As you know, plants do the photosynthesis. In rivers and streams, there are also other small organisms that are doing the photosynthetic process: they are called photosynthetic aquatic organisms and are commonly known as algae and cyanobacteria.
Algae are very important to freshwater ecosystems because they provide oxygen through photosynthesis.
The main types are: diatoms, green algae and cyanobacteria. Do some searching in google and try to explain, with your own words, what’s a diatom, a green algae and a cyanobacteria. In this picture, you can appreciate the difference, in terms of size, of all the three types:
Now that you are an expert, have a look at these pictures and classify them within: diatom, green algae and cyanobacteria.