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Zoobenthos
and macrophytes, fish
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Zoobenthos
et macrophytes
The
physical and geo-morphological features of the 77 atolls
in the French Polynesian Tuamotu archipelago show an
exceptionally high level of diversity, especially in
respect of their size, their shape, the depth of their
lagoon, the extent of the changes occurring between the
lagoon waters and the open sea, and the number an size
of any natural passes crossing the atoll rings.
These
features have little effect on the composition of the
communities inhabiting the parts of the barrier reef
which are in contact with the oceanic waters (the reef
flats and the outer slopes of the atolls). The
macrobenthic species inhabiting the barrier reefs are
actually very similar from one atoll to another; whereas
the communities inhabiting the atoll lagoons depend very
heavily on the physical and geomorphological
specificities of their surroundings. The lagoon
communities therefore vary from one atoll to another.
Those
interested can refer here to a scientific paper on this
subject.
One
of the main factors on which the structure of the lagoon
communities depends is the size of the lagoon. The
larger and deeper the lagoon, the greater the range of
coral, molluscan, echinodermal
and macroalgal species tends to be. On the other
hand, the number (per unit area) of these taxonomic
groups does not depend on the size of the lagoon. The
positive correlation found to exist between the size of
a lagoon and the diversity of the species it contains is
attributable mainly to the fact that the larger lagoons
include a much wider range of habitats, and thus provide
a large variety of organisms with suitable living
conditions. Some species of coral, echinodermata and
macroalgae settle preferentially - or even exclusively -
on the pinnacles, for example, which also explains why
the diversity of the species within these groups is
correlated with the number of pinnacles present in the
lagoon.
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In addition, the exchanges of
water occurring between the ocean and the lagoon
also strongly determine the structure of the
macrobenthic communities inhabiting the lagoon.
These fluxes mainly occur via the natural passes
or hoas,
when these are viable, as well as over the top
of the coral ring when it is completely
submerged.
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hoa
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a submerged coral ring
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The presence or absence of
viable passes does not significantly affect the number
of species present in a lagoon, but it does determine
the types of species present and the relative abundance
of the coral, echinodermal and macro-algal species
inhabiting the lagoon. In other words, some species live
only in lagoons which have passes, while others colonise
atolls which are completely self-contained. Likewise,
the amount of exchanges which take place via the hoas
and the submerged coral ring determines the
composition of the fauna
and the relative abundance of the coral and
molluscan species present
in the lagoon. Some quantitative data showing the
links between physical factors and the structure of the
communities are given in tables b1
and b2, as well as in a scientific
paper, which can be consulted by
clicking here.
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The
decisive effects of the exchanges between the
open sea and the lagoon on the composition of
the macrobenthos communities can be explained in
terms of the presence of viable passes and hoas:
these natural openings may give the species
inhabiting the outer slopes of the atoll ring
access to the lagoon, and thus facilitate their
colonisation of the lagoon waters.
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Species
which have to spend at least part of their life cycle in
the ocean waters are therefore prohibited from living in
those lagoons where there are no natural passes.
The
communities inhabiting the lagoons of the atolls with no
passes are also characterised by the fact that they
consist of only a few (sometimes only 1 or 2)
predominant species. The predominant species, which
often differ from one lagoon to another, are usually
either bivalves (Arca ventricosa, Chama
imbricata, Tridacna maxima or Pinctada
maculata), as in the lagoons on the atolls of
Pukarua, Fangataufa, Maturei Vavao, Tureia, Reao and
Pukapuka, or holothurians (Holothuria atra), as
in the Hiti
atoll lagoon.
The
lagoons on atolls with no natural passes therefore
generally give home to only a narrow range of benthic
species, often including only species which are
particularly well adapted to living under the extreme
conditions pertaining in these surroundings. This is
certainly the case in the Taiaro lagoon, where only a
few particularly resistant species can withstand the
hyper-saline waters (43 psu) (Click
here for further information).
The
last point worth mentioning is that in the lagoons which
communicate very little with the open sea, extremely
high death rates are liable to occur among the
populations as the result of dystrophic events (phytoplankton
blooms can induce a state of anoxia in the waters, which
is fatal to the lagoon communities). The occurrence of
an event of this kind has been described in the lagoons
of the atolls of Mataiva, Taiaro and Hikueru. Click
here to consult a scientific paper on this subject
Reference
sources
Adjeroud M,
1997. Long-term
changes of epibenthic macrofauna communities in a closed
lagoon (Taiaro Atoll, French Polynesia): 1972-1994. Hydrobiologia,
356: 11-19.
Adjeroud M, Andréfouët S, Payri C, in press. Mass mortality of
macrobenthic communities in the lagoon of Hikueru atoll
(French Polynesia). Coral
Reefs.
Adjeroud M, Andréfouët S, Payri C, Orempüller J, 2000. Physical
factors of differentiation in macrobenthic communities
between atoll lagoons in the Central Tuamotu Archipelago
(French Polynesia). Marine Ecology Progress Series, 196 : 25-38.
Salvat B,
1969. Dominance biologique de quelques mollusques dans
les atolls fermés (Tuamotu, Polynésie) ; Phénomène récent
– Conséquences actuelles. Malacologia, 9 : 187-189.
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