Macrobenthic Infaunal Assemblage Structure in Nearshore and Offshore Seabeds of Ghana

Abstract

The increased need for energy self-sufficiency has caused an extensive search for various sources of energy (especially, oil and gas) in the marine environment. Globally, the magnitude of impacts of the activities on the marine environment is increasing over the years and most felt on the seafloor. The study of seafloor (benthic) organisms therefore, are vital determinants of impacts on the seafloor. This thesis sought to determine the community structure of nearshore and offshore benthic macrofaunal assemblages and sediment characteristics of the two environments. It further provides a baseline of benthic macrofaunal assemblages prior to the proposed coal-powered facility at Ekumfi Aboano (nearshore), as well as, determine the current (2015) state of benthic macrofaunal assemblages in the Jubilee Fields (deep-sea). The study was carried out within the nearshore Ekumfi Aboano (av. depth–15 meters) and offshore regions (av. depth–1250 meters) of the Jubilee Fields. Sediment samples were obtained using a Van Veen grab and box corer for the nearshore and deep-sea environments respectively. Benthic macrofaunal distribution was investigated using PRIMER. Using GRADISTAT, nearshore sediment (mean grain size of 99.80 μm) was characterized as moderately sorted with very fine sand. Conversely, offshore sediment (mean grain size of 109.79 μm) was characterized as poorly sorted with very coarse silty fine sand. From the macrobenthic infauna analyses, 1113 individuals with an average density of 38 individuals/m2 were found at the nearshore environment and 2205 individuals with an average density of 47 individuals/m2 were found in the deep-sea environment. Numerical abundance showed polychaetes were 194 and 983, crustaceans were 827 and 696, molluscs were 35 and 229, echinoderms were 14 and 37 and “others” (foraminiferans, nematodes, nemerteans, sipunculids) were 43 and 260, for iv

the nearshore and offshore environments respectively. The order of dominant taxa was; crustaceans > polychaetes > molluscans > others > echinoderms–for shallow Ekumfi seabed and; polychaetes > crustaceans > molluscans > others > echinoderms–for the offshore Jubilee seabed. Species identified nearshore were 116, whereas 188 species were identified within the deep-sea environment. Both sites were evenly spread (J’ > 0.8) and showed high diversity (H’ > 4; 1-D > 0.9). Polychaetes dominated (44.58 %) offshore as arthropods dominated nearshore (74.30 %). Macrobenthic infauna abundance was higher deep-sea than nearshore and were 75 % dissimilar, possibly due to varied anthropogenic influence within the two environments.