Light Absorption Components in Onondaga Lake, New York, U.S.A.

Abstract

Features of light absorption are critical to optical aspects of water quality and in regulating the signal available in the emergent flux from the surface. Spectral characteristics, dynamics, and relationships with optically active constituents and common optical metrics, are documented for three light absorbing components in culturally eutrophic Onondaga Lake, New York, U.S.A., based on the results of a 5 year study. The absorption coefficient, a(m -1 ), is partitioned according to the additive components of colored dissolved organic matter (a CDOM ), non-algal particles (a NAP ), phytoplankton (a φ ), and water itself (a w ; known), based on laboratory measurements for near surface samples collected weekly for the spring to fall interval over the 2004-2008 period. Supporting bulk measurements included dissolved organic carbon, total suspended solids (TSS) and its organic and inorganic (FSS) fractions, the concentrations of total phosphorus and chlorophyll-a ([Chl]), downwelling irradiance profiles to determine the diffuse attenuation coefficient (K d ), the beam attenuation coefficient at 660 nm [c(660)], and Secchi disc depth. The average exponential slope values for a CDOM (0.0168 nm -1 ) and a NAP (0.0126 nm -1 ) were very similar to those reported for other case 2 systems. The shapes of a φ spectra, represented by the ratio a φ (440): a φ (676), were found to negatively depend on [Chl]. The largest component of a at 440 nm, the reference wavelength, in each of the 5 years was a CDOM (∼45 to 60 %), the smallest was a NAP (∼15 to 25 %). Progressive decreases in yearly average a φ , particularly over the last three years are documented in response to decreases in [Chl], driven by increased nutrient limitation and Daphnia grazing (2008 only). Wide short-term variations are demonstrated within years for the components of a, particularly in response to the timing of runoff and Daphnia grazing events. Strong positive relationships are reported between a NAP and both TSS and FSS, a φ (at both λ = 440 and 676 nm) and [Chl], and a p (a NAP plus a φ ) and both TSS and c(660). The summation of the absorbing components at 440 nm is demonstrated to be a strong predictor of K d . The utility of the information in supporting a model for K d and SD and advancing monitoring of water quality through measurements of surface reflectance is described.