Assessing spatio-temporal influences of hard engineering structures on coastal recession and resilience of beachfront communities and ecosystems of the central coast of Ghana, West Africa

Abstract

Governmental policies, including construction of sea defence structures and harbours to safeguard coastal communities and improve artisanal fisheries in Ghana, are still being outpaced by erosion and reduction in fish catches. This study assessed the effectiveness of coastal defence structures through bathymetry, sediment grain size characterisations, temperature and salinity profiles, plankton community structure and chlorophyll-a levels within 12.0 km transects offshore Elmina and Anomabo. Bathymetry data was collected by single beam echosounder, sediment, zooplankton, phytoplankton and seawater samples were collected using Ponar grab, 200 𝜇m, and 63 𝜇m mesh size nets, and Niskin bottle. Depth profiles were between 5.1 ± 0.5 - 25.73 ± 1.11m and 4.61 ± 0.34 - 22.29 ± 0.69 m for the Elmina and Anomabo transects. A modified exponential equation showed concave to gentle and medium-sloping seafloor. Comparative analysis revealed sediment deposition and erosion along the Anomabo transect. Seabed erosion seaward of the defense structure and deposition further offshore suggested sediment transport was more offshore than onshore of the Elmina. Sediment grain analysis confirmed sediment supply was less than been moved to greater depth offshore Elmina. Anomabo sediment size distributions showed onshore sediment deposition and neighbouring sediment supply. Temperature and salinity levels confirmed the upwelling season in July-August. Chlorophyll-a levels demonstrated oligotrophic and mesotrophic ecosystems. Plankton abundance peaks at 1.5 – 3.0 km points and suggests a shifted primary productivity community struct ure from the inner shore to deep waters offshore Elmina. Shannon-Weiner and Simpson dominance indices showed a stressed community dominated by few species. Thus, the study demonstrated seabed erosion near the defence structure and a shift in the plankton community structure and recommends continuous monitoring of the seafloor and the plankton community structure for effective management of Ghana’s nearshore ecosystem.