Carbon and nitrogen isotopic ratios of suspended particulate organic matter (SPOM) in the Black Sea water column

Carbon and nitrogen isotopic ratios (delta N-13 and delta C-13) 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 N-15 and delta C-13 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 N-15 and from -24.0 parts per thousand to -21.5 parts per thousand for delta C-13). Shelf region SPOM had higher delta N-15 and lower delta C-13 (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 N-15 values compared to SPOM, indicating fractionation during trophic transfer of nitrogen. SPOM in the oxycline increased by 3-6 parts per thousand for delta N-15, while delta C-13 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 N-15 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 N-15 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.