The Chilika Lagoon is the largest brackish water ecosystem in the Asian continent and is a Ramsar site. Here, the sorption–desorption study was standardized simulating the environmental condition of the Chilika, to ascertain whether phosphorus (P) is the source or sink for sediments of the coastal ecosystem. The sorption behaviour of sediment phosphorus was better fitting into the modified Langmuir model. The effects of salinity, pH, and temperature on phosphorus sorption were found to be better understood by nonlinear relationships. The sorption was recorded to increase at its lower concentrations with increasing of mentioned parameters, when it decreased beyond some threshold values. The maximum phosphorus sorption capacity (Q$_{max}$) was more (258 mg/kg) in the freshwater stretch of the lagoon; Northern Sector compared to the relatively saline stretches, i.e., Outer Channel (219 mg/kg) and Southern Sector (237 mg/kg) (p < 0.05). This study will help to control eutrophication by increasing P sorption.

$\bf{Highlights}$

$\bullet$ Phosphorus sorption and desorption study mark an unique for the Asian largest lagoon Chilika

$\bullet$ The behaviour of sediment phosphorus sorption was better expressed by the modified Langmuir model than by the alternative modified Freundlich model

$\bullet$ The maximum phosphorus sorption capacity (Q$_{max}$) was more in the fresh water stretch of the Chilika lagoon compared to the relatively saline stretches

$\bullet$ For management of eutrophication condition of the lagoon by increasing P sorption through restoration of the freshwater environment.