Distribution and sources of organic matter in sediments of the south-eastern Baltic Sea

Temporal and spatial distribution of sedimentary organic matter (SOM) as well as its sources in the south-eastern Baltic Sea were investigated. Organic matter was characterized by the organic carbon content, delta C-13 and delta N-15 signatures and C/N ratios. The sampling was performed in the coastal, offshore areas and in the Curonian Lagoon in the period from May 2012 to September 2014. The average elemental (C/N similar to 6.4) and isotopic composition (delta C-13 from -29.6 parts per thousand to -24.9 parts per thousand) of SOM suggested that during most of the year it was composed of both freshwater and marine phytoplanktonic material. Elevated delta N-15 values (average ranging from 5.6 parts per thousand to 6.7 parts per thousand) were detected in SOM from the Curonian Lagoon. This most likely reflected the increased isotopic signal of the Nemunas River (delta N-15 similar to 8 parts per thousand) derived from the anthropogenic input in the basin area. At the Baltic Sea offshore sites, SOM had lower delta N-15 values (delta N-15 similar to 3.5 parts per thousand on average), indicating phytoplankton blooms. A two-end member mixing model based on the carbon stable isotopic composition showed that a large proportion (similar to 90%) of SOM in the northern part of the Curonian Lagoon was of the freshwater origin. The allochthonous organic matter accounted for about 47% of SOM in the studied Baltic Sea area with the highest contribution (about 75%) within the Curonian Lagoon plume zone in the coastal waters. (C) 2016 Elsevier B.V. All rights reserved.