Within each transect, algal mats were visually identified as either orange, red, black, or green. Not all types of algal mats were found within every transect. A maximum of five algal samples of each color were collected from each transect using a #13 cork borer (diameter = 17 mm, area = 227 mm2). Algal samples were preserved in 10% formalin for laboratory identifications. In all, 141 algal samples were collected from 12 streams. Preserved algal samples were examined in the laboratory to determine species composition and relative abundance using a Nikon Diaphot phase contrast microscope. Samples were well mixed to attain an even (random) sampling of mat (some small "clumps" of algae were still present, but these did not affect the objectives of the study), and then subsamples of 2 ml were withdrawn. Each subsample was placed in a 2-ml settling chamber with a 26 mm diameter and examined at x400 magnification. To get a representative count of relative species abundance, seven random fields (a total of 0.44 mm2) needed to be examined per subsample. Within each field, algae were identified to genus and species where possible (excluding diatoms, which were counted but not identified.) Percent cover was determined in each field by measuring the length and width of all specimens using an ocular micrometer. Cell "depth" was also measured in order to estimate biovolume. All seven fields were then tallied, and the percentage of the total algal biomass was determined for each taxa. Taxonomic identifications were primarily based on the classical Antarctic literature.