Phosphorus speciation and bioavailability in the hyperdynamic gulf of Khambhat: Impacts of terrestrial inputs and...

This study assessed the effect of natural and anthropogenic phosphorus (P) loading on its speciation and bioavailability in seasonally collected suspended particulate matter (SPM) and sediments from the hyperdynamic Gulf of Khambhat (GoKh). Fractions associated with species are exchangeable P (Ex-P), iron-bound P (Fe-P), authigenic carbonate fluorapatite (CFA-P), detrital P (DeP), organic P (Or-P), and refractory P (Re-P). These were analyzed using the SEDEX (sequential extraction) method. Results indicated an increase in total particulate phosphorus (TPP) after the monsoon in GoKh, mainly due to terrestrial inputs and sediment resuspension. The dominant P fractions: De-P (11-35 %; av. 25 %), CFA-P (11-25 %; av. 20 %), and Re-P (10-29 %; av. 17 %), reflect mineral-rich detrital sources and anthropogenic influence under oxic conditions. Seasonal variations in sediment total phosphorus (TP) indicate a stable P pool, where De-P is primarily derived through lithogenic inputs and CFA-P acts as a reactive reservoir linked to in-situ transformation and skeletal debris deposition. Lower concentrations of Re-P and Or-P in sediments, compared to SPM, are attributed to oxic diagenetic conditions, poor organic matter (OM) preservation, and possible industrial discharges. Intertidal sediment cores confirm long-term P sequestration dominated by De-P and CFA-P, shaped by hydrodynamics, sediment texture, and landscape changes. The excessive P loading implied the eutrophication risk in the Gokh region caused by human-induced nutrient enrichment inputs, including the rapid expansion of blue economy activities. These anthropogenic pressures affected the bioavailability of Re-P fractions, inhibiting support to the primary productivity and raising the potential for ecological imbalance.