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R. C. Zimmerman
availability that controls seagrass production and
distribution before significant impacts occur on seagrass populations. This approach was tested as part
of a pilot study designed to develop a plan for monitoring submerged vegetation resources throughout
Puget Sound, Washington, USA (Berry et al., 2003).
The model was used to explore the maximum sustainable eelgrass density at Dumas Bay located in
the highly turbid southern region of Puget Sound.
Estimates of the submarine light environment in the
water column were obtained from calculations performed by the radiative transfer model Hydrolight
(Ver 4.2 Sequoia Scientific, Inc.) for local solar
noon on the spring equinox. Modeled water column Chl concentrations ranged from 20 to 50
mg m
−3 . Total suspended solids (TSS) concentrations ranged from 0 to 25 mg L
−1 . Model calculations of daily carbon balance produced well-behaved
second-order relationships between maximum sustainable eelgrass density and depth. These simple
relationships were then used to populate distribution maps of potential eelgrass density for different
water quality conditions. The resulting distributions
were qualitatively consistent with a field survey conducted at the site in 1995 by Norman et al. (1995)
(Fig. 9). Water column turbidity was identified as
a major factor determining eelgrass distributions in
Dumas Bay, and model predictions were more sensitive to variations in TSS than Chl. This finding is similar to the situation in other eastern Pacific estuaries
such as San Francisco Bay, where light availability is
more affected by sediment load than Chl (Alpine and
Cloern, 1988; Zimmerman et al., 1991, 1995). The
reliability of any numerical model is always limited
by the data used to parameterize the important driving variables. In this case, however, uncertainty in
shoot morphology, and shoot:root ratios in particular, represented a second-order problem with regard
to accurately modeling the eelgrass distribution.
X. Potential Impacts of Climate Change
on Seagrass Productivity
The dramatic response to CO 2 enrichment exhibited by a number of seagrass species indicates that
carbon limitation of photosynthesis may be a common feature of these submerged angiosperms (Beer
and Waisel, 1979; Millhouse and Strother, 1986; Durako, 1993; Abal et al., 1994; Invers et al., 2001).
Although the absolute kinetics of photosynthesis vs.
[CO 2 ] appear to be species specific (Invers et al.,
2001), the response generally involves a non-linear
increase in photosynthesis in response to increasing
[CO 2 ], which yields a negative response to pH (a
proxy for [CO 2 ] in seawater, Fig. 10). This effect can
be easily incorporated into the biophysical model by
defining P m as a function of pH{= f (− log[CO 2 ])}.
In the case of eelgrass, the increase in P max can be
described by:
P max = 82 exp(−0.53 · pH)
(22)
This relation sensitizes canopy productivity and
daily carbon balance calculated by Eqs. (18) to (21)
to CO 2 availability. Using a similar approach to that
described above for the analysis of water quality on
eelgrass distributions in Dumas Bay, the potential
impact of increased atmospheric [CO 2 ] on eelgrass
distribution was explored in Elkhorn Slough, California, USA, a moderately turbid estuary where eelgrass distributions are limited to less than 2 m depth
(Zimmerman et al., 1994; Zimmerman and Caffrey,
2002). Patchy meadows currently occupy about 17
ha of shallow habitat along margins of the main channel of the Slough, and these realized distributions are
consistent with model predictions of light-limited
distribution based on average water quality conditions and bathymetry (Fig. 11). Assuming present
day bathymetry and water quality, a doubled atmospheric CO 2 concentration would increase the areal
coverage of eelgrass in Elkhorn Slough to 23 ha, a
35% increase over the present distribution. In addition to increasing the density of existing beds, the
model predicts eelgrass to colonize the basin at the
top of Seal Bend, which is presently too deep for successful vegetation. Although atmospheric CO 2 concentrations are not expected to rise to a level that prevent carbon limitation of seagrass photosynthesis,
increasing the CO 2 concentration to the point where
seagrass photosynthetic performance is equivalent
to that of marine algae (pH 6.2) may yield a six-fold
increase in eelgrass coverage and greatly increase
the density of existing vegetation.
XI. Effects of Epiphytes
on Leaf Photosynthesis
Seagrass leaves are colonized by a diverse array of
epiphytes (Borowitzka et al., Chapter 19) that make
significant contributions to the overall productivity
of seagrass ecosystems (Penhale, 1977; Mazzella
R. C. Zimmerman
availability that controls seagrass production and
distribution before significant impacts occur on seagrass populations. This approach was tested as part
of a pilot study designed to develop a plan for monitoring submerged vegetation resources throughout
Puget Sound, Washington, USA (Berry et al., 2003).
The model was used to explore the maximum sustainable eelgrass density at Dumas Bay located in
the highly turbid southern region of Puget Sound.
Estimates of the submarine light environment in the
water column were obtained from calculations performed by the radiative transfer model Hydrolight
(Ver 4.2 Sequoia Scientific, Inc.) for local solar
noon on the spring equinox. Modeled water column Chl concentrations ranged from 20 to 50
mg m
−3 . Total suspended solids (TSS) concentrations ranged from 0 to 25 mg L
−1 . Model calculations of daily carbon balance produced well-behaved
second-order relationships between maximum sustainable eelgrass density and depth. These simple
relationships were then used to populate distribution maps of potential eelgrass density for different
water quality conditions. The resulting distributions
were qualitatively consistent with a field survey conducted at the site in 1995 by Norman et al. (1995)
(Fig. 9). Water column turbidity was identified as
a major factor determining eelgrass distributions in
Dumas Bay, and model predictions were more sensitive to variations in TSS than Chl. This finding is similar to the situation in other eastern Pacific estuaries
such as San Francisco Bay, where light availability is
more affected by sediment load than Chl (Alpine and
Cloern, 1988; Zimmerman et al., 1991, 1995). The
reliability of any numerical model is always limited
by the data used to parameterize the important driving variables. In this case, however, uncertainty in
shoot morphology, and shoot:root ratios in particular, represented a second-order problem with regard
to accurately modeling the eelgrass distribution.
X. Potential Impacts of Climate Change
on Seagrass Productivity
The dramatic response to CO 2 enrichment exhibited by a number of seagrass species indicates that
carbon limitation of photosynthesis may be a common feature of these submerged angiosperms (Beer
and Waisel, 1979; Millhouse and Strother, 1986; Durako, 1993; Abal et al., 1994; Invers et al., 2001).
Although the absolute kinetics of photosynthesis vs.
[CO 2 ] appear to be species specific (Invers et al.,
2001), the response generally involves a non-linear
increase in photosynthesis in response to increasing
[CO 2 ], which yields a negative response to pH (a
proxy for [CO 2 ] in seawater, Fig. 10). This effect can
be easily incorporated into the biophysical model by
defining P m as a function of pH{= f (− log[CO 2 ])}.
In the case of eelgrass, the increase in P max can be
described by:
P max = 82 exp(−0.53 · pH)
(22)
This relation sensitizes canopy productivity and
daily carbon balance calculated by Eqs. (18) to (21)
to CO 2 availability. Using a similar approach to that
described above for the analysis of water quality on
eelgrass distributions in Dumas Bay, the potential
impact of increased atmospheric [CO 2 ] on eelgrass
distribution was explored in Elkhorn Slough, California, USA, a moderately turbid estuary where eelgrass distributions are limited to less than 2 m depth
(Zimmerman et al., 1994; Zimmerman and Caffrey,
2002). Patchy meadows currently occupy about 17
ha of shallow habitat along margins of the main channel of the Slough, and these realized distributions are
consistent with model predictions of light-limited
distribution based on average water quality conditions and bathymetry (Fig. 11). Assuming present
day bathymetry and water quality, a doubled atmospheric CO 2 concentration would increase the areal
coverage of eelgrass in Elkhorn Slough to 23 ha, a
35% increase over the present distribution. In addition to increasing the density of existing beds, the
model predicts eelgrass to colonize the basin at the
top of Seal Bend, which is presently too deep for successful vegetation. Although atmospheric CO 2 concentrations are not expected to rise to a level that prevent carbon limitation of seagrass photosynthesis,
increasing the CO 2 concentration to the point where
seagrass photosynthetic performance is equivalent
to that of marine algae (pH 6.2) may yield a six-fold
increase in eelgrass coverage and greatly increase
the density of existing vegetation.
XI. Effects of Epiphytes
on Leaf Photosynthesis
Seagrass leaves are colonized by a diverse array of
epiphytes (Borowitzka et al., Chapter 19) that make
significant contributions to the overall productivity
of seagrass ecosystems (Penhale, 1977; Mazzella