Optical properties of size fractions of suspended particulate matter in littoral waters of Québec

Abstract. Mass-specific absorption (a_i^∗(λ)) and scattering (b_i^∗(λ)) coefficients were derived for four size fractions (i =  0.2–0.4, 0.4–0.7, 0.7–10, and > 10 µm, λ = wavelength in nm) of suspended particulate matter (SPM) and with samples obtained from surface waters (i.e., 0–2 m depth) of the Saint Lawrence Estuary and Saguenay Fjord (SLE-SF) during June of 2013. For the visible–near-infrared spectral range (i.e., λ = 400–710 nm), mass-specific absorption coefficients of total SPM (i.e., particulates > 0.2 µm) (hereafter a_SPM^∗) had low values (e.g., < 0.01 m² g⁻¹ at λ = 440 nm) in areas of the lower estuary dominated by particle assemblages with relatively large mean grain size and high particulate organic carbon and chlorophyll a per unit of mass of SPM. Conversely, largest a_SPM^∗ values (i.e., > 0.05 m² g⁻¹ at λ = 440 nm) corresponded with locations of the upper estuary and SF where particulates were mineral-rich and/or their mean diameter was relatively small. The variability of two optical proxies (the spectral slope of particulate beam attenuation coefficient and the mass-specific particulate absorption coefficient, hereafter γ and Svis, respectively) with respect to changes in particle size distribution (PSD) and chemical composition was also examined. The slope of the PSD was correlated with b_i^∗(550) (Spearman rank correlation coefficient ρ_s up to 0.37) and a_i^∗(440) estimates (ρ_s up to 0.32) in a comparable way. Conversely, the contribution of particulate inorganic matter to total mass of SPM (F_SPM^PIM) had a stronger correlation with a_i^∗ coefficients at a wavelength of 440 nm (ρ_s up to 0.50). The magnitude of γ was positively related to F_SPMⁱ or the contribution of size fraction i to the total mass of SPM (ρ_s up to 0.53 for i = 0.2–0.4 µm). Also, the relation between γ and F_SPM^PIM variability was secondary (ρ_s = −0.34, P > 0.05). Lastly, the magnitude of Svis was inversely correlated with a_SPM^∗(440) (ρ_s = −0.55, P = 0.04) and F_SPM^PIM (ρ_s = −0.62, P = 0.018) in sampling locations with a larger marine influence (i.e., lower estuary).