Enrichment and Bioavailability of Phosphate in a Highly Urbanized Zone of Cochin Estuary, South India

Sequential extraction (SEDEX method) was employed to explore the distribution of sediment-bound phosphorus (P) fractions and the bioavailability of P in Cochin estuary. Concentration of P fractions in surface sediments of the study area ranged as follows: Detrital P > Organic P > Authigenic P > Exchangeable P > Iron-bound P. Generally increasing trend was displayed by all the phosphate fractions toward the downstream sites due to effluent discharges from industrial, domestic, municipal, agricultural sectors and destruction of mangrove forests. The elevated levels of inorganic phosphorus (133.55–918.73 mg/kg) reflected the large-scale delivery of sediment loads associated with great floods during 2018 & 2019. Significant correlations of fine-grained sediment particles with all phosphate fractions indicated surface adsorption as a dominant distribution mechanism. Concentration of iron-bound phosphorus was controlled by adsorption on iron oxy-hydroxides. The buildup of bioavailable phosphate (range: 23.76 to 49.97%) indicate the fact that the estuarine sediments function as a sink for the nutrient. The upper and middle part of the study area recorded more severe contamination and eutrophication risk as indicated by phosphorous pollution index (range: 0.31 to 2.56).