A quantitative blueness index for oligotrophic waters: Application to Lake Tahoe, California–Nevada. (23rd October 2015)

• Record Type: Journal Article
• Authors: Watanabe, Shohei; Vincent, Warwick F.; Reuter, John; Hook, Simon J.; Schladow, S. Geoffrey
• Is Part Of: Limnology and oceanography, methods. Volume 14:Number 2(2016:Feb.)
• Page Start: 100
• Page End: 109
• Publication Date: 2015-10-23
• Subjects: Limnology -- Methodology -- Periodicals; Oceanography -- Methodology -- Periodicals; 551.48
• DOI: 10.1002/lom3.10074
• Languages: English
• ISSNs: 1541-5856

Abstract

Abstract: 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 ( B w ) 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 B w, 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 B w were determined using a hyperspectral radiometer attached to a buoy in the middle of Lake Tahoe. The B w values were highest in summer, and there was a strong inverse correlation between B w and phytoplankton concentrations as measured by in vivo chlorophyll a fluorescence. However, there was no significant correlation between B w 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.