Why are natural ecosystems more or less balanced systems? If you remove all the phytoplanktons from a pond what will happen to it?

A natural ecosystem is a balanced system because it consists of producers (green plants), herbivores (consumers I), carnivores (consumers II) and decomposers. In natural ecosystem, there is provision of constant flow of energy through food chains and recycling of the nutrients through decomposers. Thus a natural ecosystem is self-sufficient system.
The removal of phytoplanktons from a pond would disturb the pond ecosystem. Phytoplankton are the producers. They are the main organisms which store energy in the form of food for other organisms like small fishes, insect, larvae. These organisms in turn are food of big fishes. Thus removal of phytoplanktons from a pond ecosystem will stop constant flow of energy, disturbing the whole food web in the pond. First larvae and small fishes will die in want of food and ultimately fishes which feed on them will die.