Nitrous Oxide Emissions from the Gulf of Mexico Hypoxic Zone

Abstract

The production of nitrous oxide (N(2)O), a potent greenhouse gas, in hypoxic coastal zones remains poorly characterized due to a lack of data, though large nitrogen inputs and deoxygenation typical of these systems create the potential for large N(2)O emissions. We report the first N(2)O emission measurements from the Gulf of Mexico Hypoxic Zone (GOMHZ), including an estimate of the emission "pulse" associated with the passage of Tropical Storm Edouard in August, 2008. Prestorm emission rates (25-287 nmol m(-2) hr(-1)) and dissolved N(2)O concentrations (5 - 30 nmol L(-1)) were higher than values reported for the Caribbean and western Tropical Atlantic, and on the lower end of the range of observations from deeper coastal hypoxic zones. During the storm, N(2)O rich subsurface water was mixed upward, increasing average surface concentrations and emission rates by 23% and 61%, respectively. Approximately 20% of the N(2)O within the water column vented to the atmosphere during the storm, equivalent to 13% of the total "hypoxia season" emission. Relationships between N(2)O, NO(3)(-), and apparent oxygen utilization (AOU) suggest enhanced post storm N(2)O production, most likely in response to reoxygenation of the water column and redistribution of organic nitrogen. Our results indicate that mixing related emissions contribute significantly to total seasonal emissions and must therefore be included in emission models and inventories for the GOMHZ and other shallow coastal hypoxic zones.