Interannual Variability of Methane and Nitrous Oxide in the North Pacific Subtropical Gyre

The temporal variability of two important greenhouse gases, methane (CH 4 ) and nitrous oxide (N 2 O), is reported for the upper water column at Station ALOHA in the North Pacific Subtropical Gyre. Measured concentrations of N 2 O conform to predicted values with an increase in saturation during the summer period. In contrast, CH 4 is less predictable and shows an approximate 2 year transition from a state of oversaturation in surface waters to equilibrium values in 2015, implying a change in net CH 4 production. The decrease in CH 4 followed on from fluctuations in phosphate concentrations supporting the hypothesized link between microbial metabolism of phosphorus and the global biogeochemical cycle of CH 4 . At this current time, future trends in the net CH 4 production in the North Pacific Subtropical Gyre are uncertain and specifically whether the surface ocean will be a net source or sink for CH 4 .