Surface Water Quality

​Algal Blooms

What are Harmful Algal Blooms (HABs)?

Algae are microscopic, plant-like organisms that are the base of the food web in lakes, ponds, and streams. Under the right environmental conditions, algae can rapidly grow and form an algal bloom which may appear as surface scum, water discoloration, or both. Algal blooms that are formed by species that can produce toxins are known as harmful algal blooms (HABs), but can also be referred to as cyanobacteria blooms, blue-green algae blooms, cyanoHABs, or potential HABs. The toxins produced by HABs can cause adverse health effects in humans, pets, and wildlife.

The following are visual indicators of a possible HAB:

  • Discolored Water – water may appear bright green/blue-green in color.  When algae begin to die off, water may turn milky blue and produce a strong, foul odor.

​​​Surface films/discolored water.​​    Surface films/discolored water.

  • Surface Scum – some HABs form thick scums across the water surface.  They can have the appearance of spilled paint forming a film across the water's surface.  These scums can accumulate along the shoreline.

Surface scum with decaying algae.    Surface scum with decaying algae.

  • Flecks and grass clippings – some HABs accumulate into colonies observable as blue-green flecks or as clumps that resemble grass clippings.

Flecks in water.    Grass clippings in water.

Why do blooms occur?

The primary factors that cause the growth and reproduction of HABs are:

  • Sunlight
  • Relatively warm water temperatures
  • Slow-moving water
  • Nutrients (nitrogen and phosphorus)

HABs are more likely to occur in late spring to early fall when water temperatures are higher. Nutrient pollution can make blooms worse, leading to more severe HABs that occur more often. When large amounts of algae die off and decompose in a waterbody, the oxygen within the water can be depleted and can lead to fish kills.

Safety Info

It is hard to tell whether a bloom is harmful just by looking at it, so it is best to avoid the following activities around possible algal blooms:

  • Swimming, boating, kayaking, fishing, jet-skiing, water-skiing, or wading through the water
  • Touching or handling mats of algae
  • Ingesting the water
  • Using the water for washing or irrigation
  • Pets and children are at a higher risk of exposure so keep them away from water that appears discolored or scummy

"When in Doubt, Stay Out!"

If you accidentally come in contact with a HAB, take the following actions:

  • Wash thoroughly
  • Immediately seek veterinary care if your pet appears to stumble, stagger, collapse or vomit uncontrollably after being in contact with the water
  • Immediately seek medical care if a child or adult appears ill after being in contact with the water

Who Do I Contact to Report a Suspected Harmful Algal Bloom?

Active HAB Watches and Advisories

For More Information and Resources on HABS:


Questions about HABs can be directed to:

Olivia Edwa​rds

Mecklenburg County Environmental Supervisor

Matthew Phillips

Mecklenburg County Environmental Specialist II