Characteristics of the zooplankton community in the Okhotsk Sea in autumn: A comparison with the Oyashio region

Based on the vertical stratified zooplankton samples collected during the autumns (September–December) of 1996 to 1998, vertical distribution, biomass, and community structure of zooplankton and calanoid copepods in the southern Okhotsk Sea were evaluated and compared with those of the same period in the Oyashio region. In terms of fauna, zooplankton in the Okhotsk Sea are similar to that in the Oyashio region. However, their biomass, community structure and vertical distribution patterns are quite different between these two regions. Zooplankton biomass near the surface layer (0 m to thermocline) in the Okhotsk Sea was less than that of the Oyashio region. To understand the reason for this, we look at the distribution of large copepods in the mesopelagic layer in the Okhotsk Sea (this is epipelagic in the Oyashio region). Standing stocks of most zooplankton taxa were smaller in the Okhotsk Sea, while only the copepod Metridia okhotensis showed an opposite pattern. The abundance of M. okhotensis in the Okhotsk Sea was 30 times greater than that in Oyashio region, and they predominated (60% of the total copepod number) in the Okhotsk Sea. The development of a strong pycnocline in the Okhotsk Sea may be a key feature responsible for these regional differences in the zooplankton community in the Okhotsk Sea and Oyashio region.