To measure the mass and the carbon and nitrogen stable isotope ratios of the POM, 0.9–11.5 L of seawater was filtered using a pre-combusted (450°C, 6 h) glass fiber filter (pore size: 0.7 µm, GF/F, Whatman). The filtration was stopped in 2 h after sampling, and > 5 L seawater was filtered in 2 h for most of the samples. After the shipboard filtration, the samples were frozen (< –20°C). Onshore laboratory analyses were performed using methods from Kodama et al. (2021), whose preservation differed from that of Lorrain et al. (2003), while the other protocols remained the same. The carbon and nitrogen isotope ratios and their masses were determined using the same sample. The samples were exposed to HCl fumes for > 2 h to remove carbonate salt, dried in an oven (60°C) overnight and stored in a desiccator until isotope ratio measurements. After another round of oven-drying, the entire glass filter was wrapped in a tin cup. Subsequently, the carbon and nitrogen isotope ratios were measured using an Isoprime 100 isotope ratio mass spectrometer (Elementar, Langenselbold, Germany). In 2016, the colored surfaces of the glass filters were scraped off, and the carbon and nitrogen isotope ratios were measured. The absolute amounts of carbon and nitrogen in these scraped samples were different from those in the whole samples. The stable isotope ratios of carbon and nitrogen were calibrated using curves obtained from L-alanine (Shoko Science) during the measurements. The isotope compositions of carbon and nitrogen of POM sample (δ13CPOM and δ15NPOM, respectively) were expressed using Eq. (1):