Deep-sea benthic foraminiferal species diversity in the NE Atlantic and NW Arabian sea: a synthesis

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We present a synthesis of species diversity data (Fishers’ alpha index, Shannon–Wiener (log 2), ES(100), Rank 1 Dominance) for “live” (stained) foraminifera from five bathyal (1340 m depth) and abyssal (4450–4950 m depth) sites in the NE Atlantic and a 3400 m-deep site in the Arabian Sea. Three Atlantic sites (Porcupine Seabight, BIOTRANS, Porcupine Abyssal Plain) are subject to seasonal phytodetritus inputs that support low diversity populations (8–17 species). In other respects the foraminifera are highly diverse. The meiofaunal fractions (>45 or >63 μm; including fragmented and phytodetritus species) of abyssal Atlantic samples yielded >110 and >170 species in the 0–1 cm and 0–10 cm layers, respectively; the Arabian Sea sample (0–1 cm layer only) yielded 232 species. In both cases, values for diversity measures were very high. Diversity was rather lower in bathyal Porcupine Seabight samples (0–1 cm layer), which yielded <100 species. The foraminiferal macrofauna (>500 μm; Porcupine and Madeira Abyssal Plains) was also speciose (113–133 species), but diversity measures were lower and dominance higher than for the meiofauna. All assemblages contained numerous undescribed species, many belonging to poorly known monothalamous, soft-bodied taxa. Sample diversity was influenced by several factors. Combining phytodetrital and sediment populations reduced diversity and increased dominance slightly; the inclusion of deeper sediment layers and finer sieve fractions had the opposite effect. The inclusion of fragments had more impact on macrofaunal than on meiofaunal diversity, although in both cases the effect was inconsistent (either positive or negative). Porcupine and Madeira Abyssal Plain multicore samples (>63 μm fraction) contained substantially more foraminiferal species than nematode species; the numbers of foraminiferal species in boxcore samples (>500 μm fraction) were comparable to, or greater than, literature values for macrofaunal taxa such as polychaetes and isopods. Few of the more abundant species at our Atlantic abyssal plain and Arabian Sea sites are endemic. This is consistent with literature evidence that many common deep-sea foraminiferal species are cosmopolitan and implies that global foraminiferal diversity may be more modest than the high sample diversity might suggest. Calcareous foraminifera, which are well-known taxonomically and have a good fossil record, may provide a model for diversity patterns among the deep-sea benthic biota in general.