Seasonal production of nitrous oxide in a tropical estuary, off western India

Abstract

Estuaries are highly productive ecosystems, and estuarine emission of N2O contributes significantly to the global nitrogen (N) budget. Presented here are the spatio-temporal distribution and water–air flux of N2O from a monsoonal, tropical estuary, the Mandovi located off Goa, during three consecutive years of 2011, 2012 and 2013. Despite substantial input from various sources, the estuary was moderately enriched with nitrogenous nutrients (≤30μM). However, the N transformations do not seem to contribute towards considerable emissions of N2O. Nonetheless, the seasonal upwelling along the west coast of India influences the water column chemistry at the estuarine mouth, as the upwelled water intrudes ∼13 km inside the estuary, leading to significant changes in terms of N2O cycling. A high build-up of N2O in the bottom waters was observed annually during peak southwest monsoon (SWM). The highest concentrations of N2O ∼164 nM (∼2459 %, August 2011), 106 nM (∼1566 %, September 2012), and 156 nM (∼2320 %, August 2013) were recorded at the estuarine mouth that was associated with a coastally-derived upwelled water mass. However, the average concentrations of N2O during pre- and post-monsoon were 10.88 ± 2.07 nM and 11.07 ± 2.85 nM, respectively, for the three years.The water–air flux of N2O was highest during SWM and was lowest during pre- and post-monsoons. The average flux by the estuary varied from ∼3.98 - 48.46μmol m−2d−1. The average annual emission from the Mandovi estuary was estimated to be ∼5.5 × 106 g N2O y−1, which seems to be a smaller fraction when compared to coastal emissions. Our study suggests that while the Mandovi estuary remains in saturation with respect to N2O, it acts as a modest source of N2O to the atmosphere, especially during SWM.