446
14 Tides and Waves on Vegetated Coasts
Fig. 14.16a,b for a hypothetical circular reef with slope about 1:6, when waves
approach predominantly from one direction. Figure 14.16a shows the distribution of wave height at the water depth 6 m, generated by the most severe
tropical cyclone appearing with the return period t = 100 and 1000 years. On
the exposed side of the reef, wave height reaches over 5 m, while on the lee side
it is smaller then 1 m. Such wave height distribution causes a drastically different probability of survival of hard coral with a diameter D = 0.5 m, and 2.0 m
along the reef circumference. At the frontal part of the reef, these corals have
a very small chance of survival, while corals located at lee side of the reef have
a very high chance of further growth (see Fig. 14.16b). In a more complicated
situation, when the direction of incident cyclones change, the distribution of
probability of survival would be different.
14.5 Seaweeds and Seagrasses
14.5.1 Seaweeds
Seaweeds and seagrasses occur on many sheltered to fully exposed coasts, occupying rocky shores, coral reefs, and many other coastal habitats. They are the
major primary producers of coastal regions. Seaweeds are marine algae: structurally simple plants without roots, stems, or leaves, having primitive methods
of reproduction; some species are commonly found free-floating in many coastal
waters (Alongi, 1998). The rates of recruitment, growth, survival and vegetative regeneration of some microalgae, such as Sargassum, depend on a level of
sediment deposition (Umar et al., 1998).
Other seaweeds, such as kelp, thrive as forests in cool, clear waters in higher
latitudes and in subtropical areas associated with upwelling (Norway, California). Mature giant kelp plants (M acrocystis pyrifera) in Southern California
may have 100 or more fronds, and attain lengths of 15 to 50 m while attached
to the seafloor by holdfasts. The average density of typical kelp beds varies
from 6 to 12 plants per 100 m 2 (Elwany et al., 1996). Under favourable conditions, maximum rates of primary production of seaweeds can exceed rates of
production of all macrophytes, including mangroves and marsh grasses. Kelp
in wave exposed locations have long stipes and thick, narrow blades with heavily corrugated surfaces; in more sheltered areas, the stipes may be as long, but
laminar, wider and thinner (Hurd et al., 1996). Physical factors that affect kelp
growth and recruitment include water temperature, underwater light, nutrient
levels, and the concentration of suspended particles near the bottom. Therefore, the deep-water limit of kelp growth is approximately 20 m or less. The
inshore limit is mostly controlled by the largest breaking waves which cause
the holdfasts of kelps to break loose from the bottom (Seymour et al., 1989).
Other kelp species, Laminaria hyperborea, grow at several locations along
the Norwegian coast at water depths between 2 and 20 m. Usually the kelp
plant has 1-2 m height stipe, and average density of kelp forest is about 1015 plants per square metre. Fronds have the same length as the stipe. The
14 Tides and Waves on Vegetated Coasts
Fig. 14.16a,b for a hypothetical circular reef with slope about 1:6, when waves
approach predominantly from one direction. Figure 14.16a shows the distribution of wave height at the water depth 6 m, generated by the most severe
tropical cyclone appearing with the return period t = 100 and 1000 years. On
the exposed side of the reef, wave height reaches over 5 m, while on the lee side
it is smaller then 1 m. Such wave height distribution causes a drastically different probability of survival of hard coral with a diameter D = 0.5 m, and 2.0 m
along the reef circumference. At the frontal part of the reef, these corals have
a very small chance of survival, while corals located at lee side of the reef have
a very high chance of further growth (see Fig. 14.16b). In a more complicated
situation, when the direction of incident cyclones change, the distribution of
probability of survival would be different.
14.5 Seaweeds and Seagrasses
14.5.1 Seaweeds
Seaweeds and seagrasses occur on many sheltered to fully exposed coasts, occupying rocky shores, coral reefs, and many other coastal habitats. They are the
major primary producers of coastal regions. Seaweeds are marine algae: structurally simple plants without roots, stems, or leaves, having primitive methods
of reproduction; some species are commonly found free-floating in many coastal
waters (Alongi, 1998). The rates of recruitment, growth, survival and vegetative regeneration of some microalgae, such as Sargassum, depend on a level of
sediment deposition (Umar et al., 1998).
Other seaweeds, such as kelp, thrive as forests in cool, clear waters in higher
latitudes and in subtropical areas associated with upwelling (Norway, California). Mature giant kelp plants (M acrocystis pyrifera) in Southern California
may have 100 or more fronds, and attain lengths of 15 to 50 m while attached
to the seafloor by holdfasts. The average density of typical kelp beds varies
from 6 to 12 plants per 100 m 2 (Elwany et al., 1996). Under favourable conditions, maximum rates of primary production of seaweeds can exceed rates of
production of all macrophytes, including mangroves and marsh grasses. Kelp
in wave exposed locations have long stipes and thick, narrow blades with heavily corrugated surfaces; in more sheltered areas, the stipes may be as long, but
laminar, wider and thinner (Hurd et al., 1996). Physical factors that affect kelp
growth and recruitment include water temperature, underwater light, nutrient
levels, and the concentration of suspended particles near the bottom. Therefore, the deep-water limit of kelp growth is approximately 20 m or less. The
inshore limit is mostly controlled by the largest breaking waves which cause
the holdfasts of kelps to break loose from the bottom (Seymour et al., 1989).
Other kelp species, Laminaria hyperborea, grow at several locations along
the Norwegian coast at water depths between 2 and 20 m. Usually the kelp
plant has 1-2 m height stipe, and average density of kelp forest is about 1015 plants per square metre. Fronds have the same length as the stipe. The