Vertical distribution, standing stocks, and taxonomic accounts of the entire plankton community, and the estimation of vertical material flux via faecal pellets in the southern Okhotsk Sea

The faecal pellets egested by zooplankton are essential as they influence the vertical material flux in oceans. As mesozooplankton are dominant within the plankton community in the southern Okhotsk Sea during early summer, the vertical material flux via mesozooplankton is expected to be substantial. However, quantitative information on their faecal pellets is currently lacking. In this study, we evaluated the taxonomic accounts of the entire plankton community, including microplankton, mesozooplankton, and macrozooplankton, in the 0–1000 m water column. The ingestion and egestion rates of the zooplankton were also estimated. We used a fine-mesh (63 μm) plankton net along with an imaging technique (with ZooScan) to quantify the amount of in-situ faecal pellets. Furthermore, on-board experiments were conducted to estimate the faecal pellet egestion by the dominant zooplankton species. Cosmopolitan diatom species were found to dominate the microplankton biomass, whereas the large-sized calanoid copepod Metridia okhotensis, which performs nocturnal ascent diel vertical migration, dominated the mesozooplankton biomass. Two euphausiid species with different body sizes, namely the small-sized Thysanoessa inermis and the large-sized Euphausia pacifica, were found to be dominant among the macrozooplankton. The highest density and mass of faecal pellets (1888 pellets m−3, 2.96 mg C m−3) was observed in the 0–100 m layer during the daytime. Throughout the layer, the volumes of the faecal pellets peaked at 0.010–0.015 mm3, which corresponded with the size of the pellets egested by M. okhotensis in the on-board laboratory experiments. The large-sized faecal pellets (>0.2 mm3), which were inferred to be egested by euphausiids, were only observed during the night-time. Based on the on-board experiments, the faecal pellets egested by all the meso- and macrozooplankton species contained phytoplankton cells that possessed a fluorescent ability. Furthermore, cyanobacteria were the most common taxa (26–65% in number) observed in the faecal pellets. Coprophagy (feeding on faecal pellets) was observed for the calanoid copepod Gaetanus variabilis, which was collected from depths of 500–1000 m. The estimated daily ingestion rates of meso- and macrozooplankton corresponded with 21% of the standing stock of microplankton. In contrast, their daily egestion rates accounted for 40–112% of the in-situ faecal pellet masses. Faecal pellet collection by the gentle vertical towing of a fin