Ecological status and sources of anthropogenic contaminants in mangroves of the Wouri River Estuary (Cameroon)

Study Number

95

Author

Marco Fusi, Gian Maria Beone, Nicoleta Alina Suciu, Angela Sacchi, Marco Trevisan, Ettore Capri, Daniele Daffonchio, Ndongo Din, Farid Dahdouh-Guebas, Stefano Cannicci

Abstract

Mangroves are critically threatened by human activities, despite the important ecosystem functions and services they provide. Mangroves in Cameroon represent no exception to the worldwide trend of mangrove destruction especially around Douala, on the Wouri river estuary. In two sites around Douala, we assessed the presence of sterols, PAHs, PCBs, DEHP, DDT and its metabolite p,p’-DDE and potentially toxic metals in sediment samples. As a proxy of ecological quality, we measured the diversity and abundance of macrobenthos assemblages. We detected p,p’-DDE contamination, with concentrations higher than 3 μg kg−1 in 16 out of 26 samples which were attributed to recent widespread use of DDT. The detection of sterols revealed faecal contamination. Significant sensitivity of the macrobenthos to contaminants was revealed, with possible implications on the overall mangrove vulnerability to climate change and on the provision of ecosystem services to local populations.

Main Results and Conclusions

• Anthropogenic forces have resulted in a significant reduction in mangrove forest cover, as well as increased contamination in the remaining forests.
  • “Mangroves in Africa cover over 3.2 million ha, corresponding to about 20% of their global coastline coverage, with approximately 1.5 million ha located along the Atlantic Coast.” (723)
  • “As a consequence of enormous anthropogenic pressure and multiple threats, western African mangroves have declined by >25% over the past 25 years…” (723)
  • “Although mangroves contribute considerably to the social and economic well-being of the Cameroonian coastal inhabitants, their total surface area has decreased by 30% in 20 years… mainly due to rapid and uncontrolled urbanization around Douala…” (723)
  • “Douala is the largest city in Cameroon and exerts a huge pressure on the nearby mangroves, with uncontrolled sewage discharge detrimentally affecting the whole ecosystem…” (723)
• Mangroves play a central socioeconomic role in the Wouri River estuary.
  • “…the major socio-economic activity associated with mangroves for local people is in fact still artisanal fishing, with landings estimated between 76 and 106 tons per year.” (723)
  • “Fisheries play a significant role in small-scale commercial activities and they are vital in providing a source of protein and income for coastal communities” (723)
  • “Thus, the modification of both abundance and diversity of mangrove species and the deterioration of water quality, due to urban and industrial activities, will surely have detrimental consequences on the well-being of local communities.” (723-724)
• Many species have been negatively affected by the contamination in the mangrove forests.
  • “…changes in the diversity/structure of mangrove ecosystems have been reported as a response to chemical pollution (e.g. Mohamed et al., 2008), which has also been linked to a decline in some populations such as mangrove oysters and snails (e.g. Roach and Wilson, 2009) and molluscs…” (729)
  • “Our results indicate that multiple anthropogenic stressors, and in particular heavy loads of wastewater, although not resulting in the hypothesised depletion of crab abundance in Wouri River estuary mangroves, can shape their community composition.” (731)
  • “The subtle faecal contamination could triggering possible ecological bottom-up effect with relevant ecological consequences on the overall system (Hamilton, 2004), through the modification of secondary consumers primary feeding sources, such as microbenthic (Isobe et al., 2004) or infaunal macrobenthic communities (Moon et al., 2008).” (731)
  • “…mangrove sediment receiving wastewater does accumulate a remarkably higher level of long-term contaminants.” (731)
  • “…that they could potentially spread through the whole mangrove area (Agoramoorthy et al., 2008; Yi et al., 2011; Zhou et al., 2007), ultimately affecting, through the trophic chain, the secondary and tertiary consumers which are consumed by the local population.” (731)
  • “Our data clearly show that the main source of contamination in the mangrove forests surrounding Douala is represented by uncontrolled discharge of urban wastewater and the persistent, illegal and indiscriminate use of DDT. These contaminants, together with four specific heavy metals (As, Cr, Zn, Se) seem to affect the macrobenthonic assemblage of the two study sites, suggesting that Douala peri-urban mangrove is subjected to a complex patchwork of contamination. This documented inflow pollution has serious implications for ecosystem functioning and public health.” (731)