Environmental Engineering Reference
In-Depth Information
Aquatic Methods
One of the most common primary production methods used in aquatic ecosystems
dominated by fast-growing phytoplankton and benthic algae is measurement of the incor-
poration of
14
C. The technique is extremely sensitive so even very low rates of primary
production can be measured. To measure phytoplankton production, water samples are
collected, a trace amount of
14
C-labeled bicarbonate (H
14
CO
3
) is added, and the sample is
incubated in situ or in the laboratory under specified light conditions. At the end of the
incubation, the water is filtered. The
14
C captured on the filter after any residual bicarbon-
ate is removed by acidification represents primary production. The length of incubation
determines whether significant respiration of
14
C by phytoplankton occurs (
R
a
) and the
method typically measures a quantity that falls somewhere between GPP and NPP. Other
complexities need to be considered including the loss of radioactive dissolved organic
compounds, death of phytoplankton during the incubation, temporal variations of light
(e.g., daily light cycle), and possible artifacts from enclosing phytoplankton in bottles.
The incubation time, and hence measurement period, is usually a few hours, and rates are
typically extrapolated to represent daily production.
Terrestrial Methods
The phytoplankton
14
C method contrasts with approaches to forest production.
One standard approach to measuring forest primary production is through a combination
of leaf fall (foliar production) and wood production estimates. The estimate of wood pro-
duction takes advantage of the strong correlation of woody biomass with tree diameter.
Allometric equations that quantify the relationship between diameter and mass are avail-
able in the literature for many tree species. These equations are generated by harvesting
trees and measuring both morphometric characteristics and the biomass of selective com-
ponents. With these relationships, repeated measurements (usually over several years) of
tree diameters in a stand of trees can be used to calculate the accumulation of woody bio-
mass. This approach estimates NPP rather than GPP because
R
a
is occurring during the
measurement period. Foliar and wood production constitute most of the NPP in forests,
but it is important to note that this method ignores many other parts of the total NPP,
each of which can be important in some places and times. For instance, understory plants
can contribute significantly to NPP in some ecosystems. Losses to herbivory are usually
small but can sometimes be very important, as for example, during insect outbreaks or in
grasslands. Seed production can be important but is sometimes episodic (e.g., masting)
and therefore difficult to measure accurately. Root production is also part of the primary
production but is also very difficult to measure, thus most terrestrial primary production
data are presented as ANPP, above-ground net primary production. Root production can
be equal to or greater than ANPP in some ecosystems, but measurement of this process
has vexed ecologists for decades. One current method is to use a video camera to repeat-
edly measure the growth of individual roots along the face of a clear sampling tube
inserted into the ground. Although this method is promising, all root production estimates
involve great uncertainties.
