Foraminiferal microhabitats in a high marsh: Consequences for reconstructing past sea levels

The foraminiferal faunas of the high marsh of Plougoumelen, in the western part of the Gulf of Morbihan on the French Atlantic coast, have been studied in three successive sample sets taken in weekly intervals in April 2001. Each sampling day, four cores have been taken in different marsh environments, with or without vegetation, in order to determine the microhabitat of the major species and the eventual influence of a vegetation cover. Physico-chemical parameters have been surveyed during each sampling day. The foraminiferal faunas are similar to other faunas described from temperate salt marshes. We have distinguished three groups of taxa with different microhabitats: (a) Jadammina macrescens is found in comparable numbers down to about 9 cm depth; (b) shallow infaunal taxa, such as Miliammina fusca and Trochammina inflata; and (c) deep infaunal taxa, such as Haplophragmoides wilberti and Arenoparrella mexicana. Although these deep infaunal taxa have to cope with low oxygen concentrations, or even anoxia, life at depth in the sediment appears to provide ecological advantages. Our survey of the physico-chemical parameters shows an extreme variability at the surface, in comparison with much more stable conditions deeper in the sediment. These more stable conditions, demanding smaller tolerance ranges for several parameters (temperature, salinity, water content), appear to be advantageous to deep infaunal taxa. For the species J. macrescens, the microhabitat depth appears to be controlled by the water content of the sediment that strongly fluctuates in function of the meteorological conditions. During dry periods, when the water content of the sediment is lower, J. macrescens displays a much shallower microhabitat than during more humid periods. At the sediment surface, the presence of vegetation appears to have a positive influence on the density of the foraminiferal faunas by providing a shelter to desiccation during low tide, or more prolonged dry periods. Important differences exist between the live faunas found in the uppermost centimetre of the sediment, and those of the total, 9 cm deep sediment cores. Therefore, data inventories of living faunas, based on the superficial centimetre alone, can not be used as a basis for paleo-environmental reconstructions without correction for these microhabitat effects.