The absorption coefficient separated into the absorption coefficient of phytoplankton pigments ap(440) and of detritus ad(440) varied between values below the level of detection and 3 m−1 and 1.3 m−1 respectively. In addition, different particle scattering characteristics varied significantly: the particle scattering coefficient at 555 nm JAK inhibition by > 40-fold (values up to 9.3 m−1), and

the backscattering coefficient at 420 nm by almost 70-fold (values up to 0.23 m−1). Before we enter into a detailed description of particulate absorption coefficients it is worth showing the relative proportions between the absorption coefficients of particles and CDOM. Figure 3 shows the absorption budget for the non-water constituents of seawater (there, absorption is separated into components ad, aph and aCDOM). As can be seen, the absorption of non-water constituents in all our samples is dominated by CDOM at short wavelengths of light. At 350 nm and 400 nm the respective average contributions of particles (aph + ad) to the total non-water absorption (aph + ad + aCDOM) are ca 12% and 27%. But with increasing Bleomycin wavelengths the average contribution of particles increases to significant and even dominant values: it is ca 45% at 440 nm, ca 56% at 500 nm and

ca 75% at 600 nm. These contributions in individual samples also exhibit a large variability in their proportions at longer wavelengths. In this paper we focus on analyses of the variability of constituent-specific IOPs. These are optical coefficients normalized to the concentrations of certain seawater constituents. Such average values are often sought as they provide an easy way of describing the connections between biogeochemical and optical quantities. Below we show that such average values in the southern Baltic are unfortunately encumbered with a very high variability. Figures 4 and 5, and Table 2, present a summary Lck of the results of the variability

analysis of constituent-specific absorption coefficients. Figure 4a shows spectra of the mass-specific coefficient of particles ap*(λ) (i.e. the coefficient obtained by normalizing ap(λ) to SPM). Comparison of all the individual sample spectra indicates a large variability of ap* at all wavelengths. Average values of ap* and their corresponding standard deviations (SD) and coefficient of variations (CV) for seven wavelengths, chosen to cover the whole measured spectrum, are given in the first row of Table 2. Of these seven wavelengths the 440 and 550 nm bands are the ones where the variability is smallest (but still significant); the corresponding CV is 71% (the average ap* at 440 nm is 0.198 m2 g−1 and at 550 nm is about 0.065 m2 g−1). Throughout the rest of the spectrum, the variability described in terms of CV values is even higher – up to 81%.