430
B. I. van Tussenbroek, J. A. Vonk, J. Stapel et al.
annual basis, T. gratilla consumes about 24% of total seagrass production. Experimental reduction of
the abundance of Echinothrix diadema sea urchins
in Thalassia hemprichii meadows in a Kenyan coral
reef lagoon protected from fishing led to increased
bite rates on herbivory assays by parrotfishes, implying that sea urchins can reduce grazing rates of
some species of parrotfishes (McClanahan et al.,
1994).
Amongst the most important values of Thalassia-dominated seagrass meadows are their effects on
the fauna associated with these meadows. These seagrass meadows serve as a nursery ground and habitat
for fishes, crustaceans, echinoderms, and molluscs
of economic importance, thereby sustaining local
fishing economies and their food supply. They are
also rich in biodiversity (Arrivillaga and Baltz, 1999;
Prieto et al., 2000; Gell and Whittington, 2002) and
function as critical habitat and feeding grounds for
endangered species, such as the green turtle (Greenway, 1976; Williams, 1988), West Indian manatee
(Marshall et al., 2000) and dugong (de Iongh et al.,
1995; Preen, 1995), thus qualifying as priority areas
for conservation.
IX. Conclusions
During the last decade, the rapid increase in seagrass studies (see section 1.3) has continued, as is
indicated by the multitude of recent references cited
in the present work. But it is striking that most
research on tropical seagrasses, and Thalassia in
particular, has been mainly realized in first world
territories or conducted by first-world visiting scientists to tropical territories. There exists a major
need to involve scientists from tropical Western Atlantic and the Indo-Pacific in basic seagrass research.
This need is even more urgent in light of the decline of many tropical seagrass communities due to
rapid increases in exploitation and urbanization of
the many tropical coasts, particularly in South-East
Asia and the Caribbean (Green and Short, 2003).
The huge extent of Thalassia beds may give a false
impression of community resistance to the rapidly
changing conditions in these coastal areas. Yet our
knowledge concerning the foundations of these communities in still insufficient, and basic research into
the processes and interactions, which permit seagrasses to cover huge coastal areas, are essential for
conservation.
Eutrophication of coastal waters caused by human development may locally be an important nutrient input in Thalassia meadows (Touchette and
Burkholder, 2000), and it is likely that humaninduced eutrophication poses a serious threat to
Thalassia meadows (Wear et al., 1999). T. testudinum biomass and productivity are negatively
correlated with watershed nitrogen loads (Tomasko
and Lapointe, 1991; Tomasko et al., 1996), which
cause a decline in both the percentage cover and
aboveground biomass (McGlathery, 1995). In enriched beds, primary production rates, biomass, and
chlorophyll-a content of epiphytes were significantly
greater than in control beds (Wear et al., 1999),
resulting in reduction of available light (Peterson
and Heck, 2001), thereby negatively affecting the
seagrasses, which are vulnerable to light reduction
(Hemminga, 1998) and smothering by the epiphytes
(Orth and van Montfrans, 1984). Human-induced
nutrient addition may also result in changes in the
species composition of the meadows (e.g. H. wrightii
replaced T. testudinum under high-nutrient conditions: see section 6.2). For plant communities in
general, diversity tends to diminish under conditions when growth rates of all competitors are high
(Huston, 1979), and knowledge concerning the interactions and competitive displacement capacities of
the seagrasses and rooted macroalgae in relatively
diverse Thalassia meadows in oligotrophic systems
are required to timely detect critical changes in these
communities.
Colonization and population maintenance of Thalassia occur almost exclusively through vegetative
spread and branching of the rhizomes. Thus, knowledge of the development and organization of the
vegetative structure forms the basis for further understanding how these species cover thousands of
square miles of the sea bottom. Yet, since the studies of Tomlinson (see section 4.1), virtually no research has been conducted on the vegetative anatomy
and development of Thalassia, despite the fact that
Tomlinson (1974) himself stressed that many of his
observations were based on superficial examination,
and that his work was preliminary and incomplete.
Studies into the demography of shoots have been
a useful tool in understanding some aspects of the
dynamics of the shoot (ramet) populations, but information concerning dynamics of the genets such
as their vegetative extension and dispersal through
sexual propagules is scarce. Also our knowledge on
the processes of clonal growth (clonal diversity, sizes
B. I. van Tussenbroek, J. A. Vonk, J. Stapel et al.
annual basis, T. gratilla consumes about 24% of total seagrass production. Experimental reduction of
the abundance of Echinothrix diadema sea urchins
in Thalassia hemprichii meadows in a Kenyan coral
reef lagoon protected from fishing led to increased
bite rates on herbivory assays by parrotfishes, implying that sea urchins can reduce grazing rates of
some species of parrotfishes (McClanahan et al.,
1994).
Amongst the most important values of Thalassia-dominated seagrass meadows are their effects on
the fauna associated with these meadows. These seagrass meadows serve as a nursery ground and habitat
for fishes, crustaceans, echinoderms, and molluscs
of economic importance, thereby sustaining local
fishing economies and their food supply. They are
also rich in biodiversity (Arrivillaga and Baltz, 1999;
Prieto et al., 2000; Gell and Whittington, 2002) and
function as critical habitat and feeding grounds for
endangered species, such as the green turtle (Greenway, 1976; Williams, 1988), West Indian manatee
(Marshall et al., 2000) and dugong (de Iongh et al.,
1995; Preen, 1995), thus qualifying as priority areas
for conservation.
IX. Conclusions
During the last decade, the rapid increase in seagrass studies (see section 1.3) has continued, as is
indicated by the multitude of recent references cited
in the present work. But it is striking that most
research on tropical seagrasses, and Thalassia in
particular, has been mainly realized in first world
territories or conducted by first-world visiting scientists to tropical territories. There exists a major
need to involve scientists from tropical Western Atlantic and the Indo-Pacific in basic seagrass research.
This need is even more urgent in light of the decline of many tropical seagrass communities due to
rapid increases in exploitation and urbanization of
the many tropical coasts, particularly in South-East
Asia and the Caribbean (Green and Short, 2003).
The huge extent of Thalassia beds may give a false
impression of community resistance to the rapidly
changing conditions in these coastal areas. Yet our
knowledge concerning the foundations of these communities in still insufficient, and basic research into
the processes and interactions, which permit seagrasses to cover huge coastal areas, are essential for
conservation.
Eutrophication of coastal waters caused by human development may locally be an important nutrient input in Thalassia meadows (Touchette and
Burkholder, 2000), and it is likely that humaninduced eutrophication poses a serious threat to
Thalassia meadows (Wear et al., 1999). T. testudinum biomass and productivity are negatively
correlated with watershed nitrogen loads (Tomasko
and Lapointe, 1991; Tomasko et al., 1996), which
cause a decline in both the percentage cover and
aboveground biomass (McGlathery, 1995). In enriched beds, primary production rates, biomass, and
chlorophyll-a content of epiphytes were significantly
greater than in control beds (Wear et al., 1999),
resulting in reduction of available light (Peterson
and Heck, 2001), thereby negatively affecting the
seagrasses, which are vulnerable to light reduction
(Hemminga, 1998) and smothering by the epiphytes
(Orth and van Montfrans, 1984). Human-induced
nutrient addition may also result in changes in the
species composition of the meadows (e.g. H. wrightii
replaced T. testudinum under high-nutrient conditions: see section 6.2). For plant communities in
general, diversity tends to diminish under conditions when growth rates of all competitors are high
(Huston, 1979), and knowledge concerning the interactions and competitive displacement capacities of
the seagrasses and rooted macroalgae in relatively
diverse Thalassia meadows in oligotrophic systems
are required to timely detect critical changes in these
communities.
Colonization and population maintenance of Thalassia occur almost exclusively through vegetative
spread and branching of the rhizomes. Thus, knowledge of the development and organization of the
vegetative structure forms the basis for further understanding how these species cover thousands of
square miles of the sea bottom. Yet, since the studies of Tomlinson (see section 4.1), virtually no research has been conducted on the vegetative anatomy
and development of Thalassia, despite the fact that
Tomlinson (1974) himself stressed that many of his
observations were based on superficial examination,
and that his work was preliminary and incomplete.
Studies into the demography of shoots have been
a useful tool in understanding some aspects of the
dynamics of the shoot (ramet) populations, but information concerning dynamics of the genets such
as their vegetative extension and dispersal through
sexual propagules is scarce. Also our knowledge on
the processes of clonal growth (clonal diversity, sizes