Common Structures and Properties of Seagrass Beds Fringing the Coasts of the World 201
muddy sand, the bottom may be covered by a fine film of algae, mainly
diatoms, and Cyanophyta. Under estuarine conditions, Vaucheria occasionally takes part in the algal cover of the substrate.
In the permanently submerged Zostera marina beds, many detached algae
are caught in the leaf canopy and become deposited on the bottom. Along the
Atlantic coast of France, these secondary algal growths may locally comprise
10 %-20 % of the total aboveground phytomass. When the environment is
enriched with nutrients, large blankets of Chlorophyta may develop in the
intertidal zone. These are not a part of the seagrass system, but may be a
threat to them (Den Hartog 1994).
A few endophytes have occasionally been found in seagrass tissue. Of more
importance are the fungi, particularly Labyrinthula and Plasmodiophora.
Each seagrass genus seems to have its specific Labyrinthula taxon (Vergeer
and Den Hartog 1994). The most well known is L. zosterae, which has been
indicated by many authors to be the cause of the so-called wasting disease
(Den Hartog 1987; Muehlstein et al. 1991). In our opinion, it is one of the first
decomposers of senescent seagrass tissue (Den Hartog 1996). The species of
Plasmodiophora form characteristic galls on some seagrass species, but are
never abundant. An Ascomycete, Lulworthia, has also been ascribed to be a
significant decomposer of Zostera leaf tissue.
A considerable number of sessile invertebrate species occur together with
epihytic algae on seagrass leaves. There are also many vagile ones in the
seagrass canopy, particularly grazing snails. Mining organisms have rarely
been encountered in seagrass beds. However, in some Posidonia species from
southwestern Australia, minute isopods of the genera Lyseia and Limnoria
mine within the leaf parenchyma (Brearley and Walker 1995, 1996). In the
rhizomes of the tropical seagrass Thalassia hemprichii, very small teredoid
molluscs have been observed.
Some species come to graze the seagrass itself. In the tropics sirenia and
green turtles graze the seagrasses and have a considerable impact on them.
Geese and ducks have a very heavy impact on the seagrass beds in the
temperate zone. The number of species that feed directly on seagrass is
limited. Most animals depend indirectly on seagrass, feeding on the accompanying epiphytes and small faunal organisms, which, in their turn, graze the
epiphytes, and the detritus produced by the joint community of seagrass,
algae, and fauna. There are two trophic pathways, viz., grazing and detrital
pathways. The latter is by far not only the most complex, but also the most
important one.
Seagrass beds are spatially positioned between the inshore mangroves and
marshes, and the offshore coral reefs and open sea. Thus, as well as a large
number of animals that are permanent inhabitants, many others find
temporary shelter and/or food within the leaf canopy at some stage of their
life cycle or some portion of the day. In the tropics, a number of fish and sea
muddy sand, the bottom may be covered by a fine film of algae, mainly
diatoms, and Cyanophyta. Under estuarine conditions, Vaucheria occasionally takes part in the algal cover of the substrate.
In the permanently submerged Zostera marina beds, many detached algae
are caught in the leaf canopy and become deposited on the bottom. Along the
Atlantic coast of France, these secondary algal growths may locally comprise
10 %-20 % of the total aboveground phytomass. When the environment is
enriched with nutrients, large blankets of Chlorophyta may develop in the
intertidal zone. These are not a part of the seagrass system, but may be a
threat to them (Den Hartog 1994).
A few endophytes have occasionally been found in seagrass tissue. Of more
importance are the fungi, particularly Labyrinthula and Plasmodiophora.
Each seagrass genus seems to have its specific Labyrinthula taxon (Vergeer
and Den Hartog 1994). The most well known is L. zosterae, which has been
indicated by many authors to be the cause of the so-called wasting disease
(Den Hartog 1987; Muehlstein et al. 1991). In our opinion, it is one of the first
decomposers of senescent seagrass tissue (Den Hartog 1996). The species of
Plasmodiophora form characteristic galls on some seagrass species, but are
never abundant. An Ascomycete, Lulworthia, has also been ascribed to be a
significant decomposer of Zostera leaf tissue.
A considerable number of sessile invertebrate species occur together with
epihytic algae on seagrass leaves. There are also many vagile ones in the
seagrass canopy, particularly grazing snails. Mining organisms have rarely
been encountered in seagrass beds. However, in some Posidonia species from
southwestern Australia, minute isopods of the genera Lyseia and Limnoria
mine within the leaf parenchyma (Brearley and Walker 1995, 1996). In the
rhizomes of the tropical seagrass Thalassia hemprichii, very small teredoid
molluscs have been observed.
Some species come to graze the seagrass itself. In the tropics sirenia and
green turtles graze the seagrasses and have a considerable impact on them.
Geese and ducks have a very heavy impact on the seagrass beds in the
temperate zone. The number of species that feed directly on seagrass is
limited. Most animals depend indirectly on seagrass, feeding on the accompanying epiphytes and small faunal organisms, which, in their turn, graze the
epiphytes, and the detritus produced by the joint community of seagrass,
algae, and fauna. There are two trophic pathways, viz., grazing and detrital
pathways. The latter is by far not only the most complex, but also the most
important one.
Seagrass beds are spatially positioned between the inshore mangroves and
marshes, and the offshore coral reefs and open sea. Thus, as well as a large
number of animals that are permanent inhabitants, many others find
temporary shelter and/or food within the leaf canopy at some stage of their
life cycle or some portion of the day. In the tropics, a number of fish and sea