The Impacts of Micro-Topographic Changes on Mangroves in the Lower Reaches of the Benin River, Niger Delta

Mangroves of the Niger Delta: Their Importance, Threats, and Possible Restoration

“…Dredging could involve the following activities including clearing of mangrove vegetation, straightening, widening and deepening of existing creeks, and creating new canals. In the process of dredging, the dredged spoils (soil, sediment and vegetation) are typically discharged at creek banks and abandoned (Ohimain 2002(Ohimain , 2003a(Ohimain , 2003b(Ohimain , 2003c. Besides the mangroves killed via direct removal (dredging) and smothering (spoil disposal), changes in the topography, hydrology and salinity result in mangrove mortality beyond the immediate impact area (Ohimain 2004(Ohimain , 2008a(Ohimain , 2004bOhimain et al 2004Ohimain et al , 2010.…”

“…In the process of dredging, the dredged spoils (soil, sediment and vegetation) are typically discharged at creek banks and abandoned (Ohimain 2002(Ohimain , 2003a(Ohimain , 2003b(Ohimain , 2003c. Besides the mangroves killed via direct removal (dredging) and smothering (spoil disposal), changes in the topography, hydrology and salinity result in mangrove mortality beyond the immediate impact area (Ohimain 2004(Ohimain , 2008a(Ohimain , 2004bOhimain et al 2004Ohimain et al , 2010. Both permanent flooding and lack of tidal inundation kill mangroves on a large scale (Ohimain 2003a), whereas the abandoned dredged spoil soon becomes acidic and drains into the mangrove back swamps killing more mangroves (Ohimain et al 2004;Ohimain 2001Ohimain , 2004.…”

“…The environmental impacts of acidification can be overwhelming, affecting virtually all components of the ecosystem including phytoplankton (Ohimain and Imoobe 2003), zooplankton (Ohimain et al 2002), benthic invertebrates (Ohimain et al 2005), and vertebrates especially fishes (Fagbami et al 1998;Ohimain 2003a). Dredging and spoil abandonment has also been implicated in widespread hydrological changes (Ohimain 2003a), topography changes (Ohimain et al 2010), water quality changes (Ohimain et al 2008b), heavy metal pollution (Ohimain et al 2008a(Ohimain et al , 2008c, shift in vegetation types (Ohimain, 2005;Ohimain et al 2004Ohimain et al , 2005, increased erosion and siltation (Ohimain et al 2004), excessive flooding of the back swamp (Ohimain et al 2004) and coastal retreat (Eedy et al 1994).…”

“…Acidification typically results following the exposure of mangrove spoils containing pyrites Ohimain 2003aOhimain , 2003bOhimain , 2004Ohimain , 2008a, 2008b Physicochemical changes and heavy metal pollution Physicochemical changes follows dredging and exposure of pyritic dredged materials resulting in physicochemical changes such as extreme acidity and heavy metal pollution Ohimain et al 2003aOhimain et al , 2008a Maintenance dredging Maintenance dredging result in the deposition of dredged spoils upon fringing mangroves, thus killing them Ohimain 2002Ohimain , 2004 Topographic and hydrological deficiencies Changes in topography often results in altered hydrology, which do not favour mangrove recruitment, establishment and growth Ohimain et al 2010 Competition by alien species Due to altered hydrology, abandoned spoils are becoming less saline after years of weathering, which favours invasive species over mangroves Ohimain et al 2010…”