Light and Turbidity
Light exclusion from turbidity is one of the most important effects of suspended matter on algae for it interferes with their photosynthetic action depriving them of active growth. Some reports imply that plankton were thirteen times more abundant in clear and 1.5 times more abundant in moderately turbid water than in muddy ponds.
The two Rotophytons mentioned earlier, Batrachosphermum grow in cool seasons, but unlike Lemanea is apparently limited by high light sensitivity. The Rotophyta of fresh water are indeed adapted to grow at low light intensities. On the other hand, many Diatoms appear to be fairly indifferent to light in that they occur as abundantly in shaded places as in open ones. Many Chlorophyceae require a fairly high light intensity. This is indicated not only by their increase in spring and early summer, but their distribution in shaded and lit areas of stream beds.
It is indeed easy to observe the way in which conspicuous green algae disappear from a stream bed when it enters a deeply shaded reach and this phenomena of heavy mid-summer shade accounts for the fact that many species are abundant in early summer and fall and comparatively scarce in high summer.
An algal community kept at just over 18° C increased its rate of production of oxygen as light was increased, but that over 11,400 lux which is considerably less than full sunlight photosynthesis actually declined. At 8 to 10° C, the rate of photosynthesis continued to rise to the highest level of illuminating tested. It was however much lower than at the warmer temperatures. At the lowest light intensity in the higher temperature, respiration exceeded photosynthesis, a further demonstration of lower efficiency in warmer water.