Rockfall detachment susceptibility map in El Hierro Island, Canary Islands, Spain

A semi-quantitative heuristic methodology is developed to map a rockfall detachment susceptibility zonation of El Hierro Island (Canary Archipelago). The rationalized procedure, which we called non-weighted bounded indicators, is based on overlapping thematic maps of conditioning factors to mass movement, which are appropriately and individually rescaled and then composed by addition to obtain a susceptibility numerical index through a GIS. As the consistency of the geomorphological analysis depends on the expert subjective criteria and the appropriate interpretation of the landscape, the use of this methodology reduces subjectivity and quantifies the degree of susceptibility. The main factors affecting the mass movement phenomena (rockfalls events), also recognized in the field and, therefore, considered in the presented GIS arrangement, are slope, profile curvature, lithology, vegetation cover and dykes density. To calculate the slope threshold or minimum angle characteristic of rockfall source areas, mixed Gaussian slope frequency decomposition is used. The curvature index reveals stepwise areas. Qualitative geomechanical characteristics are linked to a quantitative index according to a volcanic lithological-complexes classification. Both destabilization (root-wedging) and stabilization effects are considered into the vegetation cover index. The dyke density index incorporates the bearing rock capacity decrease produced in the halo around a dyke network intrusion. Slope, curvature and vegetation indexes thresholds have been fitted following field observations. A rockfall detachment susceptibility map is obtained and classified based on the histogram maxima. The rockfall inventory, based on rockfall events reported within the island, was used for the model validation. A 12?% of the whole island shows medium to very high susceptibility.     

Seasonal occurrence and growth of benthic algae in the Patos Lagoon estuary, Brazil

The seasonal occurrence of 94 benthic algal species, including Cyanophyta, was observed on a monthly basis over one year in the Patos Lagoon estuary (32°S, 52°W), Brazil, and showed three patterns of algal periodicity. An aseasonal group of 46 species was present throughout the year and two groups of 24 species each occurred either during summer and fall or winter and spring. The seasonal growth of 55 green, brown and red algae was correlated with monthly variations of salinity, water temperature, daylength and light radiation energy, which were measured daily in the estuary. Summer/fall peak growth was observed in 15 algae most of which correlated significantly with high salinities alone or in combination with other factors during this period. Growth peak of 11 species in winter/spring was principally a function of low water temperatures alone or together with a reduced light regime.Both periodicity of benthic algae and their seasonal growth, as a more sensitive measure of their floristic affinity, demonstrated the presence of cool temperate and tropical elements in this flora typical for warm temperate biogeographic regions. Correlations between seasonal variation of algal growth and selected environmental parameters aided in the evaluation of causal factors for algal seasonality.     

In situ experimental study of reed leaf decomposition along a full salinity gradient

An experimental study on Phragmites australis leaf litter decomposition was conducted in the estuarine environment, Ria de Aveiro, Western Portugal, using the leaf-bag technique, with fine- (1 mm) and coarse-mesh (5 mm) bags. The leaf bags were placed in the field sites at day 0, covering a complete salinity gradient, and replicates were collected over time, at days 3 (leaching), 7, 15, 30 and 60. The biomass loss through the leaching phase, about 20% of the initial leaf mass, was independent of both the salinity and the bag mesh size. The biomass decay pattern along the salinity gradient varied through time and presented strong similarities between the two mesh sizes, with the remaining biomass always lower in the 5 mm mesh-size bags. At days 7 and 15, the lowest remaining biomass was observed at the head of the estuary, the preferential distribution area of P. australis. At day 30, the remaining biomass was higher in the marine area and diminished under a direct relationship with salinity, reaching the lowest value in the freshwater environment, with values ranging from 66% to 44% of the initial weight in 5 mm bags, and from 79% to 51% in 1 mm bags. The largest heterogeneity in the remaining biomass among the study areas positioned along the salinity gradient was found close to days 30 (5 mm) and 40 (1 mm). The overall results indicate that the relationship between leaf decay rate and salinity depends on the decay time considered (k₁₅, k₃₀ or k₆₀) and, for the later stages (k₆₀), also on the leaf-bag mesh size. This implies that the use of leaf litter decay rates as a functional indicator in transitional waters will need to take into consideration the factor location in the salinity gradient and leaf litter stage at which the decay rate is determined. The differences between the decay rates with the mesh size acted mainly at the level of the absolute k value and not at the level of the pattern along the salinity gradient. Even so, the data obtained at the mouth of the estuary, in the area closest to a fully marine environment, indicated that after the initial biomass loss through leaching, P. australis decayed either very slowly, in the 5 mm, or not at all, in the 1 mm mesh bags.     

Trichodesmium spp. population structure in the eastern North Atlantic subtropical gyre

Trichodesmium spp. population structure was studied based on a transect in the eastern subtropical North Atlantic in October 2006, covering a gradient from near-temperate conditions up to fully subtropical, oligotrophic waters. Trichodesmium spp. trichomes were counted and measured, distinguishing free trichomes from those forming macroscopic colonies (“puffs” or “tufts”). Moreover, for both free and colonial individuals, abundance and size structure variation were quantified from single trichome-level data. Nitrogen fixation rates were estimated using an empirical approach based on abundance observations and a heuristic approach to incorporate variation in trichome size and environmental temperature. A gradual change towards greater sizes and increased abundances was observed as physical conditions became subtropical. An extreme response was detected at one station (29.8°W, 26.3°N), in which both in situ and remote sensing data revealed the presence of a Trichodesmium bloom. Despite the presence of a mesoscale anticyclonic eddy, a close examination of prior surface conditions using altimeter data indicated that the bloom was related to the advection of southern waters. In general, our results highlighted the importance of free trichomes in Trichodesmium spp. populations in this part of the North Atlantic. Furthermore, they suggested a possible role of both size and population structure on the wide range of N₂ fixation estimates currently available in the literature.     

Intra- and inter-specific variability in total and methylmercury bioaccumulation by eight marine fish species from the Azores

We relate fish biological and ecological characteristics to total and organic mercury concentrations to determine whether accumulation is influenced by trophic level, Hg concentration in the diet, and vertical distribution. Levels of total mercury and methylmercury were determined in the muscle tissue of eight species of fish: Pagellus acarne, Trachurus picturatus, Phycis phycis, P. blennoides, Polyprion americanus, Conger conger, Lepidopus caudatus and Mora moro, caught in the Azores. All such fishes are commercially valuable and were selected to include species from a wide range of vertical distributions from epipelagic (<200 m) to mesopelagic (>300 m) environments. Methylmercury was the major form accumulated in all species, comprising an average of 88.1% of total mercury. Concentrations of mercury (total and methylmercury) increased with age, length and weight. Based on data from other studies, mercury concentrations in fish diet were estimated. Mercury levels in food ranged from 0.08 to 0.32 ppm, dry weight. Hg concentrations in the food and in muscle tissue from different species were positively correlated. Total Hg levels in the muscles were approximately nine times those estimated in food. Total mercury concentrations in muscle were positively correlated with both trophic level and median depth. Such enhanced mercury bioaccumulation in relation to depth appears to be determined primarily by concentrations in food and ultimately by water chemistry, which controls mercury speciation and uptake at the base of the food chain.     

Optimization for performance-based design under seismic demands,including social costs

Performance-based design in earthquake engineering is a structural optimization problem that has, as the objective, the determination of design parameters for the minimization of total costs, while at the same time satisfying minimum reliability levels for the specifi ed performance criteria. Total costs include those for construction and structural damage repairs, those associated with non-structural components and the social costs of economic losses, injuries and fatalities. This paper presents a general framework to approach this problem, using a numerical optimization strategy and incorporating the use of neural networks for the evaluation of dynamic responses and the reliability levels achieved for a given set of design parameters. The strategy is applied to an example of a three-story offi ce building. The results show the importance of considering the social costs, and the optimum failure probabilities when minimum reliability constraints are not taken into account.     

Turbiditic levee deposition in response to climate changes: The Var Sedimentary Ridge (Ligurian Sea)

The Var turbiditic system located in the Ligurian Sea (SE France) is an intermediate mud/sand-rich system. The particularity of the Var deep-sea fan is its single channel with abrupt bends and its asymmetric and hyper-developed levee on the right hand side: the Var Sedimentary Ridge. Long-term sediment accumulation on the Var Sedimentary Ridge makes this an ideal target for studying the link between onshore climate change and deep-sea turbidite stratigraphy. This paper focuses on the establishment of the first detailed stratigraphy of the levee, which is used to analyze the timing of overbank deposition throughout the last deglaciation. Main results indicate that high variability in turbidite frequencies and deposition rates along the Var Sedimentary Ridge are determined by two main parameters: 1) the progressive decrease of the levee height controlling the ability of turbidity currents to spill out from the channel onto the levee, and 2) climatic variations affecting the drainage basin, in particular changes in glacial condition since late Last Glacial Maximum to early Holocene. Compared to other deep-water areas, this study confirms the ability of turbiditic systems to record past climatic events on millennial timescales, and underlines the influence of European deglaciation on the observed decrease in turbidite activity in the Var canyon. The presence of a very narrow continental shelf and a single, large channel-levee system makes the Var Sedimentary Ridge a unique example of climate-controlled turbiditic accumulations.