DEEP-SEA BENTHIC FORAMINIFERAL SPECIES-DIVERSITY IN THE NE ATLANTIC AND NW ARABIAN SEA - A SYNTHESIS

We present a synthesis of species diversity data (Fishers' alpha index , Shannon-Wiener (log,), ES(100), Rank 1 Dominance) for ''live'' (stai ned) foraminifera from five bathyal (1340 m depth) and abyssal(4450-49 50 m depth) sites in the NE Atlantic and a 3400 m-deep site in the Ara bian Sea. Three Atlantic sites (Porcupine Seabight, BIOTRANS, Porcupin e Abyssal Plain) are subject to seasonal phytodetritus inputs that sup port low diversity populations (8-17 species). In other respects the f oraminifera are highly diverse. The meiofaunal fractions (> 45 or > 63 mu m; including fragmented and phytodetritus species) of abyssal Atla ntic 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) yield ed 232 species. In both cases, values for diversity measures were very high. Diversity was rather lower in bathyal Porcupine Seabight sample s (0-1 cm layer), which yielded < 100 species. The foraminiferal macro fauna (> 500 mu m; Porcupine and Madeira Abyssal Plains) was also spec iose (113-133 species), but diversity measures were lower and dominanc e higher than for the meiofauna. All assemblages contained numerous un described species, many belonging to poorly known monothalamous, soft- bodied taxa. Sample diversity was influenced by several factors. Combi ning phytodetrital and sediment populations reduced diversity and incr eased dominance slightly; the inclusion of deeper sediment layers and finer sieve fractions had the opposite effect. The inclusion of fragme nts had mon impact on macrofaunal than on meiofaunal diversity, althou gh in both cases the effect was inconsistent (either positive or negat ive). Porcupine and Madeira Abyssal Plain multicore samples ( > 63 mu m fraction) contained substantially more foraminiferal species than ne matode species; the numbers of foraminiferal species in boxcore sample s ( > 500 mu m fraction) were comparable to, or greater than, literatu re values for macrofaunal taxa such as polychaetes and isopods. Few of the more abundant species at our Atlantic abyssal plain and Arabian S ea sites are endemic. This is consistent with literature evidence that many common deep-sea foraminiferal species are cosmopolitan and impli es that global foraminiferal diversity may be more modest than the hig h sample diversity might suggest. Calcareous foraminifera, which are w ell-known taxonomically and have a good fossil record, may provide a m odel for diversity patterns among the deep-sea benthic biota in genera l. (C) 1998 Elsevier Science Ltd. All rights reserved.