Comment on bg-2022-155

Abstract

The availability of iron (Fe) and phosphorus (P) have been shown to be key factors regulating rates of nitrogen fixation in the western Subtropical Pacific. However, their relative importance at finer spatial scales between the northern South China Sea (NSCS) and the western boundary of the North Pacific is poorly constrained. Furthermore, nutrient limitation of specific diazotroph types has not yet been assessed. Here we investigated these unknowns by carrying out measurements of (i) finer scale spatial variabilities in N2 fixation rates and diazotroph abundances throughout these regions, and (ii) conducting eight additional Fe and phosphate addition bioassay experiments where both changes in N2 fixation rates and the abundances of specific diazotrophs were measured. Overall, nitrogen fixation rates were lower in the NSCS than around the Luzon Strait and the western North Pacific, which we hypothesize was due to lower Fe-to-fixed nitrogen supply ratios that decrease their competitive ability with non-diazotrophic phytoplankton. The nutrient addition bioassay experiments demonstrated that nitrogen fixation rates in the central northern South China Sea (NSCS) were co-limited by Fe and P, whereas in the western boundary of the North Pacific they were P-limited. Changes in the abundances of nifH in response to nutrient addition varied in how well they correlated with changes in nitrogen fixation rates, and the largest responses were always dominated by either Trichodesmium or UCYN-B. In general, nutrient addition had a relatively restricted impact on diazotroph community structure apart from on UCYN-B, which showed increased contribution to the diazotroph community following P addition at sites where N2 fixation rates were P-limited. We further hypothesize the importance of absolute Fe supply rates in regulating spatial variability in diazotroph community structure across the study area.