Factors controlling spatial distributions and relationships of carbon, nitrogen, phosphorus and sulphur in sediments of the stratified and eutrophic Gulf of Gdansk

• We examine changes in C, N, P and S content in sediments in the Gulf of Gdansk.
• Contents of C, N, P and S vary with depth, distance from land and bottom water oxygen.
• In the Gdansk Deep sediments accumulation of S and redox-dependent P release occur.
• Loss-on-ignition is good predictor of C, N and P in sediments in the Gulf of Gdansk.

Spatial distribution and coupling of carbon, nitrogen, phosphorus and sulphur were studied in sediments of the Gulf of Gdansk, southern Baltic Sea. We report data on sedimentary contents of total and organic carbon (TC and OC), total nitrogen (TN), total and organic phosphorus (TP and OP) and total sulphur (TS), as well as pore water concentrations of ammonium, phosphate and hydrogen sulphide. The lowest values of all the investigated parameters were observed in coastal area, while the highest were noted in sediments below halocline which is attributed to changing hydrodynamic conditions and sedimentation regimes with depth. The contents of TC, OC, TN, TP, OP and TS in sediments and their molar ratios were also noted to vary spatially in relation to distance from land and oxygen conditions in near-bottom water. As a result of high organic matter supply to the sediments caused by eutrophication we observed increased contents of TC, OC, TN, TP and OP in the upper few centimetres of sediment. Results obtained for the Gdansk Deep indicate that sediments in this area are site of accumulation of sulphur and redox-dependent phosphorus release.We analysed relationships between the investigated parameters and organic matter content in sediments measured as loss-on-ignition (LOI). Regression analysis indicated that LOI was a good estimator of contents of OC, TN and OP in the Gulf of Gdansk sediments and may be used as a screening tool when planning geochemical investigations. However, calculated equations can only be applied to LOI results obtained at similar temperature and exposure time (550 °C, 6 h). In addition to this, the method was unreliable for prediction of TS content in euxinic environments where, as in the Gdansk Deep sediments, accumulation of sulphur occurs due to the formation of iron (II) sulphides.