The first instrument used was a spectral backscattering meter (HydroScat-4;

HOBI Labs). This measured values of the volume scattering function at an angle centred at 140° and at four light wavelengths – 420, 488, 550 and 620 nm. These raw values were then used to estimate the backscattering coefficients of light in seawater bb [m− 1] at these four wavelengths, according to the method described in Maffione & Dana (1997) and in Dana & Maffione (2002). A correction for the incomplete recovery of backscattered light in highly attenuating waters (the so-called sigma-correction) was applied in accordance with the instrument User’s Manual www.selleckchem.com/products/erastin.html ( HOBI Labs 2008), using data on light absorption and attenuation coefficients measured with another optical instrument. To obtain the backscattering coefficients of particles bbp [m− 1], the theoretical backscattering coefficients of pure water were subtracted (according to Morel (1974)). The second optical instrument was a spectral absorption-attenuation meter (AC-9; WET Labs). Equipped with a 25 cm optical path length, this instrument measured the light absorption

and attenuation coefficients of all the non-water (i.e. suspended and dissolved) constituents of seawater, an [m− 1] and cn [m− 1] respectively, at nine light wavelengths (412, 440, 488, 510, 532, 555, 650, 676 and 715 nm). Corrections for in situ temperature and salinity effects on the optical properties of MTMR9 water were applied according to Pegau et al. (1997). A correction for the incomplete recovery of the scattered light in the absorption tube of the AC-9 instrument was applied according www.selleckchem.com/products/azd4547.html to Zaneveld et al. (1994) (the so-called proportional method, according to which the measured values for the longest light wavelength (715 nm in the case of our instrument) are assumed to be caused entirely by the unwanted scattering error effect, and the corrected value of absorption at this band was assumed to be 0). At this point, the reader should

note an important methodological difference between the current work and the paper of S.B. Woźniak et al. (2011) mentioned earlier. In that paper the light absorption properties of suspended particles and coloured dissolved organic matter were characterised separately, not in situ, but based on measurements of discrete seawater samples performed in a land-based laboratory using a bench-top spectrophotometer. In the current work only the in situ measured (with the AC-9 instrument) total absorption coefficient of all suspended and dissolved non-water constituents of seawater an is taken into consideration. It is relatively easy to measure the latter optical coefficient during oceanographic campaigns, so data on coefficient an are often present in different oceanographic datasets used for the calibration and validation of remote sensing algorithms.