Current is an obvious factor in running water and there are many algae that occur primarily or exclusively in streams or in similar places such as stony lake shores where there is a continuous water movement. These include Lemanea, Hildenbrandia, several species of Batrachospermum, Hydrurus, several species of Cladophora, Gongrosira, Lithoderma, and several species of Diatom.
In the southern United States, some algae are confined to the rapids and others are confined to the rapids only during the summer time. Many workers have noted that attached algae are more abundant in faster water or that certain species occur more abundantly in areas of swift flow.
There is thus an innate current demand in some species and the different species become dominant at different current speeds. The effect of current was often of greater importance than that of the quality of water.
A pertinent point to mention here is that running water, simply because it is moving, is a richer habitat than still water. The currents prevent the accumulation of a shell of depleted water around an organism by constantly presenting fresh material to its surface to replace that used up by metabolism.
This applies to plant nutrients as well as to oxygen or other dissolved substances involved in biological activity.
Since current makes water “physiologically richer,” this is an explanation for the current demand of many species and that others need current in warm weather when dissolved gas contents are low.
When dissolved gas contents (CO2 and 02) are low in running waters, it was found that Oedogonium takes up phosphorus (P32) ten times as fast from a current of 18 cm/sec as it does from still water and that the rate of uptake continues to rise up to speeds of at least 40 cm/sec. It also respires more rapidly in current so its whole metabolism is sped up.
It has been concluded then that when oxygen availability is raised by turbulent flow, the metabolism of plants is also raised and that they will thrive better the faster the flow.