Deep -Sea Reseorch , Vol. 38. Suppl 2. SI069-SIOll2 . 1991. Printed In Great Bruam. U1'JH S3 00 . 000 © 1991 Pergamon Pro" pic Microbial biomass and population diversity in the upper water column of the Black Sea DAVID F. BIRD·t and DAVID M. KARL·:j: (Re ceived 7 November 1989; in revised form 16 November 1990; accepted 3 December 1 CJ90) Abstract-Estimate s of bacterial abundance, bacterial biomass and micropl ankton biom ass in the Black Sea weremade in the upper portion of the watcr column (0-500 m) during the second leg (l4- 28 May 1988) of the U .S.-Turkey Expedition aboard R.V. Knorr . The microbial biomass maximum at the interface betweenoxygenated and sulfide-rich waters, seen in previous years in the Black Sea and generally characteristic of redoxclines in other anox icmarine habitats, was absent on this occasion. In spite of the absence of a general enrichment in total microbial or bacterial biomas s across the oxic-to-anoxic boundary, there were sharply delineated populationsof micr oorganism s living at spe cific depths with in the redoxcline. Among the microhial presen t were phot otrophic bacteria , ciliates bearing attached bacteria and putative Achromatium cells. INTRODUCTION THE presence of oxic-anoxic gradients, or redoxclines, in selected marine environments provides a potential habitat for the growth and accumulation of specialized microb ial communities (JfilRGENSEN, 1982) . Redoxclines in sea water range in dimension from microscopic (PAERL, 1985), through the narrow «3 mm) but intensely active interfaces of microbial mats (JfilRGENSEN, 1982), to regions of extensive (> 10 m) co-occurrence of both O 2 and H 2 S (SOROKIN, 1964; KARL, 1978). The region of the water column where both O 2 and H 2 S co-exist has been termed the "Cvlayer" (co-existence layer ; FASHCHUK and AVZATULLlN ,1986) . Investigat ions of the d istribution and abundance of microorganisms within the vicinity of stable Oz-H 2S redoxcl ines has generally revealed an enrichment of cell numbers and cell biomass at or slightly above the region where is first detected (H 2 S = O.5-1.uM). If the Oz-H 2 S boundary is within the euphotic zone, the microbial community may be dominated by phototrophic sulfur bacteria (SOROKIN . 1965; [NDREB0 et al. , 1979; SOREN- SEN , 1988); otherwise chemoautotrophs, includ ing H 2 S, NH: and CH 4 oxidizing bacteria, are thought to comprise a substantial portion of the microbial assemblage (SOROKIN , 1962, 1972,1983) . • School of Ocean and Earth Science and Technology, Department of Oceanograpby , Univ ersity of Hawaii, Honolulu, HI 96822. U .S .A . t Pre sent address: Department of Biological Sciences, UQAM , P.O. Box 8888, Succur sale A, Mont real , Quebec, Canad a 43C 3P8 . :j:Author to whom correspondence should be addressed. SI069