Optical characterizations and pursuit of optical closure for the western basin of Lake Erie through in situ measurements

Abstract

In situ measurements of inherent (IOPs) and apparent optical properties (AOPs), along with laboratory measurements of optically active constituents, were made at sites (n=14) in western Lake Erie following a wind event to advance the characterization of the underwater and emergent light fields of these waters and to support related IOP-based model development and testing. Modern instrumentation was used to make spectral (wavelength, λ) measurements of the IOPs of absorption [a(λ)], particulate scattering [bp(λ)], and particulate backscattering [bbp(λ)] coefficients, and the AOPs of remote sensing reflectance [Rrs(λ)], and the diffuse attenuation coefficient for downwelling irradiance [Kd(λ)]. Optical closure analyses were conducted to demonstrate the credibility of the measurements, by comparing AOP observations to predictions based on radiative transfer expressions that utilized IOP measurements as inputs. Substantial spectral variations in a and its contributing components, and more modest wavelength dependencies for bp and bbp, were documented that are consistent with observations reported for marine case 2 systems. The backscattering ratio, bbp:bp, was strongly positively related to the contribution of minerogenic particles to the overall concentration of suspended particulate material. Major spatial differences in both IOPs and AOPs were observed that were driven by the attendant differences in the concentrations and composition of the optically active constituents, but particularly minerogenic particles, mediated in part by sediment resuspension. Good optical closure between the independently measured IOPs and AOPs was achieved. Direct measurement of bbp(λ) was found to be critical to pursue closure for Rrs(λ) and thereby support related remote sensing initiatives.