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

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., 2  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 m 2  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 SPM i 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).