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The study area has been selected in the Pichavaram mangrove, at latitude of about 11020' N and longitude about 79047' E. The greater influence of continental water during rainy season (August to October) leads to low salinity, pH and Ca content. The impact of the continental water, pushed along the coast NNW direction, results in turbid and poorly salted (20) coastal waters during the rainy season. During the dry season (March - July) the salinity increases up to 34.5. Along the coast, they form intertidal mud flats with 90% of the grains smaller than 3.5ìm. In the study area, all the sediments have the same terrestrial origin and exhibit similar grain size characteristics, making negligible the influence of grain size on the distribution of foraminiferal assemblages.
When the mud flats progressively become stabilized, a high density population of Avicennia germinans grows. Olden well - stabilized flats are covered with a forest dominated by Avicennia germinans and Rhizophora racemosa. The oldest inner forest is dominated by 20 m high Avicennia trees. In the Vellar estuary, this forest passes landward to freshwater wetlands whilst it passes to dry fields along the river near Coleroon.
A total of 25 surface sediment samples were collected in February 2001, August 2002 December 2002 in stations selected to provide information on foraminiferal assemblages associated with various stages of development of the mangrove forest. Forty species have been identified in total assemblages. The species richness ranges between 0 and 20 and the density between 0 and 1800 tests in 500 ml of sediment. The highest values are in the mangrove forest, the lowest on the mud flat. Foraminiferal assemblage showed a strong seasonality.
Finally, the distribution and the growth of foraminiferal assemblage depend on: 1) stabilization of the sediment (At the time of deposition, the mud is very fluid with 100 to 200% of water, easily removed and foraminifera can not settle); 2) the relative contribution of coastal water (calcareous species dominated by Ammonia tepida and Cribroelphidium excavatum and continental waters with low Ca content (agglutinated species dominated) i) by Trochamminita irregularis in the very low salinity wet land and (ii) by Trochammina inflata associated with other agglutinated species, at the transition to the dry fields) leading to strong zonation of the assemblages and a drastic seasonal cycle; 3) the presence of canopy and litter of the mangrove forest that protect the organisms from dying and heating by wind and sun and provide trophic resources; 4) the verticle elevation that acts indirectly by influencing the time of aerial exposure and the colonization of mangrove trees. An extensive knowledge of these parameters and of the associated assemblages will make possible the interpretation of the evolution of the mud banks and of adjacent environments at different time scales, including geological times.
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