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1.
The 2010 Deepwater Horizon oil spill impacted the northern Gulf of Mexico (GOM) during the spring spawning season of Atlantic bluefin tuna (BFT). Overlap between BFT spawning habitat and surface oil in the northern GOM was examined using satellite-derived estimates of oil coverage, and spawning habitat models. Results suggested that although eggs and larvae were likely impacted by oil-contaminated waters in the eastern GOM, high abundances of larvae were located elsewhere, especially in the western GOM. Overall, less than 10% of BFT spawning habitat was predicted to have been covered by surface oil, and less than 12% of larval BFT were predicted to have been located within contaminated waters in the northern GOM, on a weekly basis. Our results provide preliminary but important initial estimates of the effects of the spill on larval BFT mortality, as concern continues over the appropriate management responses to impacts of the spill.  相似文献   

2.
梯级水坝胁迫下东江鱼类产卵场现状分析   总被引：3，自引：2，他引：1  
东江作为珠江流域重要支流之一,是香港地区以及珠江三角洲东部地区重要水源.鱼类不仅是河流生态系统重要组成也是其健康状况的重要指示生物.目前东江已经修建了一系列水坝,基于东江河流生态系统健康维持和管理的需要,有必要了解河流中鱼类产卵场的现状.据此,2010年3-10月对东江古竹江段的鱼苗(卵)进行了连续采集,目前鱼苗种类已鉴定到26种(属),主要优势种类为鰕鯱(63.0%)、赤眼鳟(16.8%)、尼罗罗非鱼(9.7%)、大眼鳜(2.0%)等;调查期间古竹江段鱼苗(卵)径流量初步估算为11.4×108尾(颗),东江鱼类主要繁殖期为4-9月.与东江历史资料及西江江段鱼类繁殖现状相比较,东江鱼类产卵场功能极度退化,不仅鱼类产卵规模已明显下降,并且东江古竹江段鱼苗中洄游性种类缺失;本次调查证明东江龙川江段原有的四大家鱼产卵场已经消失;通过典范对应分析分析表明,东江江段鱼类产卵繁殖对水文生态因子的响应程度较低,说明东江江段水文节律受梯级水坝影响程度大,偏离了鱼类产卵繁殖的生态需求.尼罗罗非鱼在东江古竹江段有暴发性繁殖现象,东江已经面临外来种的严峻威胁,分析认为梯级水坝的修建是导致罗非鱼暴发性增长的主要原因之一.根据目前东江鱼类产卵场现状,建议把鲢、鳙及广东鲂列入增殖放流的品种目录,加强人工增殖放流;从贯通鱼类洄游通道及修复产卵场的需求上考虑,建议在东江各水坝上补建鱼类过鱼通道.  相似文献   

3.
In Australia, multidecadal periods of floods and droughts have major economic consequences. Due to the short duration of Australian instrumental precipitation records, it is difficult to determine the patterns of these multidecadal periods. Proxy records can be used to create long‐term rainfall reconstructions for regions that are lacking instrumental data. However, the spatial extent over which single‐site proxy records can be applied is poorly understood. Southeast Queensland (SEQ) is an area where tree rings can be used to reconstruct long‐term rainfall patterns, but their regional representation is unknown. In this study, the spatial variability in rainfall across SEQ is investigated from 1908 to 2007 using 140 instrumental rainfall stations. Pearson correlation analysis between stations is used to create groups at the r = 0.80, 0.85, and 0.90 correlation levels, and then annual deviations from the mean are determined. These patterns indicate that rainfall is not uniform across SEQ but can be broken into 2 main spatially consistent groups. Each of these groups is broken down into several subgroups with higher correlation levels. Long‐term streamflow records are found to be correlated to rainfall patterns local to the streamflow stations, indicating that analysis of extreme events should consider spatial precipitation variability. Finally, the only currently available proxy rainfall reconstruction for the region, a 140‐year Toona ciliata tree ring width record from Lamington National Park, is compared to rainfall groups at different correlation levels across all of SEQ. The correlation between the reconstruction and the rainfall station groupings is best for the groups within which the tree‐ring record is spatially located, and this correlation improves as rainfall group correlation increases. Correlation is nearly nonexistent for groupings located at a distance from the tree‐ring site. These results demonstrate the importance of assessing the spatial variability of precipitation so that the spatial applicability of proxy records can be assessed.  相似文献   

4.
Cecilia Svensson 《水文研究》1999,13(8):1197-1215
The upper reaches of the Huai River in Central China are located in the East Asian monsoon region. Strong seasonality, as well as large interannual variability of rainfall, causes floods and an uneven supply of water. In order to conserve the water and mitigate the floods, dams and flood protection structures are constructed. Their design requires information about the rainfall. Daily observations from 1957 to 1986 from 78 rain gauges were used to study shape, orientation, movement and geographical and seasonal occurrence of storms in the 79 000 km2 study area. The rainfall characteristics were described using graphical plots, cross‐ and autocorrelation. Storms larger than 50 mm/day were found to occur from February to November, whereas storms exceeding 350 mm/day were confined to the main rainfall season from late June to mid‐August. The southern part of the study area experienced a break in the rainfall season in late July, corresponding to the seasonal northward shift of the rain belt. A weekly periodicity of 7–8 days for rainfall was found during June–July, but not during August–September. During the whole period June–September, the spatial pattern of daily rainfall revealed an elongated shape, more pronounced during June–July than August–September. The rainfall area was orientated approximately from WSW to ENE during the whole period, and showed an anticlockwise rotation of about 16° per day during June–July. The cross‐correlation analysis revealed that the rainfall area moved about 100 km/day eastward. These results and an investigation of meteorological maps indicate that the spatial correlation pattern of daily rainfall is produced by cold fronts on the Mei‐Yu front. Suggestions are made as to how to use the results for the construction of design rainfalls in the study area. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

5.
Extreme floods are the most widespread and often the most fatal type of natural hazard experienced in Europe, particularly in upland and mountainous areas. These ‘flash flood’ type events are particularly dangerous because extreme rainfall totals in a short space of time can lead to very high flow velocities and little or no time for flood warning. Given the danger posed by extreme floods, there are concerns that catastrophic hydrometeorological events could become more frequent in a warming world. However, analysis of longer term flood frequency is often limited by the use of short instrumental flow records (last 30–40 years) that do not adequately cover alternating flood‐rich and flood‐poor periods over the last 2 to 3 centuries. In contrast, this research extends the upland flood series of South West England (Dartmoor) back to ca AD 1800 using lichenometry. Results show that the period 1820 to mid‐1940s was characterized by widespread flooding, with particularly large and frequent events in the mid‐to‐late 19th and early 20th centuries. Since ca 1850 to 1900, there has been a general decline in flood magnitude that was particularly marked after the 1930s/mid‐1940s. Local meteorological records show that: (1) historical flood‐rich periods on Dartmoor were associated with high annual, seasonal and daily rainfall totals in the last quarter of the 19th century and between 1910 and 1946, related to sub‐decadal variability of the North Atlantic Oscillation and receipt of cyclonic and southerly weather types over the southwest peninsula; and (2) the incidence of heavy daily rainfall declined notably after 1946, similar to sedimentary archives of flooding. The peak period of flooding on Dartmoor predates the beginning of gauged flow records, which has practical implications for understanding and managing flood risk on rivers that drain Dartmoor. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

6.
Catchment hydrology is influenced by land‐use change through alteration of rainfall partitioning processes. We compared rainfall partitioning (throughfall, stemflow and interception) and soil water content in three land‐use types (primary forest, secondary forest and agriculture) in the Santa Fe region of Panama. Seasonal patterns were typified by larger volumes of throughfall and stemflow in the wet season, and the size of precipitation events was the main driver of variation in rainfall redistribution. Land‐use‐related differences in rainfall partitioning were difficult to identify due to the high variability of throughfall. However, annual throughfall in agricultural sites made up a larger proportion of gross precipitation than throughfall in forest sites (94 ± 1, 83 ± 6 and 81 ± 1% for agriculture, primary and secondary forests, respectively). Proportional throughfall (% of gross precipitation becoming throughfall) was consistent throughout the year for primary forest, but for secondary forest, it was larger in the dry season than the wet season. Furthermore, proportional stemflow in the dry season was larger in secondary forest than primary forest. Stemflow, measured only in primary and secondary forests, ranged between 0.9 and 3.2% of gross precipitation. Relative soil moisture content in agricultural plots was generally elevated during the first half of the dry season in comparison to primary and secondary forests. Because throughfall is elevated in agricultural plots, we suggest careful management of the spatial distribution and spread of this land‐use type to mitigate potential negative impacts in the form of floods and high erosion rates in the catchment. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

7.
Raise Beck is a mountain torrent located in the central Lake District fells, northern England (drainage area of 1·27 km2). The torrent shows evidence of several major flood events, the most recent of which was in January 1995. This event caused a major channel avulsion at the fan apex diverting the main flood flow to the south, blocking the A591 trunk road and causing local flooding. The meteorological conditions associated with this event are described using local rainfall records and climatic data. Records show 164 mm of rainfall in the 24 hours preceding the flood. The peak flood discharge is reconstructed using palaeohydrological and rainfall–runoff methods, which provide discharge values of 27–74 m3 s?1, and 4–6 m3 s?1, respectively. The flood transported boulders with b‐axes up to 1400 mm. These results raise some important general questions about flood estimation in steep mountain catchments. The geomorphological impact of the event is evaluated by comparing aerial photographs from before and after the flood, along with direct field observations. Over the historical timescale the impact and occurrence of flooding is investigated using lichenometry, long‐term rainfall data, and documentary records. Two major historical floods events are identified in the middle of the nineteenth century. The deposits of the recent and historical flood events dominate the sedimentological evidence of flooding at Raise Beck, therefore the catchment is sensitive to high magnitude, low frequency events. Following the 1995 flood much of the lower catchment was channelized using rip‐rap bank protection, re‐establishing flow north towards Thirlmere. The likely success of this management strategy in containing future floods is considered, based on an analysis of channel capacities. It is concluded that the channelization scheme is only a short‐term solution, which would fail to contain the discharge of an event equivalent to the January 1995 flood. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

8.
The native fish fauna commonly found in the drainage basins of rivers and reservoirs of Latin America, including those of the semi-arid Northeastern Brazil, are representatives of the Neotropical region. This work reports on the reproductive ecology of five commercially important and consumable native fish species, in relation to rainfall and hydrological variables of the semi-arid reservoirs in Brazil. Pluviometric precipitation, temperature, pH, dissolved oxygen and electrical conductivity of the water were registered, and maturation of fish gonads was assessed on a monthly basis. This region is characterized with short spells of rain interspersed with long dry season and rainfall seems to be the main environmental factor which modulates the timing of the spawning period of fish. Construction of reservoirs without adequate facilities for fish migration has an adverse impact on the migratory fish species which are of commercial and ecological importance to semi-arid Northeastern Brazil.  相似文献   

9.
长江流域历史水旱灾害分析   总被引：2，自引：1，他引：1  
黄忠恕  李春龙 《湖泊科学》2003,15(Z1):210-215
长江流域有丰富和长期的水旱灾害史料,最早的水灾和旱灾记载有2000余年的历史,经过系统整理和分析的历史水旱灾害资料有1000余年的旱涝型年表和500余年的旱涝分布图集.在以上资料基础上,对长江流域历史水旱灾害的地域分布特性和时间变化规律进行了初步分析:500余年历史水旱灾害的地域分布显示,流域水旱灾害总体特征是水灾重于旱灾,各级水旱灾害频率的地域分布极不均匀,存在着显著的灾害多发和少发地带,它们与自然地理环境、水系特征、气候条件和社会经济条件等因素有关;1000余年旱涝型年表分析表明,长江流域洪涝和干旱频次在时间上的非均匀分布并非完全随机,表现出多种时间尺度的年际变化特征,其中主要表现为约100a上下的大干湿气候期变化及40a左右的小旱涝期振动.  相似文献   

10.
Climate-driven alterations of hydro-meteorological conditions can change river flow regimes and potentially affect the migration behaviour of fishes and the productivity of important fisheries in the Amazon basin, such as those for the continental-scale migratory goliath catfishes (Brachyplatystoma, Pimelodidae). In this study, we investigated hydrologic responses to climate change using a hydrologic model forced with climate inputs, which integrate historical (2001–2010) observations and general circulation model (GCM) projections under the emission scenario Representative Concentration Pathway 8.5. We developed an empirical model to characterize future (2090–2099) climate-change impacts on goliath catfish spawning migrations as a function of river flow depth dynamics at the upstream elevational limit of spawning (250 m) in headwater basins of the Amazon. The model results revealed spatially variable impacts of climate change on the catfish spawning migrations. The Marañón, Ucayali, Juruá, Purus, and Madeira basins had a predicted increase in the annual mean (3–8%) and maximum (1.1–4.9%) spawning migration rate (i.e., the fraction of fish that migrate to the spawning grounds in a day), mainly due to the lengthened rising phase of flow-driven migratory events during wet seasons. The Caquetá-Japurá, Putumayo-Içá, Napo, and Blanco rivers had predicted decreases (3–7%) in the mean migration rate because of decreases in the length of the rising season of flow depth and the frequency of migratory events. The predicted timing of fish spawning migrations (quantified by the temporal centroid of migration rates) was delayed by 7–10 days in the west-central and southwest regions and was 8 days earlier in the northwest and northcentral areas, due to changes in the onset of the rising season. We established a river depth baseline that controls the onset of goliath catfish spawning migration. This depth varies between 0.9–5.6 m across study sites. We found that the estimated depth baseline was most sensitive to uncertainties in river width and cross-sectional channel shape. These results may help inform sustainable adaptation strategies for ecosystem conservation and local fisheries management in the Amazon basin.  相似文献   

11.
Equatorial Pacific sea surface temperature variations interact with processes of atmospheric circulation, creating conditions for the occurrence of El Niño–Southern Oscillation (ENSO). ENSO events represent the most important interannual phenomena affecting climate patterns worldwide and causing significant socio‐economic impacts. In the Brazilian territory, ENSO leads to an increase in drought episodes in the north‐eastern region and an increase in precipitation in the southern region, whereas the effects over the south‐east region are yet not well understood. The main goal of this study is to compare variations of isotopic composition in precipitation across the south‐east portion of the Brazilian territory during two very strong ENSO events: 1997–1998 (ENSO 1) and 2014–2016 (ENSO 2). Daily isotopic records, available from the Global Network of Isotopes in Precipitation database for ENSO 1, and samples collected during ENSO 2 were used to compare the influence of both events on the isotopic composition of precipitation. Seasonal variations indicated more depleted precipitation during the wet seasons (δ18O = ?5.4 ± 4.0‰) and enriched precipitation during the dry seasons (δ18O = ?2.8 ± 2.3‰). Observed rainfall variations were associated with atmospheric large‐scale processes and moisture transport from the Amazon region, whereas extreme values (enriched or depleted) appear to be associated with particular convective and stratiform precipitation events. Overall, more depleted isotopic composition of precipitation (δ18O = ?4.60‰) and higher d‐excess (up to +15‰) were observed during the dry season of ENSO 1 when compared with ENSO 2 dry season (δ18O‰ = ?2.80‰, d‐excess lower than +14‰). The latter is explained by greater atmospheric moisture content, particularly associated with recycling of transpiration fluxes from the Amazon region, during dry season of ENSO 1. No significant differences for δ18O and δ2H were observed during the wet season; however, d‐excess from ENSO 2 was greater than ENSO 1, due to the slightly greater atmospheric moisture content and very strong upward motion observed. Our findings highlight the opportunity that environmental isotopes offer towards understanding hydrometeorological processes, particularly, the evolution of extreme climatic events of global resonance such as ENSO.  相似文献   

12.
In this study, summer rainfall contributions to streamflow were quantified in the sub‐arctic, 30% glacierized Tarfala (21.7 km2) catchment in northern Sweden for two non‐consecutive summer sampling seasons (2004 and 2011). We used two‐component hydrograph separation along with isotope ratios (δ18O and δD) of rainwater and daily streamwater samplings to estimate relative fraction and uncertainties (because of laboratory instrumentation, temporal variability and spatial gradients) of source water contributions. We hypothesized that the glacier influence on how rainfall becomes runoff is temporally variable and largely dependent on a combination of the timing of decreasing snow cover on glaciers and the relative moisture storage condition within the catchment. The results indicate that the majority of storm runoff was dominated by pre‐event water. However, the average event water contribution during storm events differed slightly between both years with 11% reached in 2004 and 22% in 2011. Event water contributions to runoff generally increased over 2011 the sampling season in both the main stream of Tarfala catchment and in the two pro‐glacial streams that drain Storglaciären (the largest glacier in Tarfala catchment covering 2.9 km2). We credit both the inter‐annual and intra‐annual differences in event water contributions to large rainfall events late in the summer melt season, low glacier snow cover and elevated soil moisture due to large antecedent precipitation. Together amplification of these two mechanisms under a warming climate might influence the timing and magnitude of floods, the sediment budget and nutrient cycling in glacierized catchments. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

13.
Tropical river basins are experiencing major hydrological alterations as a result of climate variability and deforestation. These drivers of flow changes are often difficult to isolate in large basins based on either observations or experiments; however, combining these methods with numerical models can help identify the contribution of climate and deforestation to hydrological alterations. This paper presents a study carried out in the Tapaj?s River (Brazil), a 477,000 km2 basin in South‐eastern Amazonia, in which we analysed the role of annual land cover change on daily river flows. Analysis of observed spatial and temporal trends in rainfall, forest cover, and river flow metrics for 1976 to 2008 indicates a significant shortening of the wet season and reduction in river flows through most of the basin despite no significant trend in annual precipitation. Coincident with seasonal trends over the past 4 decades, over 35% of the original forest (140,000 out of 400,000 km2) was cleared. In order to determine the effects of land clearing and rainfall variability to trends in river flows, we conducted hindcast simulations with ED2 + R, a terrestrial biosphere model incorporating fine scale ecosystem heterogeneity arising from annual land‐use change and linked to a flow routing scheme. The simulations indicated basin‐wide increases in dry season flows caused by land cover transitions beginning in the early 1990s when forest cover dropped to 80% of its original extent. Simulations of historical potential vegetation in the absence of land cover transitions indicate that reduction in rainfall during the dry season (mean of ?9 mm per month) would have had an opposite and larger magnitude effect than deforestation (maximum of +4 mm/month), leading to the overall net negative trend in river flows. In light of the expected increase in future climate variability and water infrastructure development in the Amazon and other tropical basins, this study presents an approach for analysing how multiple drivers of change are altering regional hydrology and water resources management.  相似文献   

14.
Trends in the timing and magnitude of floods in Canada   总被引：2，自引：0，他引：2  
This study investigates trends in the timing and magnitude of seasonal maximum flood events across Canada. A new methodology for analyzing trends in the timing of flood events is developed that takes into account the directional character and multi-modality of flood occurrences. The methodology transforms the directional series of flood occurrences into new series by defining a new location of the origin. A test of flood seasonality (multi-modality) is then applied to identify dominant flood seasons. Floods from the dominant seasons are analyzed separately by a seasonal trend analysis. The Mann–Kendall test in conjunction with the method of pre-whitening is used in the trend analysis. Over 160 streamflow records from one common observation period are analyzed in watersheds with relatively pristine and stable land-use conditions. The results show weak signals of climate variability and/or change present in the timing of floods in Canada during the last three decades. Most of the significant trends in the timing of spring snowmelt floods are negative trends (earlier flood occurrence) found in the southern part of Canada. There are no significant trends identified in the timing of fall rainfall floods. However, the significance of the fall, rainfall-dominated flood season has been increasing in several analyzed watersheds. This may indicate increasing intensity of rainfall events during the recent years. Trends in the magnitude of floods are more pronounced than the trends in the timing of floods. Almost one fifth of all the analyzed stations show significant trends in the magnitude of snowmelt floods. Most of the significant trends are negative trends, suggesting decreasing magnitudes of snowmelt floods in Canada over the last three decades. Significant negative trends are found particularly in southern Ontario, northern Saskatchewan, Alberta and British Columbia. There are no significant trends in the magnitude of rainfall floods found in the analyzed streamflow records. The results support the outcomes of previous streamflow trend studies conducted in Canada.  相似文献   

15.
Global climate change is one of the most serious issues we are facing today. While its exact impacts on our water resources are hard to predict, there is a general consensus among scientists that it will result in more frequent and more severe hydrologic extremes (e.g. floods, droughts). Since rainfall is the primary input for hydrologic and water resource studies, assessment of the effects of climate change on rainfall is essential for devising proper short-term emergency measures as well as long-term management strategies. This is particularly the case for a region like the Korean Peninsula, which is susceptible to both floods (because of its mountainous terrain and frequent intense rainfalls during the short rainy season) and droughts (because of its smaller area, long non-rainy season, and lack of storage facilities). In view of this, an attempt is made in the present study to investigate the potential impacts of climate change on rainfall in the Korean Peninsula. More specifically, the dynamics of ‘present rainfall’ and ‘future rainfall’ at the Seoul meteorological station in the Han River basin are examined and compared; monthly scale is considered in both cases. As for ‘present rainfall,’ two different data sets are used: (1) observed rainfall for the period 1971–1999; and (2) rainfall for the period 1951–1999 obtained through downscaling of coarse-scale climate outputs produced by the Bjerknes Center for Climate Research-Bergen Climate Model Version 2 (BCCR-BCM2.0) climate model with the Intergovernmental Panel on Climate Change Special Report on Emission Scenarios (IPCC SRES) 20th Century Climate in Coupled Models (20C3M) scenario. The ‘future rainfall’ (2000–2099) is obtained through downscaling of climate outputs projected by the BCCR-BCM2.0 with the A2 emission scenario. For downscaling of coarse-scale climate outputs to basin-scale rainfall, a K-nearest neighbor (K-NN) technique is used. Examination of the nature of rainfall dynamics is made through application of four methods: autocorrelation function, phase space reconstruction, correlation dimension, and close returns plot. The results are somewhat mixed, depending upon the method, as to whether the rainfall dynamics are chaotic or stochastic; however, the dynamics of the future rainfall seem more on the chaotic side than on the stochastic side, and more so when compared to that of the present rainfall.  相似文献   

16.
The occurrence of devastating floods in the British uplands during the first two decades of the twenty‐first century poses two key questions: (1) are recent events unprecedented in terms of their frequency and magnitude; and (2) is climate and/or land‐use change driving the apparent upturn in flooding? Conventional methods of analysing instrumental flow records cannot answer these questions because upland catchments are usually ungauged, and where records do exist they rarely provide more than 30–40 years of data. In this paper we analyse all lichen‐dated upland flood records in the United Kingdom (UK) to establish the longer‐term context and causes of recent severe flooding. Our new analysis of torrential sedimentary deposits shows that twenty‐first century floods are not unprecedented in terms of both their frequency (they were more frequent before 1960) and magnitude (the biggest events occurred during the seventeenth–nineteenth centuries). However, in some areas recent floods have either equalled or exceeded the largest historical events. The majority of recent floods have been triggered by torrential summer downpours related to a marked negative phase of the summer North Atlantic Oscillation (NAO) between 2007 and 2012. It is of concern that historical data suggests there is far more capacity in the North Atlantic climate system to produce wetter and more prolonged flood‐rich periods than hitherto experienced in the twenty‐first century. Looking forwards, an increased likelihood of weather extremes due to climate change means that geomorphological based flood series extensions must be placed at the centre of flood risk assessment in the UK uplands and in similar areas worldwide. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

17.
Blue crab larvae are advected out of Middle Atlantic Bight estuaries immediately after spawning occurs in the estuary entrance. For the next 30 to 50 days the larvae are found offshore and mainly at the surface where they are influenced by wind-driven currents. Using a previously derived circulation model and winds from Norfolk (VA) airport, a backward trace is made from where relatively dense concentrations of megalopae were found in the Chesapeake Bight during 1983 to a point of origin (spawning).During 1983, the megalopae encountered on the shelf had their origin in Chesapeake Bay and took, at minimum, 31 to 36 days to grow to the megalopae stage. Wind forcing dominated the inner shelf region in the summer of 1983 and the resulting dispersion of Chesapeake Bay megalopae occurred briefly in the southern sector early in the season, but toward the northern sector over most of the season. Although no firm conclusions could be drawn regarding the mechanism for return, it did not seem likely that wind advection back to the point of origin would be effective.  相似文献   

18.
From September 2000 to December 2006, surface plankton samples were collected on a monthly basis, from a station located in the southern Bay of Biscay (43°37N; 1°43W France), near the deep Capbreton canyon. In this paper, the results for the ichthyoplankton assemblage are presented. Among the 62 taxa recorded, only 35 were present in the larval stage whilst only 10 were represented by their eggs. Taxa represented by both stages (eggs+larvae; N=17) had the highest abundance. The presence in the surface plankton assemblage of species, at either or both stage, is interpreted within the context of the bathymetric distribution of species. The maxima in abundance and diversity occurred in February–March, for eggs, and May–June, for larvae. This 3-month time-lag between the stages is proposed to be related to the timing of egg spawning and larval recruitment to the pelagic environment. Mean egg abundances (82.4±29.8 eggs/10 m2) were 10-fold higher than the larval abundances (7.1±1.8 larvae/10 m2). Despite pronounced monthly variability, no statistically significant decrease in either egg or larvae abundance was observed during this 6-year study period. Compared with previous published studies, this study shows that the peak in ichthyoplankton diversity occurred two months earlier. In addition, the spawning period occurred over the whole year, even during autumn and winter. Using ordination techniques, the annual sequence appearance of taxa are described at the study site: Gadiforms, Ammodytidae and Pleuronectiforms were present during the winter whilst Sparidae, Blennidae, Labridae and Gobiidae, formed the summer group. Only three species, European anchovy Engraulis encrasicolus, European pilchard Sardina pilchardus and Atlantic horse mackerel Trachurus trachurus were recorded throughout the year.  相似文献   

19.
Rain‐on‐snow events have generated major floods around the world, particularly in coastal, mountainous regions. Most previous studies focused on a limited number of major rain‐on‐snow events or were based primarily on model results, largely due to a lack of long‐term records from lysimeters or other instrumentation for quantifying event water balances. In this analysis, we used records from five automated snow pillow sites in south coastal British Columbia, Canada, to reconstruct event water balances for 286 rain‐on‐snow events over a 10‐year period. For large rain‐on‐snow events (event rainfall >40 mm), snowmelt enhanced the production of water available for run‐off (WAR) by approximately 25% over rainfall alone. For smaller events, a range of antecedent and meteorological factors influenced WAR generation, particularly the antecedent liquid water content of the snowpack. Most large events were associated with atmospheric rivers. Rainfall dominated WAR generation during autumn and winter events, whereas snowmelt dominated during spring and summer events. In the majority of events, the sensible heat of rain contributed less than 10% of the total energy consumed by snowmelt. This analysis illustrated the importance of understanding the amount of rainfall occurring at high elevations during rain‐on‐snow events in mountainous regions.  相似文献   

20.
Forest clearing and conversion to cattle pasture in the lowland Amazon region has been linked to soil compaction and increased soil water storage, which combine to diminish soil infiltration, enhance quick lateral flows and increase the stream flow response to precipitation. Quantifying the importance of quick surficial flow in response to this land use change requires identification of water sources within catchments that contribute to stream flow. Using an end member mixing analysis approach, potential contributing sources of stream flow were evaluated during an entire rainy season in a forest and a pasture watershed drained by ephemeral‐to‐intermittent streams in the south‐western Amazon. Water yield was 17% of precipitation in the pasture and 0·8% of precipitation in the forest. During the early rainy season, throughfall, groundwater, and soil water contributed 79%, 18%, and 3%, respectively, to total forest stream flow. Over the entire rainy season, throughfall, groundwater, and shallow soil water provided 57%, 24%, and 19%, respectively, of stream flow. In the pasture watershed, overland flow dominated stream flow both in the early (67%) and late (57%) rainy season, with a mean contribution of 60% overland flow, 35% groundwater, and 5% soil water. The uncertainty associated with those estimates was studied using a Monte Carlo approach. In addition to large changes in total surface flow, marked differences were found in the proportions of total stream flow in the second half of the rainy season between the forest and pasture watershed. These results suggest that (1) there is great potential for alteration of the hydrological budgets of larger watersheds as the proportion of deforested land in the Amazon increases, and (2) as more rainfall is diverted into fast flowpaths to streams in established pastures, the potential to deliver water with higher solute concentrations generated by erosion or by bypassing sites of solute removal increases. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

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