Distribution of benthic foraminifera in an oceanic (Darwinian) barrier reef lagoon, Bora Bora, French Polynesia

Abstract

Bora Bora, French Polynesia, represents the classic oceanic (Darwinian) barrier reef system, complete with a deep and extensive lagoon. The foraminiferal faunas of such oceanic barrier reef lagoons have never been studied in detail. How they relate to atoll lagoons and more broadly to coral reef lagoons in general is not known. Back-reef lagoon samples from Bora Bora were studied for their foraminiferal content. The fauna contains sparse larger symbiont bearing foraminifera, and most samples are dominated by a textulariid fauna of mainly Textularia gr. sp. 1, Textularia gr. foliacea and Textularia oceanica. Statistical analyses revealed three main assemblages, a Textularia-dominated deep central lagoon assemblage, an outer shallow lagoon assemblage characterised by abundant porcelaneous taxa, and a narrow embayment assemblage with abundant Elphidium. Although the species at Bora Bora are common to the Pacific Ocean, the Textularia-dominated assemblages are markedly different from the lagoon assemblages reported from atoll and non-oceanic barrier reef lagoons in the Pacific. These unique assemblages are hypothesised to have evolved due to interactions of the central volcanic island with the lagoon system, likely via the run-off of fine sediment and nutrients. Thereby, the prerequisites for abundant larger, symbiont-bearing foraminifera, which prefer clear, nutrient-poor waters, are largely unavailable. Despite similar substrates and facies, differences between species compositions indicate subtly different disconnected western and eastern deep lagoon environments, possibly due to differences in connectivity to the open ocean. This study provides the first detailed account of the foraminifera from the lagoon of an oceanic barrier reef lagoon.