TRACERS OF OCEAN PRODUCTIVITY
W. J. Jenkins, University of Southampton,
Southampton, UK
Copyright & 2001 Elsevier Ltd.
Introduction
Primary production is the process whereby inorganic
carbon is fixed in the sunlit (euphotic) zone of the
upper ocean, and forms the base of the marine food
pyramid. It occurs when marine phytoplankton use
sunlight energy and dissolved nutrients to convert
inorganic carbon to organic material, thereby releasing oxygen. The total amount of carbon fixed
during photosynthesis is called gross production,
whereas the amount of carbon fixed in excess of internal metabolic costs is referred to as net production. It is understood that a significant fraction of
the carbon fixed in this manner is rapidly recycled by
a combination of grazing by zooplankton and in situ
bacterial oxidation of organic material. New production is that portion of net production that is
supported by the introduction of new nutrients into
the euphotic zone. Traditionally, this has been regarded as production fueled by nitrate as opposed to
more reduced forms of nitrogen, such as ammonia
and urea. Some portion of the fixed carbon sinks out
of the euphotic zone in particulate form, or is subducted or advected away as dissolved organic material from the surface layers by physical processes.
This flux is regarded collectively as export production. The ratio of new (export) to net production,
referred to as the f-ratio (e-ratio) can vary between 0
and 1, and is believed to be low in oligotrophic (‘blue
water’), low productivity regions, and higher in eutrophic, high productivity regions. Finally, net community production is the total productivity in excess
of net community metabolic cost. On sufficiently
long space- and time-scales, it can be argued that
new, net community, and export production should
be equivalent in magnitude.
Net production has been measured ‘directly’ by
radiocarbon incubation experiments, whereby water
samples are ‘spiked’ with radiocarbon-labeled bicarbonate, and the net rate of transfer of the radioisotope into organic matter phases determined by
comparison of light versus dark incubations. Global
maps of net productivity have been constructed
on the basis of such measurements, and current
estimates indicate a global fixation rate of order 50
GT C a
À1 (1 GT ¼ 10
15 g). Rates of export, new, and
net community production are more difficult to determine directly, yet are of equal importance as determinants of biogeochemically important fluxes on
annual through centennial timescales.
Geochemical tracer techniques have been used to
make such estimates, and offer significant advantages
in that they are fundamentally nonperturbative, and
integrate over relatively large space-scales and long
time-scales. Conversely, such determinations must be
viewed from the perspective that they are indirect
measures of biogeochemical processes, and have
characteristic implicit space- and time-scales, as well
as boundary conditions, and sometimes ambiguities
and model dependence. Further, the specific tracer or
physical system used to obtain production estimates
determines the type of productivity measured. Thus
any treatment of geochemical tracer estimates must
include a discussion of these attributes.
Measuring Oceanic Productivity with
Tracers
Just a few approaches will be discussed here. Other
techniques have been used with some success, particularly with relation to particle interceptor traps,
but this section will concentrate on basic mass budgeting approaches using water column distributions
or seasonal cycling of tracers. There are three basic,
yet fundamentally independent approaches that can
be used.
1. Aphotic zone oxygen consumption rates that,
when vertically integrated, provide a net water
column oxygen demand that can then be related
stoichiometrically to a carbon export flux.
2. Seasonal timescale euphotic zone mass budgets,
particularly of oxygen, carbon, and carbon isotopes, which lead to estimates of net community
production.
3. Tracer flux-gauge measurements of physical
mechanisms of nutrient supply to the surface
ocean, which place lower bounds on rates of new
production.
These techniques, summarized in Figure 1, yield estimates of subtly different facets of biological production. On annual timescales, however, these
different modes of production should be very close to
equivalent, and hence the results of these various
181
W. J. Jenkins, University of Southampton,
Southampton, UK
Copyright & 2001 Elsevier Ltd.
Introduction
Primary production is the process whereby inorganic
carbon is fixed in the sunlit (euphotic) zone of the
upper ocean, and forms the base of the marine food
pyramid. It occurs when marine phytoplankton use
sunlight energy and dissolved nutrients to convert
inorganic carbon to organic material, thereby releasing oxygen. The total amount of carbon fixed
during photosynthesis is called gross production,
whereas the amount of carbon fixed in excess of internal metabolic costs is referred to as net production. It is understood that a significant fraction of
the carbon fixed in this manner is rapidly recycled by
a combination of grazing by zooplankton and in situ
bacterial oxidation of organic material. New production is that portion of net production that is
supported by the introduction of new nutrients into
the euphotic zone. Traditionally, this has been regarded as production fueled by nitrate as opposed to
more reduced forms of nitrogen, such as ammonia
and urea. Some portion of the fixed carbon sinks out
of the euphotic zone in particulate form, or is subducted or advected away as dissolved organic material from the surface layers by physical processes.
This flux is regarded collectively as export production. The ratio of new (export) to net production,
referred to as the f-ratio (e-ratio) can vary between 0
and 1, and is believed to be low in oligotrophic (‘blue
water’), low productivity regions, and higher in eutrophic, high productivity regions. Finally, net community production is the total productivity in excess
of net community metabolic cost. On sufficiently
long space- and time-scales, it can be argued that
new, net community, and export production should
be equivalent in magnitude.
Net production has been measured ‘directly’ by
radiocarbon incubation experiments, whereby water
samples are ‘spiked’ with radiocarbon-labeled bicarbonate, and the net rate of transfer of the radioisotope into organic matter phases determined by
comparison of light versus dark incubations. Global
maps of net productivity have been constructed
on the basis of such measurements, and current
estimates indicate a global fixation rate of order 50
GT C a
À1 (1 GT ¼ 10
15 g). Rates of export, new, and
net community production are more difficult to determine directly, yet are of equal importance as determinants of biogeochemically important fluxes on
annual through centennial timescales.
Geochemical tracer techniques have been used to
make such estimates, and offer significant advantages
in that they are fundamentally nonperturbative, and
integrate over relatively large space-scales and long
time-scales. Conversely, such determinations must be
viewed from the perspective that they are indirect
measures of biogeochemical processes, and have
characteristic implicit space- and time-scales, as well
as boundary conditions, and sometimes ambiguities
and model dependence. Further, the specific tracer or
physical system used to obtain production estimates
determines the type of productivity measured. Thus
any treatment of geochemical tracer estimates must
include a discussion of these attributes.
Measuring Oceanic Productivity with
Tracers
Just a few approaches will be discussed here. Other
techniques have been used with some success, particularly with relation to particle interceptor traps,
but this section will concentrate on basic mass budgeting approaches using water column distributions
or seasonal cycling of tracers. There are three basic,
yet fundamentally independent approaches that can
be used.
1. Aphotic zone oxygen consumption rates that,
when vertically integrated, provide a net water
column oxygen demand that can then be related
stoichiometrically to a carbon export flux.
2. Seasonal timescale euphotic zone mass budgets,
particularly of oxygen, carbon, and carbon isotopes, which lead to estimates of net community
production.
3. Tracer flux-gauge measurements of physical
mechanisms of nutrient supply to the surface
ocean, which place lower bounds on rates of new
production.
These techniques, summarized in Figure 1, yield estimates of subtly different facets of biological production. On annual timescales, however, these
different modes of production should be very close to
equivalent, and hence the results of these various
181