Phtyoplankton sample magnified by 100 times.
Phtyoplankton sample magnified by 100 times.
Richard Davidson, courtesy of U.K. Fisheries Research Service

Diatoms, Dinoflagellates, Blue Green Algae

Appearance: Most individual structures are microscopic and appear collectively as a brownish, cloudy substance in the water.
Habitat: Throughout the water column.
Seasonal appearance: All year, with blooms in the spring and fall.


Phytoplankton are single-celled, free-floating, non-swimming plants. Zooplankton, which consist of small animals and the larval forms of invertebrates and fish, together with phytoplankton make up the group called plankton. The predominant forms of phytoplankton are diatoms, golden brown algae, green algae, blue green algae, and dinoflagellates. Over 20,000 species of diatoms alone exist in the world. They have an exoskeleton composed of silica and have no means of locomotion.

Life History and Growth

Phytoplankton is the foundation of all life in the waters of Rhode Island, serving both directly and indirectly as a food source for all living animals. Their movement through water is dependent solely on currents. Blue green algae is the dominant freshwater plankton, while diatoms make up the majority of phytoplankton in water of higher salinity. Phytoplankton use the process of photosynthesis to sustain themselves, converting carbon dioxide into oxygen and organic materials. Phytoplankton are believed to produce 80 percent of the organic material in the world. A wide range of small and large fish, and invertebrates consumes these plants.

Critter Fact Occasionally, phytoplankton experience a localized, rapid growth spurt, known as a bloom, and accumulate in dense patches at the water's surface. In cases where the phytoplankton produce red pigments, the water may actually appear red. These blooms are commonly called "red tides."


Special Notes

Adapted from The Uncommon Guide to Common Life on Narragansett Bay. Save The Bay, 1998.