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

• An assessment of SOM sources in the SE Baltic Sea is provided.
• Phytoplankton is the main contributor to SOM in the research area.
• We report a retention of freshwater OM with increasing distance from the land.
• The changes in δ15N show the riverine impact and dominance of cyanobacteria.

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, δ13C and δ15N 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 ~ 6.4) and isotopic composition (δ13C from − 29.6‰ to − 24.9‰) of SOM suggested that during most of the year it was composed of both freshwater and marine phytoplanktonic material. Elevated δ15N values (average ranging from 5.6‰ to 6.7‰) were detected in SOM from the Curonian Lagoon. This most likely reflected the increased isotopic signal of the Nemunas River (δ15N ~ 8‰) derived from the anthropogenic input in the basin area. At the Baltic Sea offshore sites, SOM had lower δ15N values (δ15N ~ 3.5‰ on average), indicating phytoplankton blooms.A two-end member mixing model based on the carbon stable isotopic composition showed that a large proportion (~ 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.