Carbon and Nitrogen Isoscapes of Particulate Organic Matter in the Pacific Ocean

Abstract Large‐scale geographical distributions in nitrogen and carbon stable isotope ratios (δ 15 N and δ 13 C) of particulate organic matter (POM) are essential to understand the variation in the baseline of pelagic food webs in the Pacific Ocean, where phytoplankton production and biological N 2 fixation are highly variable because of heterogeneity of nitrate and iron supply. Here, we determined their isoscapes during summer and discussed potential factors characterizing regional ecosystems from the viewpoint of nitrogen cycling. We collected a total of 2,289 and 2,278 isotope values for δ 13 C and δ 15 N, respectively, by synthesizing previously published data with our newly measured data, and analyzed their relationships with temperature, concentrations of nitrate and chlorophyll‐ a , and N 2 fixation activity, obtained from databases. POM δ 13 C and δ 15 N regionally varied in ranges of −30 to −18‰ and −4 to 14‰, respectively. POM δ 13 C was correlated positively with temperature throughout the ocean. In contrast, POM δ 15 N was negatively correlated with nitrate concentration at high latitudes and with N 2 fixation activity at low latitudes. High values (>8‰) of POM δ 15 N were identified mainly in the marginal area of equatorial upwelling; the highest values (10–14‰) were in the subtropical Southeastern Pacific. Using the isotopic values and nitrate concentration, we classified the ecosystems into 10 groups. Our data demonstrated the distribution patterns of ecosystems with different degrees of nitrate utilization, which are presumably associated with iron supply, and ecosystems sustained by different nitrogen sources: diazotrophic nitrogen and nitrate supplied below the nitracline and/or horizontally advected.