LIVING (ROSE-BENGAL STAINED) BENTHIC FORAMINIFERA FROM THE PAKISTAN CONTINENTAL-MARGIN (NORTHERN ARABIAN SEA)

The Arabian Sea is characterized by one of the world's most pronounced oxygen minimum zones (OMZ)(< 0.1 O-2 ml/l), which impinges on the sea floor at 200-1000 m depths. The OMZ in the Arabian Sea results from ex tremely high surface water productivity and moderate thermocline venti lation. Nine box cores were taken on two parallel down-slope transects covering depths from 500 to 2000 m. From these nine box cores living (Rose Bengal stained) benthic foraminifera were studied in detail. Wit hin the upper part of the OMZ, Bolivina dilatata and Bulimina exilis a re the most abundant species. In the lower part of the OMZ, Uvigerina peregrina and B. exilis are the most abundant. Just below the OMZ, at a water depth of about 1250 m, the assemblage is typically dominated b y Rotaliatinopsis semiinvoluta and U. peregrina; in still deeper water s (1500-2000 m) Bulimina aculeata and Epistominella exigua are the mos t prominent species. Transect II was sampled three weeks after transec t I; on the average, standing stocks were four times higher in transec t II. However, down-slope species distributions are similar in the two transects, both in the 63 mu m and the 150 mu m size fractions. Also vertical (in-sediment) distributions are remarkably similar. This indi cates that standing stock differences between the two transects can be ascribed either to the effect of patchiness or, more likely, to the n ature and the amount of organic flux. We found unambiguous evidence th at in and below the OMZ many benthic foraminifera persist in suboxic t o anoxic microhabitats. This regards surface as well as subsurface hab itats. Evidently, oxygen is not a limiting factor for a considerable n umber of species. The obvious relationship between species distributio n and the OMZ might then be explained in terms of preferences for amou nt or type of organic flux. An alternative explanation involves favour able effects resulting from the absence of larger predators. The vario us models pertaining to the limiting balance between organic flux and oxygen are discussed. (C) 1998 Elsevier Science Ltd. All rights reserv ed.