Article

Environmental consequences of innovative dredging in coastal lagoon for beach restoration

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Abstract

Evidence suggests that hydraulic dredging is accompanied by considerable adverse environmental impacts on the receiving ecosystem especially on the benthos and water quality. Recently, innovative dredging is designed to minimise environmental impacts and enhance the ecological settings. Evaluations of environmental consequences of such innovative dredging are essential to quantify the ecological benefits and the associated impacts to ensure good environmental management. Congruently, innovative dredging ('design with nature' principle) in a large tropical coastal lagoon in Ghana (Keta lagoon), West Africa, was assessed Before, During and After dredging operations on spatio-temporal scales to ascertain the environmental impacts on the macrobenthic fauna, shorebirds and water quality. A total of 9091 million cubic meter of sediment was removed from the 8m stretch of the lagoon for beach nourishment, land reclamation and creation of habitat islands. The macrobenthic fauna was sampled once in 2000 (Before), 2001 (During) and 2002 (After) along seven stations (A-0 to G-0 of 1-km interval) in the dredged channel. Water quality was assessed at the subsurface and bottom layers quarterly from June, 2001 to September, 2002. The shorebirds community abundance were quantified monthly from August 2000 to 2002, but only parallel data from August-December (peak periods of shorebirds abundance) of each year (2000-2002) was used for statistical analyses.The results demonstrate that dredging had initial adverse effects on numerical abundance of macrobenthic fauna but with evidence of recovery a year after the dredging (2002). Species recorded in 2001(During Dredging) and 2002 (After Dredging) were very similar in terms of composition particularly in the wet periods, suggesting the influence of seasonal environmental factors. The abundance of the species showed significant spatio-temporal variations (p<0.05). The macrobenthic fauna was dominated by opportunistic species of the family Capitellidae. Although, Nepthys lyrochaeta revealed higher frequency of occurrence (52%), there was significant (p<0.05) decrease in abundance after dredging (2002). Conversely, Notomastus cf. latericeus depicted significant (p<0.05) increase (recovery) after dredging (2002).There was no apparent impact on coastal avifauna although numerical abundance of wader group decreased from 78% in 2000; 69% in 2001 to 51% in 2002. Conversely, terns showed increased abundance from 17% in 2000, 21% in 2001 and 47% in 2002 indicating positive impact. The shorebirds placed in the =others' category experienced peak and trough between the period (6% in 2000; 10% in 2001, and 2% in 2002). In general mean numerical abundance of the shorebirds increased from 8.8% in 2000 (Before) to 81.5% in 2002 (After) of the periods.Temporal and spatial variability occurred in the physicochemical parameters measured (e.g., salinity, total dissolved solids, total suspended solids and sulfate). However, elevated turbidity occurred in localised areas along the fetch during the dredging operation. The results of the analysis presented are pertinent to several questions, such as what are the expected ecological benefits of innovative dredging and adverse impacts on the receiving ecosystem.

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