Benthic N2 production rates (denitrification, anammox) in sand and mud sediments of the Tvärminne Archipelago, southern Finland, and accompanying environmental characteristics

Sediment samples for measuring N2 production rates and for the characterization of the sediment system were taken every month (early spring to late autumn) over 2 years from a sand and a mud sediment in the Tvärminne Archipelago at the Finnish coast of the Gulf of Finland, Northern Baltic Sea. Sediment was sampled with a HAPS bottom corer (sand) and a GEMAX twin corer (mud). Bottom water dissolved oxygen (O2) and nutrients (nitrate [NO3-], nitrate + nitrite [NO3- + NO2-], ammonium [NH4+], phosphate [PO43-]) were analysed from a sample withdrawn ~ 5 cm above the sediment surface. O2 concentrations were measured with Winkler titration; nutrient samples were 0.2 µm filtered and kept dark and cool until measurement with an autosampler (Thermo Scientific Aquakem 250). Photosynthetic active radiation (PAR) was measured close to the bottom and at the water surface with a spherical light sensor (Li-COR). The grain size of the sand sediment was analyzed with a particle detector (CILAS 1180 Naß) 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 Darcy's Law. The oxygen penetration depth (OPD) was determined by automated O2 profiling (except for May, October 2015: manual profiling) at bottom water temperature, electrode tip 100 µm (except for May 2016, Storfjärden: 250 µm). Benthic 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; 2015), and 40 (n=4), 80 (n=4), 120 (n=8) µM 15NO3- (2016) for 4h at in situ bottom water temperature and darkness. Incubations were done in diffusive cores in both mud and sand, as sand permeability was <2.5*10-12 m2 and thus significant effects of advective pore water flow on sediment biogeochemistry could be neglected (2.5*10-12 m2 was used as threshold for the onset of effects of advection in Baltic Sea coastal sediments according to Forster et al. 2003). 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).