MAGNETOTELLURIC SOUNDINGS AND THE GEOLOGICAL STRUCTURE AND TECTONICS OF NORTHERN SENEGAL, WEST AFRICA THE SENEGALO-MAURITANIAN BASIN IN

Abstract , The results of the magnetotelluric soundings provide new insights on the geological structure and tectonics of the onshore Senegalo-Mauritanian basin in northern Senegal (West Africa) . Electrical resistivities can be correlated with the major geological units. In the upper 1000 m, resistivity variations indicate a transition from freshwater saturated sediments near the surface to saline water saturated sediments above the basement. The resistivity of the sediments is dominated by their porosity resistivity of the contained fluids. The resistivity of the Cretaceous sediments is unexpectedly low, especially in the western part of the basin. The top of the metamorphic/granitic basement cannot be located on the west portion of the profile because of the feeble resistivity contrast between the impervious, compacted Aptian-Jurassic limestones and the metamorphic/crystalline rocks. The magnetotelluric estimates of the depth of the basement range from 600 m in the east to 4000 m in the west. Based on the magnetotelluric interpretation and other independent geophysical and geological information, a cross section through the and th

Size structure of phytoplankton biomass in the equatorial Atlantic Ocean

Abstract-The size structure of chlorophyll a (Chl a) and phaeopigments ((20, (10, (3, <2, and <1 ym) has been studied along three transects (4O, 23", and 35OW) covering the entire equatorial Atlantic Ocean, with special attention to the small sizes (<3 pm). Everywhere in the studied area, even in the equatorial upwelling, the bulk of Chl a is within organisms which pass through a 3 pm Nuclepore filter. The vertical distribution of the (1 pm Chl a is closely related to the depth of the nitracline. In the nitrate-depleted mixed layer, the (1 ym Chl a always dominates, and represents 71% of the total Chl a on the average. At the top of the nitracline, the < 1 ym Chl a concentration is maximum but represents only 50% of the total Chl a. In the nitrate-rich waters, whatever the depth, the percentage of (1 pm Chl a is everywhere (50% and decreases downwards. At the bottom of the photic zone, the < 1 prn Chl a values are nearly zero. When integrated over the whole euphotic layer, the < 1 ym Chl a represents about 25% of the total Chl a when nitrate is present at surface; this percentage reaches 60% when the top of the nitracline deepens to 100 m depth. Preliminary measurements of photosynthetic activity (light gradient and time course experiments) indicate that the < 1 ym fraction contains actively photosynthetic organisms but also organisms which are able to fix CO1 in the darkness in a significant proportion. The roles of sinking, nutrients, and light in the vertical distribution of picoplankton are discussed. Our results indicate that the size distribution of Chl a (and especially the relationship between abundance of picoplankton and nitrate distribution) is the same throughout the whole equatorial (MALONE, 1980). Recent studies have focused attention on a smaller size class of autotrophic organisms-the picoplankton ((2 pm, according to SIEBURTH et al., 1978). WATERBURY et al. (1979) and JOHNSON and SIEBURTH (1 979) show the existence of small unicellular cyanobacteria (procaryotes) with a widespread geographical distribution and the ability to achieve considerable densities. Recently, LI et al. (1983) in the eastern tropical Pacific, and PLATT et al. (1983) in the tropical North Atlantic (west of Azores) concluded that autotrophic picoplankton (< 1 prn) are capable of supplying 60% (range 20 to 80%) of the total primary production in the open ecosystem. These two last results are extremely important because they modify our concepts on the biological structure of the pelagic ecosystem, but they are limited in space and time. We .I

Legacy of logging roads in the Congo Basin: how persistent are the scars in forest cover?

Logging roads in the Congo Basin are often associated with forest degradation through fragmentation and access for other land uses. However, in concessions managed for timber production, secondary roads are usually closed after exploitation and are expected to disappear subsequently. Little is known about the effectiveness of this prescription and the factors affecting vegetation recovery rate on abandoned logging roads. In a novel approach we assessed logging roads as temporary elements in the forest landscape that vary in persistence depending on environmental conditions. We analyzed road persistence during the period 1986–2013 in adjacent parts of Cameroon, Central African Republic and Republic of Congo. Three successive phases of road recovery were identified on LANDSAT images: open roads with bare soil, roads in the process of revegetation after abandonment and disappeared roads no longer distinguishable from the surrounding forest. Field based inventories confirmed significant differences between all three categories in density and richness of woody species and cover of dominant herbs. We used dead-end road segments, built for timber exploitation, as sampling units. Only 6% of them were identified as being re-opened. Survival analyses showed median persistence of four years for open roads before changing to the revegetating state and 20 years for revegetating roads before disappearance. Persistence of revegetating roads was 25% longer on geologically poor substrates which might result from slower forest recovery in areas with lower levels of soil nutrient content. We highlight the contrast amongst forests growing on different types of substrate in their potential for ecosystem recovery over time after roads have been abandoned. Forest management plans need to take these constraints into account. Logging activities should be concentrated on the existing road network and sites of low soil resource levels should be spared from business-as-usual exploitation. (Résumé d'auteur

Termes de référence techniques et scientifiques, campagne agricole hivernage 87/88. Perspectives. Évaluation et suivi de la production agricole en fonction du climat et de l'environnement - ESPACE.

Les produits de la recherche agronomique et climatologique en matière de connaissance des relations entre le rendement des cultures et les conditions de sol et de pluviosité ont atteint un niveau de performance désormais compatible avec une utilisation pratique par l'ensemble des partenaires concernés par le développement rural. La disponibilité de moyens efficaces de calcul micro-informatisés, de télécommunication, d'intervention sur les terrains, autorise de plus, leur exploitation pratiquement en temps réel si des efforts d'organisation et de concertation sont consentis. Le programme ESPACE.se propose de contribuer à relever ce défi en tentant, après une campagne d'essai réalisée en 1986, de faire fonctionner, au cours de l'hivernage 1987, un dispositif d'observation et de traitement d'informations agro-météorologiques susceptibles de produire des critères objectifs d'appréciation du risque pluviométrique et de ses effets sur l'espérance de production agricole. Les informations recueillies permettront de compléter et de renforcer les actions de diagnostic déjà mises en oeuvre par les services tels que : statistiques et planifications agricoles ; cellules de suivi des Ministères techniques : groupes et réseaux associés ; programme CILSS/Agrhymet et FAO. Elles leur apporteront un éclairage spécifique, par l'analyse des de mécanismes explicatifs du comportement des cultures et la prise en compte de la diversité de leur environnement pédoclimatique. À moyen terme, l'action programmée s'inscrit, en tant que contribution agronomique, dans les orientations du programme Agrhymet, de plus en plus axées, d'une part, sur la connaissance des situations "à risque" auxquelles sont confrontés les producteurs, et d'autre part, vers le développement d 1 outils opérationnels d'appui à toute décision susceptible de minimiser les contraintes climatiques et de valoriser au mieux la ressource pluviométrique