The NE subarctic Pacific in winter .2. Biological rate processes

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, these 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 seduced 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 limited 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 dispersal 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.