Nitrogen fixation and carbon uptake in a tropical estuarine system of Goa, western India

Abstract

Biological production in aquatic systems is often nitrogen limited. Nitrogen (N2) fixation is an important contributor to nitrogen requirement of phytoplankton not only in oligotrophic but also in nutrient-replete waters. However, the importance of N2 fixation in meso-polyhaline estuaries is not well known. Here we report significant N2 fixation rates, ranging between 0.1 and 34 μmol N m−3 d−1 from the Mandovi and Zuari estuaries, two typical tropical monsoon-affected estuaries located along the Indian west coast. The estuarine waters were moderately rich in nutrients with nitrate concentration up to 5 μM in the Mandovi and 6 μM in the Zuari and PO4− up to 1 μM. Chlorophyll a concentration varied between 0.1 and 8 mg m−3 with moderate primary production (up to 262 mg C m−3 d−1). The organisms responsible for the observed diazotrophic activity in the estuaries still remain to be identified.