Phosphorus Dynamics in Water and Sediments and its Associated Hydrological Coupling with Environmental Characteristics of the Cochin Backwaters - Southwest Coast of India

Phosphorus is an essential macronutrient that plays a key role in global biogeochemical cycles. Sediment phosphorus is an active component of phosphorus cycling in an estuary whose availability and movement play important roles in eutrophication. Release of phosphorus from sediments accelerates eutrophication in an estuary but the key process responsible for this phenomenon remains unclear. Depending upon the physicochemical characteristics, sediment-bound phosphorus can function as a source or sink for phosphorus to the overlying water column of an estuary. Understanding phosphorus sorption phenomena in sediments is important in regulating the availability of phosphorus in an estuary. A spatiotemporal study on dissolved inorganic phosphorus and phosphorus fractionation in sediments within the estuarine system of Cochin that is under the influence of freshwater runoff from rivers and saline water intrusion from sea reveals a hydrological coupling process with environmental parameters (e.g., tidal currents, pH and salinity) will facilitates our understanding about the sorption phenomena that regulating phosphorus cycling and ecological functioning of this ecosystem. The various fractions of phosphorus detected in sediments were iron-bound inorganic and organic phosphorus, calcium-bound inorganic and organic phosphorus, acid-soluble organic phosphorus, and alkali-soluble organic phosphorus. The main phosphorus pool identified in sediments of the Cochin estuary is calcium and iron-bound phosphorus compound which exerts a major control on dissolved inorganic phosphate concentration present in the water column. The heavy river runoff during the monsoon considerably lowers the water column's pH and salinity, enhancing phosphorus's adsorption from water to sediments. In contrast, the seawater incursion during the pre-monsoon increases the pH and salinity of the water column which enhances the desorption of phosphorus from sediments to the water column. Thus, during the monsoon period, sediments act as a sink of phosphorus that is derived from land-based sources, while during the pre-monsoon period, they function as a source of phosphorus to the overlying water column, both of which can alter the eutrophication events in the estuary. An increase in phosphorus availability in the water column of this eutrophic system can influence ecosystem productivity by intensifying nitrogen limitation which can encourage the growth of algae and other heterotrophic organisms with high phosphorus requirements. Precipitation/flocculation of dissolved inorganic phosphate within the estuarine salinity gradients (~ 15 – 30 psu), followed by its adsorption/desorption with iron oxy-hydroxides and calcium carbonate minerals that settle towards bottom sediments governs the biogeochemical cycling of phosphorus in the Cochin backwaters.