LAUSR

Abstract Fraction composition of sediment bound phosphorous (P) in Vembanad wetland was investigated to explore the distribution, dynamics and bioavailability of P. Surface sediments collected from 30 sites in Vembanad wetland ecosystem during February 2019 were sequentially leached to determine five fractions of P and their average percentage contribution to total P were: Calcium bound phosphorous [CaCO 3 ≈ P] > Acid soluble organic phosphorous [ASOP] >Alkali soluble organic phosphorous [Alk-OP] > Iron bound phosphorous [(Fe(OOH) ≈ P] >Residual organic phosphorous [Res-OP]. All the P fractions exhibited abundance in fine grained sediments, which provide larger surface area for phosphate adsorption/ retention. Combination of phosphate with calcium under high salinity serves as the prominent mechanism for the retention of CaCO 3 ≈ P in the sediments. Spatial variability in Fe(OOH) ≈ P was regulated by pH, E h and Fe-phosphate complex formation. Inputs of organic matter (OM) enriched with P via terrestrial run-off associated with 2018 flood seemed to be the reason for the peaks levels in both ASOP and Alk-OP. Terrestrial inputs of OM to the wetland was confirmed by the enhanced levels of tannin and lignin. Res-OP was the fraction with minimum content due to diagenesis. Significant alterations in the abundance of various P fractions in sediments and bioavailability of P were due to the effect of terrestrial run-off associated with 2018 flood. Compared to previous investigations, the estimated higher content of bioavailable P (BAP) (52.88 to 85.08%) can trigger the release of P to the overlying water column and may alter the primary productivity and trophic status of the estuarine system.