It depends on the body of water. In oceans, there are planktons, water plants, macro and microinvertibrates, other animals, and more. In lakes, there are planktons, water bugs, algae, plants etc. In streams there can be lots of things as above. It really depends what and where you're looking for.
The planktons make oxygen because it breaths CO2. CO2 is 1 carbon atom and 2 oxygen atoms. So when the planktons get the CO2, it gets ONLY the carbon atom and leaves the oxygen atoms as a form as waste. The waste of the plankton is the benifeit of the animals on earth.
The ozone layer when depletes causes UV rays to enter which falls on planktons in the sea. It destroys those planktons which reduce global warming thus increasing the global warming in the environment.