290
fixed by phytoplankton may be respired during the long downwards transit of the biogenic
material, and returned to the atmosphere. This might accordingly result in lower sequestration
of carbon than expected from the influx of allochthonous nitrogenous nutrients. Because of
the predominance of grazing over sedimentation, the North Pacific should produce more
pelagic fishes than the North Atlantic (Parsons and Lalli, 1988). The background biomass pool
also constitutes a dynamic sink for carbon, which is possibly larger in the Alaskan Gyre (at
least as regards plankton) than in environments with stronger temporal variations; this living
carbon stock, however, can easily be broken down if the conditions undergo long term
changes and does not provide an efficient buffer in the global biogeochemical cycles. In
addition, because the environment is rather weakly structured in time and space, the biomass
is likely to be dispersed, which can hinder commercial exploitation of the stock (see Cushing,
1989). Fishing techniques in large areas of the Pacific Ocean are actually designed to cope
with a dispersed resource (at the price of their being highly destructive in some cases, e.g.
mammoth drift nets). One of the exceptions is the catch of salmon as they migrate back into
their native rivers; fishing in this case takes advantage of a behaviourial concentration of the
resource, which, in turn, is dependent on ecological conditions that bear no direct relationship
to the oceanic environment.
TYPE 5. PRODUCTION AND STANDING STOCK DOMINATED BY SMALL CELLS
In oligotrophic oceanic waters, picoplankton typically account for> 50 % of the chlorophyll a
biomass (e.g. Herbland and Le Bouteiller, 1981; Platt et al., 1983; Berman et aI., 1986;
Legendre et aI., 1988), with values> 90% reported by Takahashi and Hori (1984). Given
their high proportions in the biomass, the small cells account for a large proportion of the
total primary production (typically ca. 50-60%; e.g. Paerl, 1977; Herbland and Le Bouteiller,
1981; Li et al., 1983; Platt et aI., 1983). Azam et al. (1983) proposed that the small
phototrophic cells are part of a "microbial food loop", which also comprises heterotrophic
bacteria and protozoa. According to Cushing (1989), the microbial food web predominates in
stratified waters because the exudates from phototrophic picoplankton (small eukaryotic algae
and cyanobacteria) are not dispersed and can therefore be used by heterotrophic bacteria. In
the scheme of Azam et al. (1983), the numbers of picoplankton cells are controlled by
protozoa, including heterotrophic micro flagellates which are in turn exploited by ciliates; this
fixed by phytoplankton may be respired during the long downwards transit of the biogenic
material, and returned to the atmosphere. This might accordingly result in lower sequestration
of carbon than expected from the influx of allochthonous nitrogenous nutrients. Because of
the predominance of grazing over sedimentation, the North Pacific should produce more
pelagic fishes than the North Atlantic (Parsons and Lalli, 1988). The background biomass pool
also constitutes a dynamic sink for carbon, which is possibly larger in the Alaskan Gyre (at
least as regards plankton) than in environments with stronger temporal variations; this living
carbon stock, however, can easily be broken down if the conditions undergo long term
changes and does not provide an efficient buffer in the global biogeochemical cycles. In
addition, because the environment is rather weakly structured in time and space, the biomass
is likely to be dispersed, which can hinder commercial exploitation of the stock (see Cushing,
1989). Fishing techniques in large areas of the Pacific Ocean are actually designed to cope
with a dispersed resource (at the price of their being highly destructive in some cases, e.g.
mammoth drift nets). One of the exceptions is the catch of salmon as they migrate back into
their native rivers; fishing in this case takes advantage of a behaviourial concentration of the
resource, which, in turn, is dependent on ecological conditions that bear no direct relationship
to the oceanic environment.
TYPE 5. PRODUCTION AND STANDING STOCK DOMINATED BY SMALL CELLS
In oligotrophic oceanic waters, picoplankton typically account for> 50 % of the chlorophyll a
biomass (e.g. Herbland and Le Bouteiller, 1981; Platt et al., 1983; Berman et aI., 1986;
Legendre et aI., 1988), with values> 90% reported by Takahashi and Hori (1984). Given
their high proportions in the biomass, the small cells account for a large proportion of the
total primary production (typically ca. 50-60%; e.g. Paerl, 1977; Herbland and Le Bouteiller,
1981; Li et al., 1983; Platt et aI., 1983). Azam et al. (1983) proposed that the small
phototrophic cells are part of a "microbial food loop", which also comprises heterotrophic
bacteria and protozoa. According to Cushing (1989), the microbial food web predominates in
stratified waters because the exudates from phototrophic picoplankton (small eukaryotic algae
and cyanobacteria) are not dispersed and can therefore be used by heterotrophic bacteria. In
the scheme of Azam et al. (1983), the numbers of picoplankton cells are controlled by
protozoa, including heterotrophic micro flagellates which are in turn exploited by ciliates; this