Hydrophyte communities in the Tam Giang - Cau Hai lagoon

ea decline was very strong in 
1996–2010, in 1996 the area of seagrass beds 
was 2,200 ha [1], in 2003 was 1,200 ha [4], and 
remained at 1,000 ha in 2010 [3] . Currently, 
the area of seagrass distribution has increased 
significantly, about 2.037 ha (figure 4), perhaps 
due to the efforts of the project “For 
Integrated Management of Lagoon Activities 
(IMOLA) Project of Thua Thien-Hue province 
(FAO, GCP/VIE/029/ITA)” [4] to improve 
people's livelihoods by strengthening 
sustainable management of aquatic resources 
with the participation of the community in 
accordance with the socio-economic and 
production requirements of the local 
population. At the same time, the Decision 
No. 1142/QD-UBND dated June 6, 2011 of 
the People's Committee of Thua Thien-Hue 
province approved “Plan for clearance and 
reorganization of stake traps in Tam Giang - 
Cau Hai lagoon, Phu Vang district” [11], 
accordingly, implementing the zoning of stake 
trap fishing planning in the lagoon, reducing 
45% of stake traps in the whole lagoon area of 
Phu Vang district, reducing the pressure of 
exploiting stake trap fishing to gradually 
restore the ecological environment and aquatic 
resources, opening the waterways and 
migrations of aquatic species in the Tam 
Giang - Cau Hai lagoon area. It belongs to 
Thuan An town and 8 communes: Phu Thuan, 
Phu Hai, Phu Dien, Vinh Xuan, Vinh Ha, 
Vinh Phu, Phu Da and Phu Xuan. 
Some seagrass beds with large area are 
Tam Giang 5 (Con Dai) with 1,450 ha; Cau 
Hai 2 (Con Lay - Vinh Hien): 105 ha; Cau Hai 
3 - Cau Hai 4 (Vinh Giang - Ba Con): 224 ha, 
Con Co: 130 ha, Cau Hai 5 and Cau Hai 1 
(Loc Binh - Le Thien): 78 ha; Dam Sam and 
Tam Giang 5 (Con Son - Hop Chau): 60 ha; 
Con Te - Quang Thanh: 70 ha,... Freshwater 
plants are concentrated in O Lau 1, O Lau 2 
and Cau Hai 4 with a total area of 803 ha 
(table 3). If the total area of freshwater plants 
and seagrass is calculated, the distribution area 
of seagrass in Tam Giang - Cau Hai lagoon is 
over 2,840 ha. These are important habitats 
and breeding grounds for aquatic and marine 
species in this lagoon. 
Cao Van Luong et al. 
204 
Figure 4. Map of seagrass distribution in Tam Giang - Cau Hai lagoon 
Table 3. The area of some typical seagrass beds in Tam Giang - Cau Hai lagoon 
No Sites Area (ha) Species 
1 O Lau 1 
803 
Utricularia aurea, Ceratophyllum demersum, Valisneria spiralis, Blyxa aubertii, 
Myriophyllum spicatum, Potamogeton malaianus, Najas indica, Hydrylla 
verticillata,Ruppia maritima 
2 O Lau 2 
3 O Lau 3 
50 Zostera japonica, Halodule pinifolia, Valisneria spiralis, Ruppia maritima 4 O Lau 4 
5 O Lau 5 
6 Con Te 70 Zostera japonica, Halophila beccarii 
7 Dam Sam 1 
60 
Zostera japonica, Halodule pinifolia, Halodule uninervis, Ruppia maritima, 
Hydrylla verticillata 
8 Dam Sam 2 
9 Tam Giang 4 
10 Tam Giang 5 1,450 Zostera japonica, Ruppia maritima 
11 Cau Hai 5 
78 Zostera japonica, Halodule pinifolia, Halodule uninervis, 
12 Cau Hai 1 
13 Cau Hai 2 105 Zostera japonica, Halodule pinifolia, Halodule uninervis, Halophila ovalis 
14 Cau Hai 3 
224 
Zostera japonica, Halodule pinifolia, Halodule uninervis, Halophila ovalis, 
Halophila beccarii, Ceratophyllum demersum, Najas indica, Hydrylla verticillata 15 Cau Hai 4 
The spatial distribution characteristics of 
seagrass and freshwater plants in Tam Giang - 
Cau Hai lagoon are presented in table 2. In 
Tam Giang lagoon, 6 species has been 
identified (including 1 species of freshwater 
plant group), in Cau Hai: 5 species of seagrass, 
in Ha Trung - Thuy Tu: 7 species (there are 2 
species of freshwater plants). The two areas 
with the most diverse species are O Lau and 
Cau Hai lagoon with 9 species, but different in 
Hydrophyte communities in the Tam Giang 
205 
composition. O Lau has 8 species of freshwater 
plants, whereas in Cau Hai lagoon there are 6 
species of seagrass. 
The bottom topography of the central 
region of Tam Giang lagoon and Thuy Tu is 
like a basin without seagrass. Seagrasses are 
mostly distributed along the edge of the lagoon 
or on the floating dunes (figure 5), with a depth 
of 0.5–2.5 m; seagrass is also distributed at the 
sand dune inside Tu Hien estuary - the deepest 
area, with 3 species of Zostera japonica, 
Halodule pinifolia and Halophila ovalis. 
Figure 5. Distribution of seagrasses on the bottom in Tam Giang - Cau Hai lagoon [2], 
A: Quang Loi transect (Tam Giang); B: Vinh Xuan transect (Thuy Tu); C: Cau Hai transect 
The coverage and shoot density 
The highest density of shoots and coverage 
belonged to Zostera japonica with 9,905 ± 550 
shoots/m
2
, followed by Halodule pinifolia with 
6,010 ± 722 shoots/m
2
 and the lowest belonged 
to Ruppia maritima with 325 ± 17 shoots/m
2
(table 4). 
A comparison of shoot density from 2009 
to 2017 showed that there is a different 
variation among different species. In 2009, the 
shoot density of Zostera japonica reached 
8,550 shoots/m
2
, but in 2016 it was 9,905 ± 550 
shoots/m
2
 (an increase of 1.15 times). 
Similarly, the shoot density of Ruppia maritima 
increased from 200 shoots/m
2
 to 325 ± 17 
shoots/m
2
. However, in the remaining species, 
there was a slight decrease in density, in 
Halodule pinifolia from 8,734 shoots/m
2
 to 
6,010 ± 722 shoots/m
2
, in Halophila ovalis 
from 5,359 shoots/m
2
 to 3,407 ± 843 shoots/m
2
and in Halophila beccarii from 5,850 shoots/m
2
and 5,725 ± 434 shoots/m
2
 [4]. 
Cao Van Luong et al. 
206 
Table 4. The coverage and shoot density of seagrasses in Tam Giang - Cau Hai lagoon 
Species 
Cover (%) Density (shoots/m2) 
2009 2016–2017 2009 2016–2017 
Zostera japonica 5–100 90 3,000–14,100 9.905 ± 550 
Halodule pinifolia 5–90 75 5,600–11,867 6,010 ± 722 
Halodule uninervis - 25 - 1,200 ± 125 
Halophila ovalis 20–50 45 2,050–8,667 3,407 ± 843 
Halophila beccarii 50–75 90 3,550–8,150 5,725 ± 434 
Ruppia maritima 5–10 25 200 325 ± 17 
Quantity characteristics of some typical 
species 
Zostera japonica 
In the rainy season, the average length of 
Zostera japonica varies from 8.18 cm at O Lau 
3 to 20.50 cm at Tam Giang 5, the average 
length for the whole study area is 14.14 cm. 
The amount of biomass varied from 123.8 
g.fresh/m
2
 at O Lau 4 to 1,113.8 g.fresh/m
2
 in 
Tam Giang 5, the average biomass was 804.4 ± 
54.7 g.fresh/m
2
. 
Figure 6. Seasonal change in seagrass biomass in Tam Giang - Cau Hai lagoon: 
(a) Zostera japonica, (b) Halodule pinifolia, (c) Halophila ovalis 
In the dry season, the average length of 
Zostera japonica varies from 19.53 cm at Cau 
Hai 4 to 36.70 cm at Tam Giang 5, the average 
length of the whole study area is 27.29 cm. 
Similarly, biomass varied from 2,466.4 
g.fresh/m
2
 at Cau Hai 4 to 8,041.5 g.fresh/m
2
 at 
Tam Giang 5; the average was 5,355.4 ± 326.5 
g.fresh/m
2
 (figure 6a). 
Halodule pinifolia 
During the rainy season, the average length 
of Halodule pinifolia reaches the lowest value 
(8.10 cm) at Cau Hai 2 and the highest value 
(12.88 cm) at Cau Hai 4, the average length for 
the whole study area in 2009 was 10.49 cm. 
The amount of biomass varied from 387.5 
g.fresh/m
2
 at Cau Hai 2 to 475.0 g.fresh/m
2
 at 
Cau Hai 4; the average biomass was 431.3 ± 
25.8 g.fresh/m
2
. 
In the dry season, the average length of 
Halodule pinifolia changes from 9.86 cm at 
Cau Hai 4 to 17.26 cm at Cau Hai 2, the 
average length for the whole study area is 13.56 
cm. The amount of biomass varied from 650.0 
g.fresh/m
2
 at Cau Hai 4 to 1,012.5 g.fresh/m
2
 at 
Hydrophyte communities in the Tam Giang 
207 
Cau Hai 2, the average biomass was 831.3 ± 
155.3 g.fresh/m
2
 (figure 6b). 
Halophila ovalis 
This species is only distributed in areas 
with high salinity such as the vicinity of Thuan 
An estuary (Con Te, Tam Giang 5), Tu Hien 
estuary (Cau Hai 2) and even in the area of 
Truong Ha bridge (Vinh Xuan). In the rainy 
season, the average length varies from 2.96 cm 
at Cau Hai 2 to 3.59 cm at Tam Giang 5; the 
average length of the whole study area is 3.17 
cm. The biomass varied from 160.0 g.fresh/m
2
at Con Te to 280.0 g.fresh/m
2
 at Cau Hai 2, the 
average of the whole study area was 220.0 ± 
25.1 g.fresh/m
2
. 
During the dry season, the length of 
Halophila ovalis varies from 3.69 cm at Cau 
Hai 2 to 3.93 cm at Tam Giang 5; average 
length of the whole study area is 3.79 cm. The 
biomass varied from 1,600 g.fresh/m
2
 at Con 
Te to 380.0 g.fresh/m
2
 in Cau Hai 2. The 
average weight of the whole study area was 
293.3 ± 44.9 g.fresh/m
2
 (figure 6c). 
The seasonal effects on seagrass biomass and 
correlation 
Figure 7. The correlation of biomass 
of Zostera japonica 
To see the seasonal effects on seagrass 
biomass, we analyzed the correlation between 
shoot density, length and biomass of Zostera 
japonica with 120 quantitative samples. 
Applying linear equations (y = ax + b, with p 
< 0.05) gives positive correlation results 
(figure 7) and also shows that shoot density is 
a factor that has a stronger influence on 
biomass than the length. At the same time, the 
results of analyzing the above/below ground 
biomass showed that seagrass in dry season 
developed better than in rainy season with an 
average of 1.32 (i.e. 1 kg of rhizome would 
have 1.32 kg of leaf), during the rainy season 
this ratio is 0.91 (i.e. 1 kg of rhizome would 
have 0.91 kg of leaf). 
The result is consistent with the general 
ecological characteristics of tropical seagrass, 
which means that seagrasses usually grow well 
in the dry season with the low rainfall, high and 
stable salinity, few or no storms. At the same 
time, seagrasses often suffer from a decrease in 
standing density (shoots) in the rainy season 
due to high turbidity, decreased and unstable 
salinity. This result is consistent with the 
results of the study on Zostera japonica in Cua 
Dai (Quang Nam) [12, 13] and the study on 
seagrass in the Philippines by Terrados et al., 
(1998) [14]. 
CONCLUSION 
A total of 6 species of seagrass were 
identified (Halodule uninervis is recorded for 
the first time in Tam Giang - Cau Hai lagoon, 
but Halophila minor is absent) and 8 species of 
freshwater plants were also recorded 
(Potamogeton maackianus is absent). 
The total distribution area of seagrass and 
freshwater plants is over 2,840 ha. In particular, 
seagrass area has been recovering significantly 
compared to the previous study, from 1,000 ha 
in 2009 to 2,037 ha in 2017. 
Most seagrasses are distributed on the edge 
of the lagoon or on the islets with a depth of 
0.5–2.5 m. The biomass, coverage, shoot 
density and observation frequency showed that 
the Zostera japonica was the dominant species 
(the biomass of 5,355.4 ± 326.5 g.fresh/m
2
, the 
shoot density of 9,905 ± 550 shoots/m
2
 and the 
coverage of 90%). 
The analytical results showed that the 
season significantly affects the growth of 
seagrasses, most of which have a very high 
biomass in the dry season, and a decrease in the 
standing shoot density in the rainy season. 
Cao Van Luong et al. 
208 
Acknowledgements: This publication is resulted 
from the state project entitled “Assessing the 
degradation of coastal ecosystems in Vietnam 
and proposing sustainable management 
solutions” coded KC09.26/06–10; the project 
entitled “Researching scientific and legal bases 
for assessing and claiming compensation for 
damage caused by oil pollution in Vietnam’s 
waters” coded DTDL.2009G/10; the state 
project entitled “Research on solutions to 
recover ecosystems of coastal lagoons and lakes 
in the central region” coded KC08.25/11–15 
and the project entitled “Investigation into 
overall status and fluctuations of biodiversity in 
Vietnam’s coastal ecosystems” under 
Component 1 - Task No. 8 - Project 47. The 
authors express their thanks to the project team, 
executing institution and Vietnam Academy of 
Science and Technology for their supports for 
the study. 
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