Interannual variation in summer N 2O concentration in the hypoxic region of the northern Gulf of Mexico, 1985–2007
Microbial nitrous oxide (N 2O) 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 (N 2O) 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 N 2O concentrations with a tri-linear ΔN 2O/O 2 relationship based on water-column O 2 concentrations, derived from summer (July) Texas–Louisiana shelf-wide hydrographic data between 1985 and 2007. ΔN 2O (i.e., excess N 2O) was computed including nitrification and denitrification as the major microbial production and consumption pathways of N 2O. The mean modeled bottom-water N 2O 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 N 2O 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 N 2O, and nitrification is dominating N 2O production in this region. Based on the ongoing increase in the areal extent of hypoxia in the nGOM, we conclude that N 2O production (and its subsequent emissions) from this environmentally stressed region will probably continue to increase into the future.