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°N, 145°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 (r2 = 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.

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We present a conceptual model to illustrate how wind events and the annual migration and grazing of the dominant copepod Neocalanus plumchrus interact and affect the development of the spring bloom. The model was supported by observations made during 1988, 1992, and 1993. For example, in 1992, an El Niño year, the annual freshet of the Fraser River and probably the spring bloom started 1 month earlier. The bloom was interrupted by a wind event in late March. A few days later, its full recovery was interrupted by the peak in zooplankton grazing, and ambient ammonium concentrations increased. In contrast, in 1988, the annual freshet started later (mid-April), and winds remained strong throughout the same period, hindering the development of the spring bloom. The spring bloom was further suppressed by large numbers of zooplankton during April, resulting in a prolonged spring bloom. These observations indicate that interannual variations in winds and the timing of the annual freshet determine the timing and duration of the spring bloom, which in turn, determine the matching of phytoplankton to zooplankton in the Strait of Georgia. The matching or mismatching bears significant implications for food availability for juvenile fish.

The overwintering vertical stage distribution of N. plumchrus is described together with the temporal variations in total body length, dry weight, lipid and body composition in copepodite stage V (CV) and adult females. Adult males and females were found at depths of less than 200 m from January to March. Regressions of total body length to total dry weight showed significant correlations for CVs and females. Carbon and nitrogen isotopic analysis ( 6 "~ and 6I3C) indicated no temporal increase, suggesting a lack of feeding in CVs (October to December) and females (January to March) at depth and a dependency on stored lipid reserves accumulated during times of high primary production in the euphotic zone. Total lipid content per individual copepod showed significant decreases from 0.5 to 0.2 mg during the time of adult female gonadal development between January and February. A decrease in total dry mass per copepod from 0.9 to 0.4 mg also occurred during this time period. Lipid composition showed preferential retention of long-chained fatty acids until the onset of egg release in March. Significant decreases in carbon and nitrogen from January to February and, to a lesser extent, between February and March were observed, corresponding to periods of gonad maturation and egg release, respectively. Significant increase in nitrogen content suggests the possibility of bacterial ingestion by late moulting CVs. The significant decrease in C:N ratio, from 8.7 to 4.1, suggests a depletion of lipid reserves and resultant reliance on proteins as an energy source for egg release. Variations in organic content verify that the CVs remain in a state of dormancy in which minimal lipid reserves are depleted, while the onset of maturation and reproduction is the major energy consumer.