The NE subarctic Pacific in winter:II. Biological rate processes

Abstract

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.

standing stocks

“…The largest difference between winter and summer standing stocks is for mesozooplankton biomass which was more than 10-fold higher in summer than in winter, as might be expected from knowledge of zooplankton life histories in this region , Parsons & Lalli 1988. It is of interest to note that comparable levels of total standing stocks (autotrophs and heterotrophs) between summer and winter were observed even though winter rates of primary production and microzooplankton grazing were considerably lower than reported during summer (Boyd et al 1995). The implications of this are discussed in more detail in Boyd et al (1995).…”

“…It is of interest to note that comparable levels of total standing stocks (autotrophs and heterotrophs) between summer and winter were observed even though winter rates of primary production and microzooplankton grazing were considerably lower than reported during summer (Boyd et al 1995). The implications of this are discussed in more detail in Boyd et al (1995).…”

“…As the winter bacterial carbon requirement is greater than the fixed carbon in all 3 fractions of the phytoplankton (Fig. ?A), and as the turnover of phytoplankton carbon is relatively low (Boyd et al 1995), it is unlikely that sufficient carbon substrates can be supplied from phytoplankton exudation or due to sloppy feeding by grazers. Kirchman (1990) reported that heterotrophic bacteria in this oceanic region have low growth rates (due to temperature or substrate limitation) relative to the dominant autotrophs and microheterotrophs.…”

Latitudinal Differences in the Planktonic Biomass and Community Structure Down to the Greater Depths in the Western North Pacific

“…The situation may be reversed when the biological pump is weak (ocean a source of CO2) (Longhurst and Harrison, 1989). The function of the biological pump could vary from one oceanic region to another, depending on the structure of the plankton communities and abundance of the component organisms which determine the pathway of carbon through planktonic food webs, as was demonstrated in the eastern North Pacific Ocean (Booth et al, 1993;Boyd et al, 1995), western North Pacific Ocean (Shinada et al, 2001), equatorial Pacific Ocean (Ishizaka et al, 1997), subtropical Pacific Ocean (Bradford-Grieve et al, 1999), off Bermuda , North Sea (Nielsen et al, 1993), and Baltic Sea (Uitto et al, 1997). However, all these previous studies are concerned with plankton communities in the epipelagic zone, and comparable information about the plankton community structure in the mesopelagic and bathypelagic zones is extremely limited (cf.…”

Seasonal changes in the planktonic food web off Cape Esan, southwestern Hokkaido, Japan

“…Many studies have been done on the planktonic food web in offshore waters around the world, such as in the NE subarctic Pacific (Booth et al 1993, Boyd et al 1995a, b, Yamaguchi et al 2002, western North Atlantic (Harrison et al 1993), Sargasso Sea off Bermuda , Mediterranean Sea (Siokou-Frangou et al 2002) and the equator at 175°E (Ishizaka et al 1997). On the other hand, in Japan, many studies about the plankton food web have been reported from coastal waters such as the Ariake Sound (Nakamura & Hirata 2006), Uwa Sea (Nakano et al 2004), Dokai inlet (Uye et al 1998), Seto Inland Sea (Nakamura et al 1994, Uye et al 1996, Ise Bay (Uye et al 2000) and off Usujiri (Shinada et al 2005).…”