Algae
in the coral reef environment
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The nature of the
substratum for algae varies from shifting sand or gravel
to compact basaltic or calcareous rocks.
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Hard substrata type
landscape
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Soft substrata type
landscape
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Algae in the coral reef environment
Coral
reef environments are the favoured areas for benthic life, even if
the underwater vegetation is not very exuberant likely to go
unnoticed by the naturalist unfamiliar with these habitat. All the
biological groups have discovered a possibility of adaptation, and
algae in particular.
Competition
for space with other attached organisms is one of the main factors
controlling growth of the marine flora.
The
other remarkable factor in the control of growth in some plant
communities is grazing by herbivores, fish and invertebrates (molluscs
and echinoderms). In coral reef communities, herbivorous fish
account for nearly 25% of the fauna. They exert a strong predatory
pressure on the carpets of small filamentous algae, called "
turfs”, but also on the young sporelings of larger macroalgae. However, the high regenerative faculty of coralline
algae enables them to resist predation by herbivores.
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A herbivorous fish in a field
of
Caulerpa bikiniensis |
Fish are selective in the species they consume (chemical
defense in some species), so they control the specific diversity
of the algae. More, they can favour the growth of some algae, like the Pomacentrid
fish which, by their territorial behaviour, exclude predators from
their area and favour the growth of filamentous algae turf several
millimetres high (Brawley and Adey, 1977).
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With
high regenerative faculty, chemical defense system,
cooperation with others organisms, the algae show a very
good adaptation to their environment.
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Algae
production and calcification
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In addition to the direct contribution of certain
species to the trophic chain, benthic algae actively
participate in primary production with about 5.5 Kg carbon
per m2 per year. Furthermore, the exuberance of calcified
forms in the coralline environment makes algae important
contributors to reef calcification, to the order of 4.5 Kg
calcium carbonate per m2 per year (Gattuso et al., 1998). Bioconstruction is essentially carried out by coralline red
algae, while sand formation is principally attributed to
the breakdown of Halimeda (green alga).
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One
of the most numerous species of red algae in the French Polynesian
reefs.
Hydrolithon
onkodes |
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Fixed
on the soft sediments, its calcareous articles will
enrich the sands. Halimeda
macroloba |
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Ecology
of the algae on the reefs and in the lagoons
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Distribution
according to depth
Algae colonise almost any habitat as long as it is damp and
lighted, however their depth distribution is limited by their
photosynthetic ability and the depth of penetration of solar
radiation.
In
tropical regions algae Corallinaceae, have been collected at a
depth of 268 meters (Bahamas), however they are most abundant and
diversified within the first 30 meters of the water column.
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Attachment
to the substratum
Multicellular
algae are generally attached to the sustratum and make up the
benthic forms as opposed to the unicellular planktonic species.
One of the few notable cases of floating macroalgae is the
example of Sargassum in the Sargasso Sea and seasonal
agglomerations of Boodlea, Chnoospora or Hydroclathrus in
tropical lagoons. Just as remarkable is the case of unicellular
algae called zooxanthellae living in symbiosis within coral
polyps.
Generally speaking, any sufficiently long-lived solid,
living or inert surface can be colonised by algae. We speak of
epiphytic algae when they are attached to other algae,
epizoic when they are attached to animals and endolithic when
they bore and live within calcareous substrata.
The
nature of the substratum matters in terms of its structure and
texture, in the
distribution of species. Hence we distinguish species of hard
substrata that attach themselves to compact rocks, and algae
from soft bottoms, or
in others words, species colonising sediment accumulations.
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species
of hard substrata, that attach themselves to compact rocks.
Caulerpa
peltata |
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and algae from soft bottom, or in other words, species colonising
sediment accumulations.
Caulerpa
sertulariodes |
Whatever the type of substratum, its degree of cohesion (stability)
plays a fundamental role in the length of attachment. Thus,
areas with soft bottoms subject to strong hydrodynamism never
support large algal populations.
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The
effect of hydrodynamism
The
roughness of the waters, owed to the variations of the sea-level,
to the tides, to the currents and to the swells, product forces
such as the organisms are selected on their faculty to hold out
against the wrench forces.
The area where tides and waves meet, constitutes the
upper floor of the reef system, the favoured domain for brown
photosynthetic algae and calcified red algae.
The amount of exposure of habitats to hydrodynamism
allows one to distinguish exposed habitats from calm ones, as well
as a range of intermediate situations.
As a general rule, strong hydrodynamism selects
species with large basal holdfasts, whose thallus texture is
relativeley coriaceous but flexible (Sargassum) or soft,
encrusting (Lobophora variegata) or calcified (Corallinales)
forms. These adaptations are often displayed in a given species by
morphological variations such as dwarfism (Turbinaria,
Sargassum).
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Lobophora
variegata
encrusting form |
Mastophora
pacifica
lamellate form |
Similarly, in red calcified algae, massive encrusting forms are
abundant in very exposed habitats (reef crest) while lamellate
forms are rather more characteristic of calm sheltered places.
Additionally, branched forms exhibit a wide variety of form and
size of branches according to the strength of the current (Neogolithon
frutescens) and we notice a decrease in the length
of branches when there is an
increase in the force of the current (Neogoniolithon frutescens).
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The
algal communities on the atolls
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They
are presented in the chapter communities of the atolls
Click
on the link to open
it. |
References
Payri,
C., N'Yeurt A.R. & Orempüller, J. - 2001 - Algae of french
Polynesia -Algues de Polynésie Française. Edition Au Vent des îles
- tahiti, 320pp.
Payri,
C.E. & N'Yeurt, A.D.R. 1997. A revised Checklist of Polynesian
benthic Marine Algae, Australian Systematic Botany, 10: 867-910.
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