Shifts in species dominance related to spatial assemblages and variation in environmental parameters in a tropical mangrove estuary.

A shift in species dominance in the mangrove microalgae community in Cross River Estuary System (CRES), Nigeria, was carried out during the wet season to highlight the microalgae structure, identify potential indicators, and evaluate the water quality variation. Plankton samples were collected at sixteen selected sampling sites along the river. Diatoms showed the highest number of species (240 species), contributing more than 70% of the total microalgae abundance. Cluster analysis of the microalgae community delimits the CRES into three sections; upper (UCRS), middle (MCRS), and lower (LCRS) communities. The spatial shift in the microalgae community showed that the green algae (Eudorina elegans and Hylotheca dissiliens) dominated the UCRS communities, while diatoms dominated the MCRS (Polymyxus coronalis and Actinocyclus normanii) and LCRS (A. normanii and A. octonarius). The principal component analysis showed that the UCRS was influenced by turbidity and nitrate, while the MCRS and LCRS were characterized by phosphate, electrical conductivity, and salinity/total dissolved solids. The similarity percentage (SIMPER) analysis showed that Surirella tenera and Hylotheca dissiliens contributed >3.5% inter site dissimilarity between the UCRS and LCRS microalgae community. The redundancy analysis revealed that some microalgae species such as Odontella mobiliensis, Coscinodiscopsis jonesiana, A. normanii, and A. octonarius responded positively to salinity change and ammonia concentration in the estuary, while S. tenera, P. coronalis, Bacillaria paxilifer, Navicula transitans, Cyclotella meneghiniana, Humidophila contenta, Melosira granulata, Aulacoseira herzogii, Eudorina elegans, H. dissiliens, Mougeotia sp., and Dinobryon sertularia responded positively to turbidity, nitrate, and phosphate concentration. This study illustrates that CRES inhabit rich biodiversity of microalgae which some taxa could be used as indicators of the environmental changes in marine waters.

Geochemical discrimination of bulk organic matter in surface sediments of the Cross River estuary system and adjacent shelf, South East Nigeria (West Africa).

Knowledge of the sources, distribution and fate of organic matter (OM) in estuarine and adjacent shelf sediments are important for the understanding of the global biogeochemical cycles. Bulk organic carbon (C-org), total nitrogen (TN), biogenic silica (BSi), stable carbon (δ13C-org) and nitrogen (δ15N) isotopes, and sediment grain sizes were measured to study the spatial distributions and sources of sediment OM in the Cross River estuary system (CRES) and adjacent shelf. Surface sediments in the CRES were composed of clayey silt and sandy silt, while the adjacent shelf sediments were mainly silty sand. The range of the studied parameters was -28.79‰ to -22.20‰ for δ13C-org, -1.32‰-6.31‰ for δ15N, 6.7-29.2 for C-org/N ratios, 0.08%-0.33% for TN, 0.24‰-0.74‰ for BSi, and 0.47%-5.28% for C-org, and their spatial distributions showed a general decreasing trend in both the terrestrial and estuarine OM from the riverine regions to the adjacent shelf. Based on the three-end-member mixing model using the δ13C and δ15N isotopic values, ~58.01 ±â€¯15.32% of sediment OM are derived from terrestrial sources dominated by C3 vascular plants, while ~26.34 ±â€¯9.71% are attributed to estuarine sources dominated by aquatic macrophytes, and ~15.65 ±â€¯12.37% for marine plankton source. Other sources of OM identified included soils underlain C3 vascular plants and agricultural farms enriched with N, sewage, and petroleum hydrocarbons. The relationship between C-org vs. BSi, and the atomic BSi/Corg ratios suggested that diatoms also play an important role in OM sequestration in surface sediments of the CRES and adjacent shelf. The correlations of the δ13C-org and δ15N isotopic values vs. C-org/N ratios resulted in scatter plots, indicating that the distributions of sediment OM in the CRES and adjacent shelf are influenced by post depositional processes, fixed inorganic N adsorbed on fine-grained sediments, microbial degradation, as well as sediment grain size.