Toc vchp analyzer

Sea ice and seawater samples were analyzed for chlorophyll a (chl a), bacterial abundance, particulate organic carbon (POC), dissolved organic carbon (DOC), dissolved nitrogen (DN), macro-nutrients (NO₃+NO₂, PO₄, SiOH₄) and salinity. Chl a fluorescence was read on a 10AU Turner Design fluorometer calibrated using pure chl a extract (Sigma-Aldrich, Oakville, ON, Canada) and concentrations were calculated according to Parsons et al.14 . Bacterial abundance was measured by flow cytometry as detailed in Belzile et al.15 . POC was measured on a Carlo Erba NC2500 elemental analyzer (Carlo Erba Reagents, Val de Reuil, France). Dissolved organic carbon and nitrogen were measured using a Shimadzu TOC-VCHP analyzer with a TNM-1 Total Nitrogen module (Shimadzu Corp., Kyoto, Japan). Analyses were systematically checked against deep Sargasso Sea reference water from Hansell’s Certified Reference Materials (University of Miami, Miami, FL, USA). Samples for nutrient analysis were frozen at −80 °C and later analyzed on a SmartChem 200 chemistry analyzer (Unity Scientific, Brookfield, CT, USA). Additional details on the methods can be found in Michel and Niemi16 .

Concentrations of DOC and TDN were determined on a Shimadzu TOC-V_CHP analyzer via high temperature catalytic combustion. Accuracy (< 2.7% [DOC] and < 0.2% [TDN] deviation from expected value), precision (RSD%, < 4.2% [DOC] and < 2.1% [TDN]) and LOD (5 µM [DOC] and 5.8 µmol L⁻¹ [TDN]) were monitored through repeated measurements of ultrapure water and an in-house deep-water reference material (Hansell, Florida Strait, 700 m water depth, batch No. 9, 2009, 44 µmol DOC L⁻¹). For determination SPE-DOC and SPE-TDN, 100 µL of the methanolic extract were evaporated at 50 °C and re-dissolved in 10 mL of 0.01 M HCl prior to the analysis. Here, accuracy (< 3.3% [DOC] and < 1.3% [TDN]), precision (RSD%, < 4.1% [DOC] and < 5.3% [TDN]) and LOD (5.8 µmol L⁻¹ [DOC] and 3 µmol L⁻¹ [TDN]) for the respective analysis refer to the measured solutions. The actual SPE-DOC and SPE-TDN concentrations and errors were reconstructed considering the volume ratio of methanolic extract (~ 0.7 mL) to extracted sample (~ 75 mL) as well as the dilution through evaporation and re-dissolution of an extract aliquot prior to the analysis. In three individual incubations, namely one 0.1 µM Fe 3, 10 µM Fe 1 and 100 µM Fe 1, the measured SPE-TDN concentration exceeded TDN, indicating contamination and thus these samples were excluded from the evaluation of SPE-DOM on a molecular level (see indication in Tables 1, 2).