Russian Journal of Marine Biology, Vol. 29, Suppl. 1, 2003, pp. S34–S45.
Original Russian Text Copyright © 2003 by Biologiya Morya, Latypov.
1063-0740/03/2901-S $25.00 © 2003
The Gulf of Tonkin, called Bac Bo by the Vietnam-
ese, cuts deep into the continent. In the west, it borders
on the eastern Indo-Chinese coast; in the east, it borders
on the western coast of Lai Chau Peninsula and Hai
Nam Island (Fig. 1). The shallowness of the gulf and
peculiarities of its bottom relief are of special impor-
tance. The depths of the gulf and adjacent parts of the
South China Sea do not exceed 100 m. The northern
part of the gulf is a shallow-water area conﬁned with a
50-m isobath in the south. The western coast of the gulf
borders on a wide water strip with a depth below 50 m.
The bottom of the gulf is plain and is covered with soft
ground dominated by silts and silty sands with shell
fragments and organogenous matter. The central
groove of the bottom, conﬁned with 50- to 60-m iso-
baths, is covered with silty and clayey ground. Due to
the shallowness of the gulf, the water is rapidly warmed
up to 29–32
C in summer and cooled to 16
C in winter
A huge continental runoff occurring via numerous
rivers ﬂowing into the gulf plays an important role in its
hydrological regime. Thus, the Krasnaya River alone
brings into the gulf as much as 137 billion m
water and 116 million tons of suspended matter every
year . The river waters desalinate the water in the
western and northwestern parts of the gulf to 26–31‰
and form a constant runoff current with a salinity of 21–
22‰, running along the western coast southward .
The continental runoff bears a huge amount of terrige-
nous matter, forming a thick covering over the bedrock
and structures of the reef’s origin. The daily amount of
matter accumulated in the water column constitutes
, while the daily amount of matter settling
out onto the bottom varies between 16–100 g/m
typhoons, these values increase tens of times [3, 11, 32].
The Gulf of Tonkin features regular daily tides up to
3–4 m in height and strong tidal currents with a speed
of 1.5–2, and sometimes 3, knots.
In Northern Vietnam, in particular, in the Gulf of
Tonkin, the year includes four seasons, which differ
considerably in wind and precipitation regimes, as well
as in water mass dynamics [4, 31].
In winter (October–March), a pronounced current of
desalinated runoff waters is observed to run along the
western coast southward. The uppermost part of the
gulf is supplied with the warm and saline waters carried
from the southeast (the South China Sea) through the
central and eastern parts of the gulf. In the central part
of the gulf, a cyclonic gyre comprising the zone of
active water mixing is seen (Fig. 2a). In spring (May–
April), the runoff current weakens and runs closer to
the coast. The whole gulf is ﬁlled with the water carried
from south, while local cold water moves downward to
the bottom to form a bottom current running in the cen-
tral groove southward (Fig. 2b). The summertime
(June–August) is characterized by an anticyclonic cir-
culation in the center of the northern shallowness and
superﬁcial south current of warmed waters. In this
period, the river runoff increases and the bottom waters
upwell in the southwestern part of the gulf and off the
coasts of Hai Nam Island (Fig. 2c). In September, the
hydrological regime changes to its winter state, yet the
summer water circulation partly persists (Fig. 2d).
Reef-Building Corals and Reefs of Vietnam:
2. The Gulf of Tonkin
Yu. Ya. Latypov
Institute of Marine Biology FEB RAS, Vladivostok, 690041 Russia
Received December 9, 2002
—This paper deals with the history and results of the studies of reefs and coral communities of the
Gulf of Tonkin based on published and unpublished materials, including the author’s. The state of the art in the
study of reef-building scleractinian corals and reefs of this region is reported. The peculiar nature of the reefs
studied is caused by the monsoon climate in the region and river runoff waters cooled to 16–18
C, silted to
100 g/m2 per day, and freshened to 28‰ in the wintertime, i.e., conditions far from optimum for reef formation.
The silting and eutrophication of the gulf waters resulted in a change in the composition and structure of the
coral reef communities via the reduction or elimination of certain coral species. Instead of acroporids, typical
for the majority of other reefs, reef communities of the Gulf of Tonkin are dominated by poritids and faviids,
which form the framework of the reefs. These peculiarities make the reefs of the Gulf of Tonkin really unique.
the Gulf of Tonkin, reefs, reef-building corals, poritids.