Why does it help to have some microorganisms living in the pond?

One can think of a lake or a pond as a small ecosystem. In this system, there are primary producers (phytoplanktons), consumers (zooplanktons) and decomposers. The primary producers carry out photosynthesis and generate food and oxygen. Consumers, such as fishes, consume this food and carry out their life functions. When the phytoplanktons or zooplanktons die, their organic bodies are decomposed by the decomposers.

Microorganisms play the role of decomposers in this small ecosystem. They decompose the organic material in the pond and convert it to nutrients, which are returned back to the system, thus completing the nutrient cycle. Without microorganisms, the dead plants and fishes (and other organisms) will stay in the system, slowly decaying (by natural reaeration and chemical decomposition) and causing very slow nutrient recycling. Thus, microorganisms are essential to a pond or lake.

Hope this helps.