Diversity and Long-Term Dynamics of the Macrobenthos in a Mangrove Plantation and Two Natural Associations of Khanh Hoa Province, Vietnam

The rapid decline in the area occupied by mangrove forests has convinced the Vietnamese government to launch a program for conserving and restoring the mangrove ecosystems throughout the country. This program included an experimental planting of Rhizophora apiculata seedlings in Dam Bay in Khanh Hoa province. In the course of this work, the state of the macrobenthos was monitored in new mangrove plantations, and its quantitative and qualitative characteristics were compared with those in natural mangrove associations of this area. The bulk of the material was collected in three intertidal zones in the area of ​​mangrove plantations in Dam Bay. In addition, natural mangrove associations were investigated: one in Dam Bay, and the other (the most mature and largest association of the area) in the neighboring Nha Phu Bay. The natural mangrove association in Dam Bay was represented by a narrow belt of Rhizophora apiculata . The intertidal zone in the area of ​​mangrove plantations is also represented by a narrow belt of Rhizophora apiculata . Until 2004, the middle and lower zones were formed by the mudflat. In 2004, mangrove seedlings were planted in the middle intertidal zone below the mature trees of the upper zone. As the seedlings grew, they began to slow down tidal currents, and nutrients began to linger in the ecosystem. The developing crown of young trees began to give litter and shade, favorable for the development of microalgae. In addition, young trees formed a hard substrate. As a result, there was an increase in the species diversity of the macrobenthic eurybiotic species and an increase in the abundance of opportunistic species. Young trees in the middle intertidal zone and formed a single mangrove forest of a large area together with adult trees of the upper zone, which caused the appearance of specialized species of gastropods and bivalve mollusks, as well as crabs. During the monitoring period, the composition and structure of the macrobenthos of mangrove plantations were gradually approaching to those in the mature natural mangrove association in Nha Phu. However, significant differences still remain between these associations after ten years of observing the growth of mangrove seedlings. The macrobenthos of Nha Phu is represented by a large number of specialized mangrove species of gastropod mollusks, and it also has a completely different structure of the taxocene of bivalve mollusks. This is likely explained by the high floristic diversity in Nha Phu and the maturity of this mangrove association. As a result of qualitative and quantitative collection of the macrobenthos, 139 species of invertebrates were found in the three mangrove associations studied. The key groups of the macrobenthos are gastropods, bivalve mollusks, and crabs. The data obtained on the composition and diversity of the macrobenthos fit into the general pattern of the mangrove fauna of the Indo-West Pacific and Southeast Asia.