Chapter 8 Fluid Dynamics in Seagrass Ecology
217
quantity of detrital material which can remain within
the seagrass meadow or can be exported. The fate of
seagrass detritus depends, to a large extent, on the
magnitude of currents, waves, and tides (Ochieng
and Erftemeijer, 1999) and the nature of the leaves:
some leaves float on becoming detached while others sink (Zieman et al., 1979). Floating leaves are
more likely to be exported by tidal currents, but
leaves that sink and form detritus locally may also
be exported onto adjacent beaches or the deep sea
during storm events and/or spring tides (Hemminga
and Nieuwenhuize, 1990; Kirkman and Kendrick,
1997; Ochieng and Erftemeijer, 1999). It appears
that in many instances detritus remains within the
originating ecosystem, being recycled more or less
in situ (Hemminga and Nieuwenhuize, 1991; Paling, 1991). In other cases, large amounts of seagrass detritus are transported into adjacent estuaries contributing to the estuarine carbon cycle (Bach
et al., 1986; Cebrian and Duarte, 2001; Mateo et al.,
Chapter 7). Seagrass fragments have even been
found at great depths in ocean basins (>1,000 m)
where, it is postulated, they comprise an important
food source for a number of invertebrate detritivores (Menzies et al., 1967; Menzies and Rowe,
1969; Wolff, 1976, 1979; Suchanek et al., 1985),
as well as pelagic fishes and crustaceans (Williams
et al., 1987). Litter washed up onto beaches also supports a wide range of invertebrates (Kirkman and
Kendrick, 1997; Ochieng and Erftemeijer, 1999),
and the location where the litter is deposited (high
tide line or storm line) determines where invertebrates will find the highest availability of food. Litter that remains in shallow waters provides protection from erosion (Ochieng and Erftemeijer, 1999)
and a habitat for juvenile fish (Lenanton et al., 1982;
Robertson and Lennaton, 1984), but the reliability
of this habitat depends on the local hydrodynamic
conditions.
Tidal flows do not only link adjacent communities but can also isolate them. For example, in an
estuary in Kenya, organic particles efflux and reflux between mangroves and seagrasses during each
tidal cycle (Hemminga et al., 1994). During the ebb,
POM effluxes from the mangroves reaching the seagrasses; during the flood, particles resuspended in
the seagrass meadow (in part, particles generated
in the mangroves and deposited in the seagrasses)
reach the mangroves (Hemminga et al., 1994). These
particles never make it to the adjacent coral reef
due to trapping of the high turbidity plume by the
tide and onshore winds (Fig. 13; Kitheka, 1996;
Kitheka et al., 1996; Miyajima et al., 1998). Turbid waters could be detrimental to the reef-forming
coral polyps (Johannes, 1975). As a result of the
wave attenuation by the coral reef and the tidal isolation of the corals, mangroves, seagrasses and the
coral reef can co-exist just a few kilometers apart
along the Kenyan coast. This process is likely to also
apply to other reef–seagrass–mangrove associations
throughout the world.
2. Linking Seagrasses and Adjacent
Ecosystems via Wave Attenuation
Several plant and animal communities (mangroves,
marshes, corals, and oyster reefs) adjacent to seagrass meadows tend to attenuate waves, protecting
shorelines from erosion (Knutson et al., 1982; Knutson, 1988; Massel et al., 1999; M¨ oller et al., 1999).
The decrease in waves by mangroves and marshes
leads to sediment deposition (Othman, 1994) and,
consequently, reduced water turbidity. A mangrove
forest as narrow as 50–150 m can completely attenuate wave heights of up to 1 m (Othman, 1994), while
a marsh can attenuate more than 80% of the incoming wave energy (M¨ oller et al., 1999). Theoretically,
seagrasses could benefit from this wave reduction
especially during ebb flows as the water mass that
resides in the mangroves or marshes will have lower
turbidity. In turn, sub-tidal seagrass meadows adjacent to mangroves and marshes can minimize the
impact of waves on marshes and mangroves via wave
attenuation (van Katwijk, 2000).
Seagrasses also require relatively sheltered conditions in order to become established and thrive
(Fonseca and Bell, 1998; Robbins and Bell, 2000;
Koch, 2001), conditions that may have been previously found in the shelter of the once extensive
oyster reefs in Chesapeake Bay (USA) and of sand
bars in Tampa Bay (USA) (Lewis, 2002). Whereas
some wave attenuation is expected to be beneficial
for seagrass establishment, excessive wave attenuation may also be detrimental to seagrasses. When
wave energy is extremely low, sediments tend to be
relatively fine and to have elevated organic content
(Wanless, 1981; Almasi et al., 1987; Fonseca, 1996;
Fonseca and Bell, 1998), conditions which are not
always favorable to seagrasses (Koch, 2001).
217
quantity of detrital material which can remain within
the seagrass meadow or can be exported. The fate of
seagrass detritus depends, to a large extent, on the
magnitude of currents, waves, and tides (Ochieng
and Erftemeijer, 1999) and the nature of the leaves:
some leaves float on becoming detached while others sink (Zieman et al., 1979). Floating leaves are
more likely to be exported by tidal currents, but
leaves that sink and form detritus locally may also
be exported onto adjacent beaches or the deep sea
during storm events and/or spring tides (Hemminga
and Nieuwenhuize, 1990; Kirkman and Kendrick,
1997; Ochieng and Erftemeijer, 1999). It appears
that in many instances detritus remains within the
originating ecosystem, being recycled more or less
in situ (Hemminga and Nieuwenhuize, 1991; Paling, 1991). In other cases, large amounts of seagrass detritus are transported into adjacent estuaries contributing to the estuarine carbon cycle (Bach
et al., 1986; Cebrian and Duarte, 2001; Mateo et al.,
Chapter 7). Seagrass fragments have even been
found at great depths in ocean basins (>1,000 m)
where, it is postulated, they comprise an important
food source for a number of invertebrate detritivores (Menzies et al., 1967; Menzies and Rowe,
1969; Wolff, 1976, 1979; Suchanek et al., 1985),
as well as pelagic fishes and crustaceans (Williams
et al., 1987). Litter washed up onto beaches also supports a wide range of invertebrates (Kirkman and
Kendrick, 1997; Ochieng and Erftemeijer, 1999),
and the location where the litter is deposited (high
tide line or storm line) determines where invertebrates will find the highest availability of food. Litter that remains in shallow waters provides protection from erosion (Ochieng and Erftemeijer, 1999)
and a habitat for juvenile fish (Lenanton et al., 1982;
Robertson and Lennaton, 1984), but the reliability
of this habitat depends on the local hydrodynamic
conditions.
Tidal flows do not only link adjacent communities but can also isolate them. For example, in an
estuary in Kenya, organic particles efflux and reflux between mangroves and seagrasses during each
tidal cycle (Hemminga et al., 1994). During the ebb,
POM effluxes from the mangroves reaching the seagrasses; during the flood, particles resuspended in
the seagrass meadow (in part, particles generated
in the mangroves and deposited in the seagrasses)
reach the mangroves (Hemminga et al., 1994). These
particles never make it to the adjacent coral reef
due to trapping of the high turbidity plume by the
tide and onshore winds (Fig. 13; Kitheka, 1996;
Kitheka et al., 1996; Miyajima et al., 1998). Turbid waters could be detrimental to the reef-forming
coral polyps (Johannes, 1975). As a result of the
wave attenuation by the coral reef and the tidal isolation of the corals, mangroves, seagrasses and the
coral reef can co-exist just a few kilometers apart
along the Kenyan coast. This process is likely to also
apply to other reef–seagrass–mangrove associations
throughout the world.
2. Linking Seagrasses and Adjacent
Ecosystems via Wave Attenuation
Several plant and animal communities (mangroves,
marshes, corals, and oyster reefs) adjacent to seagrass meadows tend to attenuate waves, protecting
shorelines from erosion (Knutson et al., 1982; Knutson, 1988; Massel et al., 1999; M¨ oller et al., 1999).
The decrease in waves by mangroves and marshes
leads to sediment deposition (Othman, 1994) and,
consequently, reduced water turbidity. A mangrove
forest as narrow as 50–150 m can completely attenuate wave heights of up to 1 m (Othman, 1994), while
a marsh can attenuate more than 80% of the incoming wave energy (M¨ oller et al., 1999). Theoretically,
seagrasses could benefit from this wave reduction
especially during ebb flows as the water mass that
resides in the mangroves or marshes will have lower
turbidity. In turn, sub-tidal seagrass meadows adjacent to mangroves and marshes can minimize the
impact of waves on marshes and mangroves via wave
attenuation (van Katwijk, 2000).
Seagrasses also require relatively sheltered conditions in order to become established and thrive
(Fonseca and Bell, 1998; Robbins and Bell, 2000;
Koch, 2001), conditions that may have been previously found in the shelter of the once extensive
oyster reefs in Chesapeake Bay (USA) and of sand
bars in Tampa Bay (USA) (Lewis, 2002). Whereas
some wave attenuation is expected to be beneficial
for seagrass establishment, excessive wave attenuation may also be detrimental to seagrasses. When
wave energy is extremely low, sediments tend to be
relatively fine and to have elevated organic content
(Wanless, 1981; Almasi et al., 1987; Fonseca, 1996;
Fonseca and Bell, 1998), conditions which are not
always favorable to seagrasses (Koch, 2001).