Geochemical fractionation, potential bioavailability and ecological risk of phosphorus in surface sediments of the Cross River estuary system and adjacent shelf, South East Nigeria (West Africa)

Phosphorus (P) as an essential macronutrient, plays a crucial role in the regulation of biological productivity and biogeochemical cycling of other biogenic elements in aquatic environments. Different geochemical fractions of sedimentary P were separated and quantified using the sequential extraction (SEDEX) method to evaluate the abundance, sources, distribution, bioavailability and ecological risk of P in surface sediments of the Cross River estuary system (CRES) and adjacent shelf. Sediment grain size composition and bulk organic carbon were also determined. Total P (TP) ranged from 9.21-18.2 mu mol/g (average, 13.30 +/- 2.08 mu mol/g), and comprised mostly of inorganic P (IP) (IP/TP = similar to 87%). Generally, authigenic P (Ca-P) was the dominant IP fraction (Ca-P/IP = similar to 37 +/- 11.1%), representing an important long-term P reservoir. The range of organic carbon (OC) to organic P (OP) ratio (OC/OP) indicated the predominance of terrestrial OC, and that leachable OP (Le-OP) may be the major OP form in surface sediments of the study area. The partitioning and distribution of different sedimentary P fractions were controlled by the biogeo- and physicochemical conditions favorable for P transformation, sediment grain size composition and sources of sedimentary organic matter. The ratio (Fe/P) of the extractable Fe in citrate-dithionite-bicarbonate solution (Fe-CDB) to ferric oxide-bound P(Fe-P) revealed the presence of reactive Fe- matrices having strong sorption capacity for P. The contents of potential bioavailable P (BAP) (similar to 34-60% of TP) indicated that P amount in the overlying water column may increase substantially if released from surface sediments. The Fe-P was the major component of BAP, and the relationship between BAP and extractable Fe-CDB highlighted the significant impact of Fe(III) (oxyhdr)oxides on P mobility and bioavailability. The estimated P enrichment index (PEI) indicated that surface sediment is less impacted by anthropogenic P and poses less ecological risk with respect to sedimentary TP load in the study area.