Search

The speciation of heavy metals (Ni, Cr, Cu, Zn, Pb, and Cd) was studied in surface sediments of the Cross River Estuary (CRE), Gulf of Guinea, South East Nigeria. Pb (~56 %), Cd (~71 %), Zn (~67 %), and Cr (~76 %) were mainly available in non-residual phases, suggesting potential bioavailability. High contents of Ni and Cu in residual phase indicated immobilization of these metals in aluminosilicate minerals. Cd was the most polluted heavy metal with the highest bioavailability risk. Bayesian Network model results revealed that sedimentary organic carbon (OC) from terrestrial C 3 plants controlled the contents and variability of Pb and Zn, while the input of terrestrial soil OC strongly influenced Cu and Ni. However, Cd and Cr were dominantly influenced by sediment pH, while Ni was mainly influenced by sediment salinity. Strong interdependency between Cd and total nitrogen (TN) suggested that nitrogen might increase Cd bioavailability upon release from sediments. • Significant variability in heavy metals speciation occurred in surface sediments. • Cd was the most contaminated heavy metal with considerable bioavailability risk. • OC from C 3 plants and soil significantly influenced heavy metals in sediments. • Nitrogen increased Cd and Ni adsorption to surface sediments. • Estuarine sediment properties enhanced heavy metals' mobility and bioavailability. [ABSTRACT FROM AUTHOR]

The Cross River, Calabar River, and Great Kwa River together form a network of an estuary system otherwise known as the Cross River Estuary system (CRES). Studies of dissolved nutrients and water mass dynamics were conducted mainly in February 2017. Nutrients measured included NO 3 −, NO 2 −, NH 4 +, dissolved inorganic phosphate (DIP), total dissolved nitrogen (TDN), total dissolved phosphorus (TDP), dissolved organic nitrogen (DON), dissolved organic phosphorus (DOP) and dissolved silicate (DSi). The rivers were enriched with DSi and depleted of DIP. Dissolved nutrients were more elevated in the Great Kwa River than Cross River and Calabar River. The levels of DSi and DIN were less than average global concentrations, and that of river estuaries affected by anthropogenic activities. DON was the major form of TDN in the Calabar River estuary, and averagely accounted for 68–76% of TDN pool, while DOP accounted for ∼ 54–75% of TDP pool. Similarly, DON was the dominant form of TDN, and represented 75–84% of TDN pool in the Cross River estuary. In contrast, TDN was dominated by DIN, which accounted for 76–84% of TDN pool, while DIP accounted for 56–61% of TDP pool in the Great Kwa River estuary. There was a switch from DOP (67% of TDP) dominated river water to DIP (67% of TDP) dominated estuarine water in the Cross River estuary, due to enrichment of DIP in the lower Cross River estuary. The adjacent shelf was dominated by DON (58% of TDN) and DIP (70% of TDP). Based on nutrient molar ratios, primary production was strongly limited by DIP, especially in the riverine regions, and switched to DIN limitation in the adjacent shelf. Nutrients in the CRES behaved non-conservatively, and were affected by biogeochemical and physical processes. Nutrient budgets were estimated using the classical LOICZ steady−state box model. Atmospheric deposition of dissolved nutrients was very limited compared to riverine supply. The exchange time of water between the CRES and adjacent sea was ∼ 3.5 days, and may have affected biotic processes and nutrient transport. The CRES was an important source of DIP (137 × 103 moL day−1), DOP (92 × 103 moL day−1) and DON (4849 × 103 moL day−1) to the adjacent shelf, which was respectively 7, 4 and 2 times higher than the total riverine input. The ecosystem metabolism of the CRES, inferred from the ΔDIP system and ΔDIN system indicated that respiration of organic matter was higher than primary production with respiration rate of −15.7 × 106 mol C day−1 (−58 mmol C m−2 day−1), while nitrogen metabolism was −3.8 × 106 mol N day−1 (−14 mmol N m−2 d−1), suggesting that the CRES was heterotrophic, and denitrifying more than nitrogen fixation during this study. • Dissolved nutrient compositions and water mass dynamics were studied in the CRES and adjacent shelf during the dry season. • DSi and DIN were the most abundant nutrients in the rivers. • There was a shift from DIP limitation in the rivers to DIN limitation in the adjacent shelf. • The CRES is an important source of DIP, DOP and DON to the adjacent shelf. • The ecosystem metabolism indicated that the CRES is heterotrophic, and denitrifying more than nitrogen fixation. [ABSTRACT FROM AUTHOR]