Harmful Algal Blooms (HABs)

HABs begin with Cyanobacteria

Cyanobacteria, also referred to as blue-green algae, are microscopic photosynthetic organisms that occur naturally in aquatic habitats. Harmful Algal Blooms, or HABs, occur when colonies of cyanobacteria grow out of control and produce harmful toxins that can impact human and animal health. In addition, large blooms can clog the gills of fish and invertebrates, smother aquatic vegetation, and deplete oxygen as they decay.

Add the right ingredients and conditions

Harmful algal blooms can form in calm waters like ponds, lakes, slow-moving streams, and stormwater retention ponds. They generally form when conditions are just right – warm temperatures, ample sunlight, and excess levels of nutrients in the water, specifically nitrogen and phosphorus. Nitrogen and phosphorus are naturally present in waterbodies but can reach excess levels through a variety of human activities including stormwater runoff which carries fertilizers from lawns and farm fields to a waterbody, through improperly maintained septic systems, excess animal waste, and other means.

A bloom that can last

The duration of these blooms can be short lived or last for weeks to months, depending on the nutrient levels, water temperature, and whether winds or other agitation sources are present. Thus, it’s very important to pay attention to local and statewide alerts and signage or notices placed at beach entrances, boat launches, and other public areas adjacent to a waterbody.

Protect Yourself and Others from Harmful Algal Blooms – Know the Facts

Identification

Harmful algal blooms appear on the surface of the water making it discolored, often as thick green or blue-green mats or globs, spilled paint, pea soup, grass clippings, or as pinhead size particles suspended in the water. They may also appear white, gray, murky, or milky, and may collect along the shoreline. 

Knowing what is not a harmful algal bloom is equally important. If the material is long, stringy, can be lifted with a stick, is rooted or attached to plants, rocks, or sediment, or has leaves or leaf-like structures – it is not a harmful algal bloom. However, if the material appears in excess it could be an infestation of an aquatic invasive species, therefore identification of invasive species is a critical step in protecting native aquatic species and the ecosystem.

Impacts

Ecosystem

Large HABs can clog the gills of fish and invertebrates, smother aquatic vegetation, and deplete oxygen levels in water as they decay.

Human Health

Harmful algal blooms can pose serious health risks to humans and animals if ingested, inhaled, or if they touch the skin. These toxins can be present even when visible signs of HABs have dissipated. Signs of exposure can include skin rashes, respiratory issues, and gastrointestinal distress. If ingested or inhaled, symptoms may be even more severe.

*If you think you have been exposed to a harmful algal bloom, contact your local Health Department or family doctor.

Dog Health

Dogs are much more susceptible than humans to cyanobacterial poisoning. They can be exposed to toxins by drinking the affected water, eating washed up mats or scum of toxic cyanobacteria, and by direct contact with the skin. After leaving the water, dogs can also be poisoned by grooming their fur and paws.

*If you think your pet has been exposed to a harmful algal bloom, contact your veterinary professional or poison hotline immediately.

Safety

Being mindful of locations and conditions where HABs are likely to occur, exercising caution, and following all posted warnings including signs indicating unsafe water conditions are advised. If visible signs of an algae bloom are present, avoid all contact with the affected water and leave the area. Additionally, rinsing off with fresh water after recreating in surface water, even if no visible bloom is present, is always a good idea.

Prevention

Harmful algal bloom prevention begins with managing nutrient levels, specifically avoiding increases in phosphorus and nitrogen levels.  

• Consider whether fertilizer is necessary. If it is, always apply it using the “4Rs” which are: right source, right rate, right time, right place. Additionally, always follow label instructions during application.
• Pick up pet waste
• Inspect and service septic systems regularly
• Incorporate green stormwater infrastructure techniques such as rain barrels and rain gardens to help capture excess rainwater, reduce and slow water flow, and allow nutrients to infiltrate into the ground.

More about nutrients

• Natural nutrient cycles: Erosion and the water cycle contribute nutrients naturally into a waterbody. When naturally occurring in balance, nitrogen and phosphorus are critical for aquatic plant life.
• Human and animal waste inputs: Nutrients are present in human and animal waste. If not properly treated, nitrogen and phosphorus can be released into waterways via wastewater effluent. Poorly maintained septic systems can also contribute human waste and additional nutrients to surrounding waterbodies. Additionally, the nutrients in pet waste can reach a waterbody through rainwater.
• Urban and Agricultural Runoff: Overfertilizing of yards and agricultural lands can lead to excessive nutrients entering a waterway or waterbody via storm drains or as runoff.
• Legacy nutrients: Phosphorus naturally binds to underwater sediment as phosphate. If the waterbody develops a zero-oxygen zone, or anoxic zone, the chemical reaction causes phosphate to release from the sediment and into the water column. This can make managing land use for HABs frustrating because when runoff inputs of phosphorus are reduced, HABs may still form on the waterbody due to legacy phosphorus being slowly released from sediments.

Sources:

• Environmental Protection Agency (2025). Learn about Harmful Algae, Cyanobacteria and Cyanotoxins. Accessed on February 12, 2025.
• Environmental Protection Agency (2025). Nutrient Pollution. Accessed on April 24, 2025.
• National Oceanic and Atmospheric Administration (2016). What is a harmful algal bloom? Accessed on April 24, 2025.
• Pennsylvania Department of Agriculture (n.d.). Fertilizer. Accessed on April 24, 2025.