Chapter 13 Light and Photosynthesis in Seagrass Meadows
313
optical properties, along leaf axes and across leaves
of different age.
Knowledge of the instantaneous photosynthesis
rate in layer (z) allows us to calculate whole canopy
production (P c ) by summation of P(z) over all layers
(z):
P c =
z
P(z)
(19)
Daily integrated production of the canopy can be
calculated from iterative solution of the radiative
transfer and photosynthesis equations if time series
of top-of-canopy irradiance [E d (λ,0)] are available.
Alternatively, the daily integrated production can
be approximated as follows if the daily variation in
[E d (λ,0)] is sinusoidal:
P d = T
z
l(z)P max
×
1 − exp
−
0.67 · φ p · PU R(z)
P max
(20)
Here, T represents the length of the daily photoperiod and 0.67 is an empirically determined constant
(Zimmerman et al., 1996). The use of Eq. (20) requires PUR(z) to be the photosynthetically used radiation at local solar noon. The resulting photosynthesis rates can be used to determine whole plant carbon
balance by normalizing P d to the daily respiratory
demand:
Daily P : =
P d
( Leaf + + Root + + Rhizome )
(21)
The ratio of Daily P: provides a convenient index
of whole plant or canopy production. Carbon accumulates and growth is possible under light-replete
conditions that produce DailyP: > 1. Conversely,
the canopy is light limited if the DailyP: < 1.
Growth and survival under light limitation require
mobilization of stored internal reserves, reducing the
total carbon density of individual shoots and the seagrass meadow. If internal reserves are insufficient to
provide for growth and survival, shoots will die and
the meadow will thin.
The daily respiratory carbon demand, defined by
( Leaf + + Root + + Rhizome ), accounts for metabolic
consumption by both above- and below-ground
tissues. Leaf respiration is presumably constant
throughout the day, but below-ground tissues are
subjected to prolonged anoxia each night, since the
oxygen supporting below-ground metabolism is derived from leaf photosynthesis (Smith et al., 1984).
The typical metabolic response to anoxia is characterized by the Pasteur effect, in which the rate of
metabolic carbon consumption increases to provide
the required adenylate reducing power needed to sustain living cells. The anaerobic rate of metabolic
carbon consumption by roots of eelgrass, however,
slows to about 65% of the aerobic rate (Smith
et al., 1984, 1988; Smith, 1989). This phenomenon
probably occurs in other seagrass species as well,
but has not been investigated extensively. The resulting nighttime reduction in carbon demand of
below-ground tissues derived from this reverse Pasteur effect can be incorporated into the daily carbon budget by assuming that the daily period of
below-ground aerobiosis corresponds to the daily
period of irradiance-saturated photosynthesis (Zimmerman and Alberte, 1996; Zimmerman et al., 1994,
1997). The value of ( Leaf + + Root + + Rhizome ) is
also affected by the ratio of shoot:root biomass.
Typical shoot:root ratios in eelgrass range from 4
to 6, but metabolic activity of subterranean tissue decreases with distance from the meristem,
(Kraemer and Alberte, 1993). When all these factors are considered, below-ground metabolism of
eelgrass generates a daily carbon demand equivalent to 1 to 2 hours of irradiance-saturated photosynthesis. Below-ground carbon demand will be
larger in species such as turtlegrass, in which more
biomass, and therefore respiratory demand, is allocated to below-ground tissue (Fourqurean and
Zieman, 1991).
VIII. Leaf Orientation, Canopy Density,
and Self-Shading
The two-flow approximation outlined here provides a mechanistic link between photosynthesisirradiance responses of individual leaves and
productivity of seagrass canopies in natural light environments by defining the specific geometric and
optical relationships between the intact seagrass
canopy and the submarine light field. When the
geometric corrections defined by l p and ¯
µ are not
applied, P vs. E response curves produce photosynthetically saturating irradiances two to four times
higher than those reported by using a collimated light
source oriented perpendicular to the leaf surface,
313
optical properties, along leaf axes and across leaves
of different age.
Knowledge of the instantaneous photosynthesis
rate in layer (z) allows us to calculate whole canopy
production (P c ) by summation of P(z) over all layers
(z):
P c =
z
P(z)
(19)
Daily integrated production of the canopy can be
calculated from iterative solution of the radiative
transfer and photosynthesis equations if time series
of top-of-canopy irradiance [E d (λ,0)] are available.
Alternatively, the daily integrated production can
be approximated as follows if the daily variation in
[E d (λ,0)] is sinusoidal:
P d = T
z
l(z)P max
×
1 − exp
−
0.67 · φ p · PU R(z)
P max
(20)
Here, T represents the length of the daily photoperiod and 0.67 is an empirically determined constant
(Zimmerman et al., 1996). The use of Eq. (20) requires PUR(z) to be the photosynthetically used radiation at local solar noon. The resulting photosynthesis rates can be used to determine whole plant carbon
balance by normalizing P d to the daily respiratory
demand:
Daily P : =
P d
( Leaf + + Root + + Rhizome )
(21)
The ratio of Daily P: provides a convenient index
of whole plant or canopy production. Carbon accumulates and growth is possible under light-replete
conditions that produce DailyP: > 1. Conversely,
the canopy is light limited if the DailyP: < 1.
Growth and survival under light limitation require
mobilization of stored internal reserves, reducing the
total carbon density of individual shoots and the seagrass meadow. If internal reserves are insufficient to
provide for growth and survival, shoots will die and
the meadow will thin.
The daily respiratory carbon demand, defined by
( Leaf + + Root + + Rhizome ), accounts for metabolic
consumption by both above- and below-ground
tissues. Leaf respiration is presumably constant
throughout the day, but below-ground tissues are
subjected to prolonged anoxia each night, since the
oxygen supporting below-ground metabolism is derived from leaf photosynthesis (Smith et al., 1984).
The typical metabolic response to anoxia is characterized by the Pasteur effect, in which the rate of
metabolic carbon consumption increases to provide
the required adenylate reducing power needed to sustain living cells. The anaerobic rate of metabolic
carbon consumption by roots of eelgrass, however,
slows to about 65% of the aerobic rate (Smith
et al., 1984, 1988; Smith, 1989). This phenomenon
probably occurs in other seagrass species as well,
but has not been investigated extensively. The resulting nighttime reduction in carbon demand of
below-ground tissues derived from this reverse Pasteur effect can be incorporated into the daily carbon budget by assuming that the daily period of
below-ground aerobiosis corresponds to the daily
period of irradiance-saturated photosynthesis (Zimmerman and Alberte, 1996; Zimmerman et al., 1994,
1997). The value of ( Leaf + + Root + + Rhizome ) is
also affected by the ratio of shoot:root biomass.
Typical shoot:root ratios in eelgrass range from 4
to 6, but metabolic activity of subterranean tissue decreases with distance from the meristem,
(Kraemer and Alberte, 1993). When all these factors are considered, below-ground metabolism of
eelgrass generates a daily carbon demand equivalent to 1 to 2 hours of irradiance-saturated photosynthesis. Below-ground carbon demand will be
larger in species such as turtlegrass, in which more
biomass, and therefore respiratory demand, is allocated to below-ground tissue (Fourqurean and
Zieman, 1991).
VIII. Leaf Orientation, Canopy Density,
and Self-Shading
The two-flow approximation outlined here provides a mechanistic link between photosynthesisirradiance responses of individual leaves and
productivity of seagrass canopies in natural light environments by defining the specific geometric and
optical relationships between the intact seagrass
canopy and the submarine light field. When the
geometric corrections defined by l p and ¯
µ are not
applied, P vs. E response curves produce photosynthetically saturating irradiances two to four times
higher than those reported by using a collimated light
source oriented perpendicular to the leaf surface,