A quantitative blueness index for oligotrophic waters: Application to Lake Tahoe, California-Nevada

Autor: Warwick F. Vincent, John E. Reuter, S. Geoffrey Schladow, Simon J. Hook, Shohei Watanabe
Rok vydání: 2015
Předmět:
Zdroj: Limnology and Oceanography: Methods. 14:100-109
ISSN: 1541-5856
DOI: 10.1002/lom3.10074
Popis: The perceived blue color of a lake often contributes to its aesthetic appeal, and changes in blueness can be indicative of major shifts in water quality. We developed a quantitative blue water index (Bw) for natural waters, and used it to evaluate spatial and seasonal variations in ultraoligotrophic Lake Tahoe, where clarity and blueness are of ecological and economic value and a focus for lake management strategies. Spectral reflectance was measured using a profiling hyperspectral radiometer, and the values were converted to the axis values of a color space: the International Commission on Illumination L*a*b*, where L* is the lightness of color, a* ranges from green to magenta, and b* ranges from blue to yellow. The blue water index Bw, defined as negative b*, was similarly high at two offshore monitoring sites in Lake Tahoe, but much lower in a semienclosed bay and a small adjacent lake. Seasonal variations of Bw were determined using a hyperspectral radiometer attached to a buoy in the middle of Lake Tahoe. The Bw values were highest in summer, and there was a strong inverse correlation between Bw and phytoplankton concentrations as measured by in vivo chlorophyll a fluorescence. However, there was no significant correlation between Bw and Secchi depth. Blueness and visual clarity are complementary measures of the perceived optical state of natural waters, and for many lakes may provide a powerful combination of indicators for conveying water quality to the public.
Databáze: OpenAIRE
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