Assessment of Ecological Quality Status of Western Nigeria Offshore Waters (Gulf of Guinea) Using Macrobenthic Assemblage

Benthic macrofauna differs in sensitivity, tolerance, and adaptability to environmental fluctuations and disturbances induced by various anthropogenic activities. These activities pose an environmental threat to aquatic biota; thus, benthic macroinvertebrates are utilized as bioindicators of stressors. This study aimed to assess the ecological quality status (EcoQs) of Nigeria's offshore waters using macrobenthic diversity and ecological biotic indices. Bottom sediments were collected from seven (7) stations using Van-Veen grab, and Niskin water sampler for surface water onboard the RV-BAYAGBONA. The identified macrofauna communities were subjected to AZTI’s Marine Biotic Index (AMBI) to depict the ecological quality status of the environment. The physico-chemical parameters (Water temperature, dissolved oxygen, conductivity, depth, pH, alkalinity, salinity, phosphate, and nitrate) using a multi-parameter checker, HORIBA-U53. The physico-chemical parameters of the bottom water showed significant differences (p < 0.05) in DO (ST 1 – 3.9*, ST 2 – 6.31* mg/L) and pH (ST 1 – 7.9*, ST 7 – 8.02*), while the surface water showed significant differences (p < 0.05) across all stations. A total of 45 species belonging to 6 classes and 4 phyla were identified in the bottom sediments in the order Mollusca (90.98%) > Annelida (6.67%) > Sipuncula (1.57%) > Porifera (0.78%). The macrofauna community was mostly dominated by Gastropoda (ecological groupings of I and II) and Polychaete (ecological group of III). The biotic indices were ecologically grouped into I (54.3%) and II (42.5%) species that are sensitive and indifferent to pollution and group III (3.2%) tolerant species to excessive organic pollution. The biotic indices were assessed based on the level of sensitivity of the benthic community in response to anthropogenic stressors. Thus, the ecological status using AMBI, M-AMBI, and BI indicated an undisturbed marine environment (healthy EcoQs) with less anthropogenic impacts. Although periodic biomonitoring of the coastal waters is recommended, this is due to the installation of crude oil platforms for exploitation, and fishery activities deploying bottom trawling. Furthermore, complementary use of diverse indices relating to sensitivity/tolerance-based indices is highly recommended to establish a robust database on ecological grouping for diverse benthic species within the western Atlantic Ocean. Graphical Abstract: (Figure presented.)