A cluster of primitive, mainly aquatic plants that have chlorophyll-a as their primary photosynthetic pigment. Algae, unlike most other plants, are not made up of a root, stem and leaf system. Examples of algae range from unicellular plankton to meter-high seaweeds. An overproduction of algae is a concern for aquatic ecologists because very high densities of algae in lakes, rivers, reservoirs and streams can lead to ecological and human health problems.