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Harmful Algal Blooms: Red Tide vs. Blue-green Algae

Algae blooms are common in Florida waterbodies. Under the right conditions, these blooms can grow quite large and become harmful to human and ecosystem health. Recently, a harmful algal bloom (HAB) on the southwest coast of Florida has gained a lot of media attention. There are several pictures and news stories circulating on social media of dead fish and other marine animals washed ashore. Some media outlets and social media posts are incorrectly connecting these fish kills with a separate bloom occurring in Lake Okeechobee. However, these unfortunate fish kills are caused by an unrelated HAB known as red tide.

Red tide is not caused by the same organism as the Lake Okeechobee algal bloom.

Left: K. brevis, the species that causes red tide and associated fish kills. Right: The dominant cyanobacteria (Microcystis aeruginosa) responsible for the blue-green algal bloom in Lake Okeechobee, 2018.


Red Tide and Fish Kills

Harmful algal blooms that occur in the ocean and on the coast are most often caused by organisms known as dinoflagellates. Red tides are caused by a specific type of dinoflagellate, Karenia brevis (K. brevis). Most dinoflagellates live in salt water, and other kinds of dinoflagellates cause HABs along the Atlantic coast. Red tide blooms occur nearly every year in the Gulf of Mexico, and have been documented since the mid-1800s. They originate offshore in the Gulf of Mexico, and are pushed to coastal waters by winds and tides. For more information about red tide, check out this great blog post.

Statewide K. brevis concentrations as of August 3, 2018. (Source: FWC)

K. brevis produces a toxin called brevetoxin that affects the central nervous system of fish and other marine wildlife. This toxin is what causes fish kills. Toxins get in the water and the air, causing respiratory issues for swimmers and beach-goers when the blooms are near to the shore. Toxins can also cause neurotoxic poisoning in people who eat recreationally-caught shellfish. (Note: commercially caught shellfish are safe to consume). The FWC keeps an up-to-date status on red tide throughout the state.


Blue-green Algae

On the other hand, blooms in freshwater lakes and reservoirs are most commonly caused by blue-green algae (also known as cyanobacteria). There are several different kinds of blue-green algae that can cause algal blooms including: Microcystis, Anabaena, and Cylindrospermopsis. Water samples suggest that the dominant species affecting the 2018 Lake Okeechobee bloom is Microcystis aeruginosa.

Lake Okeechobee is most prone to having large cyanobacteria blooms when the weather is warm and sunny (spring through early fall).  Periods of high rainfall and tropical storms can increase the potential for blooms by stirring up nutrients in the lake bed and increasing the flow of nutrient-rich water into the lake from upstream watersheds.

Satellite Images of Lake Okeechobee, June 2018. (Source: NOAA)

Like red tide, blue-green algae can also be harmful to humans and wildlife. Some, but not all, blue-green algae types release toxins that can kill fish and wildlife. These toxins can also have negative human health impacts. Microcystis aeruginosa is known to release a toxin called microcystin, which can result in gastro-intestinal problems and possibly liver damage if contaminated water is ingested. For more information about the 2018 Lake Okeechobee algal bloom (including causes, impacts, solutions, and FAQs), check out this blog post.

2018 Algal Blooms At-a-glance

Location of Concern Southwest Florida Coast, Gulf of Mexico Lake Okeechobee, other freshwater lakes and reservoirs
Common name Red Tide Blue-green Algae
Organism Dinoflagellate Cyanobacteria
Dominant species Karenia brevis Microcystis aeruginosa
Water Salinity Salt water (ocean and nearshore coastal water) Freshwater (lakes, reservoirs)

For more information about harmful algal blooms, refer to this UF/IFAS Extension EDIS document. 

Top Banner Image Source: WESH News Channel 2

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4 Comments on “Harmful Algal Blooms: Red Tide vs. Blue-green Algae

    • Hi! Thanks for your comment! In that NCCOS 2006-2012 study, one nutrient source was found to be cyanobacteria decay, but specifically Trichodesmium decay, which is found in nutrient-poor ocean waters, not freshwater such as Lake Okeechobee. And as the study mentioned, the nutrient sources for red tide is multi and complex, making it difficult to draw a direct connection to any one source. If data for this year comes out, I will be sure to provide an update. Thanks for reading!

  1. Okay, so the water releases from Lake Okeechobee have not “caused” the red tide. Is it probable that the nutrient rich water releases are feeding and exacerbating the red tide?

    • Hi Michelle! It’s probable but currently there’s no data to confirm the nutrient sources. I’ve heard some basic confusion over the types of algae so chose to focus this post on that aspect. If you haven’t had the chance, check out questions #8-12 from this great article:. The author discusses solutions so we can better mitigate this issue in the future. Thanks for reading!