Tracing the Products of Combustion Processes in the Baltic Sea Water Column

The Baltic Sea is a semi-enclosed, shallow, brackish sea under persistent anthropogenic pressure, receiving thermogenic organic compounds as products of incomplete combustion via riverine input and atmospheric deposition. We assessed the distribution, sources, and composition of thermogenic organic matter in the Baltic Sea water column via the quantification of polycyclic aromatic hydrocarbons (PAH) and dissolved black carbon (DBC). The overall imprint of combustion processes on dissolved organic matter (DOM) molecular composition was evaluated via ultra-high resolution mass spectrometry. By combining PAH and DBC with DOM characterization, we aimed to contribute to a better understanding of the distribution and behavior of combustion products in coastal environments. Water samples were taken in the southern Baltic Sea in a salinity gradient (~6 to ~21). PAH were homogeneously distributed (average: 16.05 ± 4.44 ng L^-1) likely due to balanced sources and sinks and a long lifetime. In contrast, DBC concentrations showed significant spatial variability (average: 14.2 ± 2.8 µM), with a strong negative correlation to salinity, highlighting the input of thermogenic organic compounds from land via riverine transport to the Baltic Sea. In addition, the abundance of polycyclic aromatic compounds detected on a molecular formula level in DOM correlated with PAH diagnostic ratios. Although PAH were evenly distributed throughout the water PAH concentrations in the sea-surface microlayer increased with the contribution of biomass combustion, likely due to the use of coal during the heating season. Our study emphasizes the sources and distribution of thermogenic organic matter to the Baltic Sea, where PAH and DBC serve as proxies of different anthropogenic influences, revealing also their strengths and weaknesses. This research provides a comprehensive understanding of the sources of the long-lived portion of the carbon pool, emphasizing the role of the Baltic Sea as a catchment basin for anthropogenic pollutants and a dynamic system for the cycling of DOM.