Biologically-associated Nitrous Oxide Accumulation in the Euphotic Zone

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.