INTERDECADAL VARIATION IN DEVELOPMENTAL TIMING OF NEOCALANUS-PLUMCHRUS POPULATIONS AT OCEAN-STATION-P IN THE SUB-ARCTIC NORTH PACIFIC

A single copepod species, Neocalanus plumchrus (Marukawa), makes up mu ch of the mesozooplankton biomass in the subarctic Pacific. Its vertic al 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. A t Ocean Station P (50 degrees N, 145 degrees W), seasonal phasing of t his annual maximum has shifted dramatically between 1956 and the prese nt. Both time series observations of N. plumchrus stage composition ra tios and measurements of total upper ocean zooplankton biomass produce consistent pictures of this change. Population development was very l ate in the early 1970s (biomass maximum in mid-July to late July), ear ly in the late 1950s (late May - early June), and very early in the 19 90s (early May to mid-May). The changes in timing are strongly correla ted with large-scale year-to-year and decade-to-decade ocean climate f luctuations, as reflected by spring season temperature anomalies in th e surface mixed layer within which the juvenile copepodites feed and g row (r(2) = 0.56, development about 60 days earlier in warm than in co ld years). But the change in developmental timing is too large to be e xplained solely by physiological acceleration of individual developmen t rate. We suggest instead that the cause is interannual differences i n survival among early versus late portions of the annual copepodite c ohort.