We investigated the effects of light on benthic algal communities in four acidic lakes (Johnson Lake, McNearney Lake, Avery Lake and Garlinghouse Lake) in northern Michigan (U.S.A.) that differed in dissolved organic content. In mid-summer, a series of light filters was placed 1 cm above the sediment of each lake at a depth of 2-3 m to filter out 0%, 80% and 90% of light reaching the lake bottom. After 5 weeks, the biomass and taxonomic composition of algal communities developing under these light conditions were quantified. Light accounted for most of the variation in algal biomass (r2=0.79). High-light conditions promoted filamentous green-algal (Zygnematales) growth, while diatoms and desmids had both a greater absolute and relative abundance in lower light conditions. We hypothesize that shading by large colonies of filamentous green algae, common in acidic lakes could positively affect diatom abundance possibly by protection from UV-light or photoinhibition. Large colonies of Mougeotia sp. could also benefit by self-shading in excessive light conditions.