Environmental Engineering Reference
In-Depth Information
chemosynthetic organisms can be the main primary producers (e.g.,
Sarbu et al. 1996
).
Indeed, in the thermal vent regions of the deep sea entire ecosystems run on energy derived
from chemosynthetic microorganisms. Sulfides and other reduced compounds that emerge
with geothermal fluids from these vents are converted by microorganisms to energy through
chemosynthetic reactions. In these settings chemosynthetic primary production supports a
variety of consumers with high local biomass (
Lutz and Kennish 1993; Van Dover et al. 2002
).
Regardless of whether gross primary production is powered by chemosynthesis or photosyn-
thesis, the carbon flow pathways in
Figure 2.1
still apply and the definitions of GPP, NPP, R,
and NEP are equivalent.
MEASURING PRIMARY PRODUCTION
Methods to measure primary production vary as a function of the types of autotrophs
in an ecosystem—an algal assemblage in a lake or stream requires a different approach
compared to trees in a forest. Primary production methods also vary in terms of the pro-
cesses that are included or excluded. For example, some methods measure GPP while
other methods include autotrophic respiration and therefore measure NPP. The timescale
of the measurement may determine which of the processes in
Figure 2.1
are included. For
example, measurement of grassland primary production is often accomplished by clipping
and weighing plant material produced over a given time interval (e.g., a growing season).
This measurement represents NPP not GPP because
R
a
has occurred during the course of
the measurement. Furthermore, any losses to herbivores that were not excluded and that
occur in this period are not measured in the biomass accumulation and must either be
considered negligible or estimated to provide a correction.
In aquatic ecosystems, “bottle methods” are often used wherein a sample is collected
and incubated for a few hours to measure the uptake of radioactive inorganic carbon (
14
C)
or a change of dissolved oxygen. Other techniques include continuously monitoring chem-
ical constituents such as oxygen or pH to assess overall ecosystem respiration and primary
production. For aquatic and wetland vascular plants various harvest and morphometric
methods are used.
In terrestrial ecosystems, including fields and forests, both harvest and incremental
growth observations are used to measure production. More recently, continuous monitor-
ing of CO
2
exchange over terrestrial ecosystems has been employed to estimate production
over large areas. The actual area measured by these latter approaches varies substantially
from system to system as a function of plant cover, terrain complexity, wind, and weather
conditions.
This text does not emphasize methods but we turn now to a brief discussion of some
aquatic and terrestrial methods for measuring primary production. Our purpose is to com-
pare and contrast the methods in terms of the components of primary production
(
Figure 2.1
) that are included in or excluded by the technique. By considering the methods
we hope to deepen understanding of the process of primary production and to increase
the reader's appreciation of some of the complexities inherent in this measurement.
