Variability of light absorption coefficients by different size fractions of suspensions in the southern Baltic Sea

Abstract. Measurements of light absorption coefficients by particles suspended in seawater (ap (λ)), by phytoplankton (aph(λ)) and detritus (ad (λ)) were carried out in the Baltic Sea waters. Measurements were performed for the original (unfiltered) seawater samples and the four selected size fractions: pico-particles with diameters (0.2–2 μm), ultra-particles with diameters (2–5 μm), nano-particles with diameters (5–20 μm) and micro-particles (20–200 μm). Chlorophyll a  (Chla) concentrations and total suspended particulate matter (SPM) concentrations were determined. The proportions of particles from the size classes (micro, nano, ultra and pico) in the total light absorption by particles, phytoplankton and detritus were determined. Particles with sizes < 5 μm (i.e. pico and ultra-particles) had the largest contribution to the total particles absorption – an average of 38 % and 31 %. Particles of 5–20 μm accounted for approximately 20 % of all particles and phytoplankton and 29 % of the detritus. The contribution of large particles > 20 µm averaged 5–10 %. The average chlorophyll-specific and mass-specific light absorption coefficients, i.e. light absorption coefficients normalized to Chla and to SPM concentration, were determined for all size fractions. The determined average chlorophyll-specific light absorption coefficients ap(Chla)(λ), ad(Chla)(λ) and aph(Chla)(λ), along with standard deviations, do not allow clear separation of the individual fractions. For mass-specific light absorption coefficients, ap(SPM)(λ), ad(SPM)(λ) and aph(SPM)(λ), it is possible to distinguish between large particle fractions (microplankton – 20–200 μm) and small and medium particle fractions (0.2–20 μm).