1970s, indicated by the PDO. After the regime shift, the timing of peak spring-
community abundance was delayed by one month, from March–April to April–
May, whereas the spring–summer community peaked earlier. That resulted in an
overlap of the highest biomass period for the two communities in May.
Wintertime cooling followed by rapid summertime warming after the mid-1970s
was likely responsible for delayed initiation and early termination of the
productive season for both groups. Then in the mid-1990s the PDO and
phenology shifted back. Warm winters followed by cool summers lengthened the
productive season and again separated the life history patterns of the two groups.Fig. 16.23 (a) Time-series for the Oyashio region at depths >500 m east of Hokkaido
of mean May to July zooplankton wet weight. Bar tips are at observed values in mg m
−3, right scale; tips are rescaled as anomalies in standard deviation, left scale). Data
are from the Odate collection. (b) The Pacific Decadal Oscillation. The solid lines
indicate 10-year running means. Zooplankton biomass is low when the PDO is high
and inversely.
(^) (After Chiba et al. 2006.)
Chiba et al. suggest that decadal climatic cycles may affect winter–spring and
spring–summer differently, such that the combined changes determine annual
productivity. Clearly the period of colder winters and rapid summer warming was less