Optical properties and composition of size-fractionated suspended particulate matter samples from the surface waters of the southern Baltic Sea

Abstract

Inherent optical properties (IOPs) and various bulk composition characteristics of suspended particles were measured in original and size-fractionated surface water samples collected in different times of the year in the southern Baltic Sea. Measured IOPs included the spectral beam attenuation coefficient of particles (c_p(λ)), the spectral scattering coefficient of particles (b_p(λ)), the spectral absorption coefficients of particles (a_p(λ)), and the spectral absorption coefficients of depigmented (non-algal) particles (a_d(λ)) and phytoplankton (a_ph(λ)). The bulk composition of suspended particles' assemblages was characterized by measuring the mass concentrations of suspended particulate matter (SPM), particulate organic matter (POM), particulate inorganic matter (PIM), and phytoplankton pigments including chlorophyll a (Chla). All optical and biogeochemical measurements were performed concurrently on original (i.e., unfiltered) seawater, as well as on size-fractionated seawater samples obtained by filtration using a combination of nylon meshes and membrane filters. This allowed us to determine the contribution of three distinguished main particle size fractions – very small (VS), small (S) and combined medium and large (ML) particles - to the bulk SPM and Chla concentrations, as well as to the various optical coefficients analysed. Our original samples were characterized by a large variability in bulk particulate matter composition, with noticeable compositional differences between the distinguished main particle size fractions. The obtained biogeochemical data also indirectly suggested a marked variability in the particle size distributions occurring between individual samples from the study region. Thanks to the optical measurements performed, we documented that the contributions of the VS, S, and ML fractions to the analysed optical coefficients were largely variable between individual samples; however, in most cases, the contributions of the S fraction were the highest. We also documented clear differences between particle size fractions in the mean spectral slopes of the coefficients c_p,X(λ), and in the mean values of the SPM-specific coefficients b_p,X*(λ) and a_d,X*(λ), as well as the Chla-specific coefficients a_ph,X*^(Chla)(λ) (were the subscript X symbolically represents the selected particle size fraction - VS, S or ML). Furthermore, several examples of approximate (best-fit) statistical relationships were determined between optical characteristics of suspended matter and selected biogeochemical ratios describing changes in the proportions and composition between particle size fractions. For example, we documented the existence of relationships between SPM_ML/SPM, i.e. the ratio expressing the contribution of the ML fraction to the bulk mass concentration of suspended matter, and the spectral slope (η) of c_p(λ), or, for selected light wavelengths λ_i, the values of the coefficients b_p*(λ_i) or a_d*(λ_i). We also illustrated the relationships between Chla_ML/Chla, i.e. the ratio expressing the contribution of the ML fraction to the bulk concentration of chlorophyll a in water, and the values of the coefficient a_ph*^(Chla)(λ_i).