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Phytoplankton Blooms: The Basics

Phytoplankton Population Explosions Cause Algae Blooms

Under certain environmental conditions, canals, lakes, coastal waters and even swimming pools can experience phytoplankton 

or algae blooms. A bloom takes place when a species of phytoplankton reproduces at a rapid rate, multiplying quickly in a short 

amount of time. Sometimes more than one species blooms at 

the same time. Phytoplankton are photosynthetic organisms that 

live suspended just beneath the water’s surface. They use energy 

from sunlight and raw materials to make their own food through 

photosynthesis. Microalgae and blue-green bacteria (called cyanobacteria) are two groups of organisms that belong to the phytoplankton community. 

Blooms are often visible events. High concentrations of 

phytoplankton in the water column can cause the water to 

appear blue-green, green, brown or even red, depending upon 

the pigments found in the species experiencing the bloom. 

Pigments are substances in phytoplankton that absorb the sun’s 

energy, which is needed to drive the process of photosynthesis. 

The Right Conditions Are Needed for Phytoplankton Growth

Phytoplankton organisms are capable of absorbing the necessary substances for growth and reproduction directly from the 

surrounding water. For a species to bloom, environmental factors such as water temperature and salinity must be just right 

and essential nutrients must be available in the correct amounts. Each phytoplankton species has a different set of favorable 

conditions that promotes its growth and reproduction. 

The two most important nutrients for phytoplankton growth are the elements nitrogen (N) and phosphorus (P), which are found 

naturally in aquatic environments in various concentrations. Iron, zinc and manganese are also essential, but they are 

needed only in very small quantities. If any of the necessary nutrients are not available in the right amount for a particular species, 

growth and reproduction will be limited or non-existent in that species. But, if 

all conditions are favorable, a bloom will take place.

Many blooms are temporary events that disappear after the source 

of the nutrients is gone or when conditions are no longer favorable. For 

example, a bloom may develop after plant fertilizers containing nitrogen and 

phosphorus are carried into nearshore waters by rainfall runoff. Once the 

nutrients in the water have been depleted by the phytoplankton organisms, 

the bloom usually dissipates.

Some animals help limit or reduce phytoplankton populations by 

feeding on them. Filter-feeding oysters, scallops and sponges consume 

phytoplankton as they circulate seawater through their bodies, while 

microscopic crustaceans like copepods graze on phytoplankton in the water 

column. 

Florida Keys National Marine Sanctuary

A phytoplankton bloom in northern Florida Bay in the early 1990s.

 Photo: Florida Fish and Wildlife Conservation Commission

Different species of phytoplankton shown magnified. 

Photo: Florida Fish and Wildlife Conservation Commission

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Blooms Can Impact the Ecosystem in Several Ways

When the nutrient source is continuous and conditions remain favorable, blooms may become long-term events that 

can impact the ecosystem. Sustained blooms can block out or reduce the sunlight reaching seagrasses on the seafloor, 

which can stress or kill the plants. Since many animals 

depend on seagrass as habitat and foraging ground, 

losses can lead to changes in the fish and invertebrate 

populations. Lower seagrass cover can also reduce water 

clarity. Without the plants holding the seafloor sediments 

in place, tiny particles are free to float up in the water, 

clouding the water column and further reducing the sunlight 

that reaches seagrasses on the seafloor. The situation can 

be compounded when sediments become stirred up from the 

bottom by wind or currents, releasing trapped nutrients and 

decaying plant material that serve to further fuel the bloom. 

Long-term blooms can change the kinds and availability of plants 

at the base of the aquatic food web. Phytoplankton species serve 

as a food sources for zooplankton---tiny animals that drift in the 

water column and serve as a food source for larger species. 

During bloom events, when only certain species are abundant 

as food sources, animals that feed on the bloom species have 

plenty of food, while those that feed on other less available 

phytoplankton may not. Over time, this shift in producers at the 

base of the food web can affect the zooplankton, fish, birds and other animals in the food web. 

Low dissolved oxygen (DO) levels are sometimes found in bloom waters. Depletion of DO usually takes place during the night when 

the phytoplankton are using more oxygen than they give off in 

photosynthesis. Low DO levels can be exacerbated in residential canals and other waters because of limited circulation and exchange. 

After the bloom is over, the phytoplankton organisms die and undergo 

decomposition, a process that also removes DO from the water column. 

During some blooms, fish are seen gasping at the surface for oxygen 

and this lack of DO can be a direct cause of “fish kills” observed on the 

scene. 

A few species of phytoplankton produce poisons or toxins, which can cause 

health impacts in humans and animals. These are known as Harmful Algal 

Blooms, or HABS, and are the focus of much scientific research in coastal 

waters of the United States. The phenomenon known as “red tide” is an 

example of a HAB that is sometimes observed in Florida waters. The toxin 

produced in red tides can cause respiratory irritation in swimmers and marine animals and can make filter-feeding clams and oysters unfit for human 

consumption. 

Residents Can Help Reduce Runoff and Report Unusual Events

With continued study, scientists and ecosystem managers are working to better understand the dynamics of phytoplankton blooms. 

Coastal residents and businesses can help prevent the occurrence of some blooms by taking steps to reduce fertilizer runoff into 

nearshore waters. Although this will not prevent all blooms, it will help keep nearshore waters free of excess nutrients that can 

lead to bloom events. Residents can also help by reporting unusual events such as fish kills and phytoplankton blooms to Marine 

Ecosystem Event Response Assessment, or MEERA. The purpose of MEERA is to provide early detection of biological events in the 

Keys. For more information, visit http://isurus.mote.org/Keys/meera.phtml.

Written by: Nancy Diersing, Florida Keys National Marine Sanctuary 

Published: May 2009

Low dissolved oxygen in the water can lead to fish kills.

Photo: Florida Fish and Wildlife Conservation Commission

Bay scallops are filter-feeders that consume phytoplankton.

Photo: Florida Fish and Wildlife Conservation Commission