Cryptomonas represents a group of algae known as cryptomonads. They are common in the phytoplankton of freshwaters, but are sometimes hard to find because zooplankton love to eat them. A good time to collect cryptomonads is in late winter or early spring as pond or lake ice starts to melt. Cryptomonads respond to the increased sunlight by growing rapidly before it gets warm enough for zooplankton to grow and reproduce. These algae are so small that a few will escape the detection of summer predators and survive to produce more large populations in the cool seasons.

Phytoplankton samples from ponds and other freshwaters often include diatoms, which are coated with 2-part glass shells. The glass walls are perforated with many small holes, tubes, or slits that allow contact with the water, and which form beautiful patterns. The patterns can be seen best by using a scanning electron microscope, available in some high schools or at a local college or university.

A living diatom from a pond, viewed with a compound light microscope; note the glistening glass wall. The wall markings, which you need to see in order to correctly identify diatoms, are obscured by the golden chloroplasts and yellow oil storage droplets. This is one of a group of diatoms that have bilateral symmetry, and are known as pennate (after "feather") diatoms.

Stephanodiscus is one of a group of freshwater planktonic diatoms that has radial symmetry. Scanning electron microscopy reveals the details of its elaborate glass wall. Its name comes from the Greek words stephanos , which means crown, referring to the spines that encircle its edge, and diskos, meaning disk.

Climacostomum is one of many types of protoplankton known as ciliates because they are covered with cilia, which make their edges look somewhat fuzzy, and help them swim through the water. Ciliates that live in water that is low in mineral nutrients, and thus food prey, often have algal partners living inside them, which provide food in exchange for minerals needed by the algae.