The substrate is of sufficient importance in the ecology of algae to make it possible to refer to different types of communities.
It is fairly clear that the stability of the substrate of the substratum is important. The effect of streambed stability in reducing the numbers of algae has been noted on stones which were rolled by the water in comparison to stones that remained fixed in the stream bed.
On the whole, larger species of algae are confined to solid objects rather than to plants and the slower growing ones. Rhodophyta, are found on stable materials such as rocks and large stones.
We know little about the reasons why certain species occur on certain substrata and not on others. Different types of stone in a stream bed often acquire different amounts of flora as sometimes can be seen near bridges where alien stones, bricks and concrete are mingled with native rock.
Another example is Hildenbrandia Rivularis which is not often found in very hard water. It is suggested that this is because it is unable to grow on limestone. It is a very slow growing plant and it may be that it becomes detached from the limestone by solution of the calcium carbonate faster than it can grow to cover the surface.