Environmental Engineering Reference
In-Depth Information
by primary producers is also a key process of the global carbon cycle. Primary production
and the short- and long-term fate of this fixed carbon influences atmospheric carbon diox-
ide concentration. The study of primary production in terms of rates, controls, trophic
interactions, biogeochemical cycles, and storage of the end-products of primary production
is, therefore, central to ecosystem science.
The results of primary production are often quite evident as, for example, the rapid
growth of lawn grass during spring. In terrestrial ecosystems the accumulation of biomass
by primary producers provides important structure. For example, in forests, tree growth
leads to branch and root formation and the accumulation of wood. These structural ele-
ments are critical components affecting many physical, chemical, and biological processes
in a forest (Box 11.1). Analogous growth of marine kelp forests in the sea creates structure
and habitat that support many types of organisms.
Primary production may also be cryptic. Measurement of phytoplankton biomass day
to day in the sea or in a lake would usually reveal little variation. It would seem that no
biomass is being produced because there is no accumulation, but in this case, loss pro-
cesses such as grazing by herbivores are as rapid as the increase in phytoplankton.
Production might be high even though biomass of the phytoplankton does not change.
In contrast, when growth rates are consistently in excess of loss rates, so-called “blooms”
of phytoplankton result and can lead to massive, sometimes noxious, accumulations of
algal scums. Rather than being cryptic, these scums caused by excess primary production
are conspicuous and represent a serious environmental problem in many water bodies.
COMPONENTS OF PRIMARY PRODUCTION
Primary production is by definition a rate with units of mass per area (or volume, if
measured in water) per time. For example, primary production data are often presented
as grams carbon per square meter per day (g C m
2
2
d
2
1
). The absolute amount of plant
material produced in an ecosystem is sometimes referred to as production or yield (mass
per unit area or volume) as, for example, the total mass of corn plants generated in a field.
Time, however, is generally implicit in this use of production and yield. For example, the
production of a corn field typically refers to a mass per unit area for a growing season.
In this chapter the terms
production
and
primary production
will always refer explicitly to
rates with the time attribute of the rate specified.
Biomass is distinct from primary production. The biomass of primary producers is
mass per area or volume independent of time. Biomass is often approximately correlated
with primary production. However, it is possible as noted earlier to have low biomass but
relatively high rates of primary production as often observed in the ocean. Alternatively,
slow-growing plants may represent a substantial biomass but have relatively low rates of
primary production.
Primary production encompasses a number of processes that require definition and that
pose problems for measurement. The components of primary production are clarified by
following the flow and fates of carbon through a generalized ecosystem (
Figure 2.1
).
Primary production begins with the fixation of CO
2
into organic matter. Gross primary
production (GPP) represents this first step accounting for all the carbon dioxide fixed into
