Benthic N2 production rates, Si cycling and environmental characteristics from the Öre estuary on the Swedish coast

Sediment samples for measuring N2 production and for the characterization of the sediment system were taken in spring and summer 2015 in the Öre Estuary at the Swedish coast of the Quark Strait, northern Baltic Sea. Sediment was sampled with a HAPS bottom corer (sand) and with a GEMINI twin corer (mud). The grain size of the sand sediment was analyzed via wet sieving from the sediment layers 0-1, 1-2, 2-3 cm (spring campaign) and 0-0.5, 0.5-1.0, 1.0-1.5 cm (summer campaign); due to vertical homogeneity, these surface sediments were subsequently pooled and the sediment type was classified after Wentworth (1922). Porosity and water content were analyzed both from sediment slices and from entire core subsamples, which means that sediment in a sampling core (heights 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 Darcie's Law. The oxygen penetration depth (OPD) was determined by manual (spring) and automated (summer) profiling at bottom water temperature (electrode tip 100 µm). 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 20 cm, iD 2.3 cm) in a concentration series of 30, 60, 90, 120 µM 15NO3- (n=3) for 4-5h at in situ bottom water temperature and darkness. 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).