Temperature is an obvious ecological factor which should reveal itself in seasonal changes in the flora, but its effects are difficult to disentangle from those of light which is also low in the winter. However, there appears to be species that occur only in the tropics and so presumably need warm water. There are others that seem to require low temperatures since they occur only in winter at low altitudes, and persist for much longer in high altitude streams.

Probably, many of the species which occur in springs are cold water stenotherms. At the other end of the scale are several species which are confined to hot springs and do not survive at normal temperatures. Others, for instance the red algae, Hildenbrandia, occur only in waters that become warm in summer.

Many species, particularly of the Chlorophyceae, grow primarily in the summer time. Such are species of Oedogonium, and Cladophora Glomerata, which seem however to be limited to temperatures below 25 C. Presumably, these algae require higher temperatures or growth even though they persist as reduced structures throughout the winter.

Elevated temperatures of stream water of 2.5 to 5.5° C have been reported to cause no significant increase in plankton. However, some reports on streams subject only to thermal pollution indicated that blue-green algae became more abundant and green algae less abundant.

Studies have reported that: blue-greens favor temperatures that range from 15 to 27° C, green algae 10 to 21° C, and yellow-green algae 4 to 10° C.

Other studies cite the best growth temperature of blue-green algae as 30 to 40° C and green algae as 25 to 35° C. In mixed populations from a normal healthy stream, a large number of species of Diatoms were present at 20° C dominating the algal culture, but at 30° C only two species of diatoms were numerous and blue-green algae predominated.

Some green algae have adapted to extremes of temperature. An example is shown by those found in Arctic regions and other species have been noted in water at 45° C. The blue-green algae, Anabena and Coelosphaerium, seldom give any trouble unless water temperatures exceed 21° C. Aphanizomenon apparently prefers colder water than both Cyanophycae. It is present in some ponds even when the surface is frozen.

Diatoms appear to grow best at temperatures of 15 to 30° C. The brackish Water diatom Nitzschia Filiformis grows best at 26° and shows no growth at 34° C. The tolerant fresh water Diatom Gomphonema Parvulum grows best at 22° C and still shows considerable growth at 34° C. The least tolerance to high temperatures is shown by the sensitive fresh water Diatom Mischialenieras which grows best at 22° C and shows little or no growth at 30° C. Although the growth of Mischialenieras was inhibited at 4° C, the cells remained in good condition. For mixed populations from a normal healthy stream a large number of the Diatom species are usually present at 20° C, and dominate the culture.

To summarize temperature, many have concluded that temperature acts as a typical controlling factor governing the rate of growth by determining the rate of metabolism. The concentrations of nutrient salts shifts the temperature optimum for growth so temperature rarely plays a direct role in the acclimatization of algae, however, it has an important affect on the rate of growth.

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