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

Abstract

Large-scale geographical distributions in nitrogen and carbon stable isotope ratios (δ¹⁵N and δ¹³C) 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 N₂ 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 δ¹³C and δ¹⁵N, respectively, by synthesizing previously published data with our newly measured data, and analyzed their relationships with temperature, concentrations of nitrate and chlorophyll-a, and N₂ fixation activity, obtained from databases. POM δ¹³C and δ¹⁵N regionally varied in ranges of −30 to −18‰ and −4 to 14‰, respectively. POM δ¹³C was correlated positively with temperature throughout the ocean. In contrast, POM δ¹⁵N was negatively correlated with nitrate concentration at high latitudes and with N₂ fixation activity at low latitudes. High values (>8‰) of POM δ¹⁵N 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.