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E. W. Koch, J. D. Ackerman, J. Verduin and M. van Keulen
seagrasses are exposed to extreme light levels in the
middle of the day and intermediate light levels in
the morning and afternoon (ebb or flood), i.e. these
plants will be exposed to long hours of saturating
light. In contrast, when high tides occur in the morning and afternoon (semi-diurnal tides), the light levels in the middle of the day may be saturating (ebb
or flood), but the number of hours of saturating light
will be reduced due to the high water during the remainder of the day (Koch and Beer, 1996). These
scenarios are also complicated by turbidity (Koch
and Beer, 1996).
G. Self-Shading in Seagrass Canopies
Exposed to Tides, Currents and Waves
Although seagrass leaves contain gas-filled lacunae
and tend to become erect in the water column, the
leaves will comply with the flow generated by tidal
currents and waves. During low tide, when seagrass
leaves in the intertidal area rest on top of each other,
self-shading is at its maximum. This process is reversed as the tide returns to its full level. Under
unidirectional flows, the degree of bending in the
direction of the flow is a function of the magnitude of the current (Fonseca et al., 1982). It follows
that self-shading is expected to be highest when currents are strongest and the leaves are collapsed onto
each other. Leaf flapping also occurs under wavedominated conditions (Koch and Gust, 1999), relieving some of the self-shading as flecks of light
penetrate the leaf mass or canopy at the frequency
of flapping.
Short-term (seconds) flecks of light referred to
as “lightflecks” (Fig. 9), are also generated in areas
exposed to waves. The light that reaches the water
surface is focused at the crest of the waves and dispersed at the trough of the waves (Wing and Patterson, 1993; Fig. 9). This results in “dancing lights” in
shallow areas such as seagrass beds due to the propagation of lightflecks. The frequency of lightflecks
resembles that of the passing waves (Wing and Patterson, 1993). Although the effect of lightflecks on
seagrass productivity was never tested, productivity
is likely to be enhanced as has been demonstrated in
phytoplankton and macroalgae (Dromgoole, 1988;
Greene and Gerard, 1990; Wing and Patterson, 1993;
Wing et al., 1993).
H. Seagrass Canopies as Depositional
Environments: Not a Universal Concept
Historical evidence of seagrass beds as depositional
environments is borne out of the loss of seagrasses
(due to wasting disease or grazing) resulting in
the erosion of sediments (Rasmussen, 1977; Hine
et al., 1987). Many authors suggested that seagrass
canopies are areas where sediments deposit and accumulate (Grady, 1981; Almasi et al., 1987; Patterson and Black, 1999; Gacia and Duarte, 2001),
largely due to the reduction of velocity and turbulence intensity (e.g. Fonseca et al., 1983; Ackerman and Okubo, 1993; Worcester, 1995; Verduin
and Backhaus, 2000; Granata et al., 2001), i.e. a
reduction in stress on the sediment surface due to
reduced flow speed within the canopy that leads to
a reduction in resuspension and thus, an increase in
accumulation (Lopez and Garcia, 1998). This accumulation can be seasonal, especially during summer when seagrasses are at their maximum density,
but in winter, when the plants disappear or decrease
in density, resuspension may be greater than deposition (van Keulen and Borowitzka, 2003). Often,
the presence of seagrass rhizomes and roots in the
sediment through the winter is sufficient to stabilize the sediments. The size of the seagrass species
also affects sediment accumulation. Although bigger species may be better for sediment deposition
(Fonseca and Fisher, 1986), small seagrasses such
as Halophila decipiens and Zostera novazelandica
can still alter the sediments they colonize (Fonseca,
1985, 1989; Heiss et al., 2000).
More recent efforts have focused on the role of
seagrass canopies in trapping and retaining sediments, which appears to be related to sediment contact with leaves (Gacia et al., 1999; Agawin and
Duarte, 2002). Epiphytic layers on seagrass leaves
may contribute to the entrapment of particles in seagrass beds by increasing the roughness of the canopy
and increasing the boundary layer on the leaf surface; i.e. expanding the area in which water flow is
reduced thereby facilitating the entrapment of particles (Vermaat et al., 2000). Whereas, seagrasses are
generally viewed as agents that trap particles and
stabilize the sediments, resuspension, especially of
fine sediments with high organic content, can occur
under high wave exposure and current flow (Fonseca
et al., 1983; Fonseca and Bell, 1998). In highly waveexposed sites where seagrasses do not attenuate
E. W. Koch, J. D. Ackerman, J. Verduin and M. van Keulen
seagrasses are exposed to extreme light levels in the
middle of the day and intermediate light levels in
the morning and afternoon (ebb or flood), i.e. these
plants will be exposed to long hours of saturating
light. In contrast, when high tides occur in the morning and afternoon (semi-diurnal tides), the light levels in the middle of the day may be saturating (ebb
or flood), but the number of hours of saturating light
will be reduced due to the high water during the remainder of the day (Koch and Beer, 1996). These
scenarios are also complicated by turbidity (Koch
and Beer, 1996).
G. Self-Shading in Seagrass Canopies
Exposed to Tides, Currents and Waves
Although seagrass leaves contain gas-filled lacunae
and tend to become erect in the water column, the
leaves will comply with the flow generated by tidal
currents and waves. During low tide, when seagrass
leaves in the intertidal area rest on top of each other,
self-shading is at its maximum. This process is reversed as the tide returns to its full level. Under
unidirectional flows, the degree of bending in the
direction of the flow is a function of the magnitude of the current (Fonseca et al., 1982). It follows
that self-shading is expected to be highest when currents are strongest and the leaves are collapsed onto
each other. Leaf flapping also occurs under wavedominated conditions (Koch and Gust, 1999), relieving some of the self-shading as flecks of light
penetrate the leaf mass or canopy at the frequency
of flapping.
Short-term (seconds) flecks of light referred to
as “lightflecks” (Fig. 9), are also generated in areas
exposed to waves. The light that reaches the water
surface is focused at the crest of the waves and dispersed at the trough of the waves (Wing and Patterson, 1993; Fig. 9). This results in “dancing lights” in
shallow areas such as seagrass beds due to the propagation of lightflecks. The frequency of lightflecks
resembles that of the passing waves (Wing and Patterson, 1993). Although the effect of lightflecks on
seagrass productivity was never tested, productivity
is likely to be enhanced as has been demonstrated in
phytoplankton and macroalgae (Dromgoole, 1988;
Greene and Gerard, 1990; Wing and Patterson, 1993;
Wing et al., 1993).
H. Seagrass Canopies as Depositional
Environments: Not a Universal Concept
Historical evidence of seagrass beds as depositional
environments is borne out of the loss of seagrasses
(due to wasting disease or grazing) resulting in
the erosion of sediments (Rasmussen, 1977; Hine
et al., 1987). Many authors suggested that seagrass
canopies are areas where sediments deposit and accumulate (Grady, 1981; Almasi et al., 1987; Patterson and Black, 1999; Gacia and Duarte, 2001),
largely due to the reduction of velocity and turbulence intensity (e.g. Fonseca et al., 1983; Ackerman and Okubo, 1993; Worcester, 1995; Verduin
and Backhaus, 2000; Granata et al., 2001), i.e. a
reduction in stress on the sediment surface due to
reduced flow speed within the canopy that leads to
a reduction in resuspension and thus, an increase in
accumulation (Lopez and Garcia, 1998). This accumulation can be seasonal, especially during summer when seagrasses are at their maximum density,
but in winter, when the plants disappear or decrease
in density, resuspension may be greater than deposition (van Keulen and Borowitzka, 2003). Often,
the presence of seagrass rhizomes and roots in the
sediment through the winter is sufficient to stabilize the sediments. The size of the seagrass species
also affects sediment accumulation. Although bigger species may be better for sediment deposition
(Fonseca and Fisher, 1986), small seagrasses such
as Halophila decipiens and Zostera novazelandica
can still alter the sediments they colonize (Fonseca,
1985, 1989; Heiss et al., 2000).
More recent efforts have focused on the role of
seagrass canopies in trapping and retaining sediments, which appears to be related to sediment contact with leaves (Gacia et al., 1999; Agawin and
Duarte, 2002). Epiphytic layers on seagrass leaves
may contribute to the entrapment of particles in seagrass beds by increasing the roughness of the canopy
and increasing the boundary layer on the leaf surface; i.e. expanding the area in which water flow is
reduced thereby facilitating the entrapment of particles (Vermaat et al., 2000). Whereas, seagrasses are
generally viewed as agents that trap particles and
stabilize the sediments, resuspension, especially of
fine sediments with high organic content, can occur
under high wave exposure and current flow (Fonseca
et al., 1983; Fonseca and Bell, 1998). In highly waveexposed sites where seagrasses do not attenuate