North–south contrasts in decadal scale variations in lower trophic‐level ecosystems in the Japan Sea

Abstract Regional comparisons of interannual variations in springtime lower trophic‐level ecosystems were made for northern subarctic regions, and for southern Tsushima Current regions of the Japan Sea, based on archival hydrographic and biological data sets collected from the mid‐1960s to the early 1990s. Variations related to the Pacific Decadal Oscillation were detected for plankton biomass in both northern and southern regions, although there were regional differences with respect to mechanisms and timing. Springtime stratification increased after the late 1970s in the north, roughly coinciding with the northern Pacific regime shift in 1976/77. Stratification also increased due to warming in the south in the early 1980s, several years after the 1976/77 regime shift. Responding to the increase in stratification, springtime biomass of phytoplankton and zooplankton increased in the north and decreased in the south. Principal component analysis revealed that hydrographic conditions during spring, rather than winter, determined springtime phytoplankton biomass. In northern regions, spring phytoplankton production may be enhanced by increased light availability, due to mixed layer stabilization. In the south, where background nutrient concentration within the water column was low, increases in stratification were likely to limit nutrient supply to the surface layer, resulting in decreases in phytoplankton production. A positive relationship between phytoplankton and zooplankton biomass suggested bottom‐up control of secondary production in northern regions. The nature of the links between phytoplankton and zooplankton production was not clear in southern regions, where hydrographic conditions during winter seemed to be responsible for variations in springtime secondary production.