Benthic nutrient fluxes along the Laurentian Channel: Impacts on the N budget of the St. Lawrence marine system     

Benoît Thibodeau  Moritz F. Lehmann  Jacqueline Kowarzyk  Alfonso Mucci  Yves Gélinas  Denis Gilbert  Roxane Maranger  Mohammad Alkhatib 《Estuarine, Coastal and Shelf Science》2010

Water column concentrations and benthic fluxes of dissolved inorganic nitrogen (DIN) and oxygen (DO) were measured in the Gulf of St. Lawrence and the Upper and Lower St. Lawrence Estuary (USLE and LSLE, respectively) to assess the nitrogen (N) budget in the St. Lawrence (SL) system, as well as to elucidate the impact of bottom water hypoxia on fixed-N removal in the LSLE. A severe nitrate deficit, with respect to ambient phosphate concentrations (N*∼−10 μmol L⁻¹), was observed within and in the vicinity of the hypoxic bottom water of the LSLE. Given that DO concentrations in the water column have remained above 50 μmol L⁻¹, nitrate reduction in suboxic sediments, rather than in the water column, is most likely responsible for the removal of fixed N from the SL system. Net nitrate fluxes into the sediments, derived from pore water nitrate concentration gradients, ranged from 190 μmol m⁻² d⁻¹ in the hypoxic western LSLE to 100 μmol m⁻² d⁻¹ in the Gulf. The average total benthic nitrate reduction rate for the Laurentian Channel (LC) is on the order of 690 μmol m⁻² d⁻¹, with coupled nitrification-nitrate reduction accounting for more than 70%. Using average nitrate reduction rates derived from the observed water column nitrate deficit, the annual fixed-N elimination within the three main channels of the Gulf of St. Lawrence and LSLE was estimated at 411 × 10⁶ t N, yielding an almost balanced N budget for the SL marine system.  相似文献   

Groundwater contamination with arsenic and other trace elements in an area of the pampa,province of Córdoba,Argentina   总被引：1，自引：0，他引：1  

Hugo B. Nicolli  Jose M. Suriano  Miguel A. Gomez Peral  Luis H. Ferpozzi  Omar A. Baleani 《Environmental Geology》1989,14(1):3-16

A geochemical study of groundwater of the pampa in the province of Córdoba, Argentina, was performed; the area covered approximately 10,000 km².Physical-chemical parameters, dissolved solids, and seven trace elements were determined in 60 selected water samples. Systematic and accurate measurements of arsenic, flourine, and vanadium were performed for the first time. Three trace element contaminants not reported earlier were found: an important one, selenium, and two others of less known effects, uranium and molybdenum.Eighty-four percent of the water analyzed showed arsenic contents over 0.05 mg/L, maximum contaminant level established by the U.S. Environmental Protection Agency (1982). The frequency distribution of trace elements was analyzed, and its fit to the lognormal distribution was proved by means of the Pearson and Kolmogorov-Smirnov test; the geographic distribution of the seven trace elements was mapped and its correlation with the anion-cation composition of the water was studied.The maximum arsenic, fluorine, vanadium, and uranium contents were found in the western part of the area under study, in waters containing dominant alkali metals in the cation composition. Maximum selenium and antimony contents were found in the eastern part of the area, while molybdenum distribution does not show any relationship with the other two groups. In addition, the geographic distribution of the trace elements seems to be related to the subsurface structure, which has been inferred using interactive digital analysis of Landsat imagery. The movements of the subsoil have disturbed surface and subsurface drainage influencing the water salinity and trace element contents.In order to investigate the origin of the contamination, 54 loess samples were collected in wells at depths ranging from the surface down to the water table. This loess, which has a high proportion of volcanic components, mainly rhyolitic glass, exhibits a chemical composition corresponding to that of a dacite.The loess and the volcanic glass show anomalous contents of all contaminant trace elements, mainly arsenic and selenium. For this reason loess is considered the most important contamination source in the groundwater under study.  相似文献   

Calcite and aragonite precipitation from seawater solutions of various salinities: Precipitation rates and overgrowth compositions     

Shaojun Zhong  Alfonso Mucci   《Chemical Geology》1989,78(3-4):283-299

The reaction rate and composition of calcite and aragonite overgrowths precipitated from seawater solutions of various salinities (i.e. S=5, 15, 25, 35, 44) were determined at 25°C and 10^−2.5-atm. CO₂ partial pressure using a constant disequilibrium seeded technique. The rate data were fitted to an empirical rate law of the form:

where n is the empirical reaction order; and k is the rate constant. Calcite precipitation rates in seawater solutions do not vary appreciably as a result of salinity variations over the range investigated, while those for aragonite show an increase in going from the higher (i.e. S=35, 44) to the lower (i.e. S=5, 15, 25) salinity range. This study also confirms previously published findings that above a given saturation state (i.e. Ω_c>/2.6) aragonite precipitates more rapidly than calcite at 25°C.

The incorporation of Sr²⁺ in aragonite and Mg²⁺ in calcite overgrowths are independent of the precipitation rate. The partition coefficient of Sr²⁺ in aragonite is approximately equal to unity and is unaffected by salinity variations between 5 and 44. However, the Mg²⁺ partition coefficient in calcite, increases with decreasing salinity of the parent seawater solutions, possibly as a result of variations in the sulfate content of the solutions and solids.

The experimental results were discussed in the context of a number of geological environments.  相似文献   

Urban Seismic Risk Evaluation: A Holistic Approach   总被引：3，自引：4，他引：3  

Martha-Liliana Carreño  Omar D. Cardona  Alex H. Barbat 《Natural Hazards》2007,40(1):137-172

Risk has been defined, for management purposes, as the potential economic, social and environmental consequences of hazardous events that may occur in a specified period of time. However, in the past, the concept of risk has been defined in a fragmentary way in many cases, according to each scientific discipline involved in its appraisal. From the perspective of this article, risk requires a multidisciplinary evaluation that takes into account not only the expected physical damage, the number and type of casualties or economic losses, but also the conditions related to social fragility and lack of resilience conditions, which favour the second order effects (indirect effects) when a hazard event strikes an urban centre. The proposed general method of urban risk evaluation is multi hazard and holistic, that is, an integrated and comprehensive approach to guide decision-making. The evaluation of the potential physical damage (hard approach) as the result of the convolution of hazard and physical vulnerability of buildings and infrastructure is the first step of this method. Subsequently, a set of social context conditions that aggravate the physical effects are also considered (soft approach). In the method here proposed, the holistic risk evaluation is based on urban risk indicators. According to this procedure, a physical risk index is obtained, for each unit of analysis, from existing loss scenarios, whereas the total risk index is obtained by factoring the former index by an impact factor or aggravating coefficient, based on variables associated with the socio-economic conditions of each unit of analysis. Finally, the proposed method is applied in its single hazard form to the holistic seismic risk evaluation for the cities of Bogota (Colombia) and Barcelona (Spain).  相似文献   

Spatial organization of decadal and bidecadal rainfall fluctuations in southern North America and southern South America     

Omar Abel Lucero  Norma C. Rodriguez 《Atmospheric Research》2001,57(4)

The spatial organization of decadal and bidecadal components (fluctuations) of annual rainfall is identified in this research for two regions: (1) southern South America, and (2) southern North America (conterminous USA, southeastern Canada and northern and central Mexico). Findings indicate that these decadal and bidecadal components have highly coherent wave-like spatial organization. Two types of organization of decadal and bidecadal components of annual rainfall were identified: a train of propagating fluctuations and quasi-standing fluctuations. For decadal components, such patterns alternate in time. A widespread change in the spatial organization of decadal component of annual rainfall took place simultaneously in both continents in 1932. The bidecadal component is organized as standing fluctuations in southern North America and as travelling fluctuations in southern South America, The spatial pattern of decadal fluctuations of annual rainfall has 12- and 13-year cycles, and the spatial pattern of bidecadal fluctuations has predominantly 21- and 22-year cycles.  相似文献   

An experimental investigation of vertical vibration of model footings on sand     

Azm S. Al-Homoud  Omar N. Al-Maaitah 《Soil Dynamics and Earthquake Engineering》1996,15(7):431-445

Many free and forced vertical vibrations tests were conducted on surface and embedded models for footings on dry and moist poorly graded sand that has the following properties: D₁₀ = 0·21 mm, D₃₀ = 0·305 mm, D₆₀ = 0·423 mm, C_u = 2, C_c = 1·05, G_s = 2·66, γ_max = 1·74 cm⁻³ and γ_min = 1·4 g cm⁻³. The tests were conducted at relative density Dr = 82%. The effect of mass, area, geometry, embedment, saturation, load amplitude and frequency were studied. For this purpose square, rectangular and circular models of concrete footings were chosen. Swieleh sand was chosen as the foundation soil. Results have been obtained for models having different mass, same base shape and area; models of different base area and equal base shape and mass; and models of different base shape geometry and about equal masses and base areas.Forced vertical vibration tests results showed an increase in natural frequency and a reduction in amplitude with the increase in embedment depth, degree of saturation and footing base area. Increasing the mass of model footing resulted in a decrease in the natural frequency while the dynamic response increased. Also, results showed that the circular model footing gives low values of dynamic response in comparison to other models.Free vertical vibration test results showed an increase in damping ratio with increase in the base area of the model footing, depth of embedment and saturation of sand. On the other hand, the results showed a decrease in damping ratio with increase in the footing mass. Circular footing gives the highest value of damping ratio among other footings.Results showed that the best method for evaluating the dynamic stiffness for vertical vibration is using Dobry and Gazetas¹ (J. Geotech. Engng, ASCE, 1986, 112, 109–133) equations together with the formula proposed by Ronald and Bojan² (J. Geotech. Engng, ASCE, 1995, 121, 274–286) for evaluating the equivalent shear modulus.  相似文献   

Light requirements of seagrassesHalodule wrightii andSyringodium filiforme derived from the relationship between diffuse light attenuation and maximum depth distribution     

W. Judson Kenworthy  Mark S. Fonseca 《Estuaries and Coasts》1996,19(3):740-750

The correspondence between maximum depth of growth (Z_max) for two seagrases,Halodule wrightii andSyringodium filiforme, and the attenuation of diffuse photosynthetically active radiation (K_dPAR) were evaluated over a 3.5-yr period in the southern Indian River Lagoon, Florida. The lower limit of seagrass depth distribution was controlled by light availability. Both species grew to the same maximum depth, indicating they have similar minimum light requirements. Based on average annual values of K_dPAR, estimates of seagrass minimum light requirements ranged from 24% to 37% of the light just beneath the water surface (I_o), much hgiehr than a photic zone for many phytoplankton and macroalgae (1–5% incident light). In less transparent waters of Hobe Sound, where turbidity (NTU) and color (Pt-Co) had their highest concentrations, minimum light requirements for growth were greatest. These results suggest that more sophisticated optical models are needed to identify specific water quality constituents affecting the light environment of seagrasses. Water quality criteria and standards needed to protect seagrasses from decreasing water transparency must be based on parameters that can be routinely measured and reasonably managed.  相似文献   

Compressibility and collapse characteristics of arid saline sabkha soils     

Omar Saeed Baghabra Al-Amoudi  Sahel N. Abduljauwad 《Engineering Geology》1995,39(3-4):185-202

Arid saline soils are well-distributed over the globe, with a variety of nomenclature. Along the seaboard of the Arabian Gulf, these soils exist widely and are known as “sabkhas”. Despite the cemented and saline characteristics of the sabkha matrix, a recent investigation indicated that flooding the saline sabkha with distilled water in the conventional oedometer apparatus was incapable of producing a sudden reduction in volume and/or a significant collapse. This study proposes a modification to the conventional oedometer on undisturbed sabkha specimens to consolidating specimens under a constant head. Tests were, therefore, conducted on undisturbed sabkha specimens to assess their compressibility and collapse potential whereby percolation of water was commenced under two pressures to evaluate the role of sustained pressure on the collapse mechanisms.

Despite the low compressibility of sabkhas, results of these tests indicated that these arid, saline soils possess a high collapse potential attributable primarily to dissolution of sodium chlorides, leaching of calcium ions and soil grain adjustment. The collapse potential increases with an increase in the acting pressure at which percolation of water takes place. In contrast to other typical soils, the collapse of arid, saline soils is not instantaneous but requires sufficient volume of water to percolate in order to enhance the dissolution of the cementing agents.  相似文献   

Approaches to analyse and model changes in impacts: reply to discussions of “How to improve attribution of changes in drought and flood impacts”*     

Heidi Kreibich  Veit Blauhut  Jeroen C. J. H. Aerts  Laurens M. Bouwer  Henny A. J. Van Lanen  Alfonso Mejia 《水文科学杂志》2020,65(3):491-494

ABSTRACT

We thank the authors, Brunella Bonaccorso and Karsten Arnbjerg-Nielsen for their constructive contributions to the discussion about the attribution of changes in drought and flood impacts. We appreciate that they support our opinion, but in particular their additional new ideas on how to better understand changes in impacts. It is great that they challenge us to think a step further on how to foster the collection of long time series of data and how to use these to model and project changes. Here, we elaborate on the possibility to collect time series of data on hazard, exposure, vulnerability and impacts and how these could be used to improve e.g. socio-hydrological models for the development of future risk scenarios.  相似文献   

Consequences of a freshwater canal discharge on the benthic community and its habitat on an exposed sandy beach     

Lercari D  Defeo O  Celentano E 《Marine pollution bulletin》2002,44(12):1397-1404

This paper analyses the spatial and temporal effects of a freshwater discharge (Canal Andreoni) on the macroinfauna community and its habitat in a sandy beach of Uruguay. Bimonthly, we examined 17 environmental variables plus macroinfauna abundance, biomass, richness, evenness and diversity of three sites: Andreoni, at the canal mouth, Coronilla, at 1 km, and Barra, at 13 km from the mouth. Both univariate and multivariate techniques showed an increasing degree of perturbation towards the canal. This was reflected by abiotic and biotic differences between sites and by a consistent two-dimensional ordination of the samples. A clear seasonal pattern was found, specially accentuated at Coronilla, where the effects of the canal were stronger in winter and weaker in summer. Multivariate linking between macroinfauna and its habitat highlighted the role of salinity as explanatory variable of the observed trends.  相似文献