Ha Long bay - Cat Ba island located in the Northeast of Vietnam, in the area of Ha Long city (Quang Ninh province) and Cat Hai island district (Hai Phong city) is the largest limestone island in Vietnam. Features of biodiversity include the seven typical ecosystems of tropical limestone islands, namely tropical rainforest ecosystem, cave ecosystem, mangrove ecosystem, tidal ecosystem, coral reef ecosystem, soft-bottom ecosystem. Ha Long - Cat Ba also represents a high level of species diversity with 4,622 species of plants and animals on land and in the sea. In particular, on land there are 2,464 species, accounting for 53.3% and under the sea there are 2158 species, accounting for 46.7%. Up to 16 species and 114 species have been recorded in the Vietnam Red List and in the IUCN List, respectively. Together with hundreds of rare and precious species at national and regional levels, there are species of global conservation value, demonstrating that Ha Long bay - Cat Ba archipelago can possibly be a world heritage site according to the criteria of biodiversity (ix and x criteria).
Submerged caves and marine lakes are two iconic and common ecosystems in Ha Long-Cat Ba area. However, their biological characteristics are poorly known since very limited studies have been conducted. The aim of this study was to investigate biodiversity in these ecosystems. Three submerged caves (Hang Sang, Hang Toi and Qua Bang) and three marine lakes (Dau Be, Ang Du and Qua Bang) were chosen for study. The results showed that organism communities in submerged caves were abundant with over 142 species found. The popular groups comprised sponge and soft coral which were distributed along the length of the caves. Several species with high economic value were widespread in the caves, including stone crab Myomenippe hardwickii, flower crab Portunus pelagicus, cone snails Trochus pyramis, shoemaker spinefoot Siganus sutor, snapper Lutjanus russellii. Species that permanently live in caves were not detected. In the lakes where water is well exchanged to the sea, coral reefs were found and they usually formed a narrow band around the lakes. Sandy beaches were often observed around the lakes at the depths of 0.5-2 m, containing specialty species as clam (Sanguinolaria diphos), snout otter clam (Lutraria rhychaena), sea cucumbers with high density (in Dau Be, Qua Bang). There was no coral reef in the enclosed lake (Ang Du) because of low salinity.
Seagrasses in Truong Sa archipelago have not been intensively studied and understood, even in some obviously progressive works recently published. It is essential to carry out much more researches to fill up understanding gaps of seagrasses in the area. In 2019, within the framework of the project coded KC09.29/16–20, two surveys were conducted for three selected islands (Truong Sa Lon, Thuyen Chai and Song Tu Tay) in the area, focusing on marine ecosystems including seagrass beds. Survey results in the selected islands show eight seagrass species (one new record), the estimated total distribution area of 1,190 ha, the total reserve of biomass reaching 7,417.2 tons.dry, equivalent to 11,432.8 tons.CO2.
MỞ ĐẦUSự hình thành địa chất, địa mạo ở khu vực Hạ Long, Cát Bà đã trải qua rất nhiều quá trình sụt chìm, biển tiến và tạo núi -biển thoái, sự thay đổi nâng lên hạ xuống của các mảng kiến tạo và sự xâm thực karst kéo dài khoảng 20 triệu năm qua đã hình thành nên các hang ngầm và hồ nước mặn khi khu vực bị ngập chìm do biển dâng [1]. Mặc dù hai kiểu sinh cảnh này khá phổ biến ở khu vực song do thiếu các thiết bị và kinh nghiệm nghiên cứu nên tới năm 2003 các nhà khoa học của Viện Tài nguyên và Môi trường biển và Italia trong dự án hợp tác về bảo tồn đa dạng sinh học ven biển Việt Nam đã khảo sát một số hồ nước mặn và hang nửa ngầm. Các nghiên cứu mới tập trung vào nhóm hải miên, còn các nhóm sinh vật khác ít được
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