Biogeochemical processes as drivers of surface fCO₂ in contrasting provinces in the subarctic North Pacific Ocean

The effect of temperature, biological processes, air‐sea CO 2 exchange and vertical mixing as drivers of the seasonality of the surface water fugacity of CO 2 ( f CO 2 sw) were studied for the year 2000 in the subarctic North Pacific Ocean. The regional and seasonal variability of the surface water chemistry was studied using an extensive data set on surface water f CO 2 and nutrient concentrations in six contrasting provinces. We observed the largest seasonal amplitude for all parameters in the western provinces (Oyashio and Western Subarctic Gyre, WSG). Our study showed that biological processes and temperature were major controls for the monthly f CO 2 sw change in all provinces. The magnitude and strength of the processes showed large temporal and spatial variability. The WSG showed larger influence by biological processes and vertical mixing than the Alaska gyre (AG), where the effect caused by temperature was larger, implying different forcing of the f CO 2 change in the two gyres. Biological activity, estimated from the monthly nitrate change corrected for addition induced by vertical mixing, resulted in a net annual CO 2 loss. The net carbon loss out of the top 50 m driven by biological activity was 3 times higher in the WSG (64 g C m −2 yr −1 ) than in the AG (23 g C m −2 yr −1 ). The annual sum of the f CO 2 sw change based on all processes resulted in a CO 2 buildup in the surface waters for all provinces. Although the air‐sea CO 2 exchange was of minor importance relative to the other considered processes (4 to 13%), all provinces showed a net annual uptake of atmospheric CO 2 from 1 to 23 g C m −2 yr −1 and an average for the whole study area of about 12 (±9) g C m −2 yr −1 .