共查询到20条相似文献，搜索用时 15 毫秒
1.
Oliver Miler Ismail Albayrak Vladimir Nikora Matthew O’Hare 《Aquatic Sciences - Research Across Boundaries》2014,76(4):465-481
Biomechanical properties and morphological characteristics of stems of eight species of submerged aquatic plants were studied to analyse (1) differences between river and lake specimens, (2) seasonal differences between winter/spring and summer/autumn specimens, and (3) change of biomechanical properties and morphological characteristics along the stems. The data show that river macrophytes display not only characteristic biomechanical traits and morphological characteristics specific to their hydraulic habitats, but also distinctive temporal changes due to seasonally varying water temperature, flow velocity, and growth phase. Furthermore, the data reveal differences between lake and river specimens that could be explained by wind exposure of the lake sampling sites and the species-specific flow requirements of the river macrophytes. Biomechanical properties and morphological characteristics varied along the stem with larger cross-sections and a higher resistance against tension and bending forces at the bottom compared to the top parts, being similar for both lake and river specimens. The acquired and analysed stem biomechanical and morphological data contribute to the plant biomechanics database to underpin a wide range of studies in aquatic ecology, river and wetland management. 相似文献
2.
Vlahogianni T Dassenakis M Scoullos MJ Valavanidis A 《Marine pollution bulletin》2007,54(9):1361-1371
Mussels are used as sentinel organisms and bioindicators to evaluate the toxic effects of chemical pollutants in marine organisms, especially heavy metals, representing an important tool for biomonitoring environmental pollution in coastal areas. Antioxidant defence enzymes play an important role in cellular antioxidant defence systems and protect from oxidative damage by reactive oxygen species (ROS). Indigenous mussels Mytilus galloprovincialis of the Saronikos Gulf of Greece were used for monitoring heavy metal pollution in three polluted sites in the area and in one unpolluted site. Seasonal variations of the activity of antioxidant defence enzymes, superoxide dismutase (SOD) and catalase (CAT), as well as lipid peroxidation (LP) were measured as biomarkers in a period of three years in relation to concentrations of trace metals in their gills and mantle and compared to mussels from an unpolluted sampling site. SOD activity increased at least 2 fold at the polluted sites when compared to the control site (the high activity was recorded in the spring time). CAT activity was increased 2-3 times at the polluted sites, with high activity in the winter and spring time, compared to the control site. LP concentration was twice higher at the polluted sites, following the same seasonal pattern. Trace metals contents in mussels collected at polluted sites were 3-4 fold higher compared to the control site and showed moderate variations along the months, with a winter maximum followed by a summer pre-spawning minimum matching the seasonal trends of temperature and salinity. Our results showed that metal pollution in the Elefsis Bay (the most polluted coastal area) causes relatively medium levels of oxidative stress in tissues of mussels due to cellular oxy-radical generation. This study, which is the first in the area, showed that seasonal variations of the activity of antioxidant defence enzymes and LP concentrations in mussels can be used as potential biomarkers of toxicity for long-term monitoring in marine coastal ecosystems. 相似文献
3.
Polycyclic aromatic hydrocarbons (PAH) have been measured in mussel tissues in early spring and summer since 1993 throughout Prince William Sound (PWS) and the Gulf of Alaska (GOA). Season-specific thresholds were established at reference sites to identify ‘above background’ total PAH levels. Thresholds were estimated using one-sided 99% tolerance limits. Thresholds were similar across reference sites but differed by an order of magnitude across seasons. Trends in total PAH since 1998 were assessed for sites impacted by the 1989 Exxon Valdez oil spill or the Alyeska Marine Terminal. Summer samples exhibited no trends; early spring samples declined. In early spring, all sites were judged ‘recovered’ by 2004; in summer, one site in western Prince William Sound and two in the western GOA exceeded thresholds by 11 ng/g dry weight or less. Robust estimation methods prevented bias from observations affected by unknown releases or laboratory errors. 相似文献
4.
This article presents a comprehensive study of canopy interception in six rainforests in Australia's Wet Tropics for periods ranging between 2 and 3·5 years. Measurements of rainfall, throughfall, stemflow and cloud interception were made at sites characterized by different forest types, canopy structure, altitude, rainfall and exposure to prevailing winds. Throughfall at these sites ranged between 64 and 83% of total precipitation inputs, while stemflow ranged between 2 and 11%. At sites higher than 1000 m, cloud interception was found to contribute up to 66% of the monthly water input to the forest, more than twice the rainfall at these times. Over the entire study period, cloud interception accounted for between 4 and 30% of total precipitation inputs, and was related more to the exposure of sites to prevailing winds than to altitudinal differences alone. Over the duration of the study period, interception losses ranged between 22 and 29% of total water input (rainfall and cloud interception) at all sites except the highest altitude site on Bellenden Ker, where interception was 6% of total water input. This smaller interception loss was the result of extremely high rainfall, prolonged immersion in cloud and a sparser canopy. On a monthly basis, interception losses from the six sites varied between 10 and 88% of rainfall. All sites had much higher interception losses during the dry season than in the wet season because of the differences in storm size and rainfall intensity. The link between rainfall conditions and interception losses has important implications for how evaporative losses from forests may respond to altered rainfall regimes under climate change and/or large‐scale atmospheric circulation variations such as El Niño. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
5.
在扰动位能(PPE)理论的基础上,针对不同高度上局地环流能量转换问题,本文提出了分层扰动位能(LPPE)的概念.研究表明850hPa的分层扰动位能一阶矩(LPPE1)在热带地区为正,高纬度地区为负,200 hPa高度LPPE1在北美高纬度地区出现正值分布,100 hPa及以上LPPE1热带地区为负,高纬度为正.LPPE1冬季半球的分布与年平均相似,北半球夏季大陆上出现正的极大值.在局地,LPPE1在数值上远远大于分层扰动位能二阶矩(LPPE2)及更高阶矩,因此,LPPE的分布与LPPE1的分布相似.南海季风区低层动能的季节变化与LPPE呈现反向变化关系.相关分析表明,南海夏季风(SCSSM)与春季的LPPE1偶极型分布之间存在着显著的年际(正)相关关系,可以作为SCSSM强度的一个预报因子.春季赤道印度洋、西太平洋海表温度(SST)的负(正)异常对应春季、夏季LPPE1的南负北正(南正北负)偶极型分布,夏季(JJAS) LPPE1的偶极型分布与南海季风区动能的一致增大(减小)是两者耦合模态的主导模态,夏季南海季风区的西风增强(减弱), SCSSM增强(减弱),这是能量异常影响SCSSM的一个可能的机制. 相似文献
6.
A simple technique has been used to investigate gravity waves in the Antarctic and Arctic mesosphere. Data from two meteor radars are used, one at Rothera in the Antarctic (68°S) and one at Esrange in the Arctic (68°N). Observations have been made from 2005 to 2008 in the Antarctic and from 2000 to 2008 in the Arctic. The results reveal a seasonal behaviour with a semi-annual cycle. Over both sites wave activity maximises at the solstices and minimises at the equinoxes. Monthly-mean gravity-wave activity increases with height in all seasons except in summer when gravity-wave variances show little or no increase with height at heights below about 90 km. Above about 90 km in summer there is a rapid increase in wave activity. We suggest that this summer-time behaviour is a consequence of rapid wave growth as gravity waves ascend into the more stable air of the lower thermosphere. Differences between the Arctic and Antarctic gravity-wave field are also evident. In particular, a higher level of gravity-wave activity is observed in the Antarctic spring compared to the Arctic spring. We suggest that these inter-hemispheric differences may be the result of differences in the generation of gravity waves in the polar troposphere and stratosphere. 相似文献
7.
A.A. Silva 《Journal of Atmospheric and Solar》2011,73(17-18):2474-2481
Clouds are the main tropospheric agent in the modulation of solar ultraviolet (UV) radiation in the Earth's surface. Clouds produce both attenuation and enhancement of radiation. This work shows how the local cloud cover (LCC) can partially explain mismatches between overpass satellite and ground-based instruments in the erythemal dose rate (EDR) for an urban, tropical site in Southern Hemisphere. Best agreement occurs for the scarcer cloud cover of winter. The Ozone Monitoring Instrument (OMI) EDR is on average 15% higher than ground-based measurements for cloudless skies. This value is due to both OMI algorithm misconception about aerosols and instrumental uncertainty. 相似文献
8.
Mianrun ChenBingzhang Chen Paul HarrisonHongbin Liu 《Continental Shelf Research》2011,31(10):1075-1086
A monthly comparative study of mesozooplankton biomass and composition between a eutrophic Pearl River estuarine site (WE) and a mesotrophic coastal-oceanic site (EO) in Hong Kong waters was conducted to examine the response of mesozooplankton to nutrient-rich riverine discharge. Although the annual average mesozooplankton biomass was higher at WE than at EO, they were not statistically significant. Variations of mesozooplankton biomass at both stations followed similar trends of Chl a concentrations, with the exception of July at WE where mesozooplankton biomass was low but total Chl a was high. This mismatch may be due to the high flushing effect of the Pearl River discharge in summer and a time lag in mesozooplankton population growth. On the other hand, the composition of mesozooplankton was significantly modified by riverine discharge and eutrophication conditions. While small copepods dominated at both sites, the eutrophic estuarine water had a high abundance of barnacle and polychaete larvae, while cladocerans, bivalve larvae, gastropod larvae and chaetognaths mainly occurred at EO. Eutrophication increased the top-down role of copepods in the grazing community, revealed by an increase in the percentage of copepods in the total metazoan mesozooplankton, especially during the period of high river discharge. Moreover, mesozooplankton diversity at the two stations was similar, and they both showed relatively higher diversity during autumn and winter and lower diversity during summer, especially at WE. These results suggest that, despite high nutrient and Chl a concentrations in estuarine waters, mesozooplankton biomass were not enhanced compared to coastal waters with no river impact, possibly due to poor food quality and increased predation in the eutrophic estuarine waters. 相似文献
9.
利用美国国家环境预报中心和美国国家大气科学研究中 心（NCEP/NCAR）再分析月平均气候资料以及Xie和Arkin分析的月平均降水资料（1968～199 8年），针对索马里低空急流（SMJ）的年际变化及其对东亚夏季降水的影响问题展开了分析 研究. 结果揭示，SMJ作为最主要的越赤道气流，对两个半球间水汽输送起最关键的作用， 它把水汽从冬半球输送到夏半球. 夏季SMJ的年际变化有全球范围内的环流异常与之相联系 ，特别是东亚沿岸的波列状异常分布、南亚高压以及澳大利亚以南的偶极型异常分布；它 也同春季的北印度洋等海区的海温异常有密切关系. 研究还表明，春季SMJ的年际变化对东 亚夏季降水和大气环流有显著影响，由于SMJ影响的超前性，因此它在东亚夏季气候预测上 有重要意义. 相似文献
10.
Hexachlorocylcohexanes (HCHs) are pesticides that persist in air and water of the Northern hemisphere. To understand the spatial and temporal variability in HCH levels in estuarine surface waters we measured concentrations of two HCH isomers (-HCH and γ-HCH) at six sites in the York River estuary at bimonthly intervals for a year. Bacterial abundance and activity were also monitored using acridine orange direct counts and uptake of tritiated substrates, respectively. -HCH was consistently observed to be significantly higher in marine water compared to river water entering the estuary, suggesting that the Chesapeake Bay or Atlantic Ocean is a larger source of this compound to the York River estuary compared to riverine input. Moreover, following periods of high freshwater flow into the estuary during spring and early summer, both - and γ-HCH mixing curves indicated an additional source of these pollutants to the estuary such as land-derived runoff or groundwater discharge. In contrast, during low freshwater flow (late summer and fall) the estuary was a sink for HCHs, with γ-HCH more rapidly removed from the estuary than -HCH. During the period of low freshwater flow, concentrations of both - and γ-HCH were negatively correlated with bacterial activity. Bacterial activity as opposed to abundance appears to control HCH degradation in estuarine surface waters. 相似文献
11.
Eelgrass, Zostera marina, is the most abundant seagrass species on the coast of Korea, but most large eelgrass meadows have disappeared since the 1970s due to human-induced disturbance. To restore disturbed seagrass habitats in die-off areas, seagrass transplanting has been attempted using various methods. Here, we assessed the feasibility and efficiency of seagrass transplanting methods depending on the sediment type at the planting site. Additionally, the effect of planting time on the establishment of transplant was examined to determine the optimal season for planting. We conducted an eelgrass transplanting experiment from fall 2003 to summer 2004 at three sites with different sediment types using three planting techniques. The staple method resulted in the highest transplant survival rate (77.1-93.8%) at all three sites, but was labor intensive. Transplanting Eelgrass Remotely with Frame Systems (TERFS) method also resulted in relatively a high survival rate (58.7-69.0%) at all sites. The shell method is a newer eelgrass transplanting method in which oyster shells are used as an anchoring device, and does not require SCUBA diving for subtidal transplanting. The shell method resulted in high survival rates in muddy (81.3%) and silty sediments (76.5%), but remarkably low survival rate in sandy sediments (5.0%). The TERFS, and shell methods reduced underwater labor; thus, these methods is suitable for large-scale seagrass restoration. Eelgrass transplants planted in summer had exhibited significant mortality due to high summer water temperatures. Although transplants planted in fall to spring had relatively high survival rates, transplanting and collection of vegetative shoots are difficult in winter and spring. Therefore, fall was suggested as the most effective transplanting season off the coast of Korea. 相似文献
12.
《Limnologica》2016
We investigated the importance of meteorological and lake physical conditions for temporal, horizontal and vertical differences in the concentration of dissolved oxygen (DO) and water temperature, and the derived daily estimates of gross primary production (GPP), ecosystem respiration (R) and net ecosystem production (NEP). Our study was conducted in a subtropical and polymictic lake in Southern Brazil, during a spring–summer transition. Metabolic rates were determined from two sites using the open water oxygen technique. At the central deep site, oxygen sondes were deployed at three depths to assess patterns in vertical variability. During 10 days, an additional DO and temperature sonde was placed near the shoreline allowing us to compare metabolic differences in the surface layers between the central pelagic and littoral site. While GPP was similar, R was significantly higher at the shallower littoral site, causing NEP to be lower, although NEP was still positive. The littoral site had less diel changes in DO and higher daily variability in all metabolic rates. Variability in GPP and R at the littoral site was related to temperature, wind speed and rainfall suggesting that short-term variability in metabolic rates in shallow areas are sensitive to resuspension of sediments caused by a less stable water column. A clear vertical gradient was furthermore found for the metabolic rates at the central deep part of the lake, related to the light extinction, with highest GPP around 0.3 m and decreasing with depth, while respiration showed the inverse pattern. Below subsurface, respiration prevailed at 5.0 m depth and was uncoupled to primary production. Under conditions with high light and temperature, and low wind speeds, the mixing depth became shallower, in turn increasing the water column stability at the deep pelagic site, which resulted in higher mean light available and higher GPP in the water column. Our results confirm that deployment of sensors in different sites and depths allows for spatially, as well as temporally more representative estimates of lake metabolism. 相似文献
13.
Ozone depression in the polar stratosphere during the energetic solar proton event on 4 August 1972 was observed by the backscattered ultraviolet (BUV) experiment on the Nimbus 4 satellite. Distinct asymmetries in the columnar ozone content, the amount of ozone depressions and their temporal variations above 4 mb level (38 km) were observed between the two hemispheres. The ozone destroying solar particles precipitate rather symmetrically into the two polar atmospheres due to the geomagnetic dipole field These asymmetries can be therefore ascribed to the differences mainly in dynamics and partly in the solar illumination and the vertical temperature structure between the summer and the winter polar atmospheres. The polar stratosphere is less disturbed and warmer in the summer hemisphere than the winter hemisphere since the propagation of planetary wave from the troposphere is inhibited by the wind system in the upper troposphere, and the air is heated by the prolonged solar insolation. Correspondingly, the temporal variations of stratospheric ozone depletion and its recovery appear to be smooth functions of time in the (northern) summer hemisphere and the undisturbed ozone amount is slighily, less than that of its counterpart. On the other hand, the tempotal variation of the upper stratospheric ozone in the winter polar atmosphere (southern hemisphere) indicates large amplitudes and irregularities due to the disturbances produced by upward propagating waves which prevail in the polar winter atmosphere. These characteristic differences between the two polar atmospheres are also evident in the vertical distributions of temperature and wind observed by balloons and rocker soundings. 相似文献
14.
15.
This pilot study uses a chemical technique (KEtX) to establish vertical profiles of average monthly water temperature within river beds: the hyporheic zone at the interface between surface water and groundwater. Data are presented for two gravel-bed sites and one sand-bed site on the River Wissey, Norfolk, UK. From February to October 1992, average monthly temperatures were determined at 10cm intervals down 1 m profiles. A strong seasonal pattern was defined at all sites with hyporheic temperatures being relatively warm in winter and cool in summer. Isothermal periods occurred in March-April and September-October. Temperature ranges within the hyporheic were decreased (4.6–7.7°C) compared with those of surface waters (10 and 10.9°C). Temperature profiles were similar at all sites during winter, but the sand-bed site had relatively low temperatures at a standard depth within the hyporheic during summer and the temperature gradient became isothermal later in the autumn at this site. It is suggested that the influence of flows and substratum characteristics on temperature patterns, especially in regulated rivers, may have significant ecological implications, for example for determining the timing of salmonid fry and invertebrate emergence. 相似文献
16.
Ying Zhang Zhenbo Lv Bo Guan Yuanjin Liu Fan Li Shaowen Li Yuanqing Ma Junbao Yu Yunzhao Li 《洁净——土壤、空气、水》2013,41(10):1027-1034
Laizhou Bay, located in the northwest of Shandong Peninsula, has complex transitional environments between terrestrial and marine ecosystems. In the present study, a total of 122, 131, and 139 species were collected in spring, summer, and autumn 2011, respectively. Species constitutions of macrobenthos were grouped into four phyla, of which annelida were the most abundant phylum, the average biomass proportion of echinodermata was the lowest, and the proportion of important species for mollusca was the highest. The structure of the macrobenthic community showed significant differences between sites, and greater divergence was observed between the third site (S03) and other stations. The ABC plots showed that the biomass curve lay below the abundance curve, and the W‐statistic value was negative. The result of the BOPA index showed that two stations had moderate ecological status in spring and that there were two heavily polluted sites and one moderately polluted site in summer. The BIO‐ENV analyses indicated that the grain‐size fractions together with trace metals (Hg, Pb, Zn, Cu, and Cr) could be considered as the major environmental variables influencing the macrobenthic patterns. The results together demonstrated that the macrobenthic communities in Laizhou Bay were negatively affected, perhaps by the tremendous impact of heavy metals in the sediments. 相似文献
17.
Snow accumulation and melt were observed at shrub tundra and tundra sites in the western Canadian Arctic. End of winter snow water equivalent (SWE) was higher at the shrub tundra site than the tundra site, but lower than total winter snowfall because snow was removed by blowing snow, and a component was also lost to sublimation. Removal of snow from the shrub site was larger than expected because the shrubs were bent over and covered by snow during much of the winter. Although SWE was higher at the shrub site, the snow disappeared at a similar time at both sites, suggesting enhanced melt at the shrub site. The Canadian Land Surface Scheme (CLASS) was used to explore the processes controlling this enhanced melt. The spring‐up of the shrubs during melt had a large effect on snowmelt energetics, with similar turbulent fluxes and radiation above the canopy at both sites before shrub emergence and after the snowmelt. However, when the shrubs were emerging, conditions were considerably different at the two sites. Above the shrub canopy, outgoing shortwave radiation was reduced, outgoing longwave radiation was increased, sensible heat flux was increased and latent flux was similar to that at the tundra site. Above the snow surface at this site, incoming shortwave radiation was reduced, incoming longwave radiation was increased and sensible heat flux was decreased. These differences were caused by the lower albedo of the shrubs, shading of the snow, increased longwave emission by the shrub stems and decreased wind speed below the shrub canopy. The overall result was increased snowmelt at the shrub site. Although this article details the impact of shrubs on snow accumulation and melt, and energy exchanges, additional research is required to consider the effect of shrub proliferation on both regional hydrology and climate. Copyright 2010 John Wiley & Sons Ltd and Crown in the right of Canada. 相似文献
18.
Transplanted mussels and diffusive gradients in thin films (DGT) were used to assess levels of cadmium, copper, lead and zinc at four sites situated inside and outside of two enclosed marinas, near Melbourne, Australia. Mussels accumulated all metals except cadmium. Over one year, there were large temporal differences in tissue metal levels in mussels, but differences between sites were only apparent for copper and lead. DGT showed temporal differences of the same kind as those seen in the mussels, but appeared to have more power to discriminate between sites. DGT found higher copper levels inside marinas than outside, and high levels of zinc were found inside one marina. Levels of copper and zinc were high enough to be of environmental concern for one site. The effect of fouling on DGT measurements will have to be quantified, or avoided by the use of multiple short-term exposures, if the technique is to become a widespread in situ monitoring tool. 相似文献
19.
本文对长江中下游夏季旱涝年前期（3、4、5月的季节平均）和同期（6、7、8月的季节平均）的南半球环流作对比分析,探讨南半球环流异常与长江中下游夏季旱涝的关系. 结果表明前期和同期南半球环流均有显著差异,春季南极涛动对长江中下游旱涝的影响较夏季显著,南半球副热带高压在春、夏两季中有很好季节持续性. 因此,春季南半球环流异常可以作为长江中下游夏季旱涝主要短期气候预测因子. 南、北半球中高纬环流相互作用是长江中下游夏季降水的一个重要因素,其可能的联系机制是从南半球高纬到北半球东亚沿岸经向分布的正压遥相关. 研究中还发现在长江中下游的涝年,整个对流层中南半球春、夏两季有持续增温,这说明了南北半球的温度梯度减弱也是东亚夏季风减弱的原因之一. 相似文献
20.
《中国科学:地球科学(英文版)》2015,(3)
The warming over the Tibetan Plateau(TP) is very significant during last 30 years,but the thermal forcing has been weakened.The thermal weakening is attributed mainly to the enhancement of the TOA(top of atmosphere) outgoing radiation.This enhancement is opposite to the greenhouse-gas-induced weakening of the global mean TOA outgoing radiation and is also unable to be explained by the observed decrease of total cloud cover.This study presents the importance of cloud height change and the warming over the TP in modulating the TOA radiation budget and thus the thermal forcing during spring and summer.On the basis of surface observations and satellite radiation data,we found that both the TOA outgoing shortwave radiation and longwave radiation were enhanced during this period.The former enhancement is due mainly to the increase of low-level cloud cover,which has a strong reflection to shortwave radiation,especially in summer.The latter enhancement is caused mainly by the planetary warming,and it is further enhanced by the decrease of total cloud cover in spring,as clouds extinguish outgoing longwave radiation emitted from the land surface.Therefore,the radiative cooling enhancement and thus the thermal weakening over the TP is a response of the earth-atmosphere system to the unique change of cloud cover configuration and the rapid warming of the land surface.However,these trends in cloud cover and TOA outgoing radiation are not well represented in four reanalyses. 相似文献