Interannual variation in summer N2O concentration in the hypoxic region of the northern Gulf of Mexico, 1985-2007

Abstract

Microbial nitrous oxide (N2O) production in the ocean is enhanced under low-oxygen (O-2) conditions. This is especially important in the context of increasing hypoxia (i.e., oceanic zones with extremely reduced O-2 concentrations). Here, we present a study on the interannual variation in summertime nitrous oxide (N2O) concentrations in the bottom waters of the northern Gulf of Mexico (nGOM), which is well-known as the site of the second largest seasonally occurring hypoxic zone worldwide. To this end we developed a simple model that computes bottom-water N2O concentrations with a tri-linear Delta N2O/O-2 relationship based on water-column O-2 concentrations, derived from summer (July) Texas-Louisiana shelf-wide hydrographic data between 1985 and 2007. Delta N2O (i.e., excess N2O) was computed including nitrification and denitrification as the major microbial production and consumption pathways of N2O. The mean modeled bottom-water N2O concentration for July in the nGOM was 14.5 +/- 2.3 nmol L-1 (min: 11.0 +/- 4.5 nmol L-1 in 2000 and max: 20.6 +/- 11.3 nmol L-1 in 2002). The mean bottom-water N2O concentrations were significantly correlated with the areal extent of hypoxia in the nGOM. Our modeling analysis indicates that the nGOM is a persistent summer source of N2O, and nitrification is dominating N2O production in this region. Based on the ongoing increase in the areal extent of hypoxia in the nGOM, we conclude that N2O production (and its subsequent emissions) from this environmentally stressed region will probably continue to increase into the future.