Biologically-associated Nitrous Oxide Accumulation in the Euphotic Zone

Wan Sean Xianhui, State Key Laboratory of Marine Environmental Science, Xiamen University, China, Xiamen, China
Abstract:
The largest uncertainty and inconsistency in global marine N2O emission estimate appear in the euphotic zone, where light and phytoplankton competition inhibit nitrification, a major contributor to N2O production. The rigorous physical disturbance in the surface ocean prevents the N2O accumulation thus potentially masking the role of biological N2O production. Here we presented high vertical resolution N2O profiles in the euphotic zone from the oligotrophic South China Sea and mid-latitude northwestern Pacific Ocean. Distinctive N2O peaks, which deviated from the vertical mixing curve, were observed near the nitracline, of which the maximum of Chl-a and nitrite appeared correspondingly. The intimate association between N2O excursion and biological parameters suggests that extra source in the euphotic zone is required to support such N2O offset. By using 15NH4+ and 15NO2- tracers, we measured rates and explored the potential processes associated with N2O production. More high vertical resolution observation and process studies are urgently needed to explore the spatial-temporal distribution of euphotic zone N2O production and its controlling mechanisms to fill the knowledge gap. In addition, innovative methods are required to discern pathways biologically produced N2O in mixed layer from physical supply from deeper ocean.