A Division of the Georgia Department of Natural Resources

Harmful Algal Blooms

What are cyanobacteria?

Cyanobacteria are commonly referred to as blue green algae.  They occur naturally in waters used for recreation, fishing, and drinking water.  Under certain circumstances, these algae may grow rapidly to form dense accumulations known as blooms.  When blooms are formed by toxin-producing bacteria, it is generally referred as a harmful algal bloom (HAB).  These blooms are considered harmful because they could produce irritants and/or toxins, called cyanotoxins, which can pose health risks to humans and animals.

The presence of cyanobacteria does not necessarily mean that cyanotoxins are being produced.  Microscopic identification should be performed to determine if the algal species can produce toxins. Cyanotoxins may be present both before and after cyanobacteria are observed. 

Cyanotoxin levels should be confirmed through laboratory testing.  Algal samples can be tested to identify the type of algae present. If the algae are capable of producing toxins, the samples are tested to determine if it is producing toxin and if so, at what level. The toxins typically tested for include microcystin, cylindrospermospin and anatoxin-a. 

Visual signs of an algal bloom

Visual signs of a bloom include surface water discoloration (e.g., bright green, blue, brown, or red tint), pea soup or spilled green/blue paint appearance, reduced transparency, thick mat-like accumulation of scums, and/or dead fish. Cyanobacteria are also associated with taste and odor problems.  

Pollen season can produce waters with a similar appearance to a harmful algae bloom.  Excessive pollen is light green to yellow in color, versus the green paint-like appearance of algae blooms, as shown in these photographs. Large amounts of pollen may look alarming. However, toxins are not being produced and there are no health risks from contact with the water. 

Potential impacts of an HAB

Health risks: Cyanotoxins can cause human and animal illness through direct contact or inhalation. Depending on the species of cyanobacteria, the toxins affect the nervous system, liver, skin, or stomach. No human deaths due to cyanotoxins have occurred in the United States, though animal deaths have been widely reported. Animals such as pets, livestock, and wildlife may be exposed to cyanotoxins if they drink water from toxin-contaminated waterbodies, lick their fur after swimming in such waters, or consume toxin-containing algal scum or mats.


Health Effects from Cyanotoxin Exposure


Rash, irritation, swelling, sores Vomiting
Gastrointestinal problems Diarrhea
Respiratory problems Seizures
Fever Death
Neurologic symptoms  
Ear symptoms  
If you (or another person or animal in your care) begin to exhibit any of the symptoms listed above, contact a healthcare provider and be sure to mention the possibility of exposure to algal toxins from being in or around water containing an algal bloom.
The World Health Organization (WHO) considers toxin levels under 10 micrograms/liter to represent a low-level risk for adverse health outcomes from short-term recreational exposure. However, certain sensitive populations, such as children, the elderly and people with compromised immune systems, may still be at risk even at low concentrations and should avoid any exposure.

Drinking Water: There is potential for HABs to affect waterbodies used for water supplies; however, exposure to toxins through drinking water is uncommon. Water treatment facilities are capable of removing cyanobacteria and low levels of toxins. During a severe bloom event, water treatment facilities will take extra measures to reduce risk from contamination, and may issue drinking water advisories, if needed. It should be noted that boiling water does not remove toxins.

Environmental Damage: HABs result in dense growth that blocks sunlight from reaching water depths, making it difficult for sight-reliant species to forage. Additionally, as an HAB decays, it can deplete water of oxygen, resulting in fish kills and harm to other aquatic life.

Economic Impacts: HABs can negatively affect the economy by increasing costs associated with public health, fishery closures, reduced tourism and recreation, and HAB monitoring.

When and where do cyanobacterial blooms occur?

Cyanobacteria blooms are more common in warmer climates and tend to grow more rapidly at higher temperatures.  Most blooms occur in late summer or early fall, although they can occur at any time during the year.  Unfortunately, higher bloom occurrences during the warm summer months correspond with times of increased recreational use. Once a bloom does form, it may persist for several weeks, or it may rapidly move or dissipate within hours.

A bloom can occur at any location within a lake or reservoir, but they are most commonly reported near shorelines. Many cyanobacterial species contain gas vesicles within their cells, which enable them to float to the surface. These floating cells are then pushed via winds until they reach the shoreline, where they accumulate into visible blooms.

What should you do if you see a potential HAB?

  • If you see a bloom in a lake or reservoir, keep children and pets away from the water and limit recreational activities on the water.   Avoid contact with the water that is discolored; do not swim or wade through algal scums.

  • Do not drink, cook, or shower with untreated water from lakes, ponds, or streams.

  • Do not drive your boat, water ski or jet ski through the bloom.

  • Avoid exposure to irrigation water drawn from untreated sources.
  • Do not fish from lakes, ponds or rivers where algal scum is present.
  • Do not allow pets or livestock to swim in or drink scummy water.

  • If you or your animals come into contact with a blue-green algae bloom, wash with fresh water and soap after skin contact, and avoid swallowing or inhaling water. Wash animals' fur thoroughly before allowing them to groom themselves.
  • If you begin to exhibit any of the symptoms listed above, contact your healthcare provider and be sure to mention the possibility of exposure to algal toxins as you were in or around water.

Jar Test

The following step by step procedure can be used to make an initial assessment.


  • Clean clear jar (pint to quart size) with a screw top lid – A Mason (canning) jar or a clean store-bought pickle jar with the label removed works well.
  • For safety reasons, use rubber or latex gloves to collect a sample of water to prevent skin exposure


  1. Collect the water just below the surface of the water.  DO NOT collect sample directly from the surface, but collect just under the surface to avoid collecting just the scum on the top layer of the water.
  1. Fill the jar about ¾ of the way full with the pond water. DO NOT fill the jar completely to the top. Algae will give off gases and may cause of buildup of pressure inside the jar causing it to break.
  2. Wipe off any scum that may be on the outside of the jar.

  3. Screw the lid onto the jar.

  4. Place the jar in a cold refrigerator and leave it completely undisturbed overnight.

  5. The next day, carefully remove the jar from the refrigerator and look to see where the algae have accumulated.  IT IS VERY IMPORTANT, that you do not shake or agitate the jar in any way. If you do, this will mix the algae into the water again and you will not get test results that are usable.

  1. If the algae are all settled out near the bottom of the jar, then that is a likely indication that the lake does not have a lot of blue-green algae growing in it. (See arrow on photo.)


NO - Blue-greens (Negative)

  1. If the algae have formed a green ring around the top of the water in the jar, or just seem to be collected at the air/water divide, there is a strong possibility that the pond does have a blue-green algae community present. (See arrow on photo.)

YES - Blue-Greens (Positive)

Lab Test 

Samples may be collected and tested for the presence of cyanotoxins and algal species. Cost for testing the samples is the responsibility of the person or group who sent the sample and should be paid directly to the receiving lab.

Causes of blue green algae blooms

Elevated nutrient levels, warm temperatures, still waters, and plenty of sunlight can promote the growth of cyanobacteria.  Although cyanobacterial blooms can occur naturally, human activities play a role in increasing the frequency and intensity of blooms. Factors including pollution, alteration of water flow, climate change, and changes to the food web are all contributors.

It is likely that blooms are controlled by a combination of the following factors, as well as many others:


Physical Factors

Hydrologic Factors

Chemical Factors


Flow alterations

Nutrient loading (e.g., N and P)

Light availability


pH changes

Wind conditions

Vertical mixing

Trace metals      (i.e., Fe, Cu, Zn)


Temperature: warm water can lead to excessive blooms.

Light: intermittent exposure to high intensity sunlight can lead to excessive blooms.

Wind conditions: winds can push blooms to shorelines, where they can accumulate with great density.

Flow alterations: stagnant water can lead to excessive blooms.

Turbidity: when water is flowing slowly, turbidity or the muddiness of water may become low, which allows more sunlight to penetrate through the water column and encourages algal growth.

Vertical mixing: As water becomes warm and stagnant, vertical mixing is reduced and stratification occurs. Because cyanobacteria are able to control their depth in the water column, algae will accumulate near the surface in a stratified system, while other phytoplankton species will sink through the water column.

Nutrients: nutrients can lead to excessive blooms; runoff from lawns and farmland, wastewater discharges, and atmospheric deposition can supply excess nutrients. Heavy rains increase the likelihood of runoff with elevated nutrient levels.

pH: Changes in pH can lead to excessive blooms.

Trace metals: Metals are needed for cyanobacteria to undergo photosynthesis and respiration.


How to reduce the occurrence of algal blooms

Nutrients that cause algal blooms include phosphorus and nitrogen.  These nutrients can enter waterbodies through many natural and human sources.  Human activities including unmanaged land clearing, landscaping, farming, and motor vehicle use can contribute to increased nutrient levels.  Controlling man-made sources of nutrients can help prevent algal blooms.  The following can be implemented to help control nutrient impacts to waterbodies.

Shoreline Erosion

  • Preserve the rocks and vegetation that naturally occur along the shoreline; placing rocks for a hard armored shoreline and planting vegetation for a soft armored shoreline can prevent erosion in places on the shore where they have been removed.


  • Avoid construction within 100 feet of the shoreline, in the buffer zone, or the edge of nearshore bluffs.
  • Limit the amount of foot traffic and other recreational activities in erosion prone areas.

Street Runoff

  • Keep litter, pet wastes, leaves and debris out of street gutters and storm drains—these outlets may drain directly to streams, rivers, wetlands, and lakes.
  • Dispose of used oil, antifreeze, paints and other household chemicals properly—not in storm sewers or drains.  To locate recycling centers near you go to earth911.com.
  • Clean up spilled brake fluid, oil, grease and antifreeze. Do not hose them into the street where they can eventually reach local streams and lakes.
  • Purchase household detergents and cleaners that are low in phosphorous to reduce the amount of nutrients discharged into our lakes and streams

Lawn Clippings

  • Rake leaves, seeds, and grass clippings out of the street and gutter.  Keep in mind that the storm drain may be a direct link to the streams, rivers, and lakes.
  • Compost on site, bag for collection, or take lawn clippings to a community compost center.


  • Choose a zero-phosphorus fertilizer. Many lawns have adequate soil phosphorus and will remain healthy without adding more.
  • Test your soil to see if it needs phosphorus.  Soil samples can be sent to your local extension service office.

  • Apply lawn and garden chemicals sparingly and according to directions.
  • Sweep spilled fertilizer off paved surfaces.
  • Remember, compost and manure contain phosphorus too.

Agricultural Sources

  • Plant trees, shrubs, and grass around fields, especially those that border waterbodies, which can greatly reduce nutrient pollution.  This can help by absorbing or filtering out nutrients before they reach a water body by creating a strong buffer zone. 
  • Apply fertilizers in the proper amount, at the right time of year, and with the right method.  When applying chicken litter to the fields, follow all agriculture Best Management Practices (BMPs).
  • Implement conservation tillage that reduces how often fields are tilled, thus reducing erosion and soil compaction.  It allows soil organic matter to build up and reduces runoff. 

  • Reduce nutrient loadings that drain from agricultural fields to help prevent
  • Keep animals and their waste out of streams, rivers, and lakes to prevent putting excess nutrients in the water and to prevent erosion of stream banks.


  • Have your septic system inspected and pumped at least every three to five years, so that it operates properly.

  • Consider planting a buffer zone of vegetation to absorb any nutrients that may leak from your system, but not too close to prevent root damage.
  • Consider adding water dams to your toilet tank and installing faucet aerators and low-flow shower heads. The lower flow of water helps your septic system operate more efficiently, preventing sewage flow into your lake. 


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