Characterizations of minerogenic particles in support of modeling light scattering in Lake Superior through a two‐component approach

Abstract

The role of suspended minerogenic particles in light scattering in eastern Lake Superior and Keweenaw Bay (11 sites) during July of 2006 was evaluated with an individual particle analysis technique (scanning electron microscopy interfaced with automated image and X‐ray analyses, SAX), along with bulk measurements of particulate scattering and backscattering coefficients (bp and bbp) and chlorophyll a concentration ([Chl]). SAX measurements provided information on light‐scattering attributes of minerogenic particles, including chemical composition and particle size distribution (PSD). The data were used in Mie theory calculations for estimations of minerogenic scattering and backscattering coefficients (bm and bb,m). The bay had higher concentrations of minerogenic particles and higher values of bm, bb,m, bbp, and the backscattering ratio (bbp : bp) than the pelagic sites. Minerogenic scattering was primarily attributable to clay mineral particles in the size range of 1‐10 mm. The PSDs deviated from the (Junge) pattern of monotonic increase of particle numbers with decreasing particle size. The estimates of bm and bb,m combined with those of organic particulate components, as represented by empirical Case 1 bio‐optical models, showed reasonably good closure with bp and bbp, supporting the credibility of the SAX‐Mie approach. Variations in the bbp : bp ratio are strongly related to spatial differences in the relative contributions of phytoplankton vs. minerogenic particles to scattering, with higher values observed where the minerogenic component is more important. SAX can advance the understanding of optical variability by establishing the contributions of well‐defined minerogenic constituents that strongly influence optical properties.