New evaluation of vertical particulate organic carbon fluxes of submillimeter-sized phaeodarians in the mesopelagic twilight zone of the western North Pacific Ocean

Abstract The western subarctic North Pacific is one of the areas where substantial amounts of CO 2 are photosynthetically taken up and effectively transferred to the ocean interior. Phaeodarians (marine siliceous Rhizaria) have high biomass in the western North Pacific, but most quantitative studies of their carbon biomass and vertical carbon flux to the seafloor have been limited to phaeodarians larger than 500 µm. Here, we investigated the time-series flux of < 1 mm phaeodarian particles collected in a sediment trap deployed at a depth of 1000 m at Station K2 in the western North Pacific subarctic region from June 1, 2014 to July 6, 2015. We compared species-specific organic carbon content of phaeodarians in sediment trap samples measured using an ultra-sensitive elemental analyzer with that of living phaeodarians calculated using allometric carbon-to-volume relationships. This enabled us to estimate the remaining organic carbon content of all species of < 1 mm phaeodarians in sediment trap samples and quantified the < 1 mm phaeodarian organic carbon flux for the first time. The organic carbon flux of < 1 mm phaeodarians varied from 64 to 1364 µg m −2 d −1 during the sampling period, with a mean and standard deviation of 318 ± 266 µg m −2 d −1 . The average contribution rate to the organic carbon flux of < 1 mm phaeodarians was higher for deeper dwelling groups, with the lower intermediate dwellers (750–1000 m) accounting for 41.6%, while the surface to subsurface dwellers (0–250 m) accounted for only 0.3%. The contribution of the organic carbon flux of < 1 mm phaeodarians to that of total (sum of the < 1 mm and > 1 mm fractions) particulates ranged from 1.1% to 12.5%, with an average contribution of 3.5% during the sampling period, which was higher than the average contribution of 2.5% reported for > 1 mm phaeodarians in our previous study. The contribution of the organic carbon flux of total phaeodarians to that of total particulates ranged from 2.4 to 15.1%, with an average contribution of 6.0%. This study highlights the importance of quantifying both < 1 mm and > 1 mm phaeodarian organic carbon fluxes for understanding the marine carbon cycle.