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Understanding red tide and harmful algal blooms

August 29, 2018
By DR. BRUCE NEILL , Island Reporter, Captiva Current, Sanibel-Captiva Islander

We want to help people understand the ocean. The following is a series to inform people about red tide and harmful algal blooms currently affecting Southwest Florida.


The first thing. We really don't really know enough about red tide and the conditions that cause it to occur in high densities.

Article Photos

Dr. Bruce Neill

The name red tide is unfortunate. The water color in a red tide event is not usually red, nor does it have anything to do with the tide.

Red tide is caused by a single-celled organism, Karenia brevis. Karenia brevis is a naturally occurring species of phytoplankton classified as a dinoflagellate; it is not a plant but has the ability to carry out photosynthesis.

Karenia brevis has the ability to reproduce very rapidly in favorable conditions. Rapid reproduction is common among phytoplankton species; these episodes of rapid population expansion are called blooms.

Red tide is caused by a Karenia brevis bloom.

Karenia brevis naturally produce compounds classified as brevitoxins, a group of compounds that can be neurotoxic to animals with backbones. In humans, when ingested in high concentrations, they cause the illness neurolytic shellfish poisoning.

Brevitoxins are not actively released into the water by living Karenia brevis cells, but upon the death of the cell, these compounds are released into the ocean.

Red tide has been recorded since the 1840s in the Gulf of Mexico in Southwest Florida. Karenia brevis cells are usually present in the waters of Southwest Florida, but at very low concentrations.

The conditions favorable for Karenia brevis blooms are not well understood, but blooms are often associated with upwelling events. Upwellings are the movements of deep, nutrient-rich waters to the shallow, warm, sunlit upper levels of the ocean.

These upwelling events are associated with wind and climatic conditions that push Karenia brevis populations towards shore.

Phytoplankton population growth in the ocean is commonly limited by concentrations of nitrogen or phosphorus in the water. Human activities produce and release significant amounts of nitrogen and phosphorus - from agriculture, landscaping and sewage - these nutrients make their way into rivers, and then to the ocean.

There is no evidence that red tides are more frequent because of human-influenced discharges from lakes and rivers, but evidence indicates they last longer, and are more intense in areas of river discharges.

The estuary adjacent to Sanibel receives water from the Caloosahatchee, which is fed by Lake Okeechobee. Therefore, Sanibel is receiving water from a very large area of land. This water is nutrient-enriched by both agriculture and high density human communities which add landscaping and sewage runoff.

When Karenia brevis moves into our region, the nutrient-rich conditions from the Caloosahatchee are favorable for intense, long-lived, high concentration blooms to occur.


Shellfish filter and feed on plankton from the water; in doing so, they accumulate brevitoxins. Eating shellfish that have high concentrations of brevitoxins causes a serious illness, neurotoxic shellfish poisoning.

Commercial shellfish operations are monitored very closely in the state of Florida, and commercially harvested shellfish are safe to consume during a red tide outbreak. However, shellfish should not be harvested and/or consumed during or shortly after a red tide outbreak.

Brevitoxins can also be suspended in the air at the surf line. These molecules can irritate the back of the throat and eyes when in the air in high concentrations. Airborne molecules can also provide challenges for people with pre-existing pulmonary challenges such as asthma or COPD. The airborne effects of brevitoxins are usually in areas close to the shore and diminish quickly as one moves inland.

The amount of brevitoxin absorption through the skin is unclear, but doesn't not seem to be a significant pathway of absorption - although more research is ongoing.


Blue-green algae are bacteria are capable of carrying out photosynthesis, they are classified as cyanobacteria. Cyanobacteria are extremely diverse and different species occupy almost all habitable locations on Earth.

Many cyanobacteria are very important to our life on this planet, but some produce toxins - they are collectively known as cyanotoxins.

The cyanobacteria currently a problem for Southwest Florida are species that grow in freshwater mostly belonging to the genus Mycrocystis, the most common being Mycrocystis aeruginosa.

Mycrosystis aeruginosa is green and creates small gas spaces in its cells that cause it to float on the top of water; blooms look like a green film on the water's surface.

It thrives in nutrient-rich freshwater, has the ability to readily absorb nitrogen and phosphorus, and rapidly reproduce, allowing it to have large, rapid blooms. Because these blooms float, they shade out other algal species in the water, and monopolize nutrient resources.

The chemical produced by Mycrosystis of principal concern is microsystin. This is a small peptide chain, released upon the death of a cell; it is stable in water, and can persist weeks to months after cell death. There are many identified microsystin variants; they are hepatotoxins, which cause serious liver damage. They are also suspected to affect other organs, interfere with sperm production, and are suspected to be mutagens.

The principal known method of ingestion is by drinking water contaminated with microsystin.

Mycrocystis has been thriving in Lake Okeechobee for some time; releases from the Lake to the Caloosahatchee have "seeded" the river with these algal cells, where they continue to thrive and bloom in the nutrient rich waters.

The good news for our Gulf is that Mycrocystis does not tolerate salty conditions well. The bad news is that they will persist in our estuaries and especially the urban canals within them having intermediate salinities quite well.

Lee County has begun attempts to remove cyanobacteria blooms by skimming them from the surface of canals.

Dr. Bruce Neill is the executive director and co-founder of the Sanibel Sea School. It is a non-profit organization whose mission is to improve the ocean's future, one person at a time. For more information, visit



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