DIVERSITY AND LONG-TERM DYNAMICS OF MACROBENTHOS IN A MANGROVE PLANTATION AND TWO NATURAL ASSOCIATIONS OF THE KHANH HOA PROVINCE, VIETNAM

摘要

Rapid reduction of mangrove forests has prompted the Vietnamese government to launch a number of conservation and rehabilitation programs all over the country. One of these projects took place in Dam Bay, Khanh Hoa Province, where seedlings of Rhizophora apiculata had been planted. The present study includes a 10-year long monitoring of macrobenthos in the newly planted mangroves and a comparison of the composition and structure of the macrobenthic invertebrate assemblage of planted and local natural mangrove stands. Material was collected in three intertidal zones in a mangrove plantation and two natural mangrove associations: a small association in Dam Bay and another, large and mature association in Nha Phu Bay. The natural mangrove association in Dam Bay is a narrow belt of Rhizophora apiculata, resembling the upper intertidal zone of the mangrove plantation before 2004. Initially, the diversity and abundance of macrobenthos were low in the mangrove plantation. In 2004, in the mid intertidal zone, seaward from the mature trees of the upper intertidal zone mangrove seedlings were planted. They slowed the tidal currents with their growth and nutrients were deposited in the ecosystem. Developing trees provided litter fall, hard substrata and shadow creating suitable conditions for microalgae growth. As a consequence, the diversity and abundance growth of eurybiotic and opportunistic species occurred. In a few years, young trees and mature trees of the upper intertidal zone merged to form a sizable mangrove forest. This has led to the diversity growth of specialized mangrove gastropods, bivalves and crabs species. During the monitoring period, the composition and structure of the macrobenthic assemblage advanced towards a mature natural mangrove association at Nha Phu. But still after ten years of growth these two mangrove stands show significant differences: the specialized mangrove fauna is much more diverse at Nha Phu, while its bivalve assemblage is completely different. Possibly these distinctions are determined by the floristic diversity, structural complexity and maturity of the mangrove association at Nha Phu. A total of 139 species of macrobenthic invertebrates has been found. Key groups are gastropods, bivalves and crabs. The composition and diversity data of the present study on the investigated mangrove associations fits in the general pattern of mangrove fauna evolution of the Indo-West Pacific and Southeast Asia.