Evaluation of the Stockholm Archipelago sediments, northwestern Baltic Sea Proper, as a trap for freshwater runoff organic carbon

Time-series data on the freshwater inflow of total organic carbon (TOC) to the Stockholm Archipelago, Baltic Sea, covering four full years, showed a distinct seasonal pattern with six times higher monthly flow in winter–spring than in the summer with an annual freshwater TOC input of 28 Gg year−1. Estimation of burial flux and source apportionment of sedimentary OC in the Inner Archipelago (IA) was based on extensive sediment sampling of seismically surveyed accumulation bottom areas. The resulting IA-averaged OC content was 56±16 mg g dw−1 sediment, yielding an IA-wide bottom sediment deposition for the 1992–1995 period of 3.1 Gg year−1. The sediments in the eastern Lake Mälaren (LM), IA, and Outer Archipelago (OA) were geochemically distinguishable based on their stable carbon isotope signatures (δ13C). The δ13C values were −28.2±0.6, −25.4±1.0, and −23.6±0.3, respectively, for LM (n=15), IA (n=17), and OA (n=31). These spatial trends were consistent with the C:N ratios, which were 9.96±1.2, 9.29±0.73, and 9.13±0.22 for the same subsystems. Using a simple isotopic mass balance model, it was deduced that about one-third (39%) of the OC burial in the IA sediments was of Lake Mälaren runoff origin. Hence, a low archipelago-trapping efficiency of only 4% for the runoff TOC was found to be accumulating in the sediments underlying the approximately 30 km transect through the IA eastward of Stockholm. This scenario suggests that the organic matter and associated substances added by the Lake Mälaren outflow, passing through urban Stockholm, are largely either remineralised and released to the IA water column, or transported with the currents to more pristine offshore Baltic ecosystems.