Even a single drop of pond water may contain thousands of organisms, including different types of bacteria, protozoa, algae, rotifers, hydras and worms. Although most of the organisms that live in water are microscopic, a few are large enough to see with the naked eye.
Bacteria are some of the smallest microorganisms in the world, and they are only visible under very high magnification. Bacteria are ubiquitous, and many varieties are present in most pond water samples. Many tiny arthropods, such as copepods, also inhabit pond water and are visible to the naked eye. Large specimens may exceed 3 millimeters in length.
Protists are common, primitive organisms that live in pond water. Algae are plant-like protists, while protozoans are animal-like and are similar to very simple animals. Because they are photosynthetic, most algae are green; however, a few types are yellowish-brown. Many protozoans possess whip-like flagella or hair-like cilia that enable them to move about through the water.
Creatures called tardigrades also inhabit pond water. These tiny creatures, which are slightly less than 1 millimeter in length, are also called “water bears,” as they bear a strong resemblance to the large mammals. Tardigrades are found in water all over the world.