The microbial mat found on the sandy shoals of Douglas Lake was examined with light and electron microscopy in order to describe algal composition and assemblage microstructure. The relative contribution of the mat to lake primary productivity was also determined. In addition, the mat was employed as an in situ bioassay. The mat contained 108 algal taxa. Cyanobacteria contributed 74 to 80% of the total biovolume, whereas diatoms contributed 19 to 25% and green algae never contributed more than one percent. The mat consisted of two identifiable layers. The upper brown layer was mostly sand and fine sediments colonized by small diatoms and a few Cyanobacterial filaments. The lower green layer was a thicker entanglement of filaments, sand grains and diatoms. The dominant alga was Schizothrix calcicola. The algal mat in Douglas Lake contributed 54% of the littoral zone net primary productivity. Since the littoral zone contributed 46% of the total net primary productivity, the algal mat was responsible for 25% of the net primary productivity in Douglas Lake. The in situ bioassay of the algal mat examined the effects of acid and nitrate perturbations from the water column and sediments. Acidification eventually reduced total algal biovolume and primary productivity of the mat. However, on the first sampling date, acidification of the sediments increased primary productivity and the biovolume of Schizothrix calcicola. Nitrogen enrichment elicited no change in total algal biovolume and a slight increase in primary productivity. The abundances and biovolumes of some algal populations responded enough to distinguish between controls and treatments. Overall, the algal mat may be important in lake ecology and as a tool in environmental assessment.