Carbon and nitrogen isotopic ratios (delta(13)N and delta(13)C) of suspended particulate organic matter (SPOM) in the water column of the Black Sea were measured at a total of nine stations in September-October (autumn) 1999 and May 2001. For comparison, a station in the Mediterranean Sea and one in the Sea of Marmara were sampled in October 1999. Large-sized particle samples, as well as samples of surface sediment were also collected for N and C isotopic analysis. The results revealed important vertical and regional variations in N and C isotopic composition. Seasonal variations in SpOM delta(15)N and delta(13)C were not apparent. SPOM in the euphotic zone (EZ), oxycline, and suboxic/anoxic interface layers of the water column was characterized by distinct isotopic composition. In the EZ, the N and C isotopic ratios of SPOM were in the range typically observed for plankton-derived SPOM in the surface ocean (EZ means ranged from 2.7%. to 5.9%. for delta(15)N and from -24.0 parts per thousand to -21.5 parts per thousand for delta(13)C). Shelf region SPOM had higher delta(15)N and lower delta(13)C (EZ means of 5.9 parts per thousand and -24.0 parts per thousand. respectively). Large-sized particles (LPOM) collected by zooplankton net tows had similar to 3 parts per thousand higher delta(15)N values compared to SPOM, indicating fractionation during trophic transfer of nitrogen. SPOM in the oxycline increased by 3-6 parts per thousand for delta(15)N, while delta(13)C decreased by -2 parts per thousand to -4 parts per thousand, which may be attributed to greater lipid content. In the suboxic/anoxic interface zone, SPOM isotopic ratios (delta(15)N as low as 0.0 parts per thousand to -8.0 parts per thousand) suggest chemoautotrophic production leading to dominance of new, in situ produced organic matter. The location of the most negative delta(15)N values indicates that chemoautotrophic production is most intense at the shelf-break regions, possibly enhanced by mixing of oxygenated and nitrate-rich Mediterranean inflow waters with suboxic/anoxic Black Sea water. (c) 2006 Elsevier Ltd. All rights reserved.