Tiny aquatic organisms are microscopic organisms that live in water. Two examples of these organisms are algae and rotifers. Algae are single-celled or multi-celled organisms that range in size from 0.2 to 200 micrometers. Rotifers are microscopic animals that range in size from 0.1 to 0.5 millimeters. They are found in both freshwater and saltwater environments. Both algae and rotifers play an important role in aquatic ecosystems, providing food for other organisms and helping to maintain water quality.