Geochemical partitioning of sediment-bound phosphorous in Thrissur Kole Wetlands, Southwest India: Status of bioavailability and trophic state.

Sediment-bound phosphorous fractions were analyzed to assess the bioavailability and trophic status of Thrissur Kole wetlands. Sequential extraction followed by spectrophotometry was employed for fractionation and determination of phosphate content in each fraction. Acid soluble organic phosphorous comprising of biological molecules was the most dominant fraction [278.02 ± 11.82 mg/kg (non-monsoon); 315.77 ± 13.84 mg/kg (monsoon)] recorded during both seasons and can be sourced to allochthonous as well as autochthonous sources. Elevated content of alkali soluble organic phosphorous [162.53 ± 14.85 mg/kg (non-monsoon); 307.22 ± 14.60 mg/kg (monsoon)] reflected leaching of humic-associated phosphorous from aquatic vegetation, domestic and municipal sewage. Enrichment of calcium-bound phosphorous was due to the abundance of calcium carbonate originating through biological precipitation and inputs from guano as well as carcass of birds. Enriched organic matter adsorbed on fine-grained sediments controls the retention of organic phosphorous through organic matter-iron-phosphorous association. Intense microbial reworking on organic matter (substantiated by depleted E^h values) resulted in a sharp decline in organic phosphorous fraction. Adsorption of iron/manganese oxyhydroxides was a prominent mechanism in the retention of phosphate in sediments, endorsed by multivariate statistical analysis. Alkaline pH and reducing environment promote the release of phosphate to the water column and hence bioavailability for phytoplankton uptake. Bioavailable phosphorous constitutes a major portion of total phosphorous [average: 606.86 ± 26.24 mg/kg (non-monsoon); 651.36 ± 23.05 mg/kg (monsoon)], which implied that the wetland sediments can function as a dynamic source of phosphate to the water column, signaling eutrophication risk. [ABSTRACT FROM AUTHOR]