In over 20 yr of biological observations, with the exception of primary production data, no rate process measurements have been made during winter in the NE Pacific. In this paper, the first suite of detailed biological rate process measurements are presented. Rates of primary production, phytoplankton growth, microzooplankton grazing and bacterial production are lower during winter at Ocean Station Papa (OSP) off northwestern Canada than those observed during summer. Despite the reduced rates, there is sufficient net primary production to maintain a supply of carbon to the microbial food web during the overwintering period. As winter stocks of autotrophs and microheterotrophs at OSP have been observed on occasion to be comparable to those observed in summer, a reduced turnover of winter stocks is implied from the observed depression of biological rates. In winter, according to our limited observations, phytoplankton growth rates may be controlled by available irradiance, whereas heterotrophic bacterial production is likely to be controlled by water temperature and substrate limitation. Microzooplankton grazing rates may be primarily 11mited by temperature, since observed prey concentrations are not reduced compared to summer Despite the reduced microzooplankton grazing rates, limited winter observations suggest that their biomass remains comparable to that observed during summer. Mesozooplankton are present in low numbers in the upper water column during winter at OSP Thus, the maintenance of high microzooplankton stocks during winter, a requirement in order to prevent the development of the spring bloom, may result from low mesozooplankton grazing pressure rather than from low d~spersal due to insufficient strong mixing. The life cycle of the mesozooplankton, in addition to the shallow winter mixed layer depth in the NE subarctic Pacific, may be an important factor in the prevention of a spring bloom.

Although an extensive biological time series data set of phytoplankton and zooplankton standing stocks has been collected over a 30 yr period in the NE subarctic Pacific at Ocean Station Papa (OSP), the majority of these data were obtained before recent advances in our understanding of the structure and functioning of the manne microbial food web. In addition, recent studies did not obtain data during the winter penod. This paper provides the first contemporary biological description of the NE subarctic Pacific dunng winter Data from 2 winter cruises in the vicinity of OSP indicate that the abundance and composition of the autotrophic and heterotroph~c assemblage at these times are similar to those observed during late spring and summer by others. In contrast, winter mesozooplankton standing stocks are considerably less than observed In summer. These findings confirm the hypothesis that the relatively shallow winter mixed layer permits relatively high standing stocks of phytoplankton (20 mg C m-3) and consequently of microheterotrophs (7.1 to 13.1 mg C m-7 to subsist over the winter period. An assessment of autotrophic and heterotrophic biomass suggests that the requirements of the mesozooplankton (Stages IV to VI), microzooplankton and heterotrophic nanoflagellates may potentially be balanced by the standlng stocks of microplankton, nanoplankton/picophytoplankton, and picoplankton, respectively. However, the carbon requirements of heterotrophic bacteria cannot be balanced without invoking slower turnover times for thls pool relative to the turnover times for the dissolved organic carbon (DOC) pool.