Long‐term study of minerogenic particle optics in Cayuga Lake, New York

Abstract

The dynamics of light scattering by minerogenic particles in the upper waters of Cayuga Lake, New York, were characterized for the spring-autumn interval of 8 yr (1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006) at pelagic and nearshore sites with a scanning electron microscope interfaced with automated image and x-ray analyses (SAX). SAX results were used to estimate the minerogenic scattering coefficient ( Light scattering by particles, a fundamental process regulating radiative transfer in water (Kirk 2011), is important in determining apparent optical properties (AOPs, depend on geometry of the light field) of interest, including clarity (Preisendorfer 1986) and the remote sensing signal (Woźniak and Stramski 2004). The total scattering coefficient (b; m 21 ), corresponding to the integration of the volume-scattering function over all directions (Kirk 2011), quantifies a central feature of the light-scattering regime. b is an inherent optical property, independent of the geometry of the light field. The magnitude and spectral features of the particulate component of scattering, b p (which greatly exceeds that due to water), depend on multiple attributes of a particle population, including the number concentration (N), the particle size distribution (PSD), the composition of the individual particles and their shapes (Babin et al. 2003).The particle populations of aquatic ecosystems are heterogeneous, varying in time and space and differing greatly among systems in response to an array of drivers (Stramski et al. , 2007Peng and Effler 2011). Protocols for resolving the various components of light scattering are needed to understand these differences and dynamics, and to identify their origins and drivers, objectives that are consistent with the reductionist approach advocated by Stramski and co-workers (Stramski et al. 2001(Stramski et al. , 2007 where V is the sample volume, N m is the number of minerogenic particles in a sampled volume of water, and PA m,i is the projected area (m 2 ) of minerogenic particle i. Q bm,i depends on the complex refractive index (m i , function of composition) and size (d i ) of the particle, and the wavelength of light (l). This SAX-Mie approach supports further partitioning of b m into contributions according to size and particularly composition (e.g., clay minerals, quartz, and calcite) of particles Effler 2010, 2011). Early research with the SAX-Mie approach had appropriately focused first on testing the credibility of the forward estimates of b m for a range of particle assemblage conditions (see

Advancing two‐component partitioning of light scattering in C ayuga L ake, N ew Y ork

“…Values of the refractive indices ( m = n − in ′, where n and n ′ are the real and imaginary parts relative to water; Table ) were specified on the basis of particle type classification and literature listings (e.g., Babin et al ; Woźniak and Stramski ) and a recent study of the spectral absorbing properties of mineral particles in Lake Erie (Peng and Effler ). The n (real part, considered spectrally flat) values were consistent with earlier SAX–Mie applications (e.g., Peng et al , 2009 a ), including the long‐term study of Cayuga Lake minerogenic particle optics (Effler and Peng ). The adopted values of n ′ were lower than those used in earlier studies (e.g., 10 −4 instead of 10 −3 at 660 nm for clay minerals), based on new absorption information (Peng and Effler ).…”

“…The summary statistics for SAX observations (Table ) depict large variations in the magnitudes of PAV m , but rather uniform features of composition (percent contribution of clay minerals to PAV m , Clay%), shape (ASP), and light‐scattering efficiency factors (< Q b,m > and < Q bb,m >, see CV values). The mean PAV m , particularly for the shelf, was greater than the median, because of the effects of runoff event inputs (Effler and Peng ). The dominant contribution of clay minerals is manifested in the high values of Clay%, although their contributions in pelagic waters were somewhat diminished, reflecting the effects of brief intervals of autochthonous formation of calcite (Effler and Peng ).…”

“…Resolution of the contributions of these types, described as the “reductionist approach” (Stramski et al ), offers the opportunity to identify the drivers of optical variability among systems and dynamics within individual systems. Progress in the partitioning of b p and b bp for lacustrine waters has been reported recently (Peng et al 2009 a ; Peng and Effler ; Effler and Peng ) through a two‐component modeling approach: where b m and b o are the scattering coefficients of minerogenic and organic (of biogenic origin) particles, respectively ( b bp is partitioned analogously with the backscattering coefficients of the two components, b b,m and b b,o ). Central to this partitioning has been the development of a protocol to make forward estimates of b m and b b,m .…”

Apportionment of bioavailable phosphorus loads entering Cayuga Lake, New York

“…The intent of the guidance value, acknowledging the wide use of the concentration of TP as a metric of trophic state (Chapra, ; Cooke et al ., ), is to protect against the effects of excessive eutrophication. The exceedances on the shelf are attributable to the inclusion of observations of particularly high TP concentrations soon after runoff events for which PP associated with terrigenous minerogenic particles (mostly clay minerals) dominates (Effler and Peng, ; Effler et al ., ). The low f BAP for the PP fraction for the three local tributaries (Figures and a), together with the even lower f BAP value (1.7%) observed for a shelf sample soon after a runoff event, establishes the elevated TP concentrations on the shelf following such events are essentially uncoupled from trophic state.…”

“…Accordingly, such observations should not be integrated into assessments of the status of this area relative to the guidance value. Summer average concentrations of TP in pelagic portions of the lake, beyond the local effects of lotic inputs, where minerogenic particle concentrations remain lower (Effler and Peng, ), do not approach the guidance value (Effler et al ., , ).…”