Chapter 8 Fluid Dynamics in Seagrass Ecology
199
of molecules through the δ D is no longer the limiting factor. Instead, the capacity of biological uptake sites or enzymes to assimilate molecules that
reached the plant surface becomes limiting (Koch,
1994). In this case, the conditions are said to be kinetically (and not physically) limiting. When velocities
are at intermediate levels, around U k , a combination
of physical (δ D ) and kinetic (enzymes) limitations
may influence the uptake of nutrients (Sanford and
Crawford, 2000). At velocities below 3–5 cm s –1
(U k ), photosynthesis (i.e. carbon uptake) in Thalassia testudinum and Cymodocea nodosa is δ D limited, whereas at velocities above U k , photosynthesis seems to be limited by the kinetics of Rubisco
(Koch, 1994). Interestingly, a similar U k value was
found for the kelp Macrocystis integrifolia (Stevens
and Hurd, 1997). In contrast, some seagrass studies
were unable to detect a kinetic limitation in the assimilation of nutrients in flowing water (i.e. no U k ),
instead, assimilation was δ D limited up to the maximum velocity tested: 20 cm s –1 for Thalassia testudinum and its epiphytes (Cornelisen and Thomas,
2002) and 34 cm s –1 for Zostera marina (Fonseca
and Kenworthy, 1987). This difference may be due in
part to experimental conditions. Specifically, studies
in which assimilation was only a function of velocity
were performed with entire plants rooted in sediment
and covered by epiphytes, while the experiments in
which assimilation was a function of velocity and enzyme kinetics were done with epiphyte-free leaves,
in the laboratory. For further discussion of the role
of diffusive boundary layers on photosynthesis, see
Larkum et al., Chapter 14.
Mass transfer to seagrass leaves does not only
depend on the velocity and δ D thickness but also
on: (1) the thickness of the periphyton layer (complex of debris, mucus, bacteria, algae, small animals,
and sediment particles) on the seagrass leaves (Jones
et al., 2000), (2) the reactions occurring within the
periphyton layer (Sand-Jensen et al., 1985; Jones
et al., 2000; Cornelisen and Thomas, 2002) and (3)
the concentration of the molecules in the water column adjacent to the seagrasses-periphyton complex
(Sanford and Crawford, 2000). The water interstitial to the periphyton is expected to be static (with
the exception of occasional sweep events; Nikora
et al., 2002); therefore, δ D increases linearly with
periphyton thickness (Jones et al., 2000). Consequently, the spatial scale for diffusion of molecules
from the water column to the leaf surface is longer
and δ D -limited conditions are more likely to occur.
The critical δ D thickness at U k has been estimated to
be 98 µm and 280 µm for periphyton-free leaves
of Cymodocea nodosa and Thalassia testudinum,
respectively (Koch, 1994), whereas the δ D on artificial leaves with periphyton was quantified to be
950 µm in thickness (Jones et al., 2000). The δ D
limitation of molecules such as nitrogen, phosphorous and carbon may be further exacerbated by
the reactions occurring within the periphyton layer.
Epiphytic algae tend to assimilate biologically important molecules before they reach the seagrass
surface (Jones et al., 2000; Sanford and Crawford,
2000; Cornelisen and Thomas, 2002), thereby competing for vital nutrients (Sand-Jensen et al., 1985).
If the uptake kinetics of epiphytes is more efficient
than that of seagrasses, the microalgae could potentially outcompete the seagrasses in the uptake
of nutrients (including carbon) from the water column (Sand-Jensen et al., 1985; Beer and Koch, 1996;
Cornelisen and Thomas, 2002). According to Fick’s
first law:
J = D
C w − C s
δ D
where J is the flux of molecules, C w the concentration in the water column, and C s the concentration on the seagrass surface. δ D limiting conditions
become less important as the concentration of nutrients (C w ) in the water column increases (i.e. eutrophication). Under such eutrophic conditions, uptake is controlled by the kinetics of periphyton and
seagrasses (Sanford and Crawford, 2000). As a result, one can hypothesize that as coastal waters become more eutrophic, mass transfer-limitations may
become less important to seagrasses, but this is a
complex process as the growth of the epiphytes as
a function of the nutrient concentration also needs
to be taken into account. Additionally, when uptake
rates are δ D limited, kinetic processes become less
important and the uptake rates become a function of
the planar area of seagrasses and epiphytes exposed
to water flow.
As indicated above, the Stanton number (St), a
dimensionless number, can also be used to quantify
the efficiency of a seagrass canopy to remove
nutrients from the water, as it is the flux of a
chemical to a surface divided by its advection past
the surface (e.g. Thomas et al., 2000). St can be
obtained via direct measurements of nutrient uptake
and velocity measurements, or can be calculated.
199
of molecules through the δ D is no longer the limiting factor. Instead, the capacity of biological uptake sites or enzymes to assimilate molecules that
reached the plant surface becomes limiting (Koch,
1994). In this case, the conditions are said to be kinetically (and not physically) limiting. When velocities
are at intermediate levels, around U k , a combination
of physical (δ D ) and kinetic (enzymes) limitations
may influence the uptake of nutrients (Sanford and
Crawford, 2000). At velocities below 3–5 cm s –1
(U k ), photosynthesis (i.e. carbon uptake) in Thalassia testudinum and Cymodocea nodosa is δ D limited, whereas at velocities above U k , photosynthesis seems to be limited by the kinetics of Rubisco
(Koch, 1994). Interestingly, a similar U k value was
found for the kelp Macrocystis integrifolia (Stevens
and Hurd, 1997). In contrast, some seagrass studies
were unable to detect a kinetic limitation in the assimilation of nutrients in flowing water (i.e. no U k ),
instead, assimilation was δ D limited up to the maximum velocity tested: 20 cm s –1 for Thalassia testudinum and its epiphytes (Cornelisen and Thomas,
2002) and 34 cm s –1 for Zostera marina (Fonseca
and Kenworthy, 1987). This difference may be due in
part to experimental conditions. Specifically, studies
in which assimilation was only a function of velocity
were performed with entire plants rooted in sediment
and covered by epiphytes, while the experiments in
which assimilation was a function of velocity and enzyme kinetics were done with epiphyte-free leaves,
in the laboratory. For further discussion of the role
of diffusive boundary layers on photosynthesis, see
Larkum et al., Chapter 14.
Mass transfer to seagrass leaves does not only
depend on the velocity and δ D thickness but also
on: (1) the thickness of the periphyton layer (complex of debris, mucus, bacteria, algae, small animals,
and sediment particles) on the seagrass leaves (Jones
et al., 2000), (2) the reactions occurring within the
periphyton layer (Sand-Jensen et al., 1985; Jones
et al., 2000; Cornelisen and Thomas, 2002) and (3)
the concentration of the molecules in the water column adjacent to the seagrasses-periphyton complex
(Sanford and Crawford, 2000). The water interstitial to the periphyton is expected to be static (with
the exception of occasional sweep events; Nikora
et al., 2002); therefore, δ D increases linearly with
periphyton thickness (Jones et al., 2000). Consequently, the spatial scale for diffusion of molecules
from the water column to the leaf surface is longer
and δ D -limited conditions are more likely to occur.
The critical δ D thickness at U k has been estimated to
be 98 µm and 280 µm for periphyton-free leaves
of Cymodocea nodosa and Thalassia testudinum,
respectively (Koch, 1994), whereas the δ D on artificial leaves with periphyton was quantified to be
950 µm in thickness (Jones et al., 2000). The δ D
limitation of molecules such as nitrogen, phosphorous and carbon may be further exacerbated by
the reactions occurring within the periphyton layer.
Epiphytic algae tend to assimilate biologically important molecules before they reach the seagrass
surface (Jones et al., 2000; Sanford and Crawford,
2000; Cornelisen and Thomas, 2002), thereby competing for vital nutrients (Sand-Jensen et al., 1985).
If the uptake kinetics of epiphytes is more efficient
than that of seagrasses, the microalgae could potentially outcompete the seagrasses in the uptake
of nutrients (including carbon) from the water column (Sand-Jensen et al., 1985; Beer and Koch, 1996;
Cornelisen and Thomas, 2002). According to Fick’s
first law:
J = D
C w − C s
δ D
where J is the flux of molecules, C w the concentration in the water column, and C s the concentration on the seagrass surface. δ D limiting conditions
become less important as the concentration of nutrients (C w ) in the water column increases (i.e. eutrophication). Under such eutrophic conditions, uptake is controlled by the kinetics of periphyton and
seagrasses (Sanford and Crawford, 2000). As a result, one can hypothesize that as coastal waters become more eutrophic, mass transfer-limitations may
become less important to seagrasses, but this is a
complex process as the growth of the epiphytes as
a function of the nutrient concentration also needs
to be taken into account. Additionally, when uptake
rates are δ D limited, kinetic processes become less
important and the uptake rates become a function of
the planar area of seagrasses and epiphytes exposed
to water flow.
As indicated above, the Stanton number (St), a
dimensionless number, can also be used to quantify
the efficiency of a seagrass canopy to remove
nutrients from the water, as it is the flux of a
chemical to a surface divided by its advection past
the surface (e.g. Thomas et al., 2000). St can be
obtained via direct measurements of nutrient uptake
and velocity measurements, or can be calculated.