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 southern Baltic Sea for the original seawater samples and four size fractions: pico-particles (0.2–2 µm), ultra-particles (2–5 µm), nano-particles (5–20 µm) and micro-particles (20–200 µm). Chlorophyll a (Chl a) and suspended particulate matter (SPM) concentrations were determined. The proportions of particles from the size classes in the ap(443), aph(443) and ad(443) were determined. Pico- and ultra-particles had the largest contribution to the total particle absorption – an average of 38 % and 31 %. Particles of 5–20 µm accounted for approximately 20 % of ap(443) and aph(443) and 29 % of ad(443). The contribution of particles > 20 µm averaged 5 %–10 %. In total SPM contribution of micro-particles averaged 17 %; nano-, ultra- and pico-particles averaged 29 %, 26 % and 27 %, respectively. In total Chl a, the proportions of pico- and ultra-particles averaged 35 % each, nano-particles 16 % and micro-particles 15 %. Temporal and spatial variability of particles contributions in size classes were observed. The average chlorophyll-specific and mass-specific light absorption coefficients, i.e., light absorption coefficients normalized to Chl a or SPM, were determined for all size fractions. The chlorophyll-specific coefficients ap(Chla)(λ), ad(Chla)(λ) and aph(Chla)(λ), ± standard deviations, do not allow clear separation of the individual fractions. For mass-specific coefficients, ap(SPM)(λ), ad(SPM)(λ) and aph(SPM)(λ), it is possible to distinguish between large particles (20–200 µm) and small and medium particles (0.2–20 µm). These results will allow monitoring of suspended matter in size classes in optically complex waters of southern Baltic Sea.