291
grazing activity is accompanied by excretion of ammonium, which is used by the
photoautotrophs. Little material is exported from the nearly closed microbial food loop (e.g.
Michaels and Silver, 1988) and this system is in a quasi-steady state (e.g. Cushing, 1989).
Even if the microbial food web is largely a recycling loop, export of production is
nevertheless possible, through direct feeding by metazoans on small particles ("baleen whale"
feeding; Fenchel, 1984), incorporation of small particles into marine snow and sinking, and
their accumulation with particles of various sizes in hydrodynamic traps where they are
subject to grazing by microphagous feeders (see Fig. 2, and also Goldman, 1988). Within the
area, between SOON and 50
0
S, where coccolithophore and foraminiferan production is
significant, particulate calcium carbonate in the upper 500 m tends to peak between 10-15°C,
with low concentrations being found in the warmest waters (Bishop, 1989). The export of
carbon by calcareous tests should therefore be relatively low in warm oligotrophic waters, so
that the amount of carbon exported by the microbial food web is likely to approximate the
stoichiometric balance with the influx of allochthonous nitrogenous nutrients. Actual
sequestration of carbon is however subjected to respiratory losses similar to those described
above (Type 4), given the long and inefficient export pathways. In the oligotrophic ocean, the
sources of allochthonous nutrients (Fig. 1) include the deep ocean (upwards eddy diffusion of
N0 3 -) and the atmosphere (transport of N0 3 - and NH/NH4 +; N2 fixation). The atmospheric
nitrogen sources probably contribute mainly to the microbial food web, while upwards
diffusion may also fuel episodic blooms of large cells in the lower euphotic zone. These
episodic blooms were hypothesized by Goldman (1988; see Type 1 above), who also
suggested that phytoplankton production of Types 1 and 5 would operate quite independently
of each other in oligotrophic waters (Fig. 8).
It has already been mentioned that the production cycle in the oligotrophic ocean is in
quasi-steady state so that, according to Cushing (1989), the food chains are long and the
organisms are dispersed. Such conditions do not favour sustained exploitation of fish stocks.
Concerning the export and sequestration of carbon, the oligotrophic ocean is probably the
system where the assumptions underlying the usual steady-state/stoichiometry model for export
production (e.g. Eppley and Peterson, 1979) are best approximated. However, even in this
case, the task of estimating new production is not trivial (e.g. Platt et aI., 1989b), and the
grazing activity is accompanied by excretion of ammonium, which is used by the
photoautotrophs. Little material is exported from the nearly closed microbial food loop (e.g.
Michaels and Silver, 1988) and this system is in a quasi-steady state (e.g. Cushing, 1989).
Even if the microbial food web is largely a recycling loop, export of production is
nevertheless possible, through direct feeding by metazoans on small particles ("baleen whale"
feeding; Fenchel, 1984), incorporation of small particles into marine snow and sinking, and
their accumulation with particles of various sizes in hydrodynamic traps where they are
subject to grazing by microphagous feeders (see Fig. 2, and also Goldman, 1988). Within the
area, between SOON and 50
0
S, where coccolithophore and foraminiferan production is
significant, particulate calcium carbonate in the upper 500 m tends to peak between 10-15°C,
with low concentrations being found in the warmest waters (Bishop, 1989). The export of
carbon by calcareous tests should therefore be relatively low in warm oligotrophic waters, so
that the amount of carbon exported by the microbial food web is likely to approximate the
stoichiometric balance with the influx of allochthonous nitrogenous nutrients. Actual
sequestration of carbon is however subjected to respiratory losses similar to those described
above (Type 4), given the long and inefficient export pathways. In the oligotrophic ocean, the
sources of allochthonous nutrients (Fig. 1) include the deep ocean (upwards eddy diffusion of
N0 3 -) and the atmosphere (transport of N0 3 - and NH/NH4 +; N2 fixation). The atmospheric
nitrogen sources probably contribute mainly to the microbial food web, while upwards
diffusion may also fuel episodic blooms of large cells in the lower euphotic zone. These
episodic blooms were hypothesized by Goldman (1988; see Type 1 above), who also
suggested that phytoplankton production of Types 1 and 5 would operate quite independently
of each other in oligotrophic waters (Fig. 8).
It has already been mentioned that the production cycle in the oligotrophic ocean is in
quasi-steady state so that, according to Cushing (1989), the food chains are long and the
organisms are dispersed. Such conditions do not favour sustained exploitation of fish stocks.
Concerning the export and sequestration of carbon, the oligotrophic ocean is probably the
system where the assumptions underlying the usual steady-state/stoichiometry model for export
production (e.g. Eppley and Peterson, 1979) are best approximated. However, even in this
case, the task of estimating new production is not trivial (e.g. Platt et aI., 1989b), and the