From climate regime shifts to lower-trophic level phenology: Synthesis of recent progress in retrospective studies of the western North Pacific

Abstract

Pelagic ecosystems of the western North Pacific have experienced dramatic changes in recent decades. Prior retrospective analysis of several regions has shown that ecosystem or population changes have occurred coincident with or shortly following significant changes in ocean climate and atmospheric forcing. Here, we summarize these changes and attempt to identify specific mechanisms responsible for the changes in three regions in the western North Pacific: the Oyashio (OY), the subtropical water (ST), and the southern Japan/East Sea (JES). A cooling condition after the climate regime shift of 1976–1977 deepened the winter mixed layer depth (MLD) in both the OY and ST, but influenced lower-trophic level productivity differently between these regions. The deep MLD reduced winter phytoplankton and zooplankton biomass in the subarctic OY, presumably due to a decrease in wintertime light availability. Concurrently, it increased spring plankton biomass in the ST, presumably due to the replenishment of springtime nutrients. When a warming condition became prevalent after the 1988–1989 regime shift, the shallow winter MLD increased the winter plankton biomass in the OY, but decreased the spring plankton biomass in the ST through the same mechanism controlling light and nutrient availability. In the JES, with a complex water column structure consisting of the surface warm current and dense subsurface cold water, the hydrographic conditions and ecosystem responses could not be explained by the mechanisms for the OY and ST. We also detected cooling and warming phase shifts induced a phenological change in the OY. A model hindcast for the OY estimated an average 5-day (max. 20-day) delay in the timing of the primary production peak during the cool phase after the mid-1970s. This timing became earlier after the mid-1990s, several years after the 1988–1989 regime shift, reaching the level before the mid-1970s. The observed increase and decrease in winter and spring phytoplankton biomass, respectively, during the warm phase in the 1990s, indicate a moderate but prolonged productive season, which differs from the conventional seasonal variation pattern of the OY with an extensive but short spring bloom. This alternate phytoplankton seasonality coupled with the direct influence of a warm temperature anomaly might dramatically alter the zooplankton community structure in the OY after the 1990s, as suggested by the marked increase in the occurrence of warm-water copepods in spring. Based on these findings, we warn that retrospective analyses based on annual averages and lacking careful consideration of seasonal variation may result in erroneous findings of causes and consequences.