Assemblages of benthic foraminifera in a Holocene core from the outer area of Bahía Blanca estuary in Argentina were studied. The systematic of the group was updated, and provided 28 genera distributed among 59 species, six species with nomenclature aperta. The main species recorded were Buccella peruviana f. campsi (Boltovskoy), Ammonia beccarii (Linné), Elphidium gunteri Cole, Elphidium galvestonense Kornfeld, Elphidium articulatum d’Orbigny and Elphidium discoidale (d’Orbigny). These species allowed an estuarine environment to be determined. Qualitative and quantitative studies conducted on the faunistic content in the core allowed identification of three subenvironments closely linked to sea-level fluctuations during the mid Holocene: a lower zone (6350 yr BP) corresponding to a high intertidal environment, an intermediate zone (2460 yr BP) representing a low intertidal environment related to a system of channels and variations in environmental energy, and an upper zone characterized by the development of a high-energy littoral environment affected by the action of waves, tides and tidal currents, which underwent progressive change until it attained present-day conditions.

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Large populations of the living benthic foraminifera Haynesina germanica (Ehrenberg 1840) are reported for the first time from marsh and intertidal mud flat sediments of the Bahía Blanca estuary (Argentina). Maximum abundance of living specimens was recorded in shallow intertidal environments. The species was previously recorded from many European and North American shallow-water coastal settings, but has not been documented from Argentina. Comparative faunal assemblage analysis from dated core sample material from within the Bahía Blanca estuary shows that the species has not been present for at least the last 8,200 years. This supports the hypothesis that the species has been accidentally introduced outside its natural range as a probable result of ballast water and/or shipping activities. This study is the first report of a successful invasion of nonindigenous benthic foraminifera to the South Atlantic coast of Argentina. The introduction of nonnative foraminiferal species provides new evidence that human-mediated breaching of biogeographic barriers, will ultimately result in the biotic homogenization of foraminiferal intertidal faunal assemblages.