Terrestrial influence as a key driver of spatial variability in large benthic foraminiferal assemblage composition in the Central Indo-Pacific

Large benthic foraminifera (LBF) are important components of tropical shallow-marine ecosystems, in which they are abundant on both coral reefs and in the inter-reef areas. These protists are, similar to reef-building corals, photosymbiotic, restricted to warm (> 14 degrees C) seawater temperatures, and to benthic habitats within the photic zone. Because of their abundance, similarities to corals, short life-spans, and sensitivity to water quality, LBF are widely applied in environmental assessments of reef habitats. I review data on alive LBF assemblages within their environmental context. To be able to do so, I review the occurrences of individual species of the, in most environments, most abundant Central Indo-Pacific LBF, the Amphisteginidae, Calcarinidae, and Nummulitidae. In doing so I highlight the large uncertainties with respect to ecophenotypic and genetic variation among populations. In each of these families there appears to be substantial cryptic diversity, and thorough investigation using new morphological techniques results in higher diversity than previously known. Field observations of LBF show that nutrient availability, hydrodynamic energy, light intensity and spectrum, substrate type (sand, rubble, rubble with sand, algae, sea grass), and temperature are important environmental parameters for the occurrence of individual species. Many of these parameters are interrelated and are in some way influenced by water depth at least in part because occurrences have been related to conditions in the overlying watercolumn, and not to the microhabitat within the benthic boundary layer where the LBF live. In addition, I argue that differences in tolerance to terrestrial influence among species plays a large role in the occurrences of LBF, not only on the predominantly hard substratum reefs, but also in the predominantly sandy substratum inter-reef areas. Hence, there are distinct assemblages on reefs surrounded by deep, oceanic water, compared to terrestrially influenced reefs. Furthermore, assemblage composition is not only determined by water-quality parameters, but also by ecological dynamics of the benthic habitat on coral reefs, including shifts away from high coral cover. Abrupt changes resulting in permanent or temporal changes in the benthic habitat of coral reefs result in changes in the LBF assemblage. Especially calcarinids become more abundant with increased algal cover. Assemblage composition of benthic foraminifera, especially the weighted contribution of LBF to the total benthic foraminifera assemblage (the FoRAM Index), has been used to provide an assessment of water quality. Here I show that, in the Central Indo-Pacific, several species of LBF can occur abundantly in nearshore settings experiencing high terrestrial run-off. For a more consistent application of this method, the sampling and processing protocols (e.g., depth, sieve fractions included) should be standardised. Furthermore, differences in regional species pools require regional calibrations of the method, or finding alternative ways of simple assessments of reef quality using LBF.