Zeitschrift für Umweltanalytische Chemie

Biogeochemical Cycling of Phosphorus in the Cochin Backwaters: Southwest Coast of India

Abstract

Rejomon George, Martin GD, Gerson VJ and Nair M

Here we studied total phosphorus and its fractions in sediments from the Cochin backwaters along with dissolved inorganic phosphate, tidal currents, pH, and salinity during the monsoon and pre-monsoon periods. During the monsoon, sediments function as a sink for phosphate derived from land-based pollutant sources, whereas, during the pre-monsoon, sediment re-suspension provides a source of phosphate to the overlying water column, both with the capacity to alter eutrophication events. The heavy river runoff during the monsoon lowers the pH and salinity of the water column, thereby enhancing adsorption of phosphorus from water to sediments. In contrast, seawater incursion during the pre-monsoon increases the pH and salinity of the water column to enhance the desorption of phosphorus from sediments to the water. Iron-bound inorganic (0.48-0.78 mg g^-1) and organic phosphorus (1.04-1.30 mg g^-1), calcium-bound inorganic (0.78-1.56 mg g^-1) and organic phosphorus (1.13-1.83 mg g^-1), acidsoluble organic phosphorus (0.07-0.21 mg g^-1), and alkali-soluble organic phosphorus (0.22-0.70 mg g^-1), were detected in sediments. The main sedimentary phosphorus pool was a calcium- and iron-bound phosphorus compound. Precipitation/flocculation of dissolved inorganic phosphate within the estuarine salinity gradients (~15-30), followed by its adsorption/desorption with iron oxy-hydroxides and calcium carbonate minerals that settle in the bottom sediments govern the phosphorus biogeochemistry of the Cochin backwaters.