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 (N 2 ) fixation is an important contributor to nitrogen requirement of phytoplankton not only in oligotrophic but also in nutrient-replete waters. However, the importance of N 2 fixation in meso-polyhaline estuaries is not well known. Here we report significant N 2 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 PO 4 − 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.