Optical characterization and tests of closure for Oneida Lake, New York, USA

Abstract

Optical characteristics are a central feature of water quality and are increasingly used as a basis for monitoring concentrations of optically active constituents (OACs) through satellite-based remote sensing. This study presents a robust optical characterization of the near-surface waters of eutrophic Oneida Lake, New York, USA, that features in situ spectral measurements of inherent optical properties (IOPs) and remote sensing reflectance (Rrs(λ)), an apparent optical property (AOP) critical to remote sensing initiatives. The IOP measurements included the absorption coefficient, (a(λ)), and particulate scattering (bp(λ)) and backscattering (bbp(λ)) coefficients. Supporting measurements included Secchi depth (SD), the OACs, chlorophyll a, phycocyanin, suspended particulate material, and laboratory determinations of components of a(λ) (ax(λ)), including colored dissolved organic matter, nonalgal particles, and phytoplankton. The strong spectral patterns of a, ax, and Rrs, and more modest wavelength dependencies of bp and bbp, are documented. The credibility of the optical characterizations is demonstrated through (1) the consistency of relationships between SD−1 and bp, and ax and the OACs; (2) the extent of closure between in situ and laboratory measurements of a and ax; and (3) the extent of closure between predicted Rrs spectra, based on the observed IOP (a and bbp) spectra and application of a widely used radiative transfer relationship, and observations. Major temporal and substantial spatial variations in OACs, IOPs, SD, and Rrs(λ) are documented. Much of the variability in the OACs and SD were well predicted by system-specific empirical relationships that used Rrs values at specified wavelengths (consistent with the Medium Resolution Imaging Spectrophotometer [MERIS]) as the independent variables, supporting the potential for effective monitoring of these features through remote sensing in the future.