One of the earliest and most reliable indications of acid precipitation affecting freshwater systems is the development of littoral blooms of the filamentous green alga Mougeotia (Zygnematales, Charophyceae). Field observations of depth distribution and seasonal abundance in Little Rock Lake, an experimentally acidified seepage lake in north-central Wisconsin, suggest that Mougeotia might prefer warmer temperatures and carry out net photosynthesis at fairly low light levels. To test these hypotheses, we measured net photosynthesis and respiration at 56 combinations of irradiance and temperature in a controlled environment facility. Optimum conditions were 25°C and 300-2,300 bmol quanta m-2 s-I. Over this range, net photosynthesis averaged 40.4 mg 0, g-l h-l. Respiration rates increased with both temperature and prior irradiance. Lightenhanced respiration rates were significantly greater than dark respiration rates following irradiance 2 164 pm01 quanta m-* s-l. Mougeotia's photosynthetic response to temperature and irradiance explains aspects of its growth and distribution in acidified lakes but it does not by itself explain the magnitude of this alga's growth at low pH.Since the early 197Os, acidification of freshwater systems by acid precipitation has been increasingly recognized as a widespread and significant environmental problem. One of the earliest and most reliable indications of acid precipitation affecting freshwaters is the development of littoral blooms or mats of filamentous green algae of the family Zygnemataceae. The genus Mougeotia is one of the most common bloom-forming alga in acid waters of Europe and North America. Despite the frequency with which Mougeotia is cited as an indicator of acidification, little is known about its ecology or physiology. Why should Mougeotia rather than some other alga be so frequently associated with acidification? The most Acknowledgments