Florida algae crisis: What's the difference between red tide and blue-green algae?

Karl Schneider
The News-Press

Southwest Florida recently has experienced two types of algal blooms, both in the Gulf of Mexico and in freshwater systems like the Caloosahatchee River.

The red tide, formally known as Karenia brevis, was already in Gulf waters at the beginning of this year. It is an often recurring bloom that typically starts 10 to 40 miles offshore. Red tide isn't always red, either. It can range in color.

A tri-colored heron wades in an algal bloom along the wall at the Franklin Locks in Alva on Monday, June 25, 2018. An algal bloom has pushed up against the lock. Reports of algae on the river have been reported since last week. Most reports are east of the Franklin Locks. Some are worried that it might move west towards the estuary. The DEP is testing the algae later this week to see if it is toxic.

Blue-green algae, or cyanobacteria, is a more recent bloom affecting fresh waters in Southwest Florida. This "algae" more closely conforms in color to its name.

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With both algae blooming in the area at the same time, it's hard to know which is which.

Here's a basic breakdown of the differences from the Florida Fish and Wildlife Conservation Commission:


The most notable difference is the type of water these algae prefer. Red tide does not fare well in lower salinity waters. Most red tide blooms occur in the Gulf, but it can make its way into estuaries and bays.

Blue-green algae prefers fresh waters. Blooms most naturally occur in lakes and rivers.


Another big distinction is the cause of the blooms. Karenia brevis is naturally occurring, and there has been no direct link to agricultural runoff as the prime cause for the bloom.

However, once red tide makes it to local beaches, growth can be influenced by agricultural and urban runoff, according to Mote Marine Laboratory in Sarasota

Blue-green algae blooms have a direct relation to agricultural and urban runoff. Nutrient pollution encourages the growth of cyanobacteria.

Human effects

Once waves break open the cells of red tide, toxins are released into the air causing respiratory issues. These toxins can also accumulate in oysters and clams, leading to Neurotoxic Shellfish Poisoning in humans.

Mote Marine considers swimming in red tide to be safe, though people with respiratory illnesses could develop respiratory irritation. Mote recommends common sense when considering whether or not to swim when red tide is present. 

Blue-green algae contain a variety of toxins directly affecting humans. Ingesting cyanobacteria in water can lead to vomiting and even acute liver failure. Microcystins in the algae can also cause skin irritations from exposure.

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Marine life effects

Brevetoxins in red tide are apt to attack the central nervous system of aquatic life. These toxins are known to cause death in marine life.

Blue-green algae blooms can get large enough to block out the sun for submerged plant life. The algae also reduces the oxygen in surrounding waters, leaving little for fish and other marine life.