Porewater nutrient data, seabird fertilisation study
Porewater nutrient data from sediment sampled near the Stora Karlsö seabird colony, April 2017.
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Porewater samples were thawed, diluted 1:10 and NH4+, NOx (=NO3- + NO2-) and PO43- were analyzed on a segmented flow autoanalyzer system (ALPKEM, Flow Solution IV) following the standard methods for seawater analyses 23. Precision was ± 0.036 µmol L-1 for NH4+, ± 0.014 µmol L-1 for NOx and ± 0.016 µmol L-1 for PO43-. As there was no macrofauna in the sediment, porewater profile shapes could be easily modeled to calculate diffusive fluxes of NH4+ and PO43-, while NOx profiles did not show any generalizable trend and were not modeled. Profiles were modeled with the numerical interpretation by Berg et al. 24, which provides the best fit to a measured concentration profile assuming steady state conditions and returns diffusive fluxes between the sediment–water interface (SWI) as a function of depth. We assumed that biological diffusivity (movement of solutes due to bioturbation) was zero as macrofauna was absent, so that diffusive fluxes (J) could be calculated from the equation: J= -φ Ds δC/δx (1) where φ is sediment porosity, Ds is molecular diffusivity in sediment, C is the solute concentration determined analytically and x is sediment depth. Porosity was estimated from sediment water content, which was calculated by measuring the wet and dry weight of 5 mL sediment aliquots after drying them at 105°C. Ds was calculated according to the equations reported by Iversen and Jørgensen, (Diffusion coefficients of sulfate and methane in marine sediments: Influence of porosity. Geochim. Cosmochim. Acta 57, 571–578 (1993)).