Features of light absorption are critical to optical aspects of water quality and in regulating the signal available for remote sensing. Spectral characteristics and spatial patterns of light-absorbing components, and their relationships with optically active constituents, are documented for the Sturgeon River, Keweenaw Bay, and Lake Superior based on analyses of samples collected on two cruises (2006 and 2007, 20 sites). The absorption coefficient, a (m-1), is partitioned according to the additive components (ax) of colored dissolved organic matter (aCDOM). non-algal particles (aNAP), phytoplankton (aφ;), and water itself (aw; known). The role of minerogenic particles and their iron content in regulating aNAP is evaluated based on paired measurements by an individual particle analysis technique (Peng et al., 2009), through empirical analyses and Mie theory calculations of absorption by these particles (am). Spectral characteristics of aNAP and aφ; were consistent with those reported for other case 2 (i.e., phytoplankton not dominant) systems. However, the slope values that describe aCDOM spectra for the bay and the lake were unusually low, suggesting an atypical composition for the lake's CDOM. The dominant absorbing component in the blue wavelengths was CDOM, representing ≥ 75% of a at a wavelength of 440 nm at all sites in the 2006 survey. A general gradient in both aCDOM and aNAP extended from the Sturgeon River, through the bay, into eastern Lake Superior in that survey. Relationships between ax and optically active constituents were within the broad ranges reported for other case 2 systems. Minerogenic particles, related to their iron content, are demonstrated to be an important component of aNAP.
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Vol. 36 • No. 4