Étude de la compétitivité de complexation des métaux issus des phosphates naturels de Hahotoé-Kpogamé par les substances humiques

Le phosphate naturel de par sa formule chimique constitue une source importante de l’élément phosphore pour les plantes. Pour cela il est utilisé dans le compost pour améliorer les qualités nutritionnelles des plantes. Les analyses chimiques effectuées sur le phosphate naturel révèlent la présence des métaux lourds. Pour étudier les risques de rétrogradation de ces engrais phosphatés, notre étude s’est portée sur la complexation des substances humiques naturelles extraites des sols sur des ions métalliques tels que le calcium, cadmium et le magnésium contenus dans les phosphates naturels de Hahotoé- Kpogamé du Togo. Les résultats obtenus montrent que la quantité d’ions complexés augment avec le temps d’attaque. Le taux d’ions complexés diminue lorsque le pH de la solution d’acide humique évolue de 6,5 à 9,5. Une étude comparative de la complexation de ces trois ions révèle que le cadmium malgré sa faible teneur dans le phosphate naturel se complexe mieux que le calcium et le magnésium. Après 30 jours d’attaque du phosphate naturel nous avons obtenu un taux avoisinant les 10% pour le cadmium, 9% pour le calcium et environ 2% pour le magnésium pour une solution d’acide humique de pH=6,5.Mots-clés : complexation, métaux lourds, phosphate naturel, rétrogradation, substances humiques.Study on competitiveness of metals after complexation of phosphate rock in Hahotoè-Kpogamé by humic substancesPhosphate by its chemical constitution is an important source of the phosphorus element for plants. Thus, it is used in compost to improve the nutritional qualities of plants. Chemical analyzes of the phosphate reveal the presence of heavy metals. To assess the risk of the phosphate fertilizer downgrading, our study focused on calcium, magnesium, and cadmium ions contents in the humic substances extracted from soils fertilize with Hahotoè-Kpogamé (Togo) phosphate. The results show that the amounts of ions in the complexed humic substances increase with the attack time. The complexed ions rate decreases as the pH of the solution of humic acid changes from 6.5 to 9.5. Despite the low cadmium content in the rock phosphate, this element is more easily assimilated in complex than its two analogous. After 30 days of phosphate attack, a humic acid solution at pH = 6.5 gave a rate of about 10% for cadmium, 9% for calcium and 2% magnesium.Keywords : complexation, heavy metal, rock phosphate, downgrading, humic substances

Seasonal and inter-annual ONSET Sea Surface Temperature variability along the northern coast of the Gulf of Guinea

The aim of this study is to characterize the coastal upwelling variability at seasonal and inter-annual time scales in the northern Gulf of Guinea (NGoG) using Sea Surface Temperature (SST) collected with autonomous “ONSET” thermometers. Results show that the ONSET SST data are suitable for numerical model evaluation, and provide relevant information in addition to satellite and reanalysis data at seasonal cycle. The minor and major coastal upwellings are present in all the products. The inter-annual SST variability is more pronounced in the western part of the region (Côte d’Ivoire and Ghana) than in the eastern part (Benin and Nigeria). The pattern differences between the west and the east of the region highlight a large spatial variability of the SST in the NGoG. Indeed, the signal of the minor upwelling season is visible only in the west of the basin, namely between Cape Palmas and Cape Three Points. We also observe a well-established thermal gradient between the western and eastern parts of the basin. This gradient is increasing from west to east during the major upwelling season, and decreasing from east to west during the rest of the year. The coastal ONSET data allow to evidence higher SST anomalies than those deduced from satellite and reanalysis products. Although the cold or warm events observed in 2008, 2010 and 2012 are well detected by all products, only the ONSET data set reveal the strong negative SST anomaly observed in 2009 along the coast of Ghana and Côte d’Ivoire

Beach cusp dynamics on a reflective beach

International audienceBeach cusps are common feature of steep reflective and intermediate beaches. However very few observations reported double coupled cusp systems. Here we present a data set of observations of a beach exhibiting two sets of beach cusps. Data were collected at Grand Popo Beach (Benin, West Africa) in February 2013. Daily topographic survey along a 380m long stretch of shore allowed observing the dynamic of the two set of beach cusps. At the beginning of the field survey, we clearly observe two sets of cusps : the upper beach cusps system is relatively asymetric with a typical wavelength of about 45 m while the lower beach cusps system is relatively symmetric with a typical wave length slightly shorter (about 35 m). After two days, we measured the total destruction of the lower set of beach cusps while the upper set of beach cusps was only partially des tructed. Data suggest that destruction of the lowerbeach cusp system may be related to persistent accretions conditions and/or calm conditions but probably also to the transition from wave- driven circulation dominated by weak alongshore currents with flash and swash rips, to a second period characterized by dominant longshore current further increasing in speed, and rare swash rips. On the other hand the disappearance of the western upper beach cusp might be related to an accretion pattern and to the merging of two individual features. Our observations, consistent with previous works, suggest that beach cusps certainly arise as a result of some combination of erosion and accretion

Resilient Lagoons? Climate change, sustainability and adaptation

Lagoons are found at low-lying coastlines around the globe (Figure 1) and their associated wetlands are important dynamic environments. Ensuring the sustainability of the world’s lagoons is vital for communities, ecosystems and economies. Lagoons support highly productive ecosystems and provide critical ecosystem services, societal benefits and myriad fundamental and valuable resources that are vital for the wellbeing and livelihoods of coastal communities. Yet, the sustainability of lagoons and the communities who rely on them are under increasing pressure from a complex set of interconnected issues, including climate change, sea-level rise, pollution, poor waste management, population growth and policy approaches that favour top-down governance to the exclusion of local knowledges and priorities (Convention on Wetlands, 2021). This article summarises the latest research on lagoons using the examples of Muni Lagoon in Ghana and Lagos Lagoon in Nigeria (Figure 1). It also draws from the interdisciplinary dialogues emerging through the Global Challenges Research Fund (GCRF)-funded Resilient Lagoon Network (see website), which seeks to challenge top-down management approaches and instead prioritise participatory approaches that value local knowledges and in which coastal communities are central to resilient lagoon governanc

The Grand Popo experiment, Benin

International audienceThe first large nearshore field experiment in the G ulf of Guinea was conducted at Grand Popo Beach, Be nin, in February 2013, on an open wave-dominated micro- to meso-tidal coast, located mid-way between Cotonou and Lome harbours. The overall project aims at understanding at multi-scale (from event to interannual) the causes of the dramatic erosion observed throughout the Bight of Benin, and caused by the interaction of a large littoral drift with human engineering works. Grand Popo 2013 experiment was designed to measure the processes over the short term and to test the ability of an installed video system to monitor the evolution of this stretch of coast over the longer term. The beach, characterized by a low-tide terrace and a high tide reflective part, experiences a long swell (Hs=1.6m, Tp=16 s, oblique incidence ~15-20°). Topographic surveys showed a double beach cusp system interaction and repeated surf-zone drifter runs revealed high flash and swash rip activity driven by wave dissipation over the terrace and energetic swash dynamics at the upper reflective beach. Swash was measured over a cusp system at two locations using video poles. Wave reanalyses (ERAInterim) were used to determine the wave climate and its variability, and to quantify sediment transport. This robust methodology is thought to be replicated elsewhere in different coastal environments in West Africa, in particular with the objective to monitor various sites within the framework of the new West African Coastal Observatory

The Grand Popo beach 2013 experiment, Benin, West Africa: from short timescale processes to their integrated impact over long-term coastal evolution

The first large nearshore field experiment in the Gulf of Guinea was conducted at Grand Popo Beach, Benin, in February 2013, on an open wave-dominated micro- to meso-tidal coast, located mid-way between Cotonou and Lome harbours. The overall project aims at understanding at multi-scale (from event to interannual) the causes of the dramatic erosion observed throughout the Bight of Benin, and caused by the interaction of a large littoral drift with human engineering works. Grand Popo 2013 experiment was designed to measure the processes over the short term and to test the ability of an installed video system to monitor the evolution of this stretch of coast over the longer tenn. The beach, characterized by a low-tide terrace and a high tide reflective part, experiences a long swell (Hs=1.6 m, Tp=16 s, oblique incidence similar to 15-20 degrees). Topographic surveys showed a double beach cusp system interaction and repeated surf-zone drifter runs revealed high flash and swash rip activity driven by wave dissipation over the terrace and energetic swash dynamics at the upper reflective beach. Swash was measured over a cusp system at two locations using video poles. Wave reanalyses (ERAInterim) were used to determine the wave climate and its variability, and to quantify sediment transport. This robust methodology is thought to be replicated elsewhere in different coastal environments in West Africa, in particular with the objective to monitor various sites within the framework of the new West African Coastal Observatory