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

Abstract

Carbon and nitrogen isotopic ratios (δ15N and δ13C) 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 δ15N and δ13C 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 δ15N and from −24.0‰ to −21.5‰ for δ13C). Shelf region SPOM had higher δ15N and lower δ13C (EZ means of 5.9‰ and −24.0‰, respectively). Large-sized particles (LPOM) collected by zooplankton net tows had ∼3‰ higher δ15N values compared to SPOM, indicating fractionation during trophic transfer of nitrogen. SPOM in the oxycline increased by 3–6‰ for δ15N, while δ13C decreased by −2‰ to −4‰, which may be attributed to greater lipid content. In the suboxic/anoxic interface zone, SPOM isotopic ratios (δ15N as low as 0.0‰ to −8.0‰) suggest chemoautotrophic production leading to dominance of new, in situ produced organic matter. The location of the most negative δ15N 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.