δ13CPOM or δ15NPOM = (Rsample /Rreference – 1.0) × 1000,                                                                                                 (1) where Rsample, and Rreference are the heavy (13C and 15N) to light (12C and 14N) isotope ratios of the POM sample and reference, respectively. The reference materials were atmospheric N2 for nitrogen and Vienna Pee Dee Belemnite for carbon. Given the quality of the L-alanine, and δ13C and δ15N were rounded off to one decimal place, the precision of the analyses was within 0.2‰.