Characterizations of calcite particles and evaluations of their light scattering effects in lacustrine systems

Abstract

Characterization of autochthonously produced calcite particles, along with the associated heteronuclei, and evaluation of their effects on light scattering, are advanced for lacustrine waters. This is supported by (1) direct measurement of minerogenic particle populations from 12 northeastern U.S. lakes with scanning electron microscopy interfaced with automated image and X-ray analyses (SAX), and (2) application of the previously tested SAX−Mie-theory approach to resolve the contributions of calcite vs. allochthonous minerogenic particles to particulate scattering and backscattering coefficients (bp and bbp), and (3) comparison with organic contributions estimated through empirical bio-optics models. Intervals of substantial contributions of calcite to bp and bbp, described as “whiting events,” are demonstrated for these study lakes. Organic particles, apparently from two size ranges, likely corresponding to pico- and nanocyanobacteria, are found to serve as the primary heteronuclei for the calcite precipitation. The coating of these particles with calcite within the upper waters of the study lakes remained thin. The calculated relative increases in bbp were substantially more than for bp because of the greater effect of the higher refractive index (i.e., n) of calcite for the former. Differences in n values for calcite reported in the literature are a source of noteworthy uncertainty in associated bbp estimates. Nine years of spring−fall monitoring in Cayuga Lake, New York, documented major interannual variations in the whiting events with respect to timing, magnitude of the scattering signatures, and the sizes of heteronuclei. Whiting events can have major effects on commonly monitored optical features, including Secchi depth and remote sensing reflectance.