Assessment of trace metal contamination in mangrove ecosystems from Senegal, West Africa
Highlights
► Trace metals were analysed in sediment and harvested molluscs from African mangroves. ► A geochemical normalisation allowed for discrimination of contamination among sites. ► Metal bioaccumulation strongly according to sampling site, metal and mollusc species. ► Concerns about Cd contamination of edible shellfish from Senegal were revealed.
Introduction
African mangroves cover over 3.2 million ha corresponding to about 20% of global coverage, with approximately 1.5 million ha located along the western Atlantic coast. In consequence of the enormous pressures and threats they are submitted to within the last past decades, a decline by more than 25% of the western African mangroves has been observed over the past 25 years (UNEP, 2007). The Senegal, located in West Africa, shelters approximately 6000 km2 land-area of mangroves distributed along the Southern coast in three main areas: the low Casamance, the Sine-Saloum, and the Petite Côte. These mangroves contribute considerably to the social and economic well-being of the inhabitants of these regions. However, their total surface has been reported to decrease from 1690 km2 in 1980 to 1287 km2 in 2006 (UNEP, 2007). The government of Senegal and the international community have since taken steps to protect the biodiversity of these sites, and consequently the related socio-economic activities mainly dominated by fishing and its annex activities (from harvesting of the primary resource to commercialization of further processed fish and seafood products). Hence, the Saloum Delta and Joal-Fadiouth areas from the Sine-Saloum and the Petite Côte are of great concern in terms of conservation and management because of the traditional and well-developed fishing activities of Senegal women, particularly the harvesting of diverse mangrove invertebrates. These fisheries play a significant role in artisanal and small-scale commercial activities, and are vital in providing a source of protein and income to coastal communities (Chapman, 1987, Matthews, 2002). The overfishing and modification of water quality due to land-based activities on the marine invertebrates populations might have strong negative consequences on the well-being of local communities (Diouf, 1996, Dia, 2003). In particular, it is of main importance to determine to what extend these mangroves are exposed to anthropogenic pollution, and how it affects commercially important marine species. Since the harvest of marine invertebrates, particularly shellfish, is predominantly a female activity, the decline in these resources may have the dual effect of altering their position within their households and communities, and causing declines in households’ levels of food security and operating income (Aswani and Weiant, 2004).
Among the major pollutants from anthropogenic inputs that reach mangroves are heavy metals (MacFarlane et al., 2007). Metallic enrichment of mangrove environments arises from urban and agricultural runoff, industrial effluents, boating and recreational use of waterbodies, chemical spills, sewage treatment plants, leaching from domestic garbage dumps and mining operations (Peters et al., 1997). Heavy metal toxicity in aquatic organisms, in association with the long residence time within food chains and the potential risk of human exposure, makes it necessary to monitor the levels of these contaminants in organisms and more generally in mangrove environment. Sediment is reported to remarkably retain heavy metals (Harbison, 1986, Tam and Wong, 1995), and thus is considered as an appropriate indicator of heavy metal pollution (Idris et al., 2007). However, they are also known to become a secondary source of pollution through remobilization/resuspension processes back to the water column. Molluscs have also been widely used to monitor the quality of water ecosystems since the late 1980s because of their wide geographical distribution, sessile life-style, ease of sampling, tolerance to a range of salinities, resistance to stress, and high accumulation of a wide range of chemicals (Goldberg et al., 1978, Claisse et al., 1992, Rainbow, 1995, Cantillo, 1998). Since the major problem in understanding metal effects on aquatic biota is the high variability in metal bioavailability in the marine environment, the use of several bioindicators improve our abilities to evaluate the bioavailability and bioaccumulation of these chemicals in marine organisms.
The main objective of this work was to evaluate the contamination status of trace metals in the mangrove environment of Southern West Senegal. The levels of trace metals were determined in sediment and harvested molluscs belonging to the Sine Saloum and the Petite Côte ecosystems, considering seasonal effect. Potential ecotoxicological concerns were also investigated because of the sanitary importance of knowing the health status of exploited molluscs for human consumption and to contribute with valuable information that allows improving the harvesting activity in these regions.
Section snippets
Description of the sampling areas
Four areas were investigated in Southern West Senegal: Dionewar, Niodor and Falia in the Saloum Delta, and Fadiouth from the Petite Côte (Fig. 1).
The Saloum Delta is located in the northern part of Gambia, 150 km South of Dakar, between 13°35N and 14°15N, and between 16°W and 17°W. It is a maze of more than 200 islands covered with mangrove swamps, baobabs, kapoks, constituting a network of uncountable bolongs. The Saloum region is one of the most populated in Senegal with 67 inhabitants per km2
Trace metal contamination in mangrove surface sediments
The physico-chemical characteristics of the sampling locations and the concentrations of 12 trace elements (Li, V, Cr, Mn, Co, Ni, Cu, Zn, Ag, Cd, Pb, and Hg) and 2 major elements (Al and Fe) measured in surface sediment samples are given in Table 2. In Dionewar and Niodor, surface sediment was characterised by fine sands with a water content of 23.1 ± 1.1%, while sediment from Falia and Fadiouth corresponded to heterogeneous muddy sands with a water content of 23.0 ± 1.0% (Bodin et al., 2011). To
Acknowledgements
This study was part of a project « Femmes et Coquillages » implemented by the Fondation Internationale du Banc d’Arguin (FIBA) and financed by the MAVA – Fondation pour la Nature. The authors would like to thank the Senegalese women from Niodior, Dionewar, Falia and Fadiouth for their care and their excellent help in sampling mangrove shellfish. We are also grateful to Jean Raffray, Oumar Sadio, Augustin Diokh and Assane Thiam for their technical help at different steps, as well as Pierre Lopez
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