Description: Sediment samples for measuring N2 production rates and for the characterization of the sediment system were taken in three seasons (winter, spring, summer) from sand and mud sediments of the Vistula Estuary and the open Bay of Gdansk at the Polish coast, Southern Baltic Sea. Sediment was sampled with a HAPS bottom corer (coarse sand), a Multicorer and a Boxcorer (fine sand, mud). Porosity was analyzed both from sediment slices and from entire core subsamples, which means that sediment in a sampling core (heights 15-20 cm, iD 2.3 cm) was mixed and sub sampled, assuming vertical homogeneity; sediment was dried overnight at 105°C and calculations followed Burdige (2006). Sediment organic matter content was analyzed as loss on ignition (LOI), for which dried sediment was combusted at 550°C for 4h. Sediment permeability was measured from pooled surface sediments (~1-2 cm homogeneous surface layer) with a permeameter cell following the constant head method for laminar flow of water through granular soil; calculations were derived from Darcy's Law. The oxygen penetration depth (OPD) was determined by manual (EMB 77, AL 449) and automated (EMB 123) profiling at bottom water temperature, electrode tip 100 µm: EMB 123, AL 449 (mud) and 250 µm: EMB 77, AL 449 (sand), EMB 123 (VE05, VE49). Denitrification rates were measured with the revised isotope pairing technique (r-IPT; Risgaard-Petersen et al. 2003, 2004) that accounts for the potential contribution of anammox (anaerobic ammonium oxidation) to total N2 production. Incubations were done in acrylic cores (heights 15-20 cm, iD 2.3 cm) in a concentration series of 30, 60, 90, 120 µM 15NO3- (n=3, EMB 77, AL 449) and 40 (n=4), 80 (n=4), 120 (n=12) µM 15NO3- for 3-5h at in situ bottom water temperature and darkness. In the presence of significant advective pore water flow, an advective incubation design was used. Dw gives denitrification of nitrate from the water column; Dn gives denitrification of nitrate from sediment nitrification (coupled nitrification-denitrification). If no contribution of anammox to total N2 production was found, columns hold a zero (0). The sediment silicate content (ASi =amorphous, biogenic Si (Na2CO3-extractable), Ca-Si = easily available Si (CaCl2-extractable), Ox-Si = oxide-bound Si (extractable by acid oxalate)) was analyzed from the top sediment layer.