Cat Ba National Park contains 6 different ecosystem types typical of the limestone islands in the tropical region, including (1) tropical rain forest, (2) mangrove forest, (3) tidal flat, (4) marine lake, (5) coral reef and (6) soft bottom.
(1). Tropical rain forest ecosystem: This ecosystem includes the main part of Cat Ba National Park and covering a large area of about 15,067 ha on Cat Ba Island. It consists of the only primary forest in Cat Ba island that still exists and preserves its original state. Its area is about 1,000 ha in the central island. In this forest, there is Frog pond (Ao Ech) covering an area of 3 ha, which is a year-round fresh swamp with a dominant single tree of Salix tetrasperma and it also has Tan Man cave that is home for rare and precious animals like chamois, Cat Ba white-headed langur. The valuable Nageia fleuryi forest surrounds Ngu Lam peak, the highest point of Cat Ba National Park.
This area bears a high biodiversity with 1,588 species of higher plants (tracheophyta) in 842 genera, and 186 families belonging to 5 divisions, including: Lycopodiophyta (6 species), Equisetophyta (1 species), Pteridophyta (63 species), Pinophyta (29 species), 1,462 species of Angiospermatophyta (in which, Magnoliopsida class has 1231 species and Liliopsia class is 231 species). More than 200 terrestrial animals are also discovered in the property, including 53 species of mammal, 205 species of bird, 55 species of reptile and 25 species of amphibian. There are rare and endemic species in a small quantity such as Cat Ba white-headed langur,, chamois, white swallow, gecko, varan, etc.
Although the number of terrestrial animals is not large, there are endemic species such as Cat Ba white-headed langur. The terrestrial animals of Cat Ba National Park have the following remarkable features:
- The mammal class (Mammalia): though the mammalia is poor in terms of the quantity of species, it is typical for limestone mountain regions with high adaptation such as Cat Ba white-headed langur and Chamois.
- The bird class (Aves): There are some typical species for the sea and island regions such as white swallow, redshank and greenshank.
- The reptile class (Reptilia): The noticeable species are gecko and varan.
In addition to the specific characteristics of each group, it is noticed that they are unevenly distributed, most of which concentrate in the valleys and embayments far from the residential areas such as Re Bo Da, Nuoc Lut, Man Dop etc. Especially, the white-headed langur concentrates mainly in the mountain walls near the estuaries in Viet Hai, Lach Tau, Ong Cam embayment, etc. with the total number of about 63 individuals.
Referring to the above mentioned species, some have been protected strictly such as Catba white-head langur, chamois, varan, and gecko.
(2). Mangrove forest ecosystem: Estuary near the sea is a favorable environment for mangrove vegetation to develop, which forms a mangrove forest ecosystem typical for those countries that belong to the tropical zone. Mangrove forest ecosystem is a precious resource of the tropical coastal area in general and Cat Ba - Hai Phong in particular. This ecosystem develops steadily in the coastal areas, helping promote the diversity of genetic source and the biological productivity, and maintaining the balance and stability of the entire area. The presence of mangrove vegetation in the coastal areas and river mouths has created an attractive and typical mangrove forest ecosystem for eco-tourism.
- Species composition of mangroves: It is identified that 31 species belong to 24 families and 29 genera, of which 11 species are the main mangrove group (36% of total species), 11 species are of the salt-resistant group that join in the mangrove forest (36%), and the remaining 9 species are of the mainland source that move to the forest (28%) (Annex 1).
- Distribution of mangroves in Cat Ba - Hai Phong: Terrain and floor of Cat Ba island is very diverse and complicated, which have determined the distribution of mangroves and created attractive and distingushing landscapes for Cat Ba.
+ Distribution of mangroves on mudy flats and shells: Muddy bottom is a favorable environment for the mangroves to distribute and develop. As Phu Long and Duong Gianh (Cat Ba) are situated in the coastal areas directly influenced by fresh water from the estuaries, alluvia are brought here in fairly large quantities, enabling the mangroves to grow. In Phu Long - Cai Vieng, as many as 18 species have developed in an area up to 600 ha. And in Duong Gianh 22 species grow in the nearshore tidal flats of 18 ha. In the muddy bottom, mangroves often gather to make large flora, forming into zones like Aegiceras and Avicennia zone, followed by Kandelia, Rhizophora and Bruguera zone that is relatively pure. In the upper part of the high tide is the zone comprising many mixed species such as Aegiceras, Avicennia, Kandelia, Aegiceras, Avicennia, etc.
+ Distribution of mangroves on the stone, sand and cobble bottom: This bottom is an unfavorable environment for the development of mangrove flora. To take one example, Vuon Qua island is surrounded by the caves and rocky mountains, not near the river mouths, and the alluvia are brought here in insignificant quantity, which causes the mangrove vegetation to distribute sparsely in a small area of only 2 ha where there are 7 species having grown there. These mangroves are small and infertile, and they neither grow into large flora nor form into the zones as in the muddy bottom areas.
- Seaweed: The species of seaweed and sea grass have the unique form of existance, i.e. an attached form. They hang on to shells or trunk of mangrove trees. They are Lyngbya aestuarii (indigo seaweed); Enteromorpha prolifera, Chaetomorpha linum (green seaweed); Bostrychia, Caloglossa, Gigartina, Catenella (red seaweed).
- Bottom animals: They make good contributions to increase the biodiversity in the water body with various forms, including:
+ Group living on branches of trees has two species of snail that live on the mangrove trees and grasses, which are the permanent residents in the mangrove vegetation. These species eat leaves and epidermis of the trees.
+ Group distributing in the inlaid form comprise such species as Ostrea, and Balanus. They cling on to the trunk of trees body or to the rocks on the edge of the roots of mangrove vegetation.
+ Group living on the surface of the bottom include those which creep on the surface of the bottom and eat the biodetritus of the surface, for example, the snails in the group of gastropod molluscs.
+ Group living in the bottom are distributed under the bottom of the tidal flats covered by mangrove vegetation, most of which have a great economic value. The most typical species are mud crab (Scylla serrata), shrimp (Alpheus), shrimp (Squilla), oyster (Arcidea), soil worm (Sipunculus), and clam (Dosinia laminata).
+ Group living inside tree trunks include those which pore into dead trees, most of which belong to the Teredinidae family. They play an important role in decomposing organic materials from the mangrove trees into humus, thus offering food for all debrivous species.
+ Group migrating temporarily include young and mature individuals of the shrimp and crab group, i.e Penaeus, Metapenaeus, Scylla serrata. They migrate according to the tides to search for the food.
- Fish group: Some 90 species of fish have been discovered, that belong to 55 families related to the mangrove forest ecosystem, of which there are 51 families of brackish water fish, and 4 families of freshwater fish. Among the brackish fish, mullet usually has a high percentage, and there are goby, perch, and grunt.
- Bird group: The mangrove vegetation is a habitat, a nesting or food-seeking places for all kinds of birds. In Cat Ba - Phu Long, many birds have been discovered, including:
+ Seabird group consist of seagull, sea cormorant, sea mew, scoter and swallows. They usually follow the tides to search for the food in the mangrove flora.
+ Bird group that live in the island are usually black stork. They hide in the bushes of the mangrove trees to hunt all kinds of living creatures in the tidal areas.
+ Coastal bird group: dotterel.
+ Migration bird group: white stork, sea mew.
In adittion, reptiles and bees also live in the magrove flora.
(3). Tidal ecosystem: It includes all species living in the high tide and low tide areas. There are 3 main habitat types in the tidal ecosystem of Cat Ba, i.e. sandy beaches (sandy tidal flats), rocky tidal flats, and mixed tidal flats. In the tidal areas, two main organism groups are dominant, including zoobenthos, and seaweed/seagrass.
+ Species composition: Up to now, there have been found 658 intervebrate species living at the bottom in Cat Ba belonging to 345 genera, 146 families, 12 classes of 5 phyla: Annelida, Arthropoda, Mollusca, Echonodermata and Porifera (Appendix 2). Among these, the molluscs have the most diversified species, with 298 species, accounting for 45.3% of the total species, then comes Annelida, with 192 species, 29.2%; Crustacea with 118 species, 17.9%; 26 species of sponges, 3.9%; and echinoderms, with the fewest number of 24 species, 3.6%. By analyzing the species composition to the family-level for the entire zoobenthos, we found the results as follows:
5 families have abundant species with equal to or more than 20 species.
(4). Coral reef ecosystem: Among all groups of corals, the hard coral plays an important role in building a coral reef (called reef building coral), and the other groups have a minor or no role in reef creation, most of which only take part in the reef as the members of coral reef community. Considering the ecological role, corals are divided into two groups: reef building corals and non-reef building corals. The main difference between them is that the first group has a symbiotic algae in the soft tissue of corals with the high density (1 million cells/ cm2) (Gladfelter, 1993). The operation of such symbiotic group creates a closed ecological cycle, in which the waste of coral is used by the algae and thanks to the photosynthetic activities, it creates primary products. Of which, 60% of photosynthetic products of the symbiosis alga is provided for corals, and 40% is discharged into the surrounding water environment so as to participate in the energy and material cycle of the coral reef ecosystem. Due to the symbiosis alga, the reef building coral is only distributed in the shallow sea areas so as to ensure the light for the operation of symbiosis alga. Non-reef building corals can be distributed in the deep sea areas without a need of the light.
- Structure of coral composition: So far, 177 species of coral class have been discovered in Cat Ba – Long Chau. Of which, 166 species are hard corals Scleractinia, belonging to 13 families. And 11 other species belongs to the orders of Stolonifera, Alcyonaria and Gorgonacea. Taking into consideration the structure of corals, it shows that the Acropora genus contains the highest species number - 25 species, followed by Montipora - 14 species, Porites and Favia - 9 species each, and the remaining genera have from 1 to 7 species. While the coral fauna structure in the Southern sea is mostly based on an abundant branching coral that has larger number than any other, the group of block shaped coral in Cat Ba – Long Chau is much more diversified. The Faviidae family consists of more genege than the Acroporidae family. This can be explained that the living conditions in Cat Ba is much harder and, especially the lower temperature regime in winter prevents the larvae of narrow-temperature-scale species of the Acropodidae family from residing and developing. Horn corals and soft corals bear less value in building reef, but they contain remarkable bioactive substances. Because of the fixed residence in the fierce competitive environment, corals often produce a toxin to resist the other species and occupy further the space of distribution. These toxins are often a product produced by the symbiosis between zooxanthellae and its hosts. The developed countries have studied and used such toxin as a precious medicine to treate serious diseases. Besides, the clusters of horn corals in dark red color help beautifying the coral reefs.
- Distribution of coral reefs in Cat Ba – Long Chau: Coral reef is a natural entity on the sea bottom, having biological origin. The most important organism is the reef building coral and coralline (Coralline algae). These two groups form the foundation of the reefs. All creatures living in the coral reefs are the members of the reef community, participating in the energy transformation in the food chain of the ecosystem, and providing exploitable products for the humans. The reef building coral always require a stable living condition with suitable temperatures from 16 - 360C (optimum 25 - 300C), suitable salinity from 27-40 ‰ (optimum 34 -36‰), clean water, and hard bottom. Therefore, the coral reefs are only distributed in the shallow tropical area away from the river mouths where the water is turbid and low in salinity. Ecological surveys show that the biodiversity index (H') of the coral reefs in the property fluctuate in the range of H’ = 0.51 – 0.82, which is relatively low due to the unequal distribution of taxons. Harsh living environment poses continual threats to some species but the others that have strong competitiveness could develop quickly, creating a large distributed area of a single species. The most popular genera on the coral reefs are Galaxea, Goniopora, Porites, Pavona, Favia, Favites, Goniastrea, and Acropora. Among these, the first four genera normally dominate in most of the surveyed reefs.
The Cat Ba coral distribution on the large area firstly depends on the salinity of seawater. Less saline waters from Bach Dang river prevent corals from residing in the west part of Cat Ba island though there are rocky bottoms suitable for them to stay. In the boundary closest to the mainland, it can be found the coral in Cat Ong island south of Cat Ba island and in the farthest point of Long Chau Dong island in Long Chau island group. In the ecological surveys, some good coral reefs have been discovered, namely in Ang Tham, Cat Dua, Mui Hong, Ba Trai Dao (southeast of Cat Bat island), Dau Be – Hang Trai island group, and Long Chau island group. In other island areas, corals exists but they are scatteredly distributed or in the situation of heavy degradation.
The coral distribution under the depth level mainly depends on clear seawater levels, terrain, and bottom substances. Cat Ba waters belong to the shallow inshore area, which have been heavily influenced by the continental regime, so their clear level is low. Especially in the rainy season, the muddy bottom prevents the reef building corals from reaching to deep levels. The popular depth for them is about 5 – 6m, not exceeding the maximum level of 10m. Except for Long Chau where the water is very clear, they can distribute up to the depth of 20m. Due to the terrain of big sloping sides and the muddy bottoms closely alongside the island bases, they are normally small and cut into pieces. Coral reefs have popular width from 30 – 50m, and the largest area is not over 100 meters from the edge of the island.
In terms of the structure, they exist in two forms: typical fringing reefs in indentations of islands and atypical fringing reefs alongside narrow tidal channels or island capes where there are strong currents.
- Forms and structures of coral reefs: Islands in the property have the major forms of limestone metamorphosis. The weathered process in thousands of years ago created embayments, bays, and caves having complex forms stuck inside the mountains. In general, it can be devided into two forms: atypical fringing reefs and typical fringing reefs.
+ Type of the reef with atypical structure is the one without creating considerable carbonate accumulated sediments that originate from the coral reefs. On the reef, corals distribute sparsely and interuptedly, and sometimes they have a large block of coral. They hardly change the form of the bottom surface. Thereby, the bottom surface is mainly a ground stone in medium and average dimensions, somewhere there are dead coral pupulations. The zonation formed on such reefs does not clearly expose, nearly indicating the form of the sea bottom surface. Only the reef side is clearly shown.
+ The reefs with typical structure are often formed on the bottom ground that has a biological origin and a form zonation, so the structure is rather clear. On most part of these reefs, it can be found that there is a small inshore lagoon (an edge zone), a surface reef zone where its internal and external parts can sometimes be distinguished clearly, a reef side zone that is short but clear with horizontal, vertical ditches as well as their bottoms, and a crow reef zone or a wave breaking zone that is unclear. Still, on the boundary between the surface reef zone and the edge zone, dead corals are linked to each other by limestone seaweeds, which get stronger than those in the other sites.
Although the zonation is not shown clearly in terms of the form aspect, there is a clear zonation in the structure and composition of the coral species and other zoobenthos, and in the features of the ground material on which the coral reefs develop. These zones normally lie in parallel with the edge lines and are related to a certain depth.
+ Tidal zone (zone containing coral): Tidal zone often has the ground material of stone - gravel with fine sand sediments and coral carcass. The zone wildth is between 2 – 10m, and its average depth is about 0.5 m. On the surface of this zone, coral populations in block or covering shape are often found. Branching corals seem not exist. The coverage of coral in the low tide zone does not exceed 5%.
+ Internal coral zone: Its ground level is stone, sand and coral carcass. In this zone, microtoll forms are often found (mostly Porites, and sometimes Ganister). The zone wildth is between 5m and 20m and the depth is more than 2m. Most coral species belong to the families of Poritidae and Faviidae. There can be some bushes of shaped corals of Acropora and Montipora which distribute to become a scattered colony or sometimes they concentrate to group from 12-17 colonies/m2. Their coverage vary from 15% to 40%.
+ External reef zone: Its ground is stone and dead coral, somewhere there are some places of sandy mud and biodentritus. Characteristic of this zone is vertical and horizontal ditches on the ground of reef sediments. The zone width is 10-80 m, and the depth does not exceed 2m. In this zone, corals are distributed into a single species cover of Acropora, Goniopora, Galaxea, Sarcophyton genera, or in bushes of single or multiple species with the relative diversity of the Faviidae family. The coral coverage is about 40-60%, while in the single species cover this rate may increase up to 100%.
+ The higher part of the reef side zone: The ground level always have dead coral and corallite. This part is short and very sloping, lying in the depth of 2 – 3 m. According to the natural laws, corals create a single-species cover from the representatives of Goniopora, Acropora and Pavona genera. They cover up to 100% on the ground level.
+ The lower part of the reef side zone: The ground level is mostly dead coral, and base stones somewhere. The zone have different slopes from reef to reef, with its spread (the width) of 2-10m and the normal depth of 2-6 m. In this zone, the corals are very rich, having a high biodiversity and a morphology of coral colonies from the covering form to the branching form, and in some parts there are the single species covers of Goniopora, Lobophyllia, Galaxea, Pavona genera or their colonies with the diameter of 2m – 4m.
+ Zone of reef base: The ground level is sand mixed with organic detritus and fragments of dead coral lying on a thick muddy layer. The zone depth is about 4.5 m - 14 m. Stone coral is scatteredly distributed, including colonies in the shapes of block, semi-ball or funnel. Somewhere, horn coral colonies exist with the density of 7 colonies/1m2. Although the zoning on morphology of coral reefs is not always clear in forms (Lagoon zone, zone of reef base, etc.), the above-mentioned reefs are all zoned by substituting the respective zoobenthos communities. These communities have a stable species composition and structure in different zones of different reefs. The analysis and research on these communities have been presented in many scientific reports
- Value of coral reef ecosystem: The biggest value of the coral reefs is specially high biodiversity. Nowadays, this value is getting more prominent when the inshore environment degenerates, leading to a decrease of coastal marine resources, and many species under threat of extinction. The existence of coral reef ecosystem lays a foundation to protect genetic sources and marine biodiversity. In the world, governments have been required to specially protect the coral reefs and build the marine protected areas based on the intact coral reefs.
(5). Soft bottom ecosystem: It consists of a large seabed and its surrounding aquatic habitats. There are four typical organism groups for this ecosystem, namely zoobenthos, phytoplankton, zooplankton, and sea fish.
+ Species composition: Up to the present, the zoobenthos fauna in the soft-bottom of subtidal zone of Cat Ba consist of 340 species, 186 genera, 84 families of 4 phyla in the order of the species richness as follows:
- Mollusca Phylum consists of 162 species, 77 genera, 45 families, accounting for 7%,.
- Annelida Phylum with the class of Polychaeta comprised of 115 species, 73 genera, 23 families, accounting for 8%.
- Arthropoda Phylum with the class of Crustacea having 52 species, 31 genera and 15 families, accounting for 3%,.
- Echinodemata Phylum having 11 species, 10 genera and 5 families, accounting for 3.2%.
With the zoobenthos species number mentioned above, the soft-bottom subtidal zone is an ecosystem having a plentiful genetic source that is lower than the tidal zone but higher than the coral reef ecosystem (213 species). The Conus genus (Conidae family - Gastropoda) has the highest number of species - 11 species, followed by the genera of Eunice (Polychaeta), Drupa, and Lithophaga (Mollusca) that have 6 -7 species. The remainder of more than 20 genera have their species number from 4-6. Among the total of 84 families, 3 families have the largest number of species, namely Eunicidae - 23 species, Murricidae - 21 species, Portunidae - 16 species. 27 families have 4-15 species. The remainder of 44 families has their species number from 1-3 species.
In Cat Ba waters, the soft-bottom not only occupies large areas of the bays but also lies alternately between the stones and the coral reefs. Therefore, the soft bottom area is also a feeding area for the reef organisms. The presence of 340 species of zoobenthos proves that the soft bottom subtidal zone is a place to preserve more than 50% of the zoobenthic genetic source of the entire waters.
+ Species compostion of phytoplankton: The research results of the phytoplankton and bottom alga samples in Cat Ba waters shows that 400 species and sub-species, 105 genera of 7 algae classes have been initially identified, of which the dominant one is silica alge (Bacillariophyceae) in 222 species, and 59 genera; followed by Dinophyceae in 164 species, and 36 genera; Dictyochophyceae in 3 species, and 2 genera; Cynophyceae in 4 species, and 3 genera; Chroococceae in 1 species, and 1 genus; Chlorophyceae in 5 species, and 3 genera, and Conjugatophyceae in 1 species and 1 genus. The structural constituent of the regional phytoplankton at Cat Ba are mainly the species specific to the tropical and sub-tropical coastal area.
Remarkable genera with many species of silica algae are: Chaetoceros in 29 species, followed by Coscinodiscus in 20 species, Rhizosolenia in 20 species, Thalassiosira in 11 species, Pleurosigmain 9 species, Bacteriastrum in 8 species, and Amphora in 6 species. The rest genera range from 1 to 5 species. For the Dinophyceae class, the dominant species number is Ceratium in 29 species, Protoperidinium in 28 species, Dinophysis in 10 species, and Prorocentrum species in 7 species.
+ Density of phytoplankton: The research results obtained at the continuous sampling stations of Cat Dua and Coc Cheo in May (the rainy season) and in November (the dry season) show that density of phytoplantonic changes seasonally.
- In the rainy season: the cell density strongly fluctuates, from 25,000 cells/L to less than 5,000 cells/L. The density of phytoplankton is strongly dominated by Pseudonitzschia sp. with its density of up to104 cells/L. The density change occurs in the ascending orders, reaching to the maximum value and then decreasing.
- In the dry season: The density of phytoplankton varies from 1,000 – 10,000 cells/L. To significantly constribute to the cell density, they are Chaetoceros curvisetus, Pseudonitzschia and Bacteriastrum hyalinum.
+ Species composition: 131 species, 60 genera, 47 families, 10 orders, 7 phyla are identified in the property. Among them, the species of Copepoda has the biggest proportion of 80.7%. Zooplanton includes 4 basic ecological groups.
- Coastal zooplankton: this is the main group creating the tropical coastal zooplankton fauna. They are often capable of well adapting to the various temperatures, and salinity (20 - 30‰). In southeastern sea of Cat Ba, they make up 70-80 % of the total species, frequently occur in a large quantity or with rather high frequency in the samplings. Those are Paracalanus crasirostrix, Acartia spinnicauda, pacifica, Labidocera bipinnata, L. minuta, Centropages tenuirenis, Temora turbinata, Sagitta delicata. In addition, there are also other species that can be found with fewer frequence such as Canthocalanus pauper, Paracalanus pavus, Acrocalanus gracilis, Calanopia elliptica, Oikopleura resfuces, etc.
- Wide distributing group: these species are able to distribute widely the worldwide, from the brackish area of estuaries to the oceanic area, such as Microsetella norvegica, Oithona flumifera, Oithona similis, Oithona brevicornis, Corycaeus speciosus, etc. They make up about 20 % of the total species.
- Temperate warm-water group: there are not typical temperate species in Cat Ba like Calanus sinicus, but it is usally found Acartia erythraea, Labidocera euchaeta,
- Brackish water zooplankton: The salinity is over 20‰ in the southeastern sea of Cat Ba. However, in the rainy season, the salinity of the surface layer is sometimes down to 15‰, so some typical species of the brackish water and low salinity (some 10‰) still appear. Nevertheless, due to the temporary occurrence, their quantity is insignificant and unfrequent. They are: Schmarkeria gordiades, Pseudodiaptomus marinus, and Eurytemora sp.
- Group of salty water species: This group is adaptive to the high salinity and rather stable. The salinity is quite high, 30 – 32‰, in the dry season in Cat Ba. Usually, many salty water species appear. Typical species, sometimes in rather big quantity, are Eucalanus subcrasus, Nannocalanus minor, Undinula vulgaris and the most Acrocalanus gilber that takes up 14-27% of the total individuals.
+ Density: Because the zooplankton is characterized as a tiny body floating in the water, they are allocated widely in the sea with different densities. However, due to the studied part of the sea being not large, the individual allocation of each species is not significantly different in each surveyed point. Many species have rather big quantity in two surveyed points, such as: Acrocalanus gilber, Paracalanus pavus, P. crasiostrix, Corycaeus spesious. The allocation varies somewhat in some indicative species such as Acartia Pacifica, Undinula vulgaris, Eucalanus subcrasus.
Allocation feature of zooplankton in southeastern sea of Cat Ba is within general rule on allocating quantity of zooplankton at the coastal area in the west of the Tonkin Gulf. This is trend of uneven allocation of zooplankton among stations in the same time of sampling.
+ Density distribution
The density distribution of zooplankton in Cat Ba waters is unequal, probably due to difference of terrain, current and tidal conditions in each area.
(6). Marine lake ecosystem: It is quite a typical ecosystem type for the karst limestone islands of Cat Ba that the other places do not have. The marine lake is formally regarded as a sub-ecosystem type of Cat Ba, which was proposed by the Institute of Marine Environment and Resources in 1999 (Do Cong Thung et al., 1999). According to the published documents, there are only about 200 marine lakes all over the world, which are considered as a rare and endemic ecosystem to the host countries.