216
E. W. Koch, J. D. Ackerman, J. Verduin and M. van Keulen
These values can be affected by the density of the
bed and the vertical distribution of biomass. In dense
beds, unidirectional water flow is smoothly directed
over the top of the seagrass canopy as “skimming
flow” (Nowell and Jumars, 1984; Fonseca and Kenworthy, 1987), effectively trapping a layer of water within the canopy (Koch and Gust, 1999), i.e.
increasing the residence time. Under such conditions, nutrient concentrations within the vegetation
may be quite low (Moore et al., 1996). In contrast, a
reduction in shoot density leads to increased flow intrusion and velocity within the canopy (van Keulen,
1997). Therefore, reducing seagrass density could
permit an increase in turbulence and mixing within
the meadow, with an associated increase in nutrient
exchange and uptake, and the potential for increased
sediment resuspension. This subject needs further
attention.
VII. Hydrodynamic Processes at the
Meso-Scale Level (>1,000 m)
A. Seagrasses in the Biosphere
Seagrass meadows are one of several plant communities found in coastal areas around the world.
Marshes and mangroves line the intertidal area of
shorelines of seagrass-colonized temperate and tropical systems, respectively. The hydrodynamic functions of each of these communities can be quite
similar: attenuation of currents and waves leading
to deposition of particles and the stabilization of
the substrate (Knutson et al., 1982; Knutson, 1988;
Massel et al., 1999; M¨ oller et al., 1999). When considering these communities as a part of a larger
coastal ecosystem, interactions between seagrasses
and marshes/mangroves as well as with adjacent animal communities such as oyster and coral reefs start
to emerge (see Section VII.B).
Water masses may have traveled over extensive
distances, interacting with pelagic and benthic organisms before they reach seagrass beds. For example, during the flood tide, water masses may travel
over sediments colonized by a variety of microalgae and benthic organisms such as a coral polyps
and reefs before reaching the seagrass bed. In contrast, during ebb flow, the water that reaches seagrass
beds may have resided in an estuary, a mangrove, or a
marsh system for a period of time. Each of these plant
and animal communities tends to alter the water mass
in its chemical and/or physical properties directly or
indirectly (Bulthuis et al., 1984). When these waters are then transported into seagrasses meadows,
they also affect these plant communities. Therefore,
a link between seagrasses and adjacent plant and animal communities is expected.
B. Linking Seagrasses and Adjacent
Communities via Water Flow
1. Linking Seagrasses and Adjacent Systems
via Tidal Fluxes
Tidal flows link terrestrial, estuarine, and marine
systems. The residence time of a water mass in
a seagrass habitat (determined by the tidal fluxes)
may have a profound effect on seagrass distribution.
Short residence times (days) allow pollutants and
excess nutrients to be flushed out of a system before harming seagrasses (Kitheka et al., 1996). In
contrast, long residence times promote the accumulation of nutrients and the growth of phytoplankton
and nuisance algae while suppressing the growth of
seagrasses via low light availability (Rysgaard et al.,
1996; Herbert, 1999).
Coral reefs, seagrass beds, and mangrove forests
often co-occur in tropical coastal systems suggesting an interaction of sorts, determined by tidal flows.
For example, coastal wetlands such as marshes and
mangroves assimilate nutrients leaching from land
and thereby reduce the nutrient level reaching adjacent seagrass systems (Valiela and Cole, 2002) during ebb flows. These authors suggested that landderived N loads from 20 to 1,000 kg N ha
−1 year –1
seem to be a critical range for seagrass survival
in shallow waters. When N loads are higher, wetlands are no longer able to remove sufficient N
through denitrification, and N burial, and tidal currents will carry the excess nutrients into the seagrass beds. Excess nutrients can then lead to the
loss of the seagrasses. Therefore, adjoining plant
systems (wetlands and seagrasses) are not isolated
units but are likely to be linked (Valiela and Cole,
2002).
Seagrass beds are among the most productive systems on the planet (Dring, 1994) and experience relatively low grazing losses with most leaf production being shed (Cebrian and Duarte, 2001; Mateo
et al., Chapter 7). This amounts to a considerable
E. W. Koch, J. D. Ackerman, J. Verduin and M. van Keulen
These values can be affected by the density of the
bed and the vertical distribution of biomass. In dense
beds, unidirectional water flow is smoothly directed
over the top of the seagrass canopy as “skimming
flow” (Nowell and Jumars, 1984; Fonseca and Kenworthy, 1987), effectively trapping a layer of water within the canopy (Koch and Gust, 1999), i.e.
increasing the residence time. Under such conditions, nutrient concentrations within the vegetation
may be quite low (Moore et al., 1996). In contrast, a
reduction in shoot density leads to increased flow intrusion and velocity within the canopy (van Keulen,
1997). Therefore, reducing seagrass density could
permit an increase in turbulence and mixing within
the meadow, with an associated increase in nutrient
exchange and uptake, and the potential for increased
sediment resuspension. This subject needs further
attention.
VII. Hydrodynamic Processes at the
Meso-Scale Level (>1,000 m)
A. Seagrasses in the Biosphere
Seagrass meadows are one of several plant communities found in coastal areas around the world.
Marshes and mangroves line the intertidal area of
shorelines of seagrass-colonized temperate and tropical systems, respectively. The hydrodynamic functions of each of these communities can be quite
similar: attenuation of currents and waves leading
to deposition of particles and the stabilization of
the substrate (Knutson et al., 1982; Knutson, 1988;
Massel et al., 1999; M¨ oller et al., 1999). When considering these communities as a part of a larger
coastal ecosystem, interactions between seagrasses
and marshes/mangroves as well as with adjacent animal communities such as oyster and coral reefs start
to emerge (see Section VII.B).
Water masses may have traveled over extensive
distances, interacting with pelagic and benthic organisms before they reach seagrass beds. For example, during the flood tide, water masses may travel
over sediments colonized by a variety of microalgae and benthic organisms such as a coral polyps
and reefs before reaching the seagrass bed. In contrast, during ebb flow, the water that reaches seagrass
beds may have resided in an estuary, a mangrove, or a
marsh system for a period of time. Each of these plant
and animal communities tends to alter the water mass
in its chemical and/or physical properties directly or
indirectly (Bulthuis et al., 1984). When these waters are then transported into seagrasses meadows,
they also affect these plant communities. Therefore,
a link between seagrasses and adjacent plant and animal communities is expected.
B. Linking Seagrasses and Adjacent
Communities via Water Flow
1. Linking Seagrasses and Adjacent Systems
via Tidal Fluxes
Tidal flows link terrestrial, estuarine, and marine
systems. The residence time of a water mass in
a seagrass habitat (determined by the tidal fluxes)
may have a profound effect on seagrass distribution.
Short residence times (days) allow pollutants and
excess nutrients to be flushed out of a system before harming seagrasses (Kitheka et al., 1996). In
contrast, long residence times promote the accumulation of nutrients and the growth of phytoplankton
and nuisance algae while suppressing the growth of
seagrasses via low light availability (Rysgaard et al.,
1996; Herbert, 1999).
Coral reefs, seagrass beds, and mangrove forests
often co-occur in tropical coastal systems suggesting an interaction of sorts, determined by tidal flows.
For example, coastal wetlands such as marshes and
mangroves assimilate nutrients leaching from land
and thereby reduce the nutrient level reaching adjacent seagrass systems (Valiela and Cole, 2002) during ebb flows. These authors suggested that landderived N loads from 20 to 1,000 kg N ha
−1 year –1
seem to be a critical range for seagrass survival
in shallow waters. When N loads are higher, wetlands are no longer able to remove sufficient N
through denitrification, and N burial, and tidal currents will carry the excess nutrients into the seagrass beds. Excess nutrients can then lead to the
loss of the seagrasses. Therefore, adjoining plant
systems (wetlands and seagrasses) are not isolated
units but are likely to be linked (Valiela and Cole,
2002).
Seagrass beds are among the most productive systems on the planet (Dring, 1994) and experience relatively low grazing losses with most leaf production being shed (Cebrian and Duarte, 2001; Mateo
et al., Chapter 7). This amounts to a considerable