Chapter 8 Fluid Dynamics in Seagrass Ecology
215
1998); (ii) depth restricted (when sufficient light is
available, seagrasses colonize areas below the maximum wave penetration depth; Krause-Jensen et al.,
2003; Middelboe et al., 2003); (iii) dominated by
more robust species (e.g. Amphibolis griffithii and
Posidonia coriacea); and (iv) patchier as the disturbance of high waves may hinder the lateral expansion of some seagrass beds (Kendrick et al.,
2000; Frederiksen et al., 2004). In contrast, in sheltered waters, seagrass meadows tend to be more
continuous and are colonized by relatively more
fragile species (e.g. some Posidonia spp) (Kirkman
and Kuo, 1990). Under calm conditions, creation of
openings in meadows appears not to lead to further
wide-scale loss, although regrowth into the damaged
areas can be slow (Walker et al., 1989; Meehan and
West, 2000; Orth et al., 2002). The degree of wave
exposure can be quantified by applying the relative
wave exposure index (REI) first developed by Keddy.
This index takes into account the wind direction and
intensity and fetch and has been successfully linked
to landscape features in seagrass habitats (Fonseca
and Bell, 1998; Fonseca et al., 2000, 2002; Hovel
et al., 2002; Krause-Jensen et al., 2003; Frederiksen
et al., 2004). When wave-dissipating structures (e.g.
sand bars, sills, coral or oyster reefs) occur in the
seagrass system, the bathymetry may also have to be
taken into account in order to properly estimate the
wave exposure using the REI.
A mixture of unvegetated and densely vegetated
areas in close proximity may characterize intermediate disturbance regimes. For example, Posidonia sinuosa meadows in habitats of moderate water flow are
characterized by dense rows of plants interchanged
with strips of bare sand (Bridgwood, 2002). A similar
gradient of patterns has been reported for meadows
of Zostera marina subject to gradients in velocity
and wave energy (Fig. 12), with the proposal that
the structural integrity of the habitat would deteriorate with increasing habitat fragmentation (den Hartog, 1971; Fonseca et al., 1983; Fonseca and Bell,
1998).
Another common cause for complex seagrass
mosaic patterns is the hydrodynamically-mediated
movement of sand waves and sand dunes through
seagrass meadows (Marb` a et al., 1994; Walker et al.,
1996; Bridgwood, 2002; Paling et al., 2003; van
Keulen and Borowitzka, 2003; Frederiksen et al.,
2004). Changes in sediment height, a result of water
flow, can be significant and rapid (10’s of cm over periods of hours) (Paling et al., 2003), and larger sand
dunes can travel through meadows on a time scale
of months (Walker et al., 1996; Bridgwood, 2002;
van Keulen and Borowitzka, 2003). The degree to
which these large amounts of sediment negatively
affect the seagrasses creating unvegetated patches
depends on their tolerance for sedimentation, the
amount of sediment deposited, and the period the
plants remain buried. Sedimentation rates of 2 to
13 cm year –1 can be coped with by large (e.g. Enhalus acoroides) as well as by fast growing (e.g.
Halophila) seagrasses as well as by plants with vertical stem elongation (e.g. Cymodocea nodosa, C.
serrulata and Syringodium isoetifolium) (Vermaat
et al., 1996). Subaqueous dune migration appears
to maintain Cymodocea meadows in a continuous
state of colonization which is ultimately responsible for the characteristic patchy landscape (Marb` a
and Duarte, 1995). Even when sediments completely
cover the leaves of small, slow-growing plants, some
seagrasses are able to survive as long as the sediment is removed by currents or waves in a matter of
weeks (Halodule wrightii survived after being buried
for 2 months; Phillips, 1980). Other seagrasses such
as Posidonia oceanica are able to increase vertical
growth from 5 to 7 mm year –1 to 52 mm year –1 when
(partially) covered by sand waves (Boudouresque
and de Grissac, 1983). In contrast, Zostera marina
seems to have little or no tolerance to sedimentation
regardless of the sediment type (Mills and Fonseca,
2003).
C. Effect of Patchiness on Flow
and Flow-Related Processes
When considering processes in seagrass meadows,
spatial homogeneity is usually assumed, but in nature, patchy seagrass landscapes are more common
than homogenous ones. This patchiness may have
a major effect on physical, geological, chemical,
and biological processes. For example, gaps in the
canopy allow an accelerated influx of water into
the canopy (Granata et al., 2001), replenishing nutrients, introducing spores and propagules, eroding
sediment and increasing mixing in general. The distance currents can penetrate the edge of a seagrass
bed before becoming equilibrated with respect to
momentum has been estimated to be between 1 m
(Fonseca and Fisher, 1986) and 50 boundary layers
(assumed to be equal to the height of the canopy;
Nowell and Jumars, 1984; Granata et al., 2001).
215
1998); (ii) depth restricted (when sufficient light is
available, seagrasses colonize areas below the maximum wave penetration depth; Krause-Jensen et al.,
2003; Middelboe et al., 2003); (iii) dominated by
more robust species (e.g. Amphibolis griffithii and
Posidonia coriacea); and (iv) patchier as the disturbance of high waves may hinder the lateral expansion of some seagrass beds (Kendrick et al.,
2000; Frederiksen et al., 2004). In contrast, in sheltered waters, seagrass meadows tend to be more
continuous and are colonized by relatively more
fragile species (e.g. some Posidonia spp) (Kirkman
and Kuo, 1990). Under calm conditions, creation of
openings in meadows appears not to lead to further
wide-scale loss, although regrowth into the damaged
areas can be slow (Walker et al., 1989; Meehan and
West, 2000; Orth et al., 2002). The degree of wave
exposure can be quantified by applying the relative
wave exposure index (REI) first developed by Keddy.
This index takes into account the wind direction and
intensity and fetch and has been successfully linked
to landscape features in seagrass habitats (Fonseca
and Bell, 1998; Fonseca et al., 2000, 2002; Hovel
et al., 2002; Krause-Jensen et al., 2003; Frederiksen
et al., 2004). When wave-dissipating structures (e.g.
sand bars, sills, coral or oyster reefs) occur in the
seagrass system, the bathymetry may also have to be
taken into account in order to properly estimate the
wave exposure using the REI.
A mixture of unvegetated and densely vegetated
areas in close proximity may characterize intermediate disturbance regimes. For example, Posidonia sinuosa meadows in habitats of moderate water flow are
characterized by dense rows of plants interchanged
with strips of bare sand (Bridgwood, 2002). A similar
gradient of patterns has been reported for meadows
of Zostera marina subject to gradients in velocity
and wave energy (Fig. 12), with the proposal that
the structural integrity of the habitat would deteriorate with increasing habitat fragmentation (den Hartog, 1971; Fonseca et al., 1983; Fonseca and Bell,
1998).
Another common cause for complex seagrass
mosaic patterns is the hydrodynamically-mediated
movement of sand waves and sand dunes through
seagrass meadows (Marb` a et al., 1994; Walker et al.,
1996; Bridgwood, 2002; Paling et al., 2003; van
Keulen and Borowitzka, 2003; Frederiksen et al.,
2004). Changes in sediment height, a result of water
flow, can be significant and rapid (10’s of cm over periods of hours) (Paling et al., 2003), and larger sand
dunes can travel through meadows on a time scale
of months (Walker et al., 1996; Bridgwood, 2002;
van Keulen and Borowitzka, 2003). The degree to
which these large amounts of sediment negatively
affect the seagrasses creating unvegetated patches
depends on their tolerance for sedimentation, the
amount of sediment deposited, and the period the
plants remain buried. Sedimentation rates of 2 to
13 cm year –1 can be coped with by large (e.g. Enhalus acoroides) as well as by fast growing (e.g.
Halophila) seagrasses as well as by plants with vertical stem elongation (e.g. Cymodocea nodosa, C.
serrulata and Syringodium isoetifolium) (Vermaat
et al., 1996). Subaqueous dune migration appears
to maintain Cymodocea meadows in a continuous
state of colonization which is ultimately responsible for the characteristic patchy landscape (Marb` a
and Duarte, 1995). Even when sediments completely
cover the leaves of small, slow-growing plants, some
seagrasses are able to survive as long as the sediment is removed by currents or waves in a matter of
weeks (Halodule wrightii survived after being buried
for 2 months; Phillips, 1980). Other seagrasses such
as Posidonia oceanica are able to increase vertical
growth from 5 to 7 mm year –1 to 52 mm year –1 when
(partially) covered by sand waves (Boudouresque
and de Grissac, 1983). In contrast, Zostera marina
seems to have little or no tolerance to sedimentation
regardless of the sediment type (Mills and Fonseca,
2003).
C. Effect of Patchiness on Flow
and Flow-Related Processes
When considering processes in seagrass meadows,
spatial homogeneity is usually assumed, but in nature, patchy seagrass landscapes are more common
than homogenous ones. This patchiness may have
a major effect on physical, geological, chemical,
and biological processes. For example, gaps in the
canopy allow an accelerated influx of water into
the canopy (Granata et al., 2001), replenishing nutrients, introducing spores and propagules, eroding
sediment and increasing mixing in general. The distance currents can penetrate the edge of a seagrass
bed before becoming equilibrated with respect to
momentum has been estimated to be between 1 m
(Fonseca and Fisher, 1986) and 50 boundary layers
(assumed to be equal to the height of the canopy;
Nowell and Jumars, 1984; Granata et al., 2001).