2024 Volume 71 Issue 2 Pages 37-54
Along the southwestern coastal area of the Okhotsk Sea in Hokkaido, four water masses (Soya warm current water [SWCW], fresh surface Okhotsk Sea water [FSOSW], intermediate cold water [ICW], and Japan Sea intermediate water [JSIW]) mix horizontally and vertically during summer, resulting in characteristic oceanographic conditions, including the formation of a cold water belt (CWB) that is associated with high chlorophyll a concentration. The distribution and mixing processes of each water mass play important roles in the biological productivity of this area; however, the effects of such variations on zooplankton distribution have been poorly investigated. Therefore, in this study, we aimed to assess the relationship between the planktonic copepods and water mass distribution in the coastal area of the Okhotsk Sea in Hokkaido from May, 2017 to June 2019. Although the proportions of the two numerically dominated copepod species, Oithona similis and Pseudocalanus newmani varied by species, both were less abundant in the subsurface water masses, ICW and JSIW. However, O. similis was constantly observed in all surface water masses from SWCW to FSOSW, and mixed water mass areas, including CWB. Although P. newmani was distributed in many surface water masses, its abundance was strongly associated with the chlorophyll a concentration. Notably, a high-density patchiness (approximately 35000 ind m-3) was formed corresponding to the highest chlorophyll a concentration (6.40 mg m-3) at a station in the CWB. However, the intrinsic rate of natural increase in P. newmani abundance could not solely explain its high-density patchiness in the ephemeral and unstable water masses, such as CWB. Here, our findings suggest the influx and accumulation of populations associated with CWB formation from surrounding water masses and behavioural responses to high chlorophyll a concentrations are the main factors affecting the high-density patchiness of P. newmani.
