Environmental Engineering Reference
In-Depth Information
few times. Potential artifacts related to sample enclosure as required in the
14
C method are
eliminated. The technique is less subject to problems of extrapolation of primary produc-
tion for an ecosystem, because the method inherently provides estimates across broad
scales (e.g., an entire lake each day). This method, as for any technique, also has important
uncertainties such as the assumptions about the equivalence of day and night respiration
as well as questions of scale (e.g., what portion of an aquatic ecosystem is represented
by the oxygen measurements). Nevertheless, this type of method is greatly improving
the resolution and the accuracy of primary production measurements of ecosystems
(e.g.,
Roberts et al. 2007
).
An analogous gas-exchange approach in terrestrial ecosystems is called
eddy covariance
(sometimes called eddy flux or eddy correlation). In this method a fast-response CO
2
sen-
sor is paired with a multidirectional wind speed sensor on a tower extending above a veg-
etation canopy. The sensors measure the CO
2
concentrations associated with updrafts and
downdrafts as the turbulent air mixes into the canopy. The flux of CO
2
into and out of the
canopy on these air currents is calculated using algorithms programmed into a computer,
and the difference (efflux
influx), integrated over time, is termed the
net ecosystem
exchange
, or NEE. (By convention, NEE is negative if the net flux of CO
2
is into the canopy
and positive if the net flux is out of the canopy.) Because the release of CO
2
from the eco-
system includes both autotrophic and heterotrophic respiration, NEE is essentially an
instantaneous measurement of NEP. In some cases, nighttime measurement of NEE (CO
2
efflux at night) is used to estimate ecosystem respiration (
R
e
, including both
R
a
and
R
h
),
and GPP is calculated as the sum of NEE and
R
e
. NPP cannot be readily calculated
because it is very difficult to separate the autotrophic and heterotrophic components of
R
e
.
This method has some major advantages—its fast response allows for observation
of short-term physiological and meteorological controls on production, and it naturally
integrates over a substantial area (typically on the order of hectares) upwind of the tower.
It also allows direct measurement of NEP, which, if organic carbon losses from the system
are negligible, is a good estimate of the organic carbon accumulation rate in the ecosystem.
On the other hand, it is difficult to apply in areas where the terrain or the vegetation
canopy is uneven, and it does not measure NPP, which is very important in terrestrial
ecological studies.
REGULATION OF PRIMARY PRODUCTION
Primary production by photosynthesis obviously requires light, which attenuates rap-
idly with depth in water and from the top of the canopy to the ground on land. This limits
maximum light to upper waters of aquatic ecosystems and to terrestrial plant canopies.
Primary producers found in deeper waters and on the ground beneath canopies are
shaded and exhibit adaptations that enhance carbon fixation at low light intensities.
Primary production tends to increase with increasing light concentration up to a maxi-
mum (
Figure 2.3
) and can often be described by a saturating function (
Jassby and Platt
1976
). This relationship is useful for modeling primary production. The relationship can
be quantified by measuring primary production per unit biomass at different
light
intensities and fitting a two-parameter model
that
includes the initial slope of
the
