Distribution, biomass and diversity of benthic foraminifera in relation tosediment geochemistry in the Arabian Sea

The distribution, biomass, acid diversity of living (Rose Bengal stained) d eep-sea benthic foraminifera (> 30 mu m) were investigated with multicorer samples from seven stations in the Arabian Sea during the intermonsoonal pe riods in March and in September/October, 1995. Water depths of the stations ranged between 1916 and 4425 m. The distribution of benthic foraminifera w as compared with dissolved oxygen, % organic carbon, % calcium carbonate, a mmonium, % silica, chloroplastic pigment equivalents, sand content, pore wa ter content of the sediment, and organic carbon flux to explain the foramin iferal patterns and depositional environments. A total of six species-communities comprising 178 living species were ident ified by principal component analysis. The seasonal comparison shows that a t the western stations foraminiferal abundance and biomass were higher duri ng the Spring Intermonsoon than during the Fall Intermonsoon, The regional comparison indicates a distinct gradient in abundance, biomass, and diversi ty from west to cast, and for biomass from north to south. Highest values a re recorded in the western part of the Arabian Sea, where the influence of coastal and offshore upwelling are responsible for high carbon fluxes, Esti mated total biomass of living benthic foraminifera integrated for the upper 5 cm of the sediment ranged between 11 mg C-org m(-2) at the southern stat ion and 420 mg C-org m(-2) at the western station. Foraminifera in the size range from 30 to 125 mu m, the so-called microforaminifera, contributed be tween 20 and 65% to the abundance, but only 3% to 28% to the biomass of the fauna. Highest values were found in the central and southern Arabian Sea, indicating their importance in oligotrophic deep-sea areas. The overall abundance of benthic foraminifera is positively correlated with oxygen content and pore volume, and partly with carbon content and chlorop lastic pigment equivalents of the sediment. The distributional patterns of the communities seem to be controlled by sand fraction, dissolved oxygen, c alcium carbonate and organic carbon content of the sediment, but the critic al variables are of different significance for each community. (C) 2000 Els evier Science Ltd. All rights reserved.