Assemblages of recent intertidal mangrove foraminifera from the Akanda National Park, Gabon: sea level proxies preserved in faunal assemblages

摘要

The composition and vertical zonation of modern benthic foraminifera was studied at two intertidal mangrove environments in the Akanda National Park, Gabon. Foraminifera inhabiting the muddy substrates were found to represent a unique assemblage and are distinct from other nearshore biotas. A total of twenty one benthic species were recorded. Numerical analyses of faunal assemblages indicate that the distribution of taxa is vertically constrained, divided into two faunal zones that are elevation dependent. A low-diverse agglutinated foraminiferal assemblage, dominated by Ammotium salsum and Ammobaculites pseudospirale, was found to characterize the elevated landward field area in muddy substrates around Avicennia germinans mangroves. The assemblage is composed of agglutinated taxa only and is indicative for High Mean Water Neap Tide elevations. A moderately diverse assemblage of agglutinated and calcareous foraminifera was recovered from around Rhizomosa racemose stands and represents biotas at Mean Low Water Neap Tide elevation. The assemblage is dominated by species of Miliammina, Arenoporella mexicana, Ammotium salsum and Astrononion stelligerum. The species richness of individual assemblages was found to be tide-related and reveals a strong trend of decreasing diversity moving up the shore. These distributions resemble typical intertidal zonation patterns in mangroves and estuaries and highlight their potential for sealevel reconstructions. The study constitutes the first survey of mangrove foraminifera from Gabon.