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International Journal of Marine Science 2013, Vol.3, No.4, 23-32
http://ijms.sophiapublisher.com
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The upper part of the slope distinguished by the high
percentage of substrate covering by corals (not less
than 75%~80%) and by greater species diversity.
Patches of polyspecies settlement of corals alternated
with monospecies settlement occupying greater areas
of the bottom (Figure 4). Settlements of
Sinularia
,
Acropora
,
Montipora
and of branching forms of
Porites
dominated. In medium (transition) part of the
inner slope species of the
Pachysris
and
Merulina
genera could dominate. The base of the slope was
characterized by a lower percentage of substrate
covered by corals (20%~40%), by the lack of
extended monospecies settlements, and by the
development of large “heads” of massive corals. It
gradually transited into a sandy platform of lagoon
separating reefs from the shore.
The surveyed reefs had a very distinct axial zonation.
Their central part (the reef flat) characterized by a
homogeneous coral community, where vast fields of
branching and lamellar
Acropora
and
Montipora
pronouncedly dominated. At the southeast, south and,
especially, west margin of the axial zone, the death of
scleractinian observed. Scrap of colonies and entire
dead colonies formed a zone of sediment
accumulation in these reef areas. Finally, an extended
zone of fragments, sand, and silt sediments formed in
peripheral areas of the reef flat. All this implies that
the reef flat community was close to the later phase of
reef succession (Dollar, 1982; Sakai et al., 1986; Dai,
1993, 1996; Latypov, 1999) or, similarly to some reefs
of the Indopacific (Pichon, 1974, 1981), was in a
maturity or equilibrium stage of development.
Diversity at first increased and then gradually
decreased as more and more species became
established. Monopolists are plate-like and branching
Acropora
. The decline in diversity in older flows was
due to space limitation and competitive exclusion by
dominant species (Glynn, 1976). The geomorphological
characteristic: presence of fore reef, epi-reef, and back
reef (Figure 1, 6) complexes with characteristic
compositions of flora and fauna, and also obligatory
presence of a lagoon separating the reef from
fringing coast reefs, all enable us to attribute them
to the barrier type of reef (Coris Glossary,
http://coris.noaa.gov).
1.3 Reef of Bach Long Vi Island
Bach Long Vi Island extends for 1.6 km from
northeast to southwest at the exit from the Gulf of
Tonkin. It is surrounded by an adjoining reef with
well-defined main physiographic zones (reef crest,
reef flat, reef slope, etc.) and a small lagoon on its
southwestern side. In common, (264) species of corals
and accompanying macrobenthos identified on the
investigated reef. Representatives of the Acroporidae,
Poritidae, and Mussidae predominated among
scleractinian. Single-species aggregations of alcyonarian
Sinularia
and
Lobophyton
and the hydroid
Millepora
were fairly numerous. Of the accompanying
macrobenthos, the sea urchins
Echinostrephus molaris
,
Diadema setosum
and
Echinothrix diadema
, the
starfishes
Acantaster planci
and
Culcita novaeguineae
,
as well as the holothurian
Holothuria atra
occurred
most frequently. The bivalves
Lopha cristagalli
and
Pinctada margaritifera
were almost ubiquitous.
Asparagopsis taxiformis
,
Caulerpa racemosa
,
Turbinaria ornata
,
Padina australis
,
Sargassum
olygocystum
, and
Gracilaria
sp. predominated among
the macrophytes.
Figure 6 Bioherm on the reef Bath Long Vi (depth 10 m)
At the beginning of transect, on the northeastern part
of the reef, at a depth of 16 m is a fore reef platform or
a fore reef. This is an area of slightly silty coarse and
medium-grained sand with inclusions of blocks and
fragments of dead coral. Sparse colonies of
alcyonarian, gorgonians, and scleractinian encountered
here. A gentle slope was observed stretching 250–300 m