PHOTOSYNTHETIC RESPONSE OF STREAM PERIPHYTON TO FLUCTUATING LIGHT REGIMES

  An experiment was conducted on intact algal assemblages of stream periphyton to test their response to fluctuating and constant light regimes having the same mean intensity. The light regimes (in μmol·m⁻²·s⁻¹) were constant light at 100, light fluctuating between 50 and 150 with a period of 5 min, and light fluctuating between 10 and 460 with periods of either 4:1 or 8:2 min. Compared to the rates measured under 100 in μmol·m⁻²·s⁻¹ constant light conditions, fluctuations ranging between 50 and 150 in μmol·m⁻²·s⁻¹ with a 5-min period produced a 23% greater rate of photosynthesis. Conversely, fluctuations between 10 and 460 in μmol·m⁻²·s⁻¹ led to a 59%–74% decrease in photosynthetic activity. Detailed examination of periphytic algal responses to fluctuating light revealed that higher light intensities produced steeper photosynthesis/time slopes, but it was the combined interaction with lower light intensity that ultimately determined overall photosynthetic rate for a given light regime. This study offers compelling evidence that variable light regimes have important consequences for algal photosynthesis in natural streams.