It is clear from the discussion on the effects of light and temperature that there are seasonal changes in the aquatic flora as a whole.
This applies even in springs at constant temperatures and temperate climates where the winter flora is dominated by Diatoms and the summer flora by Chlorophytes, Cyanophytes, and Rhodophytes.
Light is therefore certainly involved. Various lists of times of occurrence of different algal species are available. Usually Diatoms dominate these lists of species and in stony streams and rivers, they are also the most abundant forms.
In streams that are dominated by melt water floods and cold winters, the seasonal cycle may become truncated by physical conditions.
For instance, in the west, the scouring actions of the floods in May and June almost clean the stones and it is late July before recovery is evident leading to a buildup of Diatoms and other algae.
During December and January, however, a decline caused by anchor and frazil ice although the number of Diatoms increased a little in early spring, they were swept away by the melt water. On stony or other solid substrate, there is a fairly clear annual cycle and in running water, the actual nature of the substratum is apparently not of great importance for many species except in respect to stability.
It can also often be seen that algae are arranged on stones and other fixed objects in a definite way in relation to the current. This is nicely shown in Figure ___ (from Hem pg 75), The illustration of the distribution of algal species on variously submerged stones.
Studies of algal colonization of substrate show heavier colonization on the upstream and downstream sides than laterally. This is undoubtedly due to the small differences in local conditions.
The upstream face of a stone is the one most likely to receive disseminates carried by the current, and it also has a thin zone of dead water because of the splitting of the main stream of the current as it passes each side of the stone. There is also a much larger dead zone of water on the downstream side, whereas the only dead water on the sides was the boundary layer. Such observations explain, at least in part, why some local distributions occur and why some species are largely confined to upstream faces in certain situations and do not alsooccur on downstream faces.