East-West Variabilities of N2 Fixation Activity in the Subtropical North Pacific Ocean in Summer: Potential Field Evidence of the Phosphorus and Iron Co-Limitation in the Western Area

In the subtropical North Pacific, the east-west gradient of iron deposition as atmospheric Asian dust strongly affects the zonal distribution of biological N-2 fixation activity in numerical models, but the in-situ relationship at a basin-scale is not well examined. We examined the trans-Pacific zonal variation in N-2 fixation activity on 23 degrees N in summer along with environmental parameters that potentially influence diazotrophy. Dissolved inorganic iron (DFe) was consistently low (<0.4 nM) throughout the transect. The atmospheric dust iron (dust-Fe) flux increased westward, whereas phosphate and labile phosphoric monoesters in the surface decreased westward. N2 fixation varied between 34.6 and 298 mu mol-N m(-2) d(-1) and was high (>200 mu mol-N m(-2) d(-1)) in the central area (150-180 degrees W). N-2 fixation rates significantly increased with dust-Fe input only in the western area (137-180 degrees E), whereas the contribution of DFe diffused from below the euphotic zone was often larger in the eastern area (120-170 degrees W). N-2 fixation was considerably low in the phosphate-depleted western area despite the excess amount of iron relative to phosphate, and it increased with the labile phosphoric monoesters stock. These indicated that N-2 fixation was primarily limited by phosphorus in the western area, though this activity also increased with iron supply from dust, likely due to phosphorusiron co-limitation. In contrast, in the phosphorus-repleted eastern area, iron supplied from dust and below the euphotic zone appeared to limit N-2 fixation. Overall, N-2 fixation in the subtropical North Pacific was likely limited by zonally different factors relating to iron and phosphorus availability. N-2 fixation, a process that converts N-2 gas into ammonia, substantially affects biological production in subtropical ecosystems, where nitrogenous nutrients are scarce. The availability of both iron and phosphorus are primary factors controlling the growth of N-2-fixing organisms. In North Pacific subtropical waters, iron input as atmospheric Asian dust is considered to control the east-west variation in N-2 fixation; however, their relationship is not well-examined. Here, we investigated the trans-Pacific distribution of N-2 fixation activity with iron and phosphorus availability. Our results show that dust input correlates with N-2 fixation in the western area (137-180 degrees E), which is in contrast with the eastern area (120-170 degrees W), where vertically diffused iron was also important. In the phosphate-depleted western area, N-2 fixation positively correlated with the proxies of phosphorus supply, suggesting phosphorus limitation. This indicates phosphorus availability substantially controls the zonal distribution of N-2 fixation in the subtropical North Pacific through co-limitation with iron. The limitations are relaxed in the central area (150-180 degrees W), likely due to elevated nutrient supply caused by bottom topography, resulting in a peak in N-2 fixation. Therefore, the dynamics of both iron and phosphorus must be considered simultaneously to predict the response of N-2 fixation to environmental changes.

Automated summary

In the subtropical North Pacific, the availability of both iron and phosphorus control the distribution of N-2 fixation. Dust input correlates with N-2 fixation in the western area, while vertically diffused iron is also important in the eastern area. In the phosphate-depleted western area, N-2 fixation is limited by phosphorus availability. Both iron and phosphorus dynamics need to be considered to predict the response of N-2 fixation.