Material cycles in the oceans influence global environmental change. In addition to physical diffusion, sedimentation and transport of faecal pallets, excreted by zooplankton feeding on microplankton in the surface layer, are known to be important in the material cycle in the ocean. Quantitative sampling of zooplankton faecal pallets is generally performed by mooring a device called a sediment trap, which collects sedimentation particles from the upper layer. However, since the sampling of faecal pallets with a sediment trap requires a mooring period of 2 to 3 days, it has remained unclear whether there are day-night differences in the amount and size of zooplankton faecal pallets in the field. In the southern Sea of Okhotsk during the summer, the research group conducted stratified day/night sampling between 0-1000 m depth with a fine-mesh zooplankton net, which can also collect faecal pallets, and analyzed the samples using an imaging scanner to develop a new method for quantitative evaluation of faecal pallets.
In the southern Sea of Okhotsk in summer, low-salinity seawater from melting sea ice is warmed by sunlight, and high-temperature, low-salinity seawater with very light specific gravity is seen on the surface layer. The underlying water is low-temperature, high-salinity brine water that is discharged during the winter ice formation process, resulting in the development of a pronounced pycnocline. The zooplankton community is dominated by krill and copepods
Metridia okhotensis, which are capable of diurnal vertical migration across this strong pycnocline. The research group quantitatively collected micro-, medium-, and large-sized zooplankton, which are the food of these zooplankton, and analyzed their community structure at the species level, as well as sedimentation and transport by faecal pallets.
