Interdecadal variation in developmental timing of Neocalanus plumchrus populations at Ocean Station P in the subarctic North Pacific

A single copepod species, Neocalanus plumchrus (Marukawa), makes up much of the mesozooplankton biomass in the subarctic Pacific. Its vertical distribution and developmental sequence are both strongly seasonal. Together, they produce a strong and narrow (<60 days duration) annual peak of upper ocean zooplankton biomass in spring and early summer. At Ocean Station P (50 degrees N, 145 degrees W), seasonal phasing of this annual maximum has shifted dramatically between 1956 and the present. Both time series observations of N. plumchrus stage composition ratios and measurements of total upper ocean zooplankton biomass produce consistent pictures of this change. Population development was very late in the early 1970s (biomass maximum in mid-July to late July), early in the late 1950s (late May - early June), and very early in the 1990s (early May to mid-May). The changes in timing are strongly correlated with large-scale year-to-year and decade-to-decade ocean climate fluctuations, as reflected by spring season temperature anomalies in the surface mixed layer within which the juvenile copepodites feed and grow (r(2) = 0.56, development about 60 days earlier in warm than in cold years). But the change in developmental timing is too large to be explained solely by physiological acceleration of individual development rate. We suggest instead that the cause is interannual differences in survival among early versus late portions of the annual copepodite cohort.