265
in a steady-state condition over the significant spatio-temporal scales; it also implies that
variations in the proportion of carbon and nitrogen assimilated (and released) by marine
plankton (see Lancelot and Billen, 1985, and references therein) only have short term
relevance and that the stoichiometric balance is conserved at the steady-state scale. However:
" ... the underlying hypothesis of a dynamic steady state for the biogeochemistry of the pelagic
region of the ocean is to be considered as no more than an idealisation of a system that, in
reality, is perturbed intermittently such that the condition of instantaneous eqUilibrium is
rarely, if ever, attained" (Platt et al., 1989b). One of the best-known marine environments
in the world, on a long-term basis, is the English Channel and the North Sea and, in addition
to the usual interannual variations, major changes over decades are widely documented there.
These include variations in the total winter stock of inorganic nutrients, yearly primary
production, zooplankton maximum standing stock (by at least one order of magnitude) and
taxonomic composition, and commercial fish stocks (see Russell et al., 1971; Southward,
1980, and references therein). The observed changes were at first considered cyclical (the
"Russell Cycle"; Cushing and Dickson, 1976), but the cyclical pattern has recently weakened
(Southward, 1983) and non-periodic changes have been described in association with
pluri-annual climatic phenomena (Cushing, 1988). With the help of historical fisheries
records, such changes can be traced back in the past for several centuries (e.g. Southward,
1974). Longer-term changes are of course associated with the glaciations, so that the proper
time-scale for a steady-state balance in production processes can be elusive indeed.
The usual steady-state/stoichiometry model does not consider that ecosystem dynamics may
have a significant influence on the export or sequestration of biogenic carbon. Given a certain
upward flux of nitrate, it is assumed that there will be a proportional downward flux of
biogenic carbon, if the fluxes are integrated over proper spatio-temporal scales (Platt et al. ,
1989b). Contrary to this usual assumption, we shall first review several cases where
ecosystems do have a significant effect on the export and sequestration of biogenic carbon (the
next four sections), and then consider these various cases within the context of a general
typology of pelagic marine ecosystems.
in a steady-state condition over the significant spatio-temporal scales; it also implies that
variations in the proportion of carbon and nitrogen assimilated (and released) by marine
plankton (see Lancelot and Billen, 1985, and references therein) only have short term
relevance and that the stoichiometric balance is conserved at the steady-state scale. However:
" ... the underlying hypothesis of a dynamic steady state for the biogeochemistry of the pelagic
region of the ocean is to be considered as no more than an idealisation of a system that, in
reality, is perturbed intermittently such that the condition of instantaneous eqUilibrium is
rarely, if ever, attained" (Platt et al., 1989b). One of the best-known marine environments
in the world, on a long-term basis, is the English Channel and the North Sea and, in addition
to the usual interannual variations, major changes over decades are widely documented there.
These include variations in the total winter stock of inorganic nutrients, yearly primary
production, zooplankton maximum standing stock (by at least one order of magnitude) and
taxonomic composition, and commercial fish stocks (see Russell et al., 1971; Southward,
1980, and references therein). The observed changes were at first considered cyclical (the
"Russell Cycle"; Cushing and Dickson, 1976), but the cyclical pattern has recently weakened
(Southward, 1983) and non-periodic changes have been described in association with
pluri-annual climatic phenomena (Cushing, 1988). With the help of historical fisheries
records, such changes can be traced back in the past for several centuries (e.g. Southward,
1974). Longer-term changes are of course associated with the glaciations, so that the proper
time-scale for a steady-state balance in production processes can be elusive indeed.
The usual steady-state/stoichiometry model does not consider that ecosystem dynamics may
have a significant influence on the export or sequestration of biogenic carbon. Given a certain
upward flux of nitrate, it is assumed that there will be a proportional downward flux of
biogenic carbon, if the fluxes are integrated over proper spatio-temporal scales (Platt et al. ,
1989b). Contrary to this usual assumption, we shall first review several cases where
ecosystems do have a significant effect on the export and sequestration of biogenic carbon (the
next four sections), and then consider these various cases within the context of a general
typology of pelagic marine ecosystems.