Spectrophotometric measurements of absorption coefficients by phytoplankton during POLARSTERN cruise PS113 along an Atlantic Transect

Water samples were taken during the transatlantic (10 May to 9 June 2018) expedition PS113 with RV Polarstern. Water samples were collected from CTD Niskin bottles at six different depths from the upper 100 to 180 m and additionally from the ship's seawater supply pumped through teflon tubing from about 11m depth during underway further described for HPLC based phytoplankton pigment data in Bracher et al. (2020). All collected samples were filtered through 25 cm Whatman GF/F filters, respectively, under low-vacuum pressure (below 200 mbar). Blank filters were collected by soaking them in 0.2um filtered seawater. Measurements for particulate and non-algal (NAP) absorption were performed directly after filtration the quantitative filtration techniques-intergrative cavity measurement device and procedure from Röttgers et al. (2016) as described in Liu et al. (2018). Phytoplankton absorption (aph) was derived by subtracting the NAP from the particulate absorption.