US Long-Term Ecological Research Network
Physiological ecology of a species of the filamentous green alga Mougeotia under acidic conditions: Light and temperature effects on photosynthesis and respiration
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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. Light enhanced 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.