ponds and lakes depend on a balance among the organisms that live there the water contaians substances called nutrients that organisms need to live and grow For example through the process of photosynthesis plants use energy from the sun to produce sugar This process releases oxygen into the water Animals that live in the water breathe oxygen when plants and animals in the water die, the process of decay uses some of the oxygen For a pond or lake to remain healthy the amount of oxygen produced and used in the lake must remain balanced