective dose E, radium equivalent activity Raeq, external hazard Hex, annual gonadal dose equivalent hazard index AGDE and excess lifetime cancer risk ELCR due to the inhalation of radon (222Rn) and consumption of radium (226Ra) in vegetation. Uranium (238U), thorium (232Th) and potassium (40K) averages were, respectively, 26, 36 and 685 Becquerel per kilogram (Bq kg?1). Soil radon (4671 Bq m?3) and radon in air (14.77 Bq m?3) were found to be less than worldwide data. Nevertheless, the average 40K concentration was 685 Bq kg?1. This is slightly higher than the United Nations Scientific Committee on the Effects of Atomic Radiation average value of 412 Bq kg?1. The obtained result indicates that some of the radiation hazard parameters seem unsavory. The mean value of absorbed dose rate (62.49 nGy h?1) was slightly higher than average value of 57 nGy h?1 (~?45% from 40K), and that of AGDE (444 μSv year?1) was higher than worldwide average reported value (300 μSv year?1). This study highlights the necessity to launch extensive nationwide radiation protection program in the mining areas for regulatory control.  相似文献   

8.
Phytosociological and habitat studies were conducted on the water and swamp vegetation of astatic water bodies within north-eastern Poland. The phytocoenoses were selected on the basis of dominance of species forming the particular communities. The analysis of 147 relevés showed the existence of 10 vegetation types: Lemnetum minoris, Spirodeletum polyrrhizae, Riccietum fluitantis, Elodeetum canadensis, Polygonetum natantis, Typhetum latifoliae, Caricetum elatae, Calletum palustris, Potentilletum palustris, Menyanthetum trifoliatae. Among the properties of water analysed water depth, , pH, total and carbonate hardness, PO43−, Ca2+, Mg2+ and Na+ were found to be most important in differentiating the habitats of the vegetation types studied. Substrate properties, which best differentiated the habitats of the associations studied were NO3−, Na+, water content, pH and total N. In spite of the wide variability of habitat conditions occurring in astatic water bodies, particular phytocoenoses distinguished on the basis of dominance of one species were associated with specific habitats. The particular phytocoenoses or groups of phytocoenoses could be good indicators of various habitat conditions that occur within astatic water bodies or changes taking place in these habitats.  相似文献   

9.
In autumn of 1966 on the northern slope of Kliuchevskoy volcano a chain of new adventive craters broke out at the height of about 2200 m. Eighty-four hours before the beginning of the eruption a swarm of preliminary volcanic earthquakes had appeared. The number of preliminary shocks was 457 with total energy of 4 × 1017 erg. With the beginning of the lava flow the earthquakes stopped and a continuous volcanic tremor appeared. The total energy of volcanic tremor amounts to 1016 erg. During the eruption numerous explosive earthquakes with the energy of 1015–1016 erg were recorded and besides the microbarograph of the Volcanostation recorded 393 explosions with an energy more than 1013 erg and their total energy was equal to 1017 erg. All together it has been formed 8 explosive craters and the lowest 9th crater was effusive. The slag cone was formed round this effusive crater, the lava effusion of basaltic-andesite composition (52,5% SiO2) tooke place from the lava boccas at the cone base and from the crater. The lava flow covered a distance of 10 km along the valley of the Sopochnoy river and descended to a height of about 800 m. The lava flow velocity at the outflow reached 800 m/hr, the lava temperature was 1050°C. The effused lava volume amounts to 0.1 km3. The eruption stopped on December 25–26, 1966.  相似文献   

10.
During the high water season, the flooding reduces environmental heterogeneity in aquatic ecosystems of the Pantanal wetland. When the water level recedes, lakes and channels are formed as individual systems. Therefore, we expected the spatial heterogeneity during the low water phase resulting in changes on biological communities leading to high phytoplankton abundance, biomass and diversity within and between habitats. To test this hypothesis, we analyzed eight freshwater systems (five oxbow lakes, two channels, and the river) during the low water period. Phytoplankton biomass, abundance, diversity (alpha, beta, gamma) and diversity metrics as richness (species per sample), Shannon diversity (H′) and evenness were measured in all systems along with nutrient concentrations, zooplankton and bacteria abundances. We found 97 species as gamma diversity. The alpha diversity was unexpectedly low in comparison to most other South American floodplain systems (38 species in river, 24 in channels and 29 in lakes). Also, the systems are relatively similar in composition (beta diversity, 28%). Connectivity differences between systems highlighted differences in phytoplankton abundance and biomass (fresh weight) ranging from 1428 ind mL−1 (river) to 3710 ind mL−1 (lakes) and from 0.71 mg L−1 (river) to 2.9 mg L−1 (lakes), respectively. However, our results did not indicate significant differences in phytoplankton species richness between the systems during the low water. Our main conclusions are that local factors may be responsible for changes on phytoplankton community and the time of isolation during the low water phase was not sufficient to promote changes in phytoplankton diversity between the habitats.  相似文献   

11.
The Queershan composite granitic pluton is located in the north of the late Paleozoic Yidun arc collision-orogenic belt, eastern Tibetan Plateau. The main rock types are coarse-grained porphyritic alkalic-monzonite granite with minor fine-grained porphyritic monzogranite and granodiorite distributed in the eastern and southwestern regions. Here we report their zircon U-Pb ages and geo- chemical data. The intrusive contact relations indicate that granodiorite was formed earlier than the alkalic-monzonite granite(105.9±1.3 Ma) and monzogranite(102.6±1.1 Ma). These suggest that the Queershan composite granitic pluton was formed through three-stage magmatic events. The alkalic-monzonite granite(105.9±1.3 Ma) and monzogranite(102.6±1.1 Ma) are characterized by high SiO2(73.5%–77.7%), K2O+Na2O(6.9%–8.5%), Ga/Al ratios(2.6–3.4) and low Al2O3(11.8%–14.5%), CaO(0.25%–1.5%), MgO(0.18%–0.69%), negative Ba, Sr and Eu anomalies, showing A-type granite affinities. The granodiorite exhibits lower SiO2, P2O5 and K2O+Na2O contents, but higher Al2O3, CaO and MgO contents than alkalic-monzonite granite and monzogranite, showing I-type granite affinity. 176Hf/177 Hf ratios of the alkalic-monzonite granite and the monzogranite are 0.282692–0.282749 and 0.282685–0.282765, respectively, and with similar ?Hf(t) values(?0.56 to 1.43 and ?0.87 to 1.90 respectively). They also present similar TDM2 model ages(1.04–1.22 and 1.07–1.2 Ga respectively), indicating they may be sourced from a similar rock source, mostly like Kangding Complex. The homogeneity of the Hf isotopic compositions and the absence of the MMEs demonstrate that little depleted mantle materials have contributed to the source. We propose that the Mesoproterozoic crust materials of the Yangtze Craton exist beneath the Yidun arc terrane and support it was a dismembered part of the Yangtze Craton. The A-type granites of Queershan composite granitic pluton are most probably related to the closure of the Bangong-Nujiang Tethys ocean.  相似文献   

12.
We expand on the empirical Green’s function deconvolution method of Ide et al. (2011) to estimate radiated energy for the six largest earthquakes worldwide over the last 10 years: 2011 M w 9.0 Tohoku-Oki, 2004 M w 9.1 Sumatra, 2010 M w 8.8 Maule, 2005 M w 8.7 Nias, 2007 M w 8.5 Bengkulu, and 2012 M w 8.6 off-Sumatra. Deconvolution of P, SV and SH components gives consistent energy results that are comparable to estimates found independently by other researchers. Apparent stress for the five great thrust earthquakes is between 0.4 and 0.8 MPa, while the 2012 off-Sumatra strike-slip earthquake has a higher apparent stress of 3 MPa, which is consistent with other studies that find a tendency for strike-slip events to be more energetic. Our results are within the spread of apparent stress from the wider global earthquake population over a large magnitude range. The azimuthal distribution of energy in each case shows signs of directivity, and in some cases, shows less energy radiated in the trench-ward direction, which may suggest enhanced tsunami potential. We find that eGfs as small as ~M 6.5 can be used for teleseismic deconvolution, and that an eGf-mainshock magnitude difference of 1.5 units yields stable results. This implies that M 8 is the minimum mainshock size for which teleseismic eGf deconvolution will work well. We propose that a database of eGf events could be used to calculate radiated energy and apparent stress of great, hazardous events in near real time, i.e., promptly enough that it could contribute to rapid response measures.  相似文献   

13.
Cross-ecosystem subsidies, such as terrestrial invertebrates and leaf litter falling into water as resources for aquatic communities, can vary across environmental gradients. We examined whether the effect of terrestrial subsidy inputs on benthic invertebrates was mediated by resident coastal cutthroat trout (Oncorhynchus clarki) in two representative streams. We experimentally manipulated the input rates (reduced, ambient) of terrestrial subsidies (terrestrial invertebrates and leaf litter) as well as the presence or absence of cutthroat trout in the two streams. The hypothesis that the reduction of terrestrial subsidies to the stream influences benthic invertebrate assemblages was supported by experimental results. The treatments of terrestrial subsidy reduction and cutthroat trout presence had a significant negative effect on benthic invertebrate community biomass and shredder biomass in East Creek with high natural terrestrial subsidy input and small amount of large wood in channel. In contrast, results from Spring Creek with low subsidy input and large amount of large wood in channel showed that only the terrestrial subsidy reduction significantly reduced the biomass of shredders. The effects of the terrestrial subsidy input and trout predation on benthic invertebrate communities varied between the two streams. Our results indicate that a subsidy effect on benthic communities can vary between nearby streams differing in canopy and habitats. This study, with the major finding of highly context-dependent effects of spatial subsidies, suggests that the interplay of resource subsidies and predators on invertebrate community assemblages can be site-specific and context-dependent on habitat features.  相似文献   

14.
Application of the silica geothermometer to over 70,000 non-thermal groundwaters from the United States has shown that there is a correlation between the average silica geotemperatures for a region (T SiO2 in °C) and the known regional heat flow (q in mW m?2) of the form: 1 $$TSiO_2 = mq + b$$ wherem andb are constants determined to be 0.67°C m2 mW?1 and 13.2°C respectively. The physical significance of ‘b’ is the mean annual air temperature. The slope ‘m’ is related to the minimum average depth to which the groundwaters may circulate. This minimum depth is estimated to be between 1.4 and 2.0 km depending on the rock type. A preliminary heat flow map based on equation (1) is presented using theT SiO2 for new estimates of regional heat flow where conventional data are lacking. Anomalously high localT SiO2 values indicate potential geothermal areas.  相似文献   

15.
Based on geochronological, petrological, stratigraphical, and sedimentological data, this paper describes the deposits left by the most powerful Holocene eruption of Chachimbiro compound volcano, in the northern part of Ecuador. The eruption, dated between 3640 and 3510 years BC, extruded a ～650-m-wide and ～225-m-high rhyodacite dome, located 6.3 km east of the central vent, that exploded and produced a large pyroclastic density current (PDC) directed to the southeast followed by a sub-Plinian eruptive column drifted by the wind to the west. The PDC deposit comprises two main layers. The lower layer (L1) is massive, typically coarse-grained and fines-depleted, with abundant dense juvenile fragments from the outgassed dome crust. The upper layer (L2) consists of stratified coarse ash and lapilli laminae, with juvenile clasts showing a wide density range (0.7–2.6 g cm?3). The thickness of the whole deposit ranges from few decimeters on the hills to several meters in the valleys. Deposits extending across six valleys perpendicular to the flow direction allowed us to determine a minimum velocity of 120 m s?1. These characteristics show striking similarities with deposits of high-energy turbulent stratified currents and in particular directed blasts. The explosion destroyed most of the dome built during the eruption. Subsequently, the sub-Plinian phase left a decimeter-thick accidental-fragment-rich pumice layer in the Chachimbiro highlands. Juvenile clasts, rhyodacitic in composition (SiO2?=?68.3 wt%), represent the most differentiated magma of Chachimbiro volcano. Magma processes occurred at two different depths (～14.4 and 8.0 km). The hot (～936 °C) deep reservoir fed the central vent while the shallow reservoir (～858 °C) had an independent evolution, probably controlled by El Angel regional fault system. Such destructive eruptions, related to peripheral domes, are of critical importance for hazard assessment in large silicic volcanic complexes such as those forming the Frontal Volcanic Arc of Ecuador and Colombia.  相似文献   

16.
Lower Cretaceous C-isotope records show intermittent negative/positive spikes, and consistent patterns of coeval chemostratigraphic curves thus document shifts that signal simultaneous responses of temporal changes in the global carbon reservoir. The standard pattern registered by the δ 13Corg and δ 13Ccarb in Lower Aptian sediments includes distinct isotope segments C1 to C8 (Menegatti et al., 1998). In the El Pui section, Organyà Basin, Spain, C-isotope segment C2 is the longest interval preceding segments C3–C6 associated with oceanic anoxic event 1a (OAE 1a), and reveals a distinct negative shift of ~1.8‰ to ~2.23‰ defining the C-isotope pattern within that interval. Total inorganic carbon (TIC), total organic carbon (TOC), δ 13Corg, microfacies, n-alkanes show no difference before, during, or after the negative inflection. The biomarkers indicate that organic matter (OM) mainly originates from algal/microbial sources because short-chain length homologues (≤nC19) dominate. nC20 through nC25 indicate some contribution from aquatic vegetation, but little from higher plants (>nC25), as also suggested by the terrestrial/aquatic ratio of n-alkanes or (TAR) = [(nC27+nC29+nC31)/(nC15+nC17+nC19)] (averages 0.085). We suggest that conjoint pulses of contemporaneous LIPs (Ontong Java) and massive explosive volcanism in northeast Asia, the Songliao Basin (SB-V), best conform to plausible causes of the negative intra-C2 carbon isotopic excursion (CIE) at that time. Because of its apparent common occurrence the intra-C2 inflection could be a useful marker harbinger to the more pronounced CIE C3, the hallmark of OAE1a.  相似文献   

17.
The contamination of organochlorine pesticides (OCPs) and hexachlorobutadiene (HCBD) in soils, plants, and terrestrial fauna from a former pesticide-producing area in Southwest China was investigated. High levels of OCP residues (ΣHCHs and ΣDDTs of 3.89–13,386 and 23.3–11,186 ng g?1 dw, respectively) were found in the soils within the producing factory, indicating that the former pesticide factory brownfield site poses a high environment risk and that effective soil remediation is needed. The OCP concentrations in soils surrounding the pesticide factory were 1–2 orders of magnitude higher than in the other agricultural areas from China. Grass, vegetables, and tree leaves from the surrounding areas of the point source showed moderate OCP pollution, which was likely to be causing potential negative impacts on terrestrial ecosystem. OCP levels in terrestrial species from the studied area including insects, free-range chickens and birds were moderate, and were generally within accepted safe levels. β-HCH was the most prevalent isomer, implying that hexachlorocyclohexane (HCH) residues were mainly derived from the degradation of technical HCH. The dichlorodiphenyltrichloroethane (DDT) profiles reflected obvious degradation of technical DDT. In the study area, the concentrations of HCBD were <0.02–5.59, 0.03–24.6 ng g?1 dw and 1.65–3.80 ng g?1 lw in soils, plants, and animals, respectively, which were relatively low and not sufficient to cause observable adverse biological effects.  相似文献   

18.
Land use related habitat degradation in freshwater ecosystems has considerably increased over the past decades, resulting in effects on the aquatic and the riparian communities. Previous studies, mainly in undisturbed systems, have shown that aquatic emergent insects contribute substantially to the diet of riparian predators. To evaluate the effect of land use on aquatic prey subsidies of riparian spiders, we performed a longitudinal study from June to August 2012 along a first order stream (Rhineland-Palatinate, Germany) covering three land use types: forest, meadow and vineyard. We determined the contribution of aquatic and terrestrial resources to the diet of web-weaving (Tetragnathidae spp.) and ground-dwelling (Pardosa sp.) riparian spiders using stable isotope analyses of aquatic emergent insects and terrestrial arthropods. The contribution of aquatic and terrestrial sources differed between Tetragnathidae spp. and Pardosa sp. as well as among land use types. Tetragnathidae spp. consumed 80–100% of aquatic insects in the meadows and 45–65% in the forest and vineyards. Pardosa sp. consumed 5–15% of aquatic insects in the forest, whereas the proportions of aquatic and terrestrial sources were approximately 50% in the meadow and vineyard. Thus, aquatic emergent insects are an important subsidy to riparian spiders and land use is likely to affect the proportion of aquatic sources in the spider diet.  相似文献   

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20.
The colour of aquatic humus increases with pH. The ‘molecular weight distribution’ of a concentrated humus sample has been studied at diffrent pH-levels and ionic strength by use of a molecular sieve (Sephadex G-25) and membrane dialysis. The fraction excluded from G-25 became greater, proportionally with increasing pH, and this was apparently not affected to any great extent by the ionic strength. Membrane dialysis of acidic, alkaline and ‘natural’ samples confirmed this change of the molecular size by pH. The colour increase of aquatic humus, caused by raising pH, is associated with an increase in the size of the humus molecules. ‘Aggregation’ and condensation are proposed as possible mechanisms.  相似文献   

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