Patterns of Phytoplankton Variability in the Tagus Estuary (Portugal)

Data were collected in the Tagus estuary from 1999–2007 on a monthly basis and combined with published results and for several previous years between 1980 and 1995, so that a comprehensive analysis could be performed over a non-continuous 27-year period. Sampling conditions and methods were similar for all datasets. Extreme wet and dry years were observed. River flow was strongly linked to phytoplankton abundance, with the highest biomass attained in dry years. The observed range of annual median Chl a was 1.8–7.6 µg L^?1 and the overall median was 3.5 µg L^?1. Dissolved inorganic nitrogen (DIN) and silicate showed a clear seasonal pattern, with a maximum in winter?spring, indicating a freshwater origin. Although wastewater treatment started in 1990, no difference was detected from 1980 to the present in terms of DIN and phosphorus. The recorded seasonal pattern for biomass with highest values in late spring–summer period is comparable to other temperate tidally influenced ecosystems. In spite of interannual differences in terms of Chl a concentration or the time of the maximum Chl a occurrence, a repeatable pattern could be identified. The mean growth development time for phytoplankton was 163 days (June 12) ranging 129–206 days (May 9–July 26) during the sampling period. No obvious changes in phytoplankton community structure were observed over time: diatoms were always the dominant group, and cryptophytes were relatively abundant throughout autumn–winter. The dominant species have remained essentially the same since 1969. River inflow, light availability, and temperature were the major factors shaping phytoplankton variability patterns. The strong influence of tidal mixing on the estuarine waters appears to lower the risk of potential eutrophication in the Tagus estuary. The lack of change in nutrients and phytoplankton biomass and composition observed in this study is an important contribution towards the assessment of natural variability versus responses to man-induced inputs in this severely anthropogenically disturbed estuary.     

Effects of Geomorphology on the Distribution of Metal Abundance in Salt Marsh Sediment

The purpose of this study is to investigate the effects of salt marsh geomorphology on the distribution of surface metal content. Surface sediment samples (46) were taken along two transects across an intertidal salt marsh island. Abundance of Ca, Al, Fe, Ni, Cr, Cu, Zn, Rb, Sr, As, and Zr vary up to 300?% with statistically significant differences along transects and between transects. Along transect, metal abundances are strongly influenced by elevation and distance from the subtidal channel. Between transects, salt marsh morphodynamics, relative marsh age, and water sources seem to affect metal distribution. These observations lead us to hypothesize that at least three scales of variability can be expected for assessing salt marsh sediment metal distributions. Larger-scale variations associated with salt marsh sediment sources and age, and another related to local salt marsh geomorphic structure. Therefore, studies that characterize salt marsh metal loading should explicitly consider the variability imposed over a range of spatial scales.     

Abiotic Limitation of Non-native Plants in the High Salt Marsh Transition Zone

Native plants in the upland to high-marsh transition zone of southern California salt marshes are mostly perennials and therefore experience the abiotic stress of low soil moisture and high soil salinity throughout much of the year. However, many annual non-native plants reproduce during the brief period of reduced salinity and increased moisture during winter rainfall. We investigated the seasonal and spatial variation in vegetation and soil properties of the transition zone using an observational study. Next, we explored the potential for managing non-native plants using a field experiment with varying timing, quantity, and frequency of salt addition treatments. The observational study showed that the distribution of non-native plants is related to changes in soil salinity and soil moisture that accompany changes in elevation, although there are variations among species. In the field experiment, salt was effective at reducing non-native plant cover, but the timing of treatment was important. Although additional work is needed to refine the salt treatments, this work supports the idea that altering abiotic conditions can effectively reduce the presence of non-native species in the upland to high-marsh transition zone.     

The pathway of mercury in contaminated waters determined by association with organic carbon (Tagus Estuary,Portugal)

Reactive dissolved Hg (Hg_R), non-reactive dissolved Hg (Hg_NR), particulate Hg (Hg_P), dissolved organic C (DOC), particulate organic C (POC), salinity and other interpretative parameters were determined in water samples collected in the North Channel and in adjacent areas of the Tagus estuary (Portugal). Higher concentrations of both dissolved and particulate Hg in the North Channel indicate a pollution source and raise the possibility of Hg escaping to adjacent areas by tidal action. This transport was confirmed by the increase of Hg_R with salinity and Hg_NR with DOC, along a longitudinal axis paralleling the North Channel. Apparently, Hg leaving this channel is progressively complexed by inorganic and organic ligands. Near the mouth of the estuary, values decreased reflecting dilution with seawater. Moreover the Hg_P:POC ratio also increased seaward, suggesting mixing with Hg enriched particles that escaped the North Channel, or incorporation of dissolved Hg species in river-derived particles. These results suggest that the pathway of anthropogenic Hg in contaminated waters may be identified by their enrichment in organic matter, both in the dissolved and particulate fraction.     

Late Quaternary valley-fill succession of the Lower Tagus Valley, Portugal

The Lower Tagus Valley in Portugal contains a well-developed valley-fill succession covering the complete Late Pleistocene and Holocene periods. As large-scale stratigraphic and chronologic frameworks of the Lower Tagus Valley are not yet available, this paper describes facies, facies distribution, and sedimentary architecture of the late Quaternary valley fill. Twenty four radiocarbon ages provide a detailed chronological framework. Local factors affected the nature and architecture of the incised valley-fill succession. The valley is confined by pre-Holocene deposits and is connected with a narrow continental shelf. This configuration facilitated deep incision, which prevented large-scale marine flooding and erosion. Consequently a thick lowstand systems tract has been preserved. The unusually thick lowstand systems tract was probably formed in a previously (30,000–20,000 cal BP) incised narrow valley, when relative sea-level fall was maximal. The lowstand deposits were preserved due to subsequent rapid early Holocene relative sea-level rise and transgression, when tidal and marine environments migrated inland (transgressive systems tract). A constant sea level in the middle to late Holocene, and continuous fluvial sediment supply, caused rapid bayhead delta progradation (highstand systems tract). This study shows that the late Quaternary evolution of the Lower Tagus Valley is determined by a narrow continental shelf and deep glacial incision, rapid post-glacial relative sea-level rise, a wave-protected setting, and large fluvial sediment supply.     

An Experimental Evaluation of Dock Shading Impacts on Salt Marsh Vegetation in a New England Estuary

Docks constructed over salt marsh can reduce vegetation production and associated ecosystem services. In Massachusetts, there is a 1:1 height-to-width ratio (H:W) dock design guideline to reduce such impacts, but this guideline’s efficacy is largely untested. To evaluate dock height effects on underlying marsh vegetation and light availability, we deployed 1.2-m-wide experimental docks set at three different heights (low (0.5:1 H:W), intermediate (1:1 H:W), and high (1.5:1 H:W)) in the high and low marsh zones in an estuary in Massachusetts, USA. We measured temperature, light, vegetation community composition, and stem characteristics under the docks and in unshaded control plots over three consecutive growing seasons. Temperature and light were lower under all docks compared with controls; both increased with dock height. Maximum stem height and nitrogen content decreased with available light. In the Spartina patens-dominated high marsh, stem density and biomass were significantly lower than controls under low and intermediate but not high docks. Spartina alterniflora, the dominant low marsh vegetation, expanded into the high marsh zone under docks. S. alterniflora aboveground biomass significantly differed among all treatments in the low marsh, while stem density was significantly reduced for low and intermediate docks relative to controls. Permit conditions and guidelines based on dock height can reduce dock impacts, but under the current guideline of 1:1 H:W, docks will still cause significant adverse impacts to vegetation. Such impacts may interfere with self-maintenance processes (by decreasing sediment capture) and make these marshes less resilient to other stressors (e.g., climate change).     

Spectral Irradiance, Phytoplankton Community Composition and Primary Productivity in a Salt Marsh Estuary, North Inlet, South Carolina, USA

We investigated spatial and temporal changes in spectral irradiance, phytoplankton community composition, and primary productivity in North Inlet Estuary, South Carolina, USA. High concentrations of colored dissolved organic matter (CDOM) were responsible for up to 84 % of the attenuation of photosynthetically available radiation (PAR). Green-yellow wavelengths were the predominant colors of light available at the two sampling sites: Clam Bank Creek and Oyster Landing. Vertical attenuation coefficients of PAR were 0.7–2.1 m^?1 with corresponding euphotic zone depths of 1.5–6.7 m. Phytoplankton biomass (as chlorophyll a [chl a]) varied seasonally with a summer maximum of 16 μg chl a l^?1 and a winter minimum of 1.4 μg chl a l^?1. The phytoplankton community consisted mainly of diatoms, prasinophytes, cryptophytes and haptophytes, with diatoms and prasinophytes accounting for up to 67 % of total chl a. Changes in phytoplankton community composition showed strongest correlations with temperature. Light-saturated chl a-specific rates of photosynthesis and daily primary productivity varied with season and ranged from 1.6 to 14 mg C (mg chl a) ^?1?h^?1 (32–803 mg C m^?3?day^?1). Calculated daily rates added up to an annual carbon fixation rate of 84 g C m^?3?year^?1. Overall, changes in phytoplankton community composition and primary productivity in North Inlet showed a strong dependence on temperature, with PAR and spectral irradiance playing a relatively minor role due to short residence times, strong tidal forcing and vertical mixing.     

长江口盐沼土壤有机质分布与矿化的空间差异

在长江口崇明东滩两类植被(互花米草、土著植被)区域分别选择一条纵向剖面,在高潮滩、中潮滩及光滩取得柱状样,利用颗粒有机碳(POC)含量、碳稳定同位素组成、土壤C/N比与颗粒组成等资料,研究盐沼有机质的分布与矿化特征。结果表明,两个纵向剖面的相同高程部位柱样之间,颗粒有机碳δ13C与POC含量的深度特征均存在显著差异;两个柱样的δ13C与POC含量的平均值均相差较大。互花米草对高潮滩柱样有机质的含量与组成均产生了明显影响,对中潮滩柱样有机质组成已有一定影响;土著植被对高潮滩柱样有机质组成的影响显著。盐沼植被对土壤有机质的分布与矿化均产生了明显影响。高潮滩柱样矿化阶段不同的有机质组分混杂,中潮滩柱样有机质的组成相对简单,矿化程度较弱。柱样粘粒含量与含水量在垂向上变化频繁,盐沼原始沉积层序对柱样的物质分布特征具有本底制约。不同高程部位柱样之间,有机质的深度分布特征以及矿化程度差异显著,盐沼碳动态受到潮滩特征性动力沉积过程的显著影响。     

Salt Marsh and Fringing Oyster Reef Transgression in a Shallow Temperate Estuary: Implications for Restoration,Conservation and Blue Carbon

The importance of intertidal estuarine habitats, like salt marsh and oyster reef, has been well established, as has their ubiquitous loss along our coasts with resultant forfeiture of the ecosystem services they provide. Furthering our understanding of how these habitats are evolving in the face of anthropogenic and climate driven changes will help improve management strategies. Previous work has shown that the growth and productivity of both oyster reefs and salt marshes are strongly linked to elevation in the intertidal zone (duration of aerial exposure). We build on that research by examining the growth of marsh-fringing oyster reefs at yearly to decadal time scales and examine movement of the boundary between oyster reef and salt marsh at decadal to centennial time scales. We show that the growth of marsh-fringing reefs is strongly associated to the duration of aerial exposure, with little growth occurring below mean low water and above mean sea level. Marsh-shoreline movement, in the presence or absence of fringing oyster reefs, was reconstructed using transects of sediment cores. Carbonaceous marsh sediments sampled below the modern fringing oyster reefs indicate that marsh shorelines within Back Sound, North Carolina are predominantly in a state of transgression (landward retreat), and modern oyster-reef locations were previously occupied by salt marsh within the past two centuries. Cores fronting transgressive marsh shorelines absent fringing reefs sampled thinner and less extensive carbonaceous marsh sediment than at sites with fringing reefs. This indicates that fringing reefs are preserving carbonaceous marsh sediment from total erosion as they transgress and colonize the exposed marsh shoreline making marsh sediments more resistant to erosion. The amount of marsh sediment preservation underneath the reef scales with the reef’s relief, as reefs with the greatest relief were level with the marsh platform, preserving a maximum amount of carbonaceous sediments during transgression by buffering the marsh from erosional processes. Thus, fringing oyster reefs not only have the capacity to shelter shorelines but, if located at the ideal tidal elevation, they also keep up with accelerating sea-level rise and cap carbonaceous sediments, protecting them from erosion, as reefs develop along the marsh.     

Wave Attenuation at a Salt Marsh Margin: A Case Study of an Exposed Coast on the Yangtze Estuary

To quantify wave attenuation by (introduced) Spartina alterniflora vegetation at an exposed macrotidal coast in the Yangtze Estuary, China, wave parameters and water depth were measured during 13 consecutive tides at nine locations ranging from 10 m seaward to 50 m landward of the low marsh edge. During this period, the incident wave height ranged from <0.1 to 1.5 m, the maximum of which is much higher than observed in other marsh areas around the world. Our measurements and calculations showed that the wave attenuation rate per unit distance was 1 to 2 magnitudes higher over the marsh than over an adjacent mudflat. Although the elevation gradient of the marsh margin was significantly higher than that of the adjacent mudflat, more than 80% of wave attenuation was ascribed to the presence of vegetation, suggesting that shoaling effects were of minor importance. On average, waves reaching the marsh were eliminated over a distance of ∼80 m, although a marsh distance of ≥100 m was needed before the maximum height waves were fully attenuated during high tides. These attenuation distances were longer than those previously found in American salt marshes, mainly due to the macrotidal and exposed conditions at the present site. The ratio of water depth to plant height showed an inverse correlation with wave attenuation rate, indicating that plant height is a crucial factor determining the efficiency of wave attenuation. Consequently, the tall shoots of the introduced S. alterniflora makes this species much more efficient at attenuating waves than the shorter, native pioneer species in the Yangtze Estuary, and should therefore be considered as a factor in coastal management during the present era of sea-level rise and global change. We also found that wave attenuation across the salt marsh can be predicted using published models when a suitable coefficient is incorporated to account for drag, which varies in place and time due to differences in plant characteristics and abiotic conditions (i.e., bed gradient, initial water depth, and wave action).     

Holocene flooding history of the Lower Tagus Valley (Portugal)

The present paper aims to reconstruct the Lower Tagus Valley flooding history for the last ca. 6500 a, to explore the suitability of pollen‐based local vegetation development in supporting the reconstruction of flooding history, and to explain fluvial activity changes in terms of allogenic (climate, human impact) and autogenic (system intrinsic) processes. The flooding history has been determined by cored sedimentary records located ～18 km apart in distal, low‐energy backswamps on both sides of the Tagus channel. In these low‐energy backswamps, fine‐grained sediment layers deposited from suspended load of overbank flood water reflect periods with multiple overbank floods. By means of a multi‐proxy approach (sedimentology, magnetic susceptibility, grain size, loss‐on‐ignition, carbonate content and pollen), sedimentary and environmental changes were identified. At both sites, synchronous lithological intervals accumulated, suggesting a common origin for the changes in fluvial activity since ca. 6500 cal. a BP. Based on lithological changes, three phases of high fluvial activity (6500–5500, 4900–3500 and 1000–0 cal. a BP) and two phases of low fluvial activity (5500–4900 and 3500–1000 cal. a BP) were identified. Two periods with dominant allogenic controls on fluvial activity in the Lower Tagus Valley were identified: relative sea level (6500–5500 cal. a BP) and human impact (1000–0 cal. a BP). During the intermediate period, changes in fluvial activity may have been caused by climate (5500–1000 cal. BP), but unambiguous correlations are difficult to make. This is due to the way allogenic controls are translated through the fluvial system, the geomorphological differences between upstream and downstream studies and autogenic processes. The comparison of local vegetation development and flooding phases as reconstructed using sedimentology shows a limited added value of using local palynology as a proxy for fluvial activity. Copyright © 2010 John Wiley & Sons, Ltd.     

Middle Terrace Deposits of the Tagus River in Alpiarça, Portugal, in Relation to Early Human Occupation

The stratigraphy and sedimentology of the Q3 middle terrace alluvial sequence in the lower Tagus river valley, Portugal, were studied near the village of Alpiarça, approximately 40 km upstream from the estuarine area. Two main stratigraphic units were recognized, separated by an important uncomformity. The Lower Gravels unit (LG) consists of intercalations of medium to coarse gravel deposits, mainly quartzitic, with coarse sandy matrix, organized in tabular bodies. The overlying Upper Sands unit (US) consists of tabular sandy channel deposits and overbank fines, the latter containing well-developed paleosols and backswamp deposits, showing a general aggrading trend, apparently with varying rates; available data indicate that deposition of the US took place under temperate climatic conditions. Within US deposits are several paleolithic archaeological sites, the lower ones in the alluvial stratigraphy being Middle Acheulian, whereas those embedded in overlying deposits are, from bottom to top, Upper Acheulian and Micoquian. Some of these sites have been recently excavated. The quartzite artifacts were apparently abandoned by early humans on the flood plain surface during deposition of the US unit and were subjected to limited reworking during their incorporation in the alluvium. TL/OSL dating of sandy-silty sediments, though imprecise, support archaeological evidence pointing to an age of 150,000 to 70,000 yr B.P. for the US unit.     

Plasticity,Not Adaptation to Salt Level,Explains Variation Along a Salinity Gradient in a Salt Marsh Perennial

Evolutionary ecologists have long been intrigued by the fact that many plant species can inhabit a broad range of environmental conditions and that plants often exhibit dramatic differences in phenotype across environmental gradients. We investigated responses to salinity treatments in the salt marsh plant Borrichia frutescens to determine if the species is responding to variation in edaphic salt content through phenotypic plasticity or specialized trait response. We grew seedlings from fruits collected in high- and low-salt microhabitats, assigned seedlings to high- and low-salt treatments in a greenhouse, and measured traits related to salt tolerance. All traits were highly plastic in response to salinity. Plants from the two microhabitats did not differ in trait means or respond differently to the treatments. These results suggest that environmental differences between the two microhabitats are not creating genotypes adapted to high and low salt levels. In addition, despite evidence for variation in allozyme markers in this population, there was no significant genotypic variation (family effect) in any of the trait means measured across microhabitats. There was variation in plasticity for only leaf Na and leaf B concentration. The high degree of plasticity for all traits and the lack of differences among microhabitats across the salinity gradient suggest plasticity in many traits may be fixed for this species.     

137Cs剖面的沉积信息提取——以香港贝澳湿地为例

通过建立模型,对采自香港大屿山岛贝澳湿地三个站位的沉积样品的137Cs实测剖面与模型计算剖面的比较分析,得出三个站位的沉积速率分别为0.24 cm/a、0.21 cm/a、0.34 cm/a,除P1站位外,P2、P3站位由于受到潮水的作用,表层沉积物受到侵蚀,侵蚀量分别为4.8 cm和7 cm。该结果与常用的时标方法所的结果进行了比较,P1站位由上述模型所得的结果与常用的时标方法所得的结果一致。可以看出,在稳定的沉积环境下,常用的确定时标的方法可以较准确地计算出沉积速率,而在堆积与侵蚀变化的沉积环境下,常用的确定时标的方法就存在一定的缺陷。该工作为137Cs剖面的沉积信息提取进行了有益的尝试。但上述模型中,由于采用日本东京地区的137Cs大气沉降通量记录替代香港地区的137Cs大气沉降通量记录,并没有考虑 137Cs的扩散效应,所以该模型还有待进一步的改进。     

基于地面激光扫描的典型海岸带盐沼潮滩地形反演

海岸带潮滩地形具有快速变化的特点;在盐沼植被覆盖的潮滩区域,地面激光扫描仪（terrestrial laser scanning,TLS）获取地形数据受到地表植被的影响。为了研究TLS在盐沼潮滩使用的地形测量精度与盐沼植被种类、盖度的关系,本文以芦苇群落、白茅群落、互花米草群落、海三棱藨草群落4种典型海岸带盐沼植被群落为研究对象,在移动窗口法的基础上辅以聚类分析的植被滤除算法,分别从点云原始数据中恢复地表地形地貌特征。研究结果表明：1）植被盖度越高,TLS反演地形精度越低,两者负相关。2）不同植被的激光穿透能力不同：盖度大于50%时,激光无法穿透原始盖度分别为70%、65%、65%的白茅群落、互花米草群落、海三棱藨草群落,均方根误差（root-mean-square error,RMSE）分别为22.0、22.0、8.6 cm;盖度等于50%时,白茅群落、海三棱藨草群落、芦苇群落和互花米草群落的RMSE分别为16.0、6.6、4.5、5.7 cm;盖度小于50%时,芦苇群落、互花米草群落、海三棱藨草群落地形反演精度小幅度提高,白茅群落地形反演精度提升较为明显。3）在盐沼潮滩地区使用TLS反演地形时,增加TLS架设高度、对同一区域多方位反复扫描可能有助于提高地形反演效果。     

渤海西部河口潮间带区海水及沉积物中重金属研究

通过对渤海湾西部永定新河河口潮间带区海水及沉积物中重金属含量及污染状况的研究,认为海水中重金属含量平均值的大小顺序为Zn＞Cu＞Pb＞Hg＞Cd;因海水中重金属输入径的不同,不同潮期海水中重金属含量存在差异:大潮期海水中重金属Cu,Pb和Hg的含量大于小潮期海水中的含量,而重金属Zn和Cd的含量则小于小潮期海水中的含量;因受海水中悬浮物含量、种类及河口区成淡水混合发生絮凝作用等的影响,海水中重金属含量在垂向上也存在差异.无论是大潮期还是小潮期,海水中重金属舍量总是表层略大于底层;按一类水质标准对海水中重金属污染状况的分析,认为研究区海水已局部受到重金属污染,污染最严重的是重金属Zn,其次为重金属Pb,Hg和Cu,不存在重金属Cd污染;沉积物中重金属舍量平均值的大小顺序与海水中的相同,说明海水与沉积物中重金属循环处于相对平衡状态.沉积物中重金属含量平均值均在渤海湾环境背景值范围内,且是渤海海域含量最低的区域.经过对沉积物重金属污染状况的分析,研究区沉积物重金属污染轻微.     

Some Morphological Aspects and Hydrological Characterization of the Tagus Floods in the Santarém Region, Portugal

In Portugal, few studies have been made of the historical floods of the RiverTagus (the longest in the Iberian Peninsula). This fact led to the study of theLower Tagus (Santarém region) using written historical documents,cartographic documents and hydrological data, consisting mainly of waterlevel records. With the support of the historical documents and the analysisof all the maps, it was possible to verify that the human intervention has beenfundamental in the morphological changes of the Tagus' channel. It becameaware that the river changed from a braided to a single channel with alternatebars. From the hydrological data, return periods for the floods were determined,using the Pearson Type 3 distribution. The selection of the most important floods,from 1855 to 1998, enabled to build a ``flood hazard scale' for this region.     

长江口盐沼滩面发育对有机碳深度分布的制约

通过对长江口崇明东滩高潮滩、中潮滩以及光滩柱状样的有机碳含量与碳稳定同位素组成（δ13C）、粒度组成等的测定,研究盐沼有机碳深度分布特征与形成机制。结果表明,盐沼土壤颗粒有机碳（POC）主要赋存于粒径小于0.016 mm的颗粒中,POC含量对粒径在0.002~0.004 mm区间的颗粒含量变化最敏感,说明盐沼POC主要来自长江径流悬移质,这与有机碳稳定同位素结果一致。土壤POC含量与不同粒径区间颗粒含量相关关系表明,高潮滩与中潮滩柱样的泥沙级配较为接近;光滩柱样POC含量与不同粒径区间颗粒含量相关关系特征与高、中潮滩柱样的基本类似,主要不同表现在粒径大于0.016 mm的粗颗粒,这很可能受控于盐沼不同高程部位动力沉积过程。盐沼植被对高、中潮滩柱样POC的贡献相当可观,个别层段高达55.6%;植被对土壤POC的贡献受到滩面过程的明显制约。滩面动力沉积过程形成盐沼垂向上独特的沙、泥纹层构造,其优良的封堵效能显著影响土壤有机碳的垂向分布。盐沼滩面动力沉积过程是塑造有机碳深度分布特征的关键因素。     

Nature：沿海富营养化导致盐沼消失

2012年10月,Nature期刊发表了一篇题为《海富营养化促使盐沼消失》（Coastal eutrophication as a driverof salt marsh loss）的文章。文章指出,盐沼是一种具有很高生产力的沿海湿地,可以为人类提供重要的生态系统服务（例如沿岸城市的暴风雨防护、养分去除和碳封存）。尽管采取了很多保护措施,     

Geophysical methods applied to fault characterization and earthquake potential assessment in the Lower Tagus Valley, Portugal

The study region is located in the Lower Tagus Valley, central Portugal, and includes a large portion of the densely populated area of Lisbon. It is characterized by a moderate seismicity with a diffuse pattern, with historical earthquakes causing many casualties, serious damage and economic losses. Occurrence of earthquakes in the area indicates the presence of seismogenic structures at depth that are deficiently known due to a thick Cenozoic sedimentary cover. The hidden character of many of the faults in the Lower Tagus Valley requires the use of indirect methodologies for their study. This paper focuses on the application of high-resolution seismic reflection method for the detection of near-surface faulting on two major tectonic structures that are hidden under the recent alluvial cover of the Tagus Valley, and that have been recognized on deep oil-industry seismic reflection profiles and/or inferred from the surface geology. These are a WNW–ESE-trending fault zone located within the Lower Tagus Cenozoic basin, across the Tagus River estuary (Porto Alto fault), and a NNE–SSW-trending reverse fault zone that borders the Cenozoic Basin at the W (Vila Franca de Xira–Lisbon fault). Vertical electrical soundings were also acquired over the seismic profiles and the refraction interpretation of the reflection data was carried out. According to the interpretation of the collected data, a complex fault pattern disrupts the near surface (first 400 m) at Porto Alto, affecting the Upper Neogene and (at least for one fault) the Quaternary, with a normal offset component. The consistency with the previous oil-industry profiles interpretation supports the location and geometry of this fault zone. Concerning the second structure, two major faults were detected north of Vila Franca de Xira, supporting the extension of the Vila Franca de Xira–Lisbon fault zone northwards. One of these faults presents a reverse geometry apparently displacing Holocene alluvium. Vertical offsets of the Holocene sediments detected in the studied geophysical data of Porto Alto and Vila Franca de Xira–Lisbon faults imply minimum slip rates of 0.15–0.30 mm/year, three times larger than previously inferred for active faults in the Lower Tagus Valley and maximum estimates of average return periods of 2000–5000 years for M 6.5–7 co-seismic ruptures.