The study of foraminiferal distribution trends in the coastal environments of Southwest Nigeria sector of the Gulf of Guinea
This study established the distribution trends of foraminiferal assemblages in relation to the parameters influencing the trends in the coastal environments of some parts of southwest Nigeria sector of the Gulf of Guinea. This was with a view to characterizing the surface sediments in the various environments using benthic foraminiferal population. Two hundred surface-sediment samples were collected from Ikate beach, Tarkwa bay, Lagos harbour, Lagos lagoon, Badagry creek, Yewa river and Badagry beaches in southwest, Nigeria. Bottom water was collected for pH and salinity measurements; and water depths measured where necessary. Sediment samples were used for granulometric, chemical and foraminiferal analyses. Granulometric analysis involved the calculation of grain size percentages and generation of probability cumulative frequency curves from which various textural parameters were calculated. Aqua regia digestion of <75 µm sediment fractions was done for 0.5 g of relevant samples and Inductively Coupled Plasma-Optical Emission Spectrometer (ICP-OES) was used to measure concentration of trace metals. Also, 30 g was taken from each sample and washed over 63 µm sieve for foraminiferal analyses. The foraminifera were picked, stored in cellules, identified, counted, described and assigned to families, genera and species. The data generated were subjected to various multivariate analyses using the Paleontological Statistics (PAST) software package (including two similarity indices, the “Q” and “R” modes) and abundance variation of the foraminifera. The identified assemblages in decreasing order of abundance were Textularia sagittula, Ammonia beccarii, Eponides cribrorepandus, Quinqueloculina padana and Hanzawaia boueana (Ikate beach assemblage); Hanzawaia boueana, Textularia sagittula, Florilus atlanticus, and Meloni padana (Tarkwa bay and Lagos harbour assemblages); Ammonia beccarii, Florilus atlanticus, Hanzawaia boueana, and Textularia sagittula (Lagos lagoon assemblage); Textularia sagittula, Eponides cribrorepandus, Hanzawaia boueana, Florilus atlanticus, Quinqueloculina padana and Q. vulgaris (Badagry beach assemblage). Textularia sagittula, seemed to register most abundant occurrence throughout the study area but confined to only sand dominated substrates in the microhabitats of Lagos lagoon. The Total Foraminiferal Number (TFN) was highest at Lagos harbour and lowest at Badagry beach based on favourable sediment texture identified in Lagos harbour. The harbour sediment was mainly fine sand whereas the Badagry beach was mostly coarse sand. The trace metal concentrations were below background levels at the Tarkwa bay and Lagos harbour but constituted moderate pollution in Lagos lagoon, Badagry creek and Yewa river. Water depth and pH seemed to have significance on few stations in Lagos harbour and lagoon. Salinity also played prominent role but was masked by the nature of the substrates at the northwestern end of Lagos lagoon where mono-specific assemblage was encountered. The non-saline to extremely low salinity condition imposed as a result of the overwhelming fluvial activities might account for the non-recovery of foraminifera in sediments of Badagry creek and Yewa river. Also, low pH made these environments uninhabitable for foraminifera as tests of calcareous hyaline and porcellaneous were easily decalcified at pH <7.5. The study concluded that the processes that defined the substrates strongly correlated with the species abundance and variation, and hence determined the pattern of distribution of foraminifera.