Dähnke, Kirstin; Ankele, Markus; Brandt, Peter (2021): Nutrients and nutrient isotope data in South Equatorial Atlantic Ocean water samples collected during the Meteor cruise M131 in 2016 [dataset]. PANGAEA, https://doi.org/10.1594/PANGAEA.929671

R/V Meteor cruise M131 carried out a physical oceanography research program with a biogeochemical sampling component in the South Equatorial Atlantic Ocean and eastern boundary upwelling region off Angola and Namibia. The program was part of the EU collaborative project PREFACE (“Enhancing prediction of tropical Atlantic climate and its impacts”) and the sampling for and analysis of nutrient data were linked to BMBF collaborative project GENUS (“Geochemistry and Ecology of the Namibian Upwelling System”). CTD data of the expedition M131 are archived under doi:10.1594/PANGAEA.910994. The bottle files corresponding to water samples analysed for nutrient concentrations and nitrate isotopic composition were provided by Gerd Krahmann (GEOMAR). Depths and data for temperature, salinity, oxygen concentrations labeled “CTD“ in the table are values from calibrated CTD sensors at closure of the bottles, numbers for fluorescence are uncalibrated. Water samples were taken by Maria-Elena Vorrath during M131 and were analyzed after shipment in the laboratory of Kirstin Dähnke at Helmholtz-Zentrum Geesthacht. Nutrient concentrations were measured with an AutoAnalyzer 3 system (Seal Analytics) using standard colorimetric methods by Markus Ankele. Nitrate was determined after reduction to nitrite, followed by a reaction with sulfanilamide to form a red azo dye (Grasshoff and Anderson 1999). Phosphate was measured after formation of a blue antimony-phosphorous colour complex, according to Murphy and Riley (1962). Ammonium was measured fluorometrically based on Holmes et al. (1999). The relative errors of duplicate sample measurements were below 1.5% for NOx and phosphate concentrations, and below 0.3% for ammonium and silicate. The detection limit was <0.5 µmol kg−1 for NOx, > 0.1 µmol kg−1 for phosphate, >0.013 µmol kg−1 for ammonium and >0.016 µmol kg−1 for silicate.

Delta15N_NO3 and Delta18O_NO3 were determined in the laboratory at Helmholtz-Zentrum Geesthacht (Kirstin Dähnke) with the denitrifier method (Sigman et al. 2001; Casciotti et al. 2002). The isotopic composition was determined with a GasBench II coupled to a Delta Plus XP mass spectrometer (ThermoFinnigan). Replicate measurements were performed, and two international standards (IAEA-N3, Delta15N = 4.7‰, Delta18O=25.6‰ and USGS 34 (Delta15N=-1.8‰, Delta18O=-27.9‰; Böhlke et al. 2003), were measured with each batch of samples. To correct for exchange with oxygen atoms from water, a bracketing correction was applied (Sigman et al. 2009). The standard deviation for standards and samples was 0.2‰ for Delta15N and 0.4‰ for Delta18O.