An objective of the Long-Term Ecological Research (LTER) project in the McMurdo Dry Valleys is to understand processes regulating productivity, biomass, and distribution of the stream communities using a combination of long-term monitoring, in situ experiments, and modeling. Algal mat and moss communities that grow in and along the margins of antarctic streams become active during a short period in the austral summer when temperatures and meltwater are sufficient to promote growth. Some streams are known to support high biomass (2-400 milligrams of chlorophyll-a per square meter), but production rates are at the low range for freshwater communities (Vincent et al. 1993). Removal processes, such as wind, flood scouring, and grazing by protozoans and micrometazoans, may regulate biomass accumulations since light and nutrients are not limiting factors for algal growth (Howard-Williams and Vincent 1989). Additional controlling factors may include variable streamflow, freeze-thaw events, and winter dessication. In turn, the mats are likely to influence downstream soil and lake ecosystems by removing and transforming nutrients.

Spectroradiometry may be useful in accomplishing several LTER goals, including assessing distribution, biomass, and nutreint status of the stream communities. The ability of a plant to reflect or absorb light is dependent on its morphological and chemical characteristics which, in turn, are a function of plant development, health, and growing conditions. The relation between spectral reflectivity and plant status makes spectroradiometry a potential tool for studying ecological features of plant populations. In this article, we explore the use of close-range remote-sensing techniques for assessing algal and moss communities of the streams within the McMurdo Dry Valleys.

In January 1994, we measured spectral and pigment characteristics of the six dominant algal and moss communities of the Canada Stream in the Lake Fryxell basin, Taylor Valley. The assemblages are identified here according to their color: orange-colored, red-colored, green-colored, or black-colored algae, and green or black moss. Taxonomic identification is currently in progress; however, previous studies indicate that the algal mats are dominated by cyanobacteria (Vincent et al. 1993). Spectral-reflectance measurements were taken from each assemblage by using a handheld spectroradiometer (Model PSII, Analytical Spectral Devices, Inc.), which measures in 512 bands of about 1.4-nanometer (nm) width between 1000-1400 hours during cloud-free periods. Spectra were taken 5 centimeters (cm) above each sample resulting in a circular field of view of 0.2-cm diameter. Algae and mosses were briefly removed from the stream to obtain spectra because flowing water complicated and reduced the spectral signal. Chlorophyll-a and carotenoids were analyzed using the trichromatic method (Strickland and Parsons 1968).

Green-colored moss and red-colored, orange-colored, and green-colored algae exhibited reflectance patterns typical of vegetation; the greatest reflectance occurred in the near infrared (NIR, 700-800 nm) and absorption in the blue (400-500 nm) and red (600-700 nm) regions of the electromagnetic spectrum (figure). Absorption in the blue region is likely due to carotenoids, which absorb maximally in the the 400-550-nm range (Vincent et al. 1993), whereas absorption in the red region corresponds to the maximum chlorophyll-a absorption at 680 nm. Chlorophyll-a concentrations in these assemblages ranged between 5 and 8.8 micrograms per square centimeter (ug/cm²) and carotenoids between 4.5 and 11.2 ug/cm² (table). Although all phototrophic algae contain chlorophyll-a, they can be distinctly colored by other pigments. This range of pigmentation contributes to the variation in spectral signatures observed in the Taylor Valley mats. For example, the red-colored algae are distinguished by having an additional, small absorption feature centered at 630 nm, possibly due to the pigment phycocyanin, which absorbs maximally at 620 nm (Vincent et al. 1993).

Spectral patterns of the black-colored algae and black-colored moss were markedly different from that of the other four stream assemblages (figure). Reflectance gradually increased from 400 to 900 nm and despite the high chlorophyll concentrations found in the black-colored algae (41.8 ug/cm²; table) the absorption feature usually associated with chlorophyll at 680 nm is not obvious compared to other algal types (figure). Chlorophyll-a absorption is masked due to the high absorption of all spectral bands by the combined "black" pigments of this assemblage, leaving no reflectance peaks (green and NIR) to contrast with the high red absorption. Black-colored moss, which was not as darkly pigmented as the black-colored algae, reflected higher at all wavelengths than the black-colored algae and displayed a slight chlorophyll-a absorption feature. Chlorophyll-a and carotenoid concentrations for black-colored moss were 16.8 and 32.3 ug/cm², respectively (table).

Spectral analysis has been used to estimate biomass and productivity in aquatic vegetation (e.g., Dewey et al. 1993; Penuelas et al. 1993). Several indexes have been correlated with biomass, including the normalized difference vegetative index (NDVI; NDVI=ratio of maximum NIR:minimum reflectance; table), with highly variable results due to vegetative type and influences of background substrate. The use of a narrow-band NDVI as a biomass predictor of the Canada Stream assemblages was less than satisfactory. No relationship between the NDVI and pigment analysis was apparent (table). Variable vegetation type (algae vs. mosses) and three-dimensional structure of the assemblage may be factors contributing to the lack of predictive power of the NDVI. Also, the general lack of contrast in spectral reflectance from the black-colored algae and black-colored mosses makes it difficult to apply classical vegetation indexes, such as the NDVI, which are based on contrast.

The spectral data illustrate the potential for using spectral reflectance patterns, spectral biomass indices, and changes in the near infrared reflectance as tools for studying ecological features of the algal and moss communities in and along dry valley streams. The acquisition of ecologically meaningful spectral data will require a thorough investigation of the relationships between spectral features and biomass, physiological status, pigment content, three-dimensional structure of the assemblage, production, and nutrient status of dry valley stream communities.

This work was supported by National Science Foundation grant OPP 92-11773 and an Institutional Project Assignment grant from the Desert Research Institute, Reno, Nevada. We thank the Desert Research Institute for the use of the ASD PSII Spectrometer, M. Anthony for chlorophyll analysis, and D.M. McKnight for field assistance and advice. Use of trade names in this article is for identification purposes only and does not constitute endorsement by the U.S. Geological Survey.

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