EC 1631    Published July 2009
December 2017

Cyanobacteria are a type of photosynthetic bacteria commonly referred to as blue green algae. They are an ancient life form found in ocean, freshwater and terrestrial environments. Cyanobacteria can grow as single-celled organisms, as a colony that may look like strands, or bunched together in mats or spherical clusters. Viewing an individual cell requires a microscope, but larger colonies are visible to the naked eye.

Like many bacteria, some cyanobacteria can produce toxins harmful to humans, pets, livestock and wildlife. When cyanobacteria begin to grow rapidly this can result in a bloom, which is a dense and often scummy accumulation of cyanobacteria cells and colonies. During blooms, the cyanobacteria become the dominant life in their environment. This is also the time when the largest number of toxins are found in water, representing the greatest threat to public health.

Cyanobacteria and their toxins have been detected in many lakes, rivers and reservoirs across Oregon including in the Portland metro area, along the coast, in the Willamette Valley, in the Cascades, and in southern and eastern Oregon. Over the past ten years, several government agencies have been involved with sampling and responding to blooms at the local, state and federal levels.

Cyanobacteria’s importance to Oregon

Blooms can affect the quality of drinking water, natural resources and recreational environments. This may result in potential harm to public health and could burden local economies. Microcystin is the most commonly detected cyanobacteria toxin in Oregon lakes and reservoirs. It will be discussed throughout this publication.

Over the past five years, the Oregon Department of Human Services (DHS) has listed 46 health advisories warning the public about the presence of blooms associated with toxins of concern to health.

To find out which lakes have had advisories in the past, or to inquire about current advisories, contact DHS at 877-290-6767 or visit:

Some scientists are concerned that the frequency, intensity and distribution of cyanobacteria blooms are increasing over time. Human activities, such as increased nutrient loading to lakes, modification of water flow, climate change and the introduction of new or invasive species are thought to contribute to cyanobacteria blooms.

The recognition of harmful blooms over the past decade has led to measures designed to protect public health and natural resources. Reports of potentially toxic blooms and public health advisories have been increasing in Oregon during that time. Whether this reflects greater awareness of the problem or an actual increase in the frequency and intensity of blooms is unclear.

The best way to determine if toxins are present in lake water is to take a water sample and test it using accepted methods. Testing for such toxins can be expensive, but new options are being developed.

A less-costly alternative is to determine the species of cyanobacteria in the water. This information can be used to find out whether the species is known to have produced toxins in the past. Many types of cyanobacteria are not known to produce toxins so finding them would not require a health advisory.

How to know if a bloom is toxic

Monitoring efforts can be difficult, especially in large areas with many lakes. Sampling them all would require extensive time and money. Not every bloom can be tested, which means knowing how to identify cyanobacteria blooms is important. Toxic and non-toxic blooms can look like scum, strands or mats of plant-like material, and foam on water. Colors vary. They are commonly green or bright green, blue, brown, or red, and can at times be mistaken for antifreeze, dye or paint in the water. Often, the levels of toxin are highest near the shoreline and in dried mat-like material on the shore. It is important for you, your family and your pets to avoid water with active blooms.

Figure 1 shows an accumulation of cyanobacteria scum along the shoreline of an Oregon lake. The bright green coloration is common, as are the striating lines of cyanobacteria farther from the shore. Before scums form, clumps of cyanobacteria colonies are visible (figure 2). This stage can resemble pollen and other environmental agents. Figure 3 shows a dried scum mat. Notice the bluish tint of this material. Dried scum may contain high toxin concentrations, so it is important to keep your family and pets away from them as well.

In Oregon, blooms have lasted from a few days to several months. All blooms will eventually die. When they do, the water can change color and develop a strong smell, but will look normal again eventually. When blooms start to die, the cyanobacterial cells burst open and microcystin mixes into the water. This is a dangerous time for recreational and drinking water exposure.

Cyanobacteria and nutrients

Cyanobacteria are photosynthetic bacteria. Like other kinds of phytoplankton, they require light and nutrients such as phosphorous and nitrogen. Historically, these nutrients were present at low levels in lakes and streams, which limited cyanobacterial growth. Human activities including logging, commercial fishing, agriculture, detergent use and sewage discharge, have increased the nutrient content of Oregon waters. These added nutrients help phytoplankton grow into blooms.

Lakes where this is happening are called eutrophic. A eutrophic system has high levels of otherwise limiting nutrients and reduced oxygen levels during the summer.

Plants and animals can also affect the growth and composition of cyanobacteria, especially invasive species that disrupt the balance of an environment. Generally, fish increase the nutrient load in lakes, which in turn increases cyanobacteria growth. Research has indicated that some invasive mussels and clams selectively feed on non-toxic phytoplankton, which then favors the growth of toxin-producing cyanobacteria.

The eutrophication of drinking water sources is an economic concern as well as a public health concern. Though not all cyanobacteria produce toxins, blooms often give water an offensive odor and a foul taste. Treating water to eliminate nuisance taste and odors is expensive. Treating water that contains toxin-producing cyanobacteria is more important. In this case, the water must not be chlorinated before it is filtered because doing so will break the cyanobacteria cells, causing the toxin to be released into the water. Filtering out the organism is easier than filtering out the toxin.

As eutrophication becomes more widespread in Oregon, water treatment operators may need to adjust their treatment methods to address cyanobacteria blooms.

Public health concerns

Excessive exposure to cyanobacteria and their toxins leads to a variety of human health effects. The three main exposure routes for any chemical substance are: ingestion or swallowing; contact with the skin or eyes; and inhalation.

In Oregon the most dangerous form of exposure to microcystins is through drinking contaminated water. Most large drinking water systems have the technology and expertise to address cyanobacteria blooms. However, smaller systems and homes that use surface water may not be able to.

Contaminated water may also be ingested during recreational activities. Some of these exposure pathways are outlined in table 1.

Contact with the cells of cyanobacteria may result in rash-like skin reactions. Typically the reactions will go away without medical attention. The condition can be more severe in areas where cyanobacteria cells are trapped against skin and clothing, or in areas that had longer periods of contact with the contaminated water. The reaction is thought to result from irritation caused by complex structures on the cell walls of cyanobacteria called lipopolysaccharides.

Exposure to microcystins has been associated with many symptoms including sore throat, blistered mouth, ear and eye irritation, abdominal pain, fever and flu-like feelings. Microcystin is also a known liver toxin. But, typically, a higher level of exposure is needed to cause liver damage than to produce other symptoms. People with liver disease are more susceptible to microcystins. Pregnant women, infants and young children, the elderly and medically fragile individuals also should be considered particularly susceptible.

Accounts of cyanobactera-related deaths in pets, wildlife and livestock have been reported in Oregon (Jacoby and Kann 2007). Exposure to toxins found in dried bloom material on the shoreline is of special concern. Some dogs are thought to have died after grooming this material from their coats.

In studies where plants were irrigated with water containing microcystin, the plants were ruined and toxins were found in the edible portions of some of the crops. Water from toxin-containing blooms should not be used to irrigate food or feed crops.

Is it safe to eat fish caught in a toxin-producing bloom? In general, the livers of exposed fish contain the largest amounts of microcystin. But, studies have shown microcystin in the fillets of fish at levels of concern for people who eat large quantities. This is especially true for fish that live in lakes year round. Fish that migrate to the ocean, such as salmon and steelhead, typically have very low or no detectable levels of microcystin in their fillets.

If you choose to eat fish caught during a bloom, discard the skin, fats and organs, which are most likely to contain toxin. Microcystins are resistant to heat, so cooking is unlikely to reduce microcystin levels in the fillet. Shellfish are known to accumulate microcystin as well. Avoid eating whole freshwater shellfish during active blooms.

Sampling for cyanobacteria

Before monitoring takes place, water quality managers should review the history of the water body and surrounding land use. This may help in choosing appropriate sample sites and in understanding the potential causes of the blooms.

If possible, monitoring water quality before a bloom occurs may help identify conditions that trigger a bloom. Once a bloom occurs, the monitoring plan should shift to address potential public health concerns.

Under these circumstances, collecting samples that represent the worst-case scenario for public health is important. Such measures might mean taking a single sample off a dock or on the shoreline of a popular swimming area.

After water samples have been collected, a number of different endpoints can be analyzed. If the water body does not have a history of blooms, identifying the cyanobacteria species or using various water quality measures may be sufficient. For water bodies that have had confirmed cyanobacteria blooms in the past, or if rapid growth is noted, consider testing for toxins. ELISA (enzyme-linked immunosorbent assay) is a popular method for detection. This test offers a numerical result and is fairly sensitive. To contact a laboratory that can identify cyanobacteria or measure levels of toxin call DHS at 877-290-6767.

The Oregon Department of Environmental Quality publication Guidance for Assessing and Characterizing Toxic Cyanobacteria Blooms, ( offers more details about sample collection and how to develop a monitoring plan.

Learn more

Many information resources about cyanobacteria are available to the public and professionals.

The World Health Organization offers an overview, Toxic Cyanobacteria in Water: A Guide to their Public Health Consequences, Monitoring and Management:

Oregon State University Extension Service: can assist with toxicological, agricultural, and environmental issues associated with blooms

Find current advisories and track blooms at the Oregon Department of Human Services (Public Health) Harmful Algal Bloom Surveillance Program. Call 877-290-6767 or visit:


Jacoby, J.M., and J. Kann. 2007. The occurrence and response to toxic cyanobacteria in the Pacific Northwest, North America. Lake and Reservoir Management, 23: 123-143.

Stone, D., and W. Bress. 2007. Addressing public health risks for cyanobacteria in recreational freshwaters: the Oregon and Vermont framework. Integrated Environmental Assessment and Management, 3: 137-143.

World Health Organization. 1999. Toxic cyanobacteria in water: a guide to their public health consequences, monitoring and management. London: E&FN Spon.

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