Parameterization of phytoplankton spectral absorption coefficients in the Baltic Sea: general, monthly and two-component variants of approximation formulas

This paper presents approximate formulas (empirical equations) for parameterizing the coefficient of light absorption by phytoplankton aph(λ) in Baltic Sea surface waters. Over a thousand absorption spectra (in the 350–750&thinsp;nm range), recorded during 9 years of research carried out in different months of the year and in various regions of the southern and central Baltic, were used to derive these parameterizations. The empirical material was characterized by a wide range of variability: the total chlorophyll a concentration (Tchl a) varied between 0.31 and 142&thinsp;mg&thinsp;m−3, the ratio of the sum of all accessory pigment concentrations to chlorophyll a ( ∑ Ci∕Tchl a) ranged between 0.21 and 1.5, and the absorption coefficients aph(λ) at individual light wavelengths varied over almost 3 orders of magnitude. Different versions of the parameterization formulas were derived on the basis of these data: a one-component parameterization in the classic form of a power function with Tchl a as the only variable and a two-component formula – the product of the power and exponential functions – with Tchl a and  ∑ Ci∕Tchl a as variables. We found distinct differences between the general version of the one-component parameterization and its variants derived for individual months of the year. In contrast to the general variant of parameterization, the new two-component variant takes account of the variability of pigment composition occurring throughout the year in Baltic phytoplankton populations.</p