Changes in phytoplankton assemblages may significantly alter elemental cycles. However, the differing contributions of phytoplankton functional types to the biological pumps have not been explored. This study aims to evaluate the sinking process of phytoplankton to the bathypelagic layer at the functional-type level. We collected sinking particles using sediment traps moored at 387 m and 890 m depths from June to August 2022 in the Sea of Japan and morphologically divided them into aggregates and fecal pellets (ellipsoidal, cylindrical, spherical, and tabular). The carbon flux of sinking particle types was measured, and the phytoplankton assemblages in every sinking particle type were investigated with 16S rRNA gene amplicon sequencing. The proportion of the phytoplankton-origin amplicon sequence variant (ASV) per total 16S gene sequence reads numbers in aggregates was 5.76 ± 0.496% (median ± interquartile range, n = 6) at 890 m, which was significantly lower than that in the ellipsoidal fecal pellets (8.96 ± 6.00%, n = 64) at 890 m depth. The number and sizes of the ellipsoidal pellets, considered appendicularian-origin, significantly increased with depth, and bigger ellipsoidal pellets were richer in phytoplankton ASVs. Diatom Chaetocerotales were the dominant phytoplankton group in each sample type and depth, except in the ellipsoidal and cylindrical fecal pellets at 890 m depth, where cyanobacteria Synechococcus was dominant. This suggests that phytoplankton, including Synechococcus, is effectively transported to the bathypelagic layer via the mesopelagic appendicularians repackaging process, while diatom Chaetocerotales effectively sink, regardless of the sinking processes.

Carbon and nitrogen dynamics in the Sea of Japan (SOJ) are rapidly changing. In this study, we investigated the carbon and nitrogen isotope ratios of particulate organic matter (δ13CPOM and δ15NPOM, respectively) at depths of ≤ 100 m in the southern part of the SOJ from 2016 to 2021. δ13CPOM and δ15NPOM exhibited multimodal distributions and were classified into four classes (I–IV) according to the Gaussian mixed model. A majority of the samples were classified as class II (n = 441), with mean ± standard deviation of δ13CPOM and δ15NPOM of –23.7 ± 1.2‰ and 3.1 ± 1.2‰, respectively. Compared to class II, class I had significant low δ15NPOM (-2.1 ± 0.8‰, n = 11), class III had low δ13CPOM (-27.1 ± 1.0‰, n = 21), and class IV had high δ13CPOM (-20.7 ± 0.8‰, n = 34). All the class I samples, whose δ15NPOM showed an outlier of total data sets, were collected in winter and had comparable temperature and salinity originating in Japanese local rivers. The generalized linear model demonstrated that the temperature and chlorophyll-a concentration had positive effects on δ13CPOM, supporting the active photosynthesis and phytoplankton growth increased δ13CPOM. However, the fluctuation in δ15NPOM was attributed to the temperature and salinity rather than nitrate concentration, which suggested that the δ15N of source nitrogen for primary production is different among the water masses.  These findings suggest that multiple nitrogen sources, including nitrates from the East China Sea, Kuroshio, and Japanese local rivers, contribute to the primary production in the SOJ.

Mesozooplankton biomass plays a key parameter in the recruitment processes of fish and biogeochemical processes. Four decadal observations in the coastal Sea of Japan, the marginal sea of the North Pacific, indicate that wet weight-based mesozooplankton biomass is controlled by both environment-induced bottom-up and predatory-induced top-down processes. Interannual variations in mesozooplankton biomass using a generalized linear model approach showed a decrease in biomass during the 1980s, followed by a rapid increase in the early 1990s, and a gradual decrease in the 2010s. These interannual variations were the mirror image of the small pelagic planktivorous fish biomass. The difference in zooplankton biomass from the previous year was negatively correlated with the difference in small pelagic planktivorous fish biomass, which was supported by a Granger causality analysis. Therefore, the results of this study indicate that top-down control is one of the main causes of long-term variations of zooplankton biomass in the ocean.