Meiobenthos of a mangrove intertidal area of Nha Trang Bay, Vietnam, South China Sea: quantitative distribution and community structure with special reference to free-living nematodes.
Meiobenthic studies were performed in an intertidal area in the Be river estuary (Nha Trang Bay, Vietnam). The study area is a fragment of riverine type mangroves heavily damaged by human impact, including timber cutting and wasting. There are three biotopes situated in the middle intertidal zone: a fringe of Rhizophora stylosa, a bush of Avicennia aff. alba behind it, and muddy sand with fiddler crabs (Uca spp.), which is free of mangrove plants. Three replicate samples of meiobenthos were collected in each biotope, each sample was subdivided into two layers: 0-1 and 1-4 cm. The abundance of metazoan meiobenthos varied from 735 indiv./10 cm2 in the Uca spp. biotope to 244 indiv./10 cm2 beneath the Rhizophora trees. Six taxonomic groups of high rank were found among the meiofauna: Nematoda, Copepoda (Harpacticoida), Oligochaeta, Turbellaria, Kinorhyncha, and Foraminifera (Allogromiida). Spatial variability of meiobenthos and its key taxa was estimated, and spatial distribution regularities of free-living nematode species were described. Nematodes constituted 90-95% of all meiobenthic organisms in the samples. A total of 48 species of free-living nematodes were found in the investigated mangrove intertidal area. About 90% of total meiobenthos inhabited the upper 0-1 cm of the sediments. In terms of species composition and set of dominants, the nematode community is comprised of three local assemblages: one of them inhabits the uppermost centimeter in the Uca and Avicennia biocenoses; the second assemblage occupies the upper sediment layer in the Rhizophora stand; a less abundant but specific assemblage of several nematode species occurs in the subsurface sediments at all three sites.