Growth inhibition by high light intensities in algae from lakes undergoing acidification

Blooms ofChrysochromulina breviturrita Nich. (Prymnesiophyceae) have been found to be restricted to lakes above pH 5.5 even though the alga is able to tolerate pH 4.0 in laboratory culture. A possible explanation is the increased transparency in acidifying lakes and a sensitivity ofC. breviturrita to high light intensities. A comparison was made withMougeotia sp., a filamentous green alga which co-occurs in moderately acidic lakes and has a similar pH tolerance range. This alga forms dense, floating mats or amorphous clouds in the upper littoral zone, where it would be exposed to full sunlight irradiances. In cultures ofC. breviturrita, prolonged exposures to 1600 µE · m^–2 · s^–1 (I₀) resulted in reductions in cell yield which were dependent age at the onset of exposure to high light intensity. Only cultures exposed to high light intensities during late stationary phase were able to recover to control levels and no recovery occurred if these cultures were nitrogen deficient.Mougeotia was more tolerant of both high light intensity and nitrogen limitation during the recovery period. The inability ofC. breviturrita to recover from the effects of high light intensity during nitrogen deprivation may be particularly important in small, stratified lakes which are undergoing acidification. The slow rate of vertical circulation, and increasing transparency, would prolong exposure of the alga to the high irradiance levels of nutrient-deficient epilimnetic waters. This suggests that the geographic distribution ofC. breviturrita may be explained in part by the increasing light intensities in lakes undergoing acidification.