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 <i>Rhizophora stylosa</i>, a bush of <i>Avicennia</i> aff. <i>alba</i> behind it, and muddy sand with fiddler crabs (<i>Uca </i>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 cm<sup>2</sup> in the <i>Uca</i> spp. biotope to 244 indiv./10 cm<sup>2</sup> beneath the <i>Rhizophora </i>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 <i>Uca </i>and <i>Avicennia</i> biocenoses; the second assemblage occupies the upper sediment layer in the <i>Rhizophora</i> stand; a less abundant but specific assemblage of several nematode species occurs in the subsurface sediments at all three sites.