Influence of hypoxia on phosphorus cycling in Alappuzha mud banks, southwest coast of India

Abstract

The southwest coast of India is known to experience two concurrent processes during the southwest monsoon, namely coastal upwelling and mud banks at selected stretches such as that off Alappuzha. The mud bank sediments were found to contain very high concentrations of phosphorus (ave. 2255 ± 278 μg/g), and the present study measured changes in concentration of elements such as carbon, phosphorus, iron, and manganese within the Alappuzha mud banks, to understand the influence of upwelling on the cycling of these elements. The results showed that coastal upwelling led to hypoxic conditions within the mud bank region during the peak southwest monsoon, which induced significant changes in the phosphorus fractions in the sediments. There was an enrichment of iron, caused by the efflux of reactive Fe from the subsurface sediment layers and depletion of manganese, due to reductive dissolution of Mn oxides and subsequent release of Mn. The low molar ratios of total organic carbon to total nitrogen (9 ± 1) and total organic carbon to organic phosphorus (118 ± 57) generally indicated that the organic matter was relatively fresh and derived from marine plankton. Sedimentary phosphorus also exhibited wide fluctuations in its geochemical forms during hypoxia. Five geochemical forms of P such as exchangeable P (Ex-P), Fe and Mn bound P (Fe-P), P from Al oxides, non-reducible Fe bound P and organic P (Non Red-P), calcium bound P (Ca-P) and refractory organic P (ROP) were extracted, adopting sequential extraction procedure. The percentage contribution of various P forms were 2%–11% Ex-P, 4%–25% Fe-P, 4%–10% Non Red-P, 60%–78% Ca-P and 4%–12% ROP. The fractionation study revealed that precipitation of apatite and vivianite are the significant pathways of phosphorus enrichment in mud bank sediments. The study indicates that the enrichment of labile P fractions could be mediating the enhancement in biological production in the Alappuzha mud banks during the southwest monsoon.