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Tropical intertidal sediments often contain porewater of relatively high salinity, especially in areas exposed to
longer periods without seawater inundation and high evaporation. Such an area exists on the west coast of Zanzibar:
a high intertidal mangrove plateau, flooded only during spring high tides, with sediment porewater salinities
commonly exceeding 100 ppt. A field survey was conducted in this area to examine variations in population density
of major meiofaunal taxa and the assemblage structure of free-living marine nematodes during spring-neap tidal
cycles. Samples were taken on seven occasions for two months, starting from the end of the rainy season. Porewater
salinity remained high throughout the sampling period, ranging from 89 to 160 ppt. Neither spring tide inundation
nor heavy rains lowered the salinity markedly. The meiofauna consisted only of four taxa, present on all sampling
occasions: nematodes, harpacticoid copepods, plathyhelminthes and chironomids. Densities in surface sediments
(0–5 cm) were low compared to other mangrove areas, ranging from 271 to 656 animals 10 cm−2 with nematodes
dominant on all sampling occasions (58–87%). Density fluctuations could not be explained by the effects of spring
tide inundation, but the meiofauna showed significant correlations with grain size and organic material. Despite
the wide range of salinity, only the numbers of chironomids were negatively correlated with increased salinity.
Nematode species diversity was low in all samples, although altogether 28 species were recorded in the samples.
Four species occurred in more than 50% of the samples (Microlaimus sp. (100%), Metalinhomoeus sp. (76%),
Daptonema sp.l (56%), Chromadorina sp. (56%)) while 12 species were found only in one or two samples.
Multidimensional scaling ordination (MDS) of the nematode species abundance data indicated little effects of
spring tide inundation on the assemblage structure, but rather a successive change from wet to dry season with a
reduction in species diversity and increased numbers of the dominant nematode species Microlaimus sp. |
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