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C. den Hartog and R.C. Phillips
urchins perform a diurnal migration from adjacent coral reefs to nocturnal
feeding grounds of the seagrass beds, and vice versa. There are also seasonal
visitors that shelter and feed during their migratory movements in the
seagrass beds. Finally, there are a great number of juvenile fish species which
migrate from the mangrove and marshes to the seagrass beds to find refuge
and food there before migrating to offshore habitats (Thayer et al. 1984).
The number of structural elements in the intertidal beds is smaller than in
the sublittoral counterparts. Some of these differences depend not only on the
degree of tolerance to desiccation, but also depend on salinity and temperature fluctuations. A more important part, however, is often played by the
dominant seagrass species itself, as its activity rhythm, and thus the temporal
development of the bed, determines to a high degree whether a species can
inhabit the seagrass bed permanently or not. In the case that species can only
live temporarily in the seagrass bed it is of importance that they can shift to
other communities where they meet favourable conditions. The richness of
species is thus largely dependent on the ecological quality of other habitats in
the immediate surroundings. It leads to the conclusion that large seagrass
beds may not be richer in species than small beds that form mosaics or that
border on other communities.
9.4 Seagrass Production
There are many data available on the functioning of seagrass beds, particularly of the performance of the dominant species. Most seagrass species form
vast dense meadows in shallow coastal areas. They are documented as
belonging to the richest and most productive ecosystems in the world,
rivalling in productivity cultivated tropical agricultural crops (Zieman and
Wetzel 1980). The physical stability and shelter supplied by the seagrass bed
structure provide the basis for a highly productive ecosystem (Wood et al.
1969; Fonseca et al. 1998). The ability of seagrasses to exert a major influence
on the marine seascape is due to a large extent on their rapid growth and high
net productivity (Tables 9.3,9.4).
The biomass of the leaves varies, depending on water depth, substrate,
nutrient availability, and season. Zieman (1975) reported that leaves of
Thalassia testudinum constituted 15 %-22 % of the total dry weight of the
plants, but this proportion could vary between 10 % and 45 %. Most of the
biomass is in the sediment and is often difficult to sample because of the
penetration of the roots (Zieman 1972,1975; Zieman and Wetzel 1980). Most
of the roots are found in the upper sediment layers. Zieman (1972) found in
south Florida that the roots of T. testudinum were able to penetrate to 4 m
depth in the sediment to the underlying bedrock. In annual populations, the
C. den Hartog and R.C. Phillips
urchins perform a diurnal migration from adjacent coral reefs to nocturnal
feeding grounds of the seagrass beds, and vice versa. There are also seasonal
visitors that shelter and feed during their migratory movements in the
seagrass beds. Finally, there are a great number of juvenile fish species which
migrate from the mangrove and marshes to the seagrass beds to find refuge
and food there before migrating to offshore habitats (Thayer et al. 1984).
The number of structural elements in the intertidal beds is smaller than in
the sublittoral counterparts. Some of these differences depend not only on the
degree of tolerance to desiccation, but also depend on salinity and temperature fluctuations. A more important part, however, is often played by the
dominant seagrass species itself, as its activity rhythm, and thus the temporal
development of the bed, determines to a high degree whether a species can
inhabit the seagrass bed permanently or not. In the case that species can only
live temporarily in the seagrass bed it is of importance that they can shift to
other communities where they meet favourable conditions. The richness of
species is thus largely dependent on the ecological quality of other habitats in
the immediate surroundings. It leads to the conclusion that large seagrass
beds may not be richer in species than small beds that form mosaics or that
border on other communities.
9.4 Seagrass Production
There are many data available on the functioning of seagrass beds, particularly of the performance of the dominant species. Most seagrass species form
vast dense meadows in shallow coastal areas. They are documented as
belonging to the richest and most productive ecosystems in the world,
rivalling in productivity cultivated tropical agricultural crops (Zieman and
Wetzel 1980). The physical stability and shelter supplied by the seagrass bed
structure provide the basis for a highly productive ecosystem (Wood et al.
1969; Fonseca et al. 1998). The ability of seagrasses to exert a major influence
on the marine seascape is due to a large extent on their rapid growth and high
net productivity (Tables 9.3,9.4).
The biomass of the leaves varies, depending on water depth, substrate,
nutrient availability, and season. Zieman (1975) reported that leaves of
Thalassia testudinum constituted 15 %-22 % of the total dry weight of the
plants, but this proportion could vary between 10 % and 45 %. Most of the
biomass is in the sediment and is often difficult to sample because of the
penetration of the roots (Zieman 1972,1975; Zieman and Wetzel 1980). Most
of the roots are found in the upper sediment layers. Zieman (1972) found in
south Florida that the roots of T. testudinum were able to penetrate to 4 m
depth in the sediment to the underlying bedrock. In annual populations, the