Characterizations of the light-scattering attributes of mineral particles in Lake Ontario and the effects of whiting

Abstract

Light-scattering attributes of minerogenic particles from the upper waters of Lake Ontario, collected lake-wide from pelagic waters in late 2007 summer and early 2008 spring cruises and over the summer interval at a near-shore site in 2008, were characterized by scanning electron microscopy interfaced with automated image and X-ray analyses (SAX). SAX results were used to estimate minerogenic scattering and backscattering coefficients ( b m and b b,m) through Mie theory. Two minerogenic particle regimes with respect to light scattering were resolved: (1) clay mineral dominance and (2) dominance by ‘whiting’ (CaCO 3 precipitate) in late summer in portions of the lake. Clay minerals made noteworthy and important contributions to overall particulate scattering and backscattering coefficients ( b p and b bp, respectively) in spring and early summer. Dramatic increases in values of b p, and particularly b bp, as well as decreases in Secchi disk depth (SD), were observed during whiting from the associated large increases in b m and b b,m. Features of these events were the primary drivers of the spatial patterns in late summer and temporal differences observed for scattering and SD. Particles in the size range of 1–10 μm were responsible for minerogenic scattering during stratification, but those with sizes > 10 μm made noteworthy contributions at certain sites during spring turnover. The credibility of the SAX-Mie estimates of b m and b b,m was supported by the extent of optical closure obtained with paired bulk measurements of overall b p and b bp (2007 summer cruise), and independent estimates of organic particulate scattering and backscattering through empirical bio-optical models.