Detection and Quantification of Snow Algae with an Airborne Imaging Spectrometer

Seasonal snowfields in semiarid regions, such as the western United States, provide a habitat for microbial life (9), as well as the primary regional freshwater supply. Snow can host an abundant microbial community supported by phytoplankton collectively termed snow algae (7, 8). Microbial processes, such as heterotrophy (1), photosynthesis (10), and nutrient cycling (9), occur in melting snow and are important factors in estimating carbon budgets and CO2 flux (16). However, few carbon flow models consider these activities in the snow meltwater column. Additionally, variations in snow algal biomass and species composition may reflect regional environmental or climate changes (21). While there have been numerous reports that have quantified snow algae at the plot scale in terms of cells per milliliter (11, 19, 21), there have been no direct estimates of algal biomass at the snowfield or watershed scale. This is primarily because standing crops of snow algae are not uniformly distributed, making biomass estimates problematic. Remote sensing through imaging spectroscopy offers the capacity to analyze spectral reflectance features that are related to snow algal concentration at a spatial resolution commensurate with the spatial variability of surface cover in alpine basins. Imaging spectrometers, such as the National Air and Space Administration/Jet Propulsion Laboratory Airborne Visible Infrared Imaging Spectrometer (AVIRIS), have improved the ability of remote sensing to quantify surface cover properties (5). The use of airborne imaging spectroscopy to estimate phytoplankton abundance has provided valuable information concerning algal population dynamics and primary production in freshwater and marine water (13). Chlamydomonas nivalis is the most prevalent alga found in snowfields in the Sierra Nevada of California (18). During a bloom, nonmotile algal resting spores (aplanospores) of C. nivalis impart a deep red color to snow. Germination that results in motile (flagellated) cells of C. nivalis occurs only in saturated snow since the algae require liquid water to move around snow grains and position themselves vertically according to irradiance levels and spectral composition (6). C. nivalis spores are spherical and have radii that range from 20 to 50 μm. During a bloom, most cells lie near the snow-air interface, but cells can be found down to a depth of 10 cm (18). In the Sierra Nevada of California, snow algae are found in old, wet snowfields at elevations over 3,000 m (18). Thomas and Duval (19) demonstrated that there is a significant negative correlation between snow albedo and algal cell concentration but also found that decreases in albedo due to algal snow did not contribute to a significant decrease in the mean albedo of snowfields. This work addresses multiple unexplored issues: (i) to document and analyze the reflectance spectrum of algal snow and its relationship to algal concentration, (ii) to develop a model that relates algal concentration to the reflectance spectrum of algal snow, and (iii) to demonstrate the ability of remote sensing with an imaging spectrometer to detect and quantify the spatial distribution of algal concentrations in alpine snow. We addressed these problems with data collected during a field campaign in the Sierra Nevada of California in the summer of 2000. These data include direct measurements of snow algal concentration (in cells per milliter), snow spectral reflectance, and high-spatial-resolution data from the AVIRIS.