Microalgae, cyanobacteria, chlorophyll, photosynthesis, ocean color, blooms, primary production, food webs, carbon dioxide, oxygen, nutrients, upwelling, marine snow, harmful algal blooms, climate change, and aquatic ecosystem monitoring
Phytoplankton
Phytoplankton are microscopic, photosynthetic organisms that drift in water, form the base of many aquatic food webs, and help move carbon through the ocean.
What phytoplankton are
Phytoplankton are tiny photosynthetic organisms that drift in oceans, lakes, rivers, and estuaries. They include many kinds of microalgae and cyanobacteria. Most are too small to see individually, but together they can reshape the color and chemistry of entire water bodies.
How they grow
Like plants on land, many phytoplankton contain chlorophyll and use sunlight to turn carbon dioxide and water into organic matter. They also need nutrients such as nitrogen, phosphorus, and iron. When light, nutrients, and water conditions line up, populations can grow quickly.
Blooms
A bloom is a rapid increase in phytoplankton abundance. Some blooms are natural seasonal events that feed food webs. Others become harmful when they produce toxins, block light, clog fish gills, or decay in ways that strip oxygen from the water.
Food webs
Phytoplankton are primary producers: they make new organic matter at the base of aquatic food webs. Zooplankton graze them, small fish eat zooplankton, and larger animals build from that energy. A change in phytoplankton timing or species can ripple upward.
Carbon and oxygen
Phytoplankton take in carbon dioxide during photosynthesis and release oxygen. Some of the carbon they fix returns quickly to surface waters through respiration and grazing. Some sinks as marine snow, helping move carbon into deeper water.
What controls them
Nutrients often arrive through upwelling, river flow, mixing, dust, or runoff. Light depends on depth, water clarity, season, ice cover, and cloudiness. Temperature and grazing matter too; phytoplankton live inside a moving balance of growth and loss.
How scientists track them
Researchers measure chlorophyll, collect water samples, identify species under microscopes, sequence DNA, deploy sensors, and use satellite ocean color. Satellites are powerful, but they mostly see the surface, so field measurements remain essential.
Why it matters
Phytoplankton are small enough to miss and important enough to change the planet. They connect sunlight to seafood, oxygen, marine snow, carbon storage, harmful algal blooms, and the way ocean ecosystems respond to climate change.