Evidence for a seismic activity mainly constituted of hybrid events at Cayambe volcano,Ecuador. Interpretation in a iced-domes volcano context

The high activity level of Hybrid Events (HE) detected beneath the Cayambe volcano since 1989 has been more thoroughly investigated with data from a temporary array. The unusual HE spectral content allows separating a high-frequency signal riding on a low-frequency one, with a probable single source. HEs are interpreted as high frequency VT events, produced by the interaction between magmatic heat and an underground water system fed by thaw water from the summital glacier, which trigger simultaneous low-frequency fluid resonance in the highly fractured adjacent medium. Pure VTs are interpreted as ‘aborted’ HEs occurring probably in the oldest and coldest part of the volcano complex. To cite this article: B. Guillier, J.-L. Chatelain, C. R. Geoscience 338 (2006).     

Formation of U-depleted rhyolite from a basanite at El Hierro, Canary Islands

Phonolite and trachyte are the felsic magmas of the alkaline magma suites, which characterize the Canary Islands. The October 2011 submarine eruption off El Hierro, the westernmost island, nevertheless, produced a small volume of rhyolitic magma. The rhyolite occurred as highly vesicular, white coloured pumices enveloped in and mingled with darker coloured basanitic pumice. The basanitic pumice is relatively crystal poor with a few euhedral olivines (mostly Fo_77–79), clinopyroxenes and Fe-rich spinels, whereas very rare olivine of same composition is found together with equally rare Fe-sulphide and FeTi-rich oxides in the rhyolite. The Fe–Mg exchange equilibrium in the oxides permits to calculate an equilibrium temperature of 970–890 °C for the rhyolite, in agreement with quartz-melt equilibrium at ca. 930 °C. A striking mineralogical feature of the rhyolite is the presence of rounded to contorted grains of milky quartz, which are xenocrysts incorporated and partly dissolved into the magma. Analyses of residual volatile concentrations in the glasses show that the rhyolite melt was highly degassed, whereas the basanitic glass still has important halogen concentrations. Trace element patterns of the mafic glasses and their elevated incompatible element concentrations are typical of the western Canary Island basanites. In contrast, the trace element composition of the rhyolite shows surprisingly low concentrations for all elements except the most incompatible ones (e.g. Rb, Ba, K and Th). All other measured LILE, HFSE and REE have significantly lower concentration than the basanitic counterpart that can be explained by fractionation of accessory phases (1 % apatite, 1 % sphene and 0.1 % zircon). Surprisingly, low U concentration is presumably related to elevated oxygen fugacity in the rhyolite, causing U to be in a hexavalent state, and fluxing of F-rich gas leading to volatilization of UF₆, known to emanate at low temperature. The results suggest that a gas-rich basanitic melt remobilized a small volume of stagnant rhyolitic melt formed by incorporation of approximately 10 % quartz-rich sediment into a late differentiate of trachytic composition. Sediments at the interface of an old oceanic crust adjacent to a continental shield and younger volcanic island are likely to act as magma traps were sediment assimilation may alter the mantle-derived magma composition. Quartz assimilation thus explains the production of rhyolite magma in a volcanic island characterized by an alkaline magma series from primitive basanites to trachytes.     

Revisiting Peruvian anchovy (Engraulis ringens) trophodynamics provides a new vision of the Humboldt Current system

The Peruvian anchovy or anchoveta (Engraulis ringens) forages on plankton and is a main prey for marine mammals, seabirds, fish, and fishers, and is therefore a key element of the food web in the Humboldt Current system (HCS). Here, we present results from the analysis of 21,203 anchoveta stomach contents sampled during 23 acoustic surveys over the period 1996–2003. Prey items were identified to the genus level, and the relative dietary importance of different prey was assessed by determination of their carbon content. Variability in stomach fullness was examined relative to the diel cycle, the distance from the coast, sea surface temperature, and latitude, using generalized additive models (GAMs). Whereas phytoplankton largely dominated anchoveta diets in terms of numerical abundance and comprised >99% of ingested prey items, the carbon content of prey items indicated that zooplankton was by far the most important dietary component, with euphausiids contributing 67.5% of dietary carbon followed by copepods (26.3%). Stomach fullness data showed that anchoveta feed mainly during daytime between 07h00 and 18h00, although night-time feeding also made a substantial contribution to total food consumption. Stomach fullness also varied with latitude, distance from the coast, and temperature, but with substantial variability indicating a high degree of plasticity in anchoveta feeding behaviour. The results suggest an ecological role for anchoveta that challenges current understanding of its position in the foodweb, the functioning of the HCS, and trophic models of the HCS.     

The impact of overfishing and El Niño on the condition factor and reproductive success of Peruvian hake, Merluccius gayi peruanus

In this study we examine physiological responses of Peruvian hake (Merluccius gayi peruanus) to changes in their population structure and physical environment during the period 1971–2004. In particular, we assess the relative investment of energy in growth and/or reproduction of small (20–35 cm) and large (35–50 cm) hake. We calculated the (i) condition (Fulton’s K), (ii) gonad and (iii) gut fullness indices for 42,761 female hakes sampled from commercial landings; these indices indicate fish somatic, reproductive and feeding condition, respectively. Using Generalized Additive Models we then examined potential relationships between these indices and sea surface temperature anomalies and date. Drastic energy exhaustion and a decrease in female hake fecundity were observed during El Niño events. The long-term trend showed a general increase in condition factor and a decrease in gonad index for large hake between 1971 and 2004. Small hake exhibited a different trend with an increase in reproductive activity, which was accompanied by an earlier maturation. We hypothesise that the observed low investment of energy in reproduction by large female hake might be related to the lack of large males, due to a sex-selective fishery and the impact of El Niño. We suggest that fishing diminished hake reproductive capacity, modified the sex ratio in favour of females and increased population vulnerability to environmental stress, in particular to the El Niño. The impact of multidecadal variability and predators like the squid, Dosidicus gigas, remain unresolved until longer time series become available.     

Objectives of fisheries management: case studies from the UK, France, Spain and Denmark

The main objectives of fisheries management are generally similar throughout the world. These are often stated in policy documents such as the Common Fisheries Policy and the Magnuson–Stevens Fishery Conservation and Management Act. However, at the local level often the key objectives of management are more detailed, characterised by both the overriding management structure and the status and type of fishery concerned. In this paper, we consider case study fisheries from the UK, France, Spain and Denmark to compare some of the various types of fisheries and fisheries management systems that exist in the European Union. From this, we define the key objectives for each management system.     

Seasonal succession of cyanoprokaryotes in a hypereutrophic oligo-mesohaline lagoon from the South of France

The Bolmon lagoon (South of France) is an oligo-mesohaline coastal lagoon that has undergone intense eutrophication in the past decades, resulting from a strong concentration of human activities in its drainage basin. Consequently, it exhibits some characteristics typical of an advanced trophic state; namely, the disappearance of submerged vegetation, the permanently intense phytoplankton growth and the recurrence of cyanoprokaryote blooms. As cyanoprokaryote dominance in south-temperate saline lagoons is little reported, we carried out this study in order to understand the seasonal variations in the phytoplankton composition and biomass, and to analyse the influence of environmental parameters such as salinity, nutrients and climate on the seasonal succession of species. In this lagoon, the phytoplankton was permanently dominated by cyanoprokaryotes, probably because of high availability of nutrients, low light penetration in the water column and frequent turbulent mixing induced by wind. The two most abundant species Planktothrix agardhii (in winter–spring) and Pseudanabaena limnetica (in summer) have low light requirements and are well adapted to a high mixing frequency, which defines the S1 functional group in Reynolds' typology for phytoplankton. Although widely studied in north-temperate lakes, blooms of these typically freshwater species are almost unreported in the Mediterranean area, especially in brackish ecosystems that are not their normal habitat. In the Bolmon lagoon, all their requirements for nutrients, light and mixing are satisfied and they seem to cope with a moderate presence of salt but P. agardhii was less competitive than P. limnetica at highest salinities, the latter being probably more halophytic. Contrary to the observations in lakes located at higher latitudes, the Mediterranean climate seems to induce a typical seasonal pattern of succession characterised by the dominance of P. agardhii (winter) – Chroococcales (spring) – Pseudanabaenaceae (summer) – P. agardhii (autumn, winter). The warm temperatures seemed to have a major influence on the phytoplankton succession, being responsible for the survival of Planktothrix during winter and its rapid and intense development in early spring. Intense mixing and high irradiance in summer promoted the development of Pseudanabaenaceae, as reported in another south-temperate lagoon, the Albufera of Valencia (Spain). The ecological success of Oscillatoriales observed in the Bolmon lagoon is a perfect example of a shift to the “turbid stable state” as proposed for freshwater shallow lakes only. Our work demonstrated that hypereutrophic Mediterranean lagoons can function very similarly to shallow lakes at higher latitudes; but the warmer climate and higher irradiances are probably responsible for differences in the seasonal pattern of species dominance.     

Assessment of the effects of hydrocarbon contamination on the sedimentary bacterial communities and determination of the polar lipid fraction purity: Relevance of intact phospholipid analysis

This in situ study reports on the relevance of intact phospholipids as new biomarkers and highlights the limits of the classical fatty acid analysis after the hydrolysis of the polar lipid fraction. The analysis of polar lipid fractions revealed significant contributions of marine macrophytes with either non-phospholipid compounds (sulfoquinovosyl diacylglycerol) or with both non-bacterial phospholipid classes (phosphatidylcholine) and non-bacterial phospholipid molecular species (some molecular species of phosphatidylglycerol). The analysis of intact phospholipids showed also the predominance of gram-negative bacteria in sediments whereas the analysis of fatty acids alone cannot confirm such information. Lastly, as regard to the effects of petroleum hydrocarbons on the sedimentary bacterial communities, we observed a higher degree of saturation of the fatty acyl chains of the phospholipids extracted from sediment exposed to a massive and chronic crude oil contamination. Such a result was interpreted as an adaptive response resulting in higher membrane rigidity of the bacterial communities.     

Site effects of the Roio basin, L��Aquila

During the microzonation studies of the April 6th, 2009 L??Aquila earthquake, we observed local seismic amplifications in the Roio area??a plane separated from L??Aquila city center by mount Luco. Six portable, digital instruments were deployed across the plain from 15 April to mid-May 2009. This array recorded 152 aftershocks. We analyzed the ground motion from these events to determine relative site amplification within the plain and on surrounding ridges. Horizontal over vertical spectral ratio on noise data (HVSRN), aftershock recordings (HVEQ) and standard spectral ratio (SSR) showed amplifications at 1.3 and 4.0?Hz on quaternary deposits. Seismic amplifications in the frequency range of 4 and 6?Hz were also observed on a carbonate ridge of Colle di Roio, on the northwestern border of the plateau. A small amplification was noticed near the top of mount Luco, another rocky site. Large discrepancies in the amplification levels between methods have been observed for these sites, but the HVSRN, HVEQ and SSR gave similar results at the stations located in the Roio plain. On the rocky sites, the SSR was more reliable than the HVSRN at estimating the transfer function of the site, even if the resonance frequency seemed to be well detected by the latter method.