Articles | Volume 14, issue 23
Research article
29 Nov 2017
Research article |  | 29 Nov 2017

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

Gholamreza Mohammadpour, Jean-Pierre Gagné, Pierre Larouche, and Martin A. Montes-Hugo

Abstract. Mass-specific absorption (ai(λ)) and scattering (bi(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 aSPM) 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 aSPM 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 bi(550) (Spearman rank correlation coefficient ρs up to 0.37) and ai(440) estimates (ρs up to 0.32) in a comparable way. Conversely, the contribution of particulate inorganic matter to total mass of SPM (FSPMPIM) had a stronger correlation with ai coefficients at a wavelength of 440 nm (ρs up to 0.50). The magnitude of γ was positively related to FSPMi or the contribution of size fraction i to the total mass of SPM (ρs up to 0.53 for i = 0.2–0.4 µm). Also, the relation between γ and FSPMPIM variability was secondary (ρs = −0.34, P > 0.05). Lastly, the magnitude of Svis was inversely correlated with aSPM(440) (ρs = −0.55, P = 0.04) and FSPMPIM (ρs = −0.62, P = 0.018) in sampling locations with a larger marine influence (i.e., lower estuary).

Short summary
The mass-specific absorption coefficients of total suspended particulate matter (aSPM*) had relatively low (high) values in areas of of the St. Lawrence Estuary influenced by marine (freshwater) waters and dominated by large-sized (small-sized) and organic-rich (mineral-rich) particulates. The inorganic content of particulates was correlated with size-fractionated aSPM* values at a wavelength of 440 nm and the spectral slope of aSPM* as computed within the spectral range 400–710 nm.
Final-revised paper