The C:N ratio of POM occasionally exhibited outliers, due to which, the mean and standard deviations (SD) of the C:N ratio were calculated and the samples whose C:N ratio differed by > 3 × SD from the mean value were eliminated. After elimination, we re-calculated the mean and SD of the C:N ratio, and the samples whose C:N ratio differed by > 3 × recalculated SD from the recalculated mean value were eliminated. Then, the samples lacking environmental data were removed. As a result, six samples were removed, and 507 samples were used in this study. Among these samples, 101 samples were obtained along the SI-line.