Latitudinal distribution of nitrogen isotopic composition in suspended particulate organic matter in tropical/subtropical seas

Natural nitrogen isotopic composition (δ15N) of suspended particulate organic matter (POM) and nitrogen fixation rates via 15N2 assay were measured in surface waters along 120° E from 30° N to 30° S in the Asian marginal seas (the East/South China Seas and the Sulu/Celebes/Java Seas) and the northeastern Indian Ocean in November–December 2005 and March 2006. The POM δ15N values ranged from−1.8 to 12.2‰ with an average of 3.6‰ and showed a decreasing trend towards the equator in both hemispheres. In parallel, the measured N2 fixation rates showed an increase from the subtropical to the tropical seas. This implies that a higher contribution of 15N-depleted POM was derived from enhanced N2 fixation. Water temperature and the stability of water column were partly responsible for the observed variations in nitrogen fixation. The large-scale spatial variations in suspended POM δ15N and N2 fixation rates suggest that the suspended POM δ15N may be a potential indicator of nitrogen fixation in surface waters in tr...