Macrofaunal Spatial Patterns in Relationship to Environmental Variables in the Richibucto Estuary,New Brunswick,Canada

Estuarine macrobenthos respond to a variety of environmental gradients such as sediment type and salinity, and organic enrichment. A relatively new influence, organic loading from suspended bivalve culture, has the potential to alter this response. A study on soft-bottom macrobenthic communities was carried out in the Richibucto estuary (46°40′N, 64°50′W), New Brunswick, Canada, with samples collected from 18 stations in late September and early October 2006. The site consisted of a large tidal channel originating upstream in a small river. The channel was punctuated by bag culture of oysters along its length. A total of 88 species were recorded. The mean values of abundance, species richness, and diversity (H′) of macrofauna were 11,199 ind. m⁻² (ranged from 4,371 to 19,930 ind. m⁻²), 23.4 species grab⁻¹ and 3.29 grab⁻¹, respectively. In general species richness and H′ increased from the upper estuary to the estuarine mouth. Multivariate analyses clearly exhibited the spatial distribution in community structure, which coincided with the locations along the estuary (the upper, the lower and the mouth), as well as inside and outside the channel. Species richness and diversity H′ showed strong positive correlations with salinity (21.2–25.2 ppt), and abundance was positively correlated with water depth (1.0–4.5 m). Abundance and species richness were negatively correlated with both of silt–clay fraction (3.3–24.8%) and sorting (σ_I). Species richness was also negatively correlated with organic content (1.9–12.7%). The BIO-ENV analyses identified silt–clay fraction, σ_I and salinity as the major environmental variable combination influencing the macrofaunal patterns, and silt–clay fraction as the single best-correlated variable.     

Spatial Heterogeneity Created by Burrowing Crabs Affects Human Impact on Migratory Shorebirds

We show that, as previously described, there are conflicts between areas used by people and shorebirds. However, we propose that burrowing organisms add complexity to the shorebird–human interaction that should be taken into account for management and conservation. This is because SW Atlantic mudflats are dominated by high densities (up to 60 crabs m⁻²) of a strong bioturbator, the burrowing crab Neohelice granulata (=Chasmagnathus granulatus). These crabs affect the habitat use and foraging performance of shorebird species. The two-banded plover Charadrius falklandicus feed more in burrow areas whereas the yellowlegs Tringa spp. and the white-rumped sandpiper Calidris fuscicollis tend to avoid burrow areas. We evaluated the effect of human recreational activities on shorebirds, taking into account shorebird–crab interaction, during shorebird migratory periods (November to April) of 2001, 2002, and 2003. This study was performed at the Mar Chiquita coastal lagoon (37° 40′ S, Argentina). Results showed that people only rarely used the extensive soft bottom intertidals dominated by burrowing assemblages of N. granulata because the soft sediments make walking difficult. Therefore, human impact is reduced for the two-banded plover. However, for several migratory shorebirds that also avoid burrow areas, these areas act as suboptimal alternative areas when human impact is important in their preferred habitats. When human activity is low, shorebirds remain foraging in the area but they decrease their feeding intakes. The extensions of burrow areas differ among estuaries and are likely to generate between-estuaries differences as stopover sites for shorebirds. Also, other species that form large burrowing assemblages can have a similar impact on shorebird–human interactions.     

Light Requirements for Growth and Survival of Eelgrass (<Emphasis Type="Italic">Zostera marina</Emphasis> L.) in Pacific Northwest (USA) Estuaries

We developed light requirements for eelgrass in the Pacific Northwest, USA, to evaluate the effects of short- and long-term reductions in irradiance reaching eelgrass, especially related to turbidity and overwater structures. Photosynthesis-irradiance experiments and depth distribution field studies indicated that eelgrass productivity was maximum at a photosynthetic photon flux density (PPFD) of about 350–550 μmol quanta m⁻² s⁻¹. Winter plants had approximately threefold greater net apparent primary productivity rate at the same irradiance as summer plants. Growth studies using artificial shading as well as field monitoring of light and eelgrass growth indicated that long-term survival required at least 3 mol quanta m⁻² day⁻¹ on average during spring and summer (i.e., May-September), and that growth was saturated above about 7 mol quanta m⁻² day⁻¹. We conclude that non-light-limited growth of eelgrass in the Pacific Northwest requires an average of at least 7 mol quanta m⁻² day⁻¹ during spring and summer and that long-term survival requires a minimum average of 3 mol quanta m⁻² day⁻¹.     

填土表面倾斜时的主动土压力计算

支护结构工程中,当坡顶填土表面倾斜时,无法采用常规的土压力理论进行计算。探讨了《建筑边坡工程技术规范》存在的问题,提出了推荐方法。通过算例验证了推荐方法,具有计算简单、计算结果合理的优点,可供工程设计中参考使用。     

肋柱式锚杆支挡结构的徐变次内力分析

采用老化徐变理论和等效弹性模量法,对肋柱式锚杆支挡结构的徐变影响提出了具体计算方法,并通过实例计算了支挡结构的徐变次内力。计算结果表明,在逆作法条件下,随着开挖深度的加大,岩土侧向压力也增大,使后期施工的混凝土构件产生的徐变次内力也随之增大;考虑徐变后使结构设计更加安全可靠,尤其坡脚处后期浇筑的混凝土构件,更要关注徐变的影响。     

印度洋岸边某电站工程的岩土工程问题初探

从海岸地带的地理地质环境入手,针对燃煤电站工程的特点,就工程建设中的几个主要岩土工程问题,分析探讨了其成因、特点和对工程建设的制约与影响程度．提出了解决这些问题的对策和建议。     

某基坑工程土钉支护工作性状试验研究

通过对广州天河荟雅苑基坑工程土钉支护进行试验和研究,分析了土钉支护体系的工作性状,并为同类工程的设计提供了一定的依据。     

Fluid Geochemistry as Indicator of Tectonically-Related,Deep Water Circulations in the Sardinian Rift-Campidano Graben (Italy): New Insights from Environmental Isotopes

The EC funded Geochemical Seismic Zonation program (EEC GSZ Project 1996–1998) chose Sardinia as a low-seismicity site, in which the relationships between fluid geochemistry and seismo-tectonics had to be investigated and results compared with outcomes from other selected high-seismicity sites. A first article, examining the role of fault segmentation and seismic quiescence on the geochemical composition of groundwaters and gases, has already been presented (Angelone et al. 2005). This article deals with environmental isotopes which, together with selected hydrochemical data, give hints on tectonically-related fluid circulations. Four water-dominated hydrothermal systems were considered, all located along regional fault systems and discharging groundwaters belonging to the Na–HCO₃ and Na–Cl facies. In the considered systems, groundwater circulation takes place, principally, in the Palaeozoic Crystalline Basement (PCB), with the exception of the Logudoro system, where hydrological circuits develop in the Mesozoic Carbonate Platform (MCP). The high CO₂ contents, the non-attainment of fluid-rock equilibrium and the large lithological variability prevent the construction of a unique hydrogeological–geochemical conceptual model. In this case, stable isotopes provide a useful tool to describe the origin of fluids and their subterranean movements. Stable isotopes of water, integrated with hydrochemical data, indicate that fluids are derived from three main end members. The dominant component is a relatively recent local meteoric water; the second one is marine water; and the third one is a fossil freshwater, depleted in heavy isotopes with respect to modern rains. The latter end member entered the aquifer system in the past, when climatic conditions were greatly different from today. At least two circulation systems can be recognised, namely a shallow cold system and a deep hydrothermal system, as well as two distinct hydrological processes: (1) gravity-controlled descent of cold water towards greater depths and (2) convection linked to a thermal gradient, causing deep fluids to rise up from the hydrothermal reservoir towards the surface. The highly variable δ¹³C_TDIC values suggest the presence of two distinct CO₂ sources, namely, a biogenic one and a thermogenic one. The relation between the isotopic compositions of CO₂ and He indicates an increased mantle signature in uprising CO₂-rich fluids.