The algal ecology of peatlands and the periphyton communities which are abundant in these habitats are relatively understudied. I performed independent in situ manipulations of light and pH in an intermediate fen in northern lower Michigan in order to examine the response of the native algal community, especially the response of desmids, to different levels of pH and different quantities and qualities of light. Levels of pH were manipulated in enclosures from the control pH level of 5.0 to an acid treatment (pH=4.0) and a neutral treatment (pH=7.0). There were no significant differences in species richness. Chlorophyll a concentration was greater in the acid treatment than the control. Closterium polystichum, a Cosmarium species, Peridinium inconspicuum, Synedra acus and unicellular desmids as a group were most abundant in the neutral treatment relative to the acid treatment. The abundance of a species of Mougeotia, chrysophytes and a species of Oedogonium were greater in the acid treatment relative to the control. Increases in Zygnemataceae may account for the increase in chlorophyll a. There was no strong response of the desmid community. Light levels were manipulated at the sediment surface with a series of filters that blocked 90%, 60% and 5% of the ambient light. There was no difference in levels of chlorophyll a as a result of light manipulation. Spirogyra sp. increased in abundance in the 90% shaded treatment relative to the control. Filamentous desmids were more abundant in the control relative to the dark treatment. There was a non-significant increase of bluegreen algae in the dark treatment. Changes in community structure may be due to differences in light-harvesting proteins in different algal groups.