Spatial variability of nitrous oxide in the Minho and Lima estuaries (Portugal)

Célia Gonçalves,Maria José Brogueira

doi:10.3989/scimar.04637.26a

Abstract

Nitrous oxide (N2O) is a potent long-lived greenhouse gas and estuaries represent potentially important sources of this biogas to the atmosphere. In this work, we analyse the first N2O data obtained in the Minho and Lima estuaries, and the processes and environmental factors that may regulate its production in these systems. In September 2006, N2O attained values of up to 20.0 nmol L–1 in the upper reaches of the Lima estuary and the river was, apparently, the main source of biogas to the system. In Minho N2O reached a maximum of 14.4 nmol L–1 and nitrification appeared to contribute to the enhancement of N2O. In the upper estuary, the relatively high concentrations of nitrification substrate NH4+, the positive correlations found between N2O level above atmospheric equilibrium (ΔN2O) and apparent oxygen utilization and NO2–, and the negative correlations between ΔN2O and NH4+ and pH can be interpreted as in situ N2O production through pelagic nitrification. Principal component analysis gave evidence of considerable differences between upper estuaries, particularly in terms of higher N2O in Lima and NH4+ in Minho. Surface waters of both estuaries were always N2O-supersaturated (101-227%) and estimated N2O emissions from Minho and Lima were 0.28 Mg N2O-N yr–1 and 0.96 Mg N2O-N yr–1, respectively, which represent a reduced fraction of N2O global emission from European estuaries.

Highlights

In the last few decades, the study of N2O has acquired greater importance due to its contribution to global climate change
The maximum value was reached in the upper estuary, suggesting that the Lima River was the main source of N2O to the estuarine system
In the Minho estuary, concentrations varied from 8.6 to 14.4 nmol L–1 and by contrast with those observed in Lima were above the conservative mixing line, pointing to the existence of N2O sources within the estuary

Summary

Introduction

In the last few decades, the study of N2O has acquired greater importance due to its contribution to global climate change. N2O is an important long-lived greenhouse gas in terms of radiative forcing (0.17±0.03 W m–2) (Myhre et al 2013) and represents the major anthropogenic contributor to stratospheric ozone destruc-. It has a long atmospheric lifetime of 131±10 years (Prather et al 2012) and its global warming potential is 310 times greater than that of carbon dioxide, in a time horizon of 100 years. In 2011 atmospheric N2O levels (324.2±0.1 ppb) exceeded the pre-industrial levels (270±7 ppb) by about 20% (Myhre et al 2013), largely due to increased agricultural activity and industry

Methods

Results

Discussion

Conclusion