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Harmful Algae Blooms (HABs)

Harmful Algal Blooms (HABs): What they are and why they matter

Brent Thomas

September 16, 2019

Have you ever heard of harmful algal blooms, also known as HABs? Many people have not, but everyone should be aware of what they are and their implications. HABs are a serious concern due to the fact that they can produce toxins in lakes, rivers, and ponds (surface waters), and are capable of causing illness, irritation, and even death, in pets, livestock, and humans. On top of producing toxins, HABs also bring about treatment challenges for public water systems that use surface water. They can create taste and odor issues in drinking water and increase the frequency needed to clean filters in water treatment plants.


HABs can be observed just about anywhere there is water. They have been observed in lakes, ponds, stormwater retention basins, rivers, streams, and reservoirs, all over the world. In Ohio alone, extensive HABs have been observed in Lake Erie, the Ohio River, and many inland water bodies. The Indiana Department of Environmental Management (IDEM) has issued an advisory for several lakes and waterways across the state due to HABs, and the waters of Utah Lake are under shore-to-shore public health advisory for the same reason. Three dogs recently died from toxic algae after swimming in a North Carolina pond, and HABs have also been observed recently in places like Sacramento, Portland, and Central Park, New York.


HABs are made up of cyanobacteria. Cyanobacteria, also referred to as blue-green algae, are microscopic organisms that naturally occur in surface water. These organisms multiply to form harmful algal blooms. Sometimes the cyanobacteria are distributed throughout the water, but often times, cyanobacteria float to form scums on or near the surface of the water. The scums are created by the cells of cyanobacteria grouping together to grow in colonies. Most HABs in the state of Ohio appear greenish or black, but they can appear in a wide variety of colors, including blue-green, purple, red, white, or brown.


Cyanobacteria differ from true algae, or green algae, which is commonly found in water bodies but is generally not harmful. True algae and cyanobacteria both use chlorophyll for photosynthesis, but for different reasons. True algae are essentially plants, while cyanobacteria are photosynthesizing bacteria. Cyanobacteria and true algae are two very different organisms and for that reason, they should not be treated the same. There are no known harmful toxins released by dying true algae, but cyanobacteria can contain harmful cyanotoxins within the cell wall that can be released during cell growth or death.


Understanding what triggers HABs is the first step in reducing their occurrence and impact. HABs can be minimized or avoided by reducing the nutrients and pollutants that make it into the water.


Where do they come from?

A few of the biggest factors that contribute to HABs include excess nutrients (phosphorus or nitrogen), sunlight, warmer temperatures, low-water or low-flow conditions, and low salinity. Nutrients that contribute to HABs and other algal blooms (mostly phosphorus and nitrogen) come from many sources. Some of the most prevalent sources include:


  • Agriculture
  • Lawn fertilizers
  • Wastewater treatment plants
  • Sewer overflows
  • Leaking septic systems
  • Precipitation

Preventing and Treating HABs

The toxins the cyanobacteria produce are known as cyanotoxins. Typically, the most effective way to remove cyanotoxins is while they are still encased within the cyanobacteria cells. Once these toxins are released from the cells, they become much more difficult to remove. With that in mind, when a cyanobacteria cell dies, those toxins can also be released. This is a concern when disrupting the cells in water treatment plants and in algaecide treatments of water bodies.


Conventional water treatment processes are relatively effective at removing whole cyanobacteria cells, but not so effective at removing the toxins that have been released from the cells. Additional chemical processes are needed to remove these toxins, such as activated carbon. Algaecides, like copper sulfate and hydrogen peroxide, can also be applied to the drinking water source to effectively control the growth of algae and cyanobacteria before it reaches the treatment plant.


Ohio is known for its high monitoring of treatment plants. Ohio water systems are required to submit a Notice of Intent (NOI) to Ohio EPA Division of Surface Water and obtain coverage under the pesticide general permit prior to applying algaecide to a source of drinking water.


It is noted by the EPA that treatment should be applied at the early stages of a bloom when cyanobacteria cell counts are still low (<10,000 cells/ml). There are two major reasons for this:

  • This is when the potential for cyanotoxin release is not probable or low
  • If the treatment is applied at the early stages of a bloom and toxic compounds are released into the water, they can be removed effectively during the treatment processes


The most prevalent cyanotoxin from HABs is microcystin, and this is tested for in Ohio public drinking water systems. Water treatment plants are required by law to conduct microcystin and cyanobacteria screenings. The frequency and type of monitoring is based on various criteria, mostly focused on past occurrences, and these screenings occur on either a weekly, biweekly, or monthly basis. Plants with past issues have to come up with a general plan for future prevention and treatment. You can find all of the screening requirements here.


Here is how the safety of drinking water is determined based on microcystin concentrations:


  • The level where water becomes unsafe to drink for children under 6 and sensitive populations is 0.3 microcystins (ug/L)
  • The level where water becomes unsafe to drink for children 6 and older and adults is 1.6 microcystins (ug/L)
  • The level where water becomes unsafe to use is 20 microcystins (ug/L)


The test results in both drinking water sources and in recreational lakes are publicly available on Ohio EPA’s website.


To learn more information about HABs, regardless of your state, and the measures being taken to make our water safer to drink and to use, email us at