Absorption coefficient spectra (median) of phytoplankton during POLARSTERN cruise PS93.2

Absorption coefficients by phytoplankton (aph) were measured on water samples collected from underway AC-S flow-through system outflow and CTD. Measurements were performed using a dual-beam UV/VIS spectrophotometer as described in Liu et al. (2018) and Liu et al. (2019).

1. About sampling: a) underway samples were collected from unfiltered AC-S outflow (depth: 11m); CTD samples with 5-6 depths were collected on upcast. b) vacuum pressure for filtration: maximum 200 hPa c) samples were shock frozen in liquid nitrogen and then stored in -80°C until measurement after cruise. d) blank filters were collected by soaking them in 0.2um filtered seawater. e) replicates were not collected.
2. About measurements: a) Instrument: dual-beam UV/VIS Spectrophotometer (Cary 4000, Varian Inc., Palo Alto, CA, USA) equipped with a 150 mm integrating sphere (external DRA-900, Varian, Inc. and Labsphere Inc., made from Spectralon (TM)). b) wavelength range of scan and resolution: 300-850 nm, 1 nm. c) slit band width: 2 nm. d) scan speed: 150 nm min−1 e) Sample filters were extracted with 95% methanol for the absorption measurement of NAP. f) The baseline of the spectrophotometer was set with a dry piece of a blank filter. Filters were wetted with purified water and measured regularly after every fifth sample filter served as references.
3. About data analysis: a) interpolated wavelength range and resolution: 350-750 nm, 1nm. b) pathlength amplification factor (beta correction factor): 4.5 (Röttgers and Gehnke 2012; Röttgers et al., 2014). c) Null-point scattering correction at 750 nm is applied.