Optical characterization of Lake Champlain: Spatial heterogeneity and closure

Abstract

A robust optical characterization of the underwater and emergent light fields of Lake Champlain was conducted for sites (n=11) throughout the lake in August 2011, based on in situ measurements with modern instrumentation and laboratory measurements of optically active constituents (OACs) and components (ax) of the absorption coefficient (a). Inherent optical property (IOP) measurements included a, ax, and the particulate scattering and backscattering coefficients. Metrics of apparent optical properties (AOPs) included Secchi depth, the diffuse attenuation coefficients for downwelling [Kd(λ)] and scalar (K0) irradiance and remote sensing reflectance [Rrs(λ)]. The credibility of the measurements is demonstrated through: (1) consistency of relationships between OACs and IOPs and AOPs, (2) the approach toward equivalence of laboratory and field measurements, and (3) the extent of closure of predictions of Kd(λ) and Rrs(λ), based on IOP measurements and radiative transfer expressions, with paired observations of these AOPs (average differences of 9.4 and 19.3%). Wide spatial differences in OACs, and the resulting IOPs and AOPs, are documented throughout the bounds of the lake and are the result of its morphologic complexity and differing external loading. The lake is a complex case 2 system, with uncoupled variations in OACs and ax over the bounds of the lake. Both empirical and radiative transfer expressions are used to predict changes in AOPs in response to hypothetical changes in OACs.