Mercury Contamination in Local Fish

Mercury Contamination in Local Fish

Imperiled (verb) – put at risk of being harmed, injured, or destroyed.


In 2021 the Florida Department of Environmental Protection classified 44 area waterways in the Pensacola Bay System as imperiled.  Such designations are based on an environmental parameter making it unhealthy for one reason or another.  When we think of an unhealthy body of water, many times we think of sewage.  There are nine bodies of water in the Pensacola Bay System classified as imperiled due to the fecal bacteria concentrations within.  There are another seven for bacteria levels high enough to close them for shellfish harvesting.  This is a total of 16 bodies of water having bacteria issues (36% of the 44 designations).

Closed due to bacteria.
Photo: Rick O’Connor


Low dissolved oxygen and fish kills is another parameter we think of.  There are four waterways designated imperiled due to high nutrients (a cause of hypoxia and fish kills), and one for low dissolved oxygen readings itself.  This is a total of five (11% of the 44 designations).

Dead redfish on the eastern shore of Mobile Bay.
Photo: Jimbo Meador


But you may be surprised to learn that 23 of the 44 imperiled water bodies (52%) are designated based on the mercury content of the fish sampled there.


Most people are aware of the mercury issue in fish.  Many of those living in the Pensacola Bay area are aware of this issue locally, but they may not be aware that with the 2021 designations, it is the primary reason for many listed.  To be fair, it is not that mercury issues are increasing, it may be more that there are 97 waterways in the Pensacola Bay System being considered for delisting in 2021 and those are listed for a variety of other issues.  What it is stating is that with the 44 that remain imperiled, mercury is the primary cause.


We have all heard of mercury in fish, but where is it coming from?  What health problems does it cause?  And is there anything that can be done to make these bodies of water healthier?


Mercury is a naturally occurring element on the periodic table.  It is element #80, meaning it has 80 protons and electrons, one of the larger naturally occurring elements.  It is a silver-colored liquid at room temperature, one of only two naturally occurring elements in the liquid phase at these temperatures – the other being bromine.  It is sporadically found throughout the earth’s crust, usually combined with other elements.  There are two forms of mercury – mercury (I) and mercury (II) – indicating the number of cations available for sharing or transferring in compound bonding.  Mercury (II) is more common in nature.



The element has been of interest to humans for centuries.  There are records of it buried beneath the Mayan pyramids, though we are not sure how it was used, and it was used in Chinese medicine centuries ago.  The Spanish used it to help extract silver from mines during their colonial period around the world.  It was also used in separating fir from skin in felt hat making in the 19th century.  Hatters who used this method eventually had neurological problems and became known as “mad hatters”, an idea used in Lewis Carol’s Alice’s Adventures in Wonderland.


In more modern times it has been used in fillings for tooth cavities (including my own) and preserving specific vaccines.  Being a good conductor of electricity and not of heat, it is used in numerous electrical components, fluorescent lighting, and batteries.  Some cultures used it to help “whiten their skin” and a common use is in the processing and production of certain industrial chemicals.  Today, due to the toxic properties of mercury, many of these uses are no longer.

Fluorescent lighting contains mercury.


Mercury is obtained for these uses by mining their ores.  The most sought after ore is cinnabar, a red-colored rock found around the world.  Mercury (II) sulflide (HgS) is a common compound found in cinnabar.  When heated and oxidized it will produce sulfur dioxide and elemental mercury.


HgS + O2 à Hg + SO2


Cinnabar is the most common ore mined for mercury.
Photo: Classic Crystal



The problem with mercury is that it is toxic, and some forms of mercury are more toxic than others.  The element is known to cause brain, kidney, and lung issues.  It also can weaken the immune system.  It is most known for the neurological problems it causes.  Sensory impairment, lack of motor skill coordination, psychotic reactions, hallucinations, tremors and spasms have all been connected to exposure to mercury.  There are concerns with the neurological development within the fetus if exposed to mercury and many of the health advisories target women of childbearing age who are pregnant or considering it.  They have included the very young and the very old in their recommendations that these members of the population do not eat more than 6 ounces of fish (or shellfish) that have high mercury contamination.


Mercury contamination in fish.
Image: BBC



The organic forms of mercury, dimethylmercury and methylmercury, are the more toxic forms.  These are introduced to the environment both naturally and from human activity.  Once in the aquatic environment they are absorbed by the phytoplankton (microscopic plants in aquatic environments).  Methylmercury accumulates in lipids (fats) within the cell at relatively low concentrations (phytoplankton are not large).  However, they are not passed by the creature.  The slightly larger zooplankton (microscopic animals) feed on the phytoplankton and accumulate the mercury they have stored.  Feeding on a lot of these, they accumulate even more mercury.  The zooplankton are consumed by small fish, who eat a lot and accumulate even more mercury.  Then the mid-sized fish consume them, and the larger fish consume those, and on and on.  The top predators have accumulated enough methylmercury to be hazardous to human health IF they are consumed by people.  This process of increasing the concentration of mercury through the food chain is known as biomagnification – “magnifying the problem”.


So, which fish are of concern?


Based on the Florida Department of Health for freshwater systems in Escambia County.

  • Bluegill, Channel catfish, Largemouth bass, Long-eared sunfish, Red-eared sunfish, Spotted sunfish and Warmouth from the Escambia River system – you should not eat more than one/week.
  • Do not eat chain pickerel or largemouth bass – and do not consume more than two red-eared sunfish from Crescent Lake.
  • Lake Stone near Century FL – no more than two bluegill and sunfish per week and no more than one largemouth bass each week.
  • From the Perdido River do not eat more than two bluegill or sunfish each week and do not eat largemouth bass from the Perdido River.
  • The same species and regulations apply for the Yellow River system as well.

The following marine species are of concern….

Almaco jack, Atlantic spadefish, Atlantic croaker, Weakfish (trout), Black drum, Black grouper, Blackfin tuna, Bluefish, Cobia, Dolphin, Pompano, Gafftop catfish, Gag, Greater amberjack, Gulf flounder, Hardhead catfish, King mackerel, Ladyfish, Lane snapper, Bonito, Mutton snapper, Pigfish, Red grouper, Red snapper, Sand seatrout, Scamp, Shark, Sheepshead, Snowy grouper, Southern flounder, Southern kingfish, Spanish mackerel, Spot, Striped mullet, Vermillion snapper, Wahoo, White mullet, Yellow-edge grouper, and Yellowfin tuna.


In each case it is not recommended eating more than two servings a week.  For a few, it is recommended that the most vulnerable people mentioned earlier not at ANY… Those would include Blackfin tuna, Cobia, King mackerel, Bonito, and Shark.


It is recommended that NO ONE eat king mackerel over 31 inches and any shark species over 43 inches in length.


I guess as you look at this list, you see fish species that you like.  This list can lead folks to think… “I am just not going to eat seafood”.  This would be a mistake.  The Department of Health has found there are essential vitamins and nutrients provided be seafood that are missing if you do not eat them.  They found additional problems in fetal development when seafood protein was left out of the mothers’ diet.  So, the response would be… eat other seafood species you do not see on this list… or, if you see something you do like, no more than 1-2 6-ounce servings per week.


So, is there anything we can do about the mercury issue in our bay system?


Well, to have the biggest impact you will need to determine the biggest source.  33% of the mercury in our environment comes from natural sources, such as volcanic eruptions.  We can do nothing about volcanic eruptions, or other natural sources, so we will need to look at anthropogenic (human) sources.


The larger sources would be anthropogenic, which account for 67% of the known mercury in the environment, focusing on these can make a large impact.  Coming in at No.1 – producing electricity by burning coal.  This accounts for 65% of the anthropogenic sources.  Moving away from burning coal would make a huge difference.  But that is easier said than done.  Mining and burning coal are important for the economy of many communities.  It is one of the cheaper methods of producing much needed electricity.  But in addition to producing mercury compounds during the heating process, many other toxic compounds are produced and released as well – not to mention the amount of greenhouse gases produced during this process.  Hence the name “dirty coal”.  There are other methods of producing electricity and the solution would be to convert not only the power plants to these methods, but the coal dependent communities to this line of work.  This one step would make a big difference.

Power plant on one of the panhandle estuaries.
Photo: Flickr


At a much smaller scale, mining for gold produces 11% of the mercury from the mine tailings, cement production (7%), and incinerating garbage (3%).  Though not a large player in this game, reducing the amount of solid waste burned each year would help reduce the mercury issue.


The takeaway here is that the number of imperiled waterways in the Pensacola Bay System have been reduced over recent years and we will look at this in another article.  But for those that remain, mercury is the prime reason.  It is also important to understand that mercury is a naturally occurring element and can not be broken down, so we have what we have – but, we can stop adding to the problem.  Third, eating some seafood each week is good for you.  You will just need to select species that are not problems or watch how much you eat if you prefer some of the listed species.


For more information on the 2021 imperiled waterways list visit


For more on the seafood safety species lists visit


Other sources for this article included:


Wikipedia –


Miller, G.T., Spoolman, S.E. 2011. Living in the Environment. Brooks and Cole Cengage Learning. Belmont CA. pp. 674.

The Recent Fish Kill Near Pensacola Was Due to Stratification… Say What?

The Recent Fish Kill Near Pensacola Was Due to Stratification… Say What?

This began with a call from one of my volunteers who was checking salinity at Shoreline Park.  She reported the salinity, but also reported to smell of dead fish – though she could not see them.  I visited Shoreline Park the following day on another project and could smell it as well.  There was a large amount of dead seagrass washed ashore from a recent storm and I thought this may be the cause of the smell because I did not see the dead fish either.


When I got home, I checked the FWC fish kill database.  It reported a redfish kill in Pensacola Bay.  It is unusual to see a kill of only one species.  Many times, these are releases from fishing activity, particularly bait, and thought this must be the case – FWC did not mention the cause.  I let the volunteer know and asked to keep an eye out.


I reported this to the Escambia County Division of Marine Resources to (a) let them know, and (b) to find out if they had any idea of cause.  They replied that the location was incorrect.  The kill was actually near Galvez Landing on Innerarity Point.  He (Robert Turpin) had visited the site and did find any dead fish.  I have a lot of volunteers over that way so asked each to take a look.  They did not see any dead fish.  I asked them to keep an eye out and collect a dead fish if they saw one for testing.  Often when a large fish kill occurs, and it is only one species, the suspect cause is disease.  Tissue samples could confirm this.


And then came another call.

This time it was from one of our Master Naturalist who lives on the eastern shore of Mobile Bay.  He wanted to know what was up with all of the dead redfish along the shore of the bay.  He sent photos and his beach was littered with them.  I reached out to Mississippi/Alabama Sea Grant to see if they knew what was going on.  They had heard about the situation and knew the Alabama Department of Natural Resources was collecting samples.  The Gulf Islands National Seashore then reported large numbers of dead redfish along the National Shores property in Mississippi, something was up.

Dead redfish on the eastern shore of Mobile Bay.
Photo: Jimbo Meador

I eventually got word from Dr. Marcus Drymon at the Dauphin Island Sea Lab.  They had a team working on this.  Their team reported that stratification of the Gulf had created a hypoxic (low dissolved oxygen) layer on the bottom and the male “bull redfish” had gathered for breeding and died.


So, we are back to our title – what is stratification and how did this cause the fish kill?


Stratification is the layering of the water.  Less dense water will sit atop the more dense.  Water temperature or salinity can cause this density difference and layering.  Colder and/or saltier water is denser and will form the bottom layer.  If you have high winds, it will mix the water and the stratification disappears.  Tides and currents can affect this as well.


What they believe happened recently was excessive amounts of rainfall created a large layer of freshwater to move from Mobile Bay into the open Gulf.  The combination of tides and wind allowed a stratified layer to form.  The oxygen that marine life uses is dissolved into the water at the surface and referred to as dissolved oxygen (DO).  If the system is stratified, then the oxygen dissolved at the surface will not reach the bottom and hypoxia (low DO) can happen.  They this is what happen.  It just so happens that the large male redfish (bull reds) had congregated just offshore for breeding.  They are more sensitive to low DO than the smaller females and any juveniles.  So, the males died.  To answer the question as to why other fish did not die (what you typically see in a DO related fish kill) – the numbers were not mentioned by there was one reference to 4.0 ppm.  This is the high threshold of hypoxia.  Many fish can tolerate at this concentration, but the male redfish could not.


So, that is what we think happened.  The perfect storm of the demise of a group of male redfish just off of Mobile, and the carcasses drifted to other locations.

Maintaining your septic system: Should you use additives?

Maintaining your septic system: Should you use additives?

A conventional septic system is composed of a septic tank and a drainfield, where most of the wastewater treatment takes place. Image: US EPA

Why do you need to maintain a septic system?

Conventional septic systems are made up of a septic tank (a watertight container buried in the ground) and a drain field, or leach field. In the septic tank, solids settle on the bottom (the sludge layer), and oils and grease float to the top and form a scum layer. The liquid wastewater, which is in the middle layer of the tank, flows out through perforated pipes into the drainfield, where it percolates down through the ground. Most wastewater treatment takes place in the drainfield.

Solids settle to the bottom of a septic tank (sludge), oils and greases float to the top (scum) and wastewater (effluent) flows out of the tank into the drainfield for further treatment. Image: Soil and Water Science Lab, UF GREC.

Although bacteria continually work on breaking down the organic matter in your septic tank, sludge and scum will build up, which is why a system needs to be cleaned out periodically. If not, sludge and scum can flow into the drainfield clogging the pipes and sewage can back up into your house. Overloading the system with water also reduces its ability to work properly by not leaving enough time for material to separate out in the tank, and by flooding the system.

Should you use additives in your septic system?

Septic systems do not need any additives to function properly and treat wastewater. Although there are many commercial microbiological and enzyme additives sold on the market that claim to enhance bacterial populations and reduce the time between septic system pumping, there really isn’t any peer-reviewed scientific literature that shows that these additives are effective in doing what they claim.

In Florida, the Department of Health (DOH) reviews commercially sold additives to ensure that they are safe to use in septic systems. DOH clearly states that although products are approved, it does not mean that this is an endorsement or a product recommendation. Approval simply means that as required by Florida law, the additive doesn’t interfere with septic system function and that when an additive is used, the effluent (wastewater) leaving the septic system meets Florida’s water quality standards. Only products in compliance with this law can be sold or used in septic systems in Florida. You can find a list of approved products and more information regarding additives on the Florida Department of Environmental Protection (FDEP) septic system website. Access the list of products directly here (updated 10/18/2021).

How can you properly care for your septic system?

The best way to keep your system functioning properly is to follow some common-sense practices.

  1. Only flush human waste and toilet paper down the toilet. Wet wipes do not break down in the septic system even though the packaging labels them as septic-safe!

    Be mindful of what you put down sinks and flush down toilets. All drains in your home lead to the septic tank. Image: A. Albertin

  2. Think at the sink. Avoid pouring oil and fat down the kitchen drain. Avoid excessive use of harsh cleaning products and detergents which can affect the microbes in your septic tank (regular weekly cleaning is fine). Prescription drugs and antibiotics should never be flushed down the toilet.
  3. Limit your use of the garbage disposal. Disposals add organic matter and additional water to your septic system, which results in the need for more frequent pumping.
  4. Take care at the surface of your tank and drainfield. Don’t drive vehicles or heavy equipment over the system. Avoid planting trees or shrubs with deep roots that could disrupt the system or plug pipes.
  5. Conserve water. Reduce the amount of water pumped into your septic tank through water conservation practices like (1) repairing leaky faucets, toilets, and pipes, (2) installing, low-flush toilets, low-flow showerheads and faucet aerators, and (3) only running the washing machine and dishwasher when full.
  6. Have your septic system pumped by a certified professional. The general rule of thumb is every 3-5 years, but it will depend on household size, the size of your septic tank, how much wastewater you produce and what you flush down your toilet.

Even when conventional septic systems are well maintained, they are still a source of nutrients, particularly nitrogen, to groundwater. They were designed from a public health perspective to remove pathogens, not nutrients.

For more information on septic systems, visit the UF/IFAS septic system website and FDEP’s septic system website.


Fecal Bacteria in the Bay… What Can You Do?

Fecal Bacteria in the Bay… What Can You Do?

For many in the Pensacola Bay area, water quality is a top concern.  Excessive nutrients, heavy metals, and fecal bacteria from run-off have all been problems.  In recent years fecal bacteria has become a major concern, forcing both health advisories at public swimming areas, and shellfish harvesting closures.  In a report from the Florida Department of Environmental Protection last modified in 2021, 43 sites in the Pensacola Bay area were verified as impaired and 11 of those (22%) were due to high levels of fecal bacteria1.

Closed due to bacteria.
Photo: Rick O’Connor


As the name implies, these are bacteria associated with the digestive tract and find their way into waterbodies via animal waste.  Animal waste can harbor pathogenic organisms and contribute to algal blooms which leads to hypoxia (low dissolved oxygen in the water), and potential fish kills.  As bad as hypoxia and fish kills can be, it is the health issue from the pathogens that are often the larger concern.

The spherical cells of the “coccus” bacteria Enterococcus.
Photo: National Institute of Health


In saline waters, the fecal bacterium Enterococcus is the species used for indication of animal waste.  This bacterium is more tolerant of salt water than E. coli, or other fecal bacteria, and a better choice as an indicator for this reason.  Enterococcus is found in the intestines for birds and mammals and enters waterways through their feces.  Waterfowl, pets, and livestock can all be sources, but it is human waste that many point to when the bacteria counts are over the environmental thresholds set.  Human waste enters the waterways either by septic or sewer overflows.  It is the septic systems we will look at in this article.

A conventional septic system is made up of a septic tank (a watertight container buried in the ground) and a drain field. Image: Soil and Water Science Lab UF/IFAS GREC.

How the septic system works…


The process of course begins when you flush.  The wastewater leaves the commode and enters a pipe which leads to the septic tank outside.  Here the wastewater separates.  The solid waste will settle to the bottom forming a layer called sludge.  Fats, oils, and grease float on water and form a top layer called scum.  The untreated wastewater settles in the middle.  This wastewater will drain from the septic tank into a series of smaller pipes and leach into a drain field.  The drain field should be made of large grain material, like sand or gravel, that allows the filtration of the water as it dissipates into the environment.  On paper this system should work well, and often does, but you can see where problems can occur.


  • Was the septic system placed in the correct area? Often as a homeowner you have no control over where the tank is placed but there are regulations on this, and they should have been followed.
  1. Is there sufficient drain material for the effluent to effectively drain and filter (plenty of sand/gravel)?
  2. Is the site too close to the water table? Saturated ground will not allow for proper filtration and can create layers of untreated water to settle near the surface creating foul odors and leach into local waterways.  Over the last decade the Pensacola area has seen an increase in the annual amount of rain.  This increase can turn what was a suitable location into one that is no longer.

Again, many homeowners have no control over the placement of the septic but doing your due diligence when purchasing a home, you can do.  Checking the situation of the septic can save you a lot of problems down the road.

1-gallon container provided free to dispose of your oil and grease.
Photo: Rick O’Connor

The FOG gallon containers are found in these metal cabinets placed around the county.
Photo: Rick O’Connor

  • What are you flushing down the commode? Here you DO have control.
  1. Excessive amounts of fats, oils, and grease will increase the scum layer, and this can enter the pipes leading to the drain field causing clogging of those. Clogs can cause backups into the tank and leaks near the top as well as backups into your home.  Not only can these overflows create problems in the home, but they can also end in our waterways creating water quality problems.  So, what do you do with the oil and grease left over from cooking?  In Escambia County, the Emerald Coast Utility authority provides what they call the FOG program2.  This program provides free 1-gallon plastic jugs to place your oil and grease in.  You can find these in metal cages at locations around the county, there is one at the extension office.  You take it home, fill it up, and return it for a new one.
  2. Disposable wipes can be flushed, but they do not degrade. Excessive amounts of disposal products flushed into your septic system will certainly create clogs in the drainpipes and backups into the lawn, waterways, and your home.  Do not flush these!  Toss them into the trash can.
  3. Interestingly we are learning that milk will solidify after pouring down the drain. It forms solid chunks resembling concrete and can also create backups and overflows.  This is relatively newly discovered problem.  One suggestion is to pour unwanted milk on your garden, but you do not want to flush it down the drain to your septic tank.
  4. There has been discussion on chemical products marketed to clean your septic. Many of our experts believe that these can alter the good microbes within the tank that breakdown the solid waste layer, the sludge.  This is not something you want.
  5. The last one is water itself. Not that you cannot flush water down the drain, but excessive amounts can create situations where either the scum or the sludge layers reach the drainpipes and form clogs.  Do not use excessive amounts of water from different sources in your home at the same time.  Do not overload the system.


  • Driving over the septic tank or drain field.

Most understand that this can cause problems.  It can compact the soil being used for the drain field, thus making it less effective at draining and filtering.  It can also cause cracks in both the tank and the drainpipes, which can create leaks that allow untreated wastewater to travel away from the drain field.  Be careful where you use heavy vehicle traffic on your lawn.


  • Pumping out your septic system.

This is something few people do.  The untreated wastewater should flow into the drain field and percolate through the sand/gravel bed.  However, the scum and sludge do not drain and need to be pumped periodically.  The recommended cycle for pump outs is once every 3-5 years.  Again, this is something most homeowners do not do until backup problems occur.  We do recommend having your system pumped on that cycle.


  • Convert to a sewer system.

This is not an option for everyone, and we know for some in which it is, they are not eager to do so.  But converting to a sewer takes the maintenance issue off the homeowner and onto the local utility.  The conversion can be expensive but, in some communities, there are cost share programs to help with this.  Check with your local utility for more information.

If placed, used, and maintained properly septic systems can last 25-30 years and not be a major problem for local waterways.  Converting to a sewer system can help, but there are also problems here.  We will address those in our next post.




1 Florida Department of Environmental Protection. 2021. Verified Lists for Group 4 Basins Cycle 2 – Pensacola Bay.


2 Fats, Oils, and Grease (FOG).  Emerald Coast Utility Authority (ECUA).

Iron-oxidizing bacteria: an unusual natural phenomenon

Iron-oxidizing bacteria: an unusual natural phenomenon

Iron-oxidizing bacteria produces an orange color and oily sheen in the floodplains of Congaree National Park, South Carolina. Used with permission from Karen Jackson, ©2020, Clemson University

“Someone dumped oil in the creek behind my house!” I had dozens of people call with that exclamation when I was a field inspector for the Florida Department of Environmental Protection’s (FDEP) wetlands compliance program. A significant portion of the job entailed responding to concerns and complaints from citizens regarding damage to wetland areas. In the field, I would come across an oily film along creeks in rural, near-pristine conditions in northern Holmes County and in heavily populated neighborhoods in the tourist hot spots of Destin and Panama City. The first time I saw it, I was taken aback. A shiny, rainbow sheen is something you might expect in an oil-soaked parking lot, not a relatively untouched body of water.

The reaction between iron, native bacteria, and oxygen can produce this orange sheen and filamentous material in streams and groundwater (as it exits the soil). Photo credit: Carrie Stevenson, UF IFAS Extension

Thankfully, an experienced colleague explained the workings of iron-oxidizing bacteria to me, and I was able to allay the fears of all those frantic homeowners. All the places I’ve ever seen evidence of iron bacteria on the water were adjacent to wetlands with some level of iron in the soil. The bacteria essentially “eat” ferrous iron, which is common and able to react with other elements in oxygen-free (anaerobic) environments. Wetlands are classic examples of anaerobic soils, and the mucky conditions of a stream floodplain are ideal for iron bacteria. These are naturally occurring, harmless bacteria that gain energy by breaking down iron available in the soil. In addition to the oily film, side effects of iron-oxidizing bacteria can include a swampy odor, a reddish filament, or red chunks of iron. In large amounts, these byproducts can clog wells if present in pipes. This can be problematic and prevent water flow, but the iron and bacteria are not threats to human health

A colleague with Escambia County recently responded to a homeowner call about bright orange water flowing out of their front yard. While not the typical creek location, environmental conditions were absolutely suited for this phenomenon. Their neighborhood is situated adjacent to a large wetland area, and several of the homes have French drains in the backyards that drain out to the street. During heavier rainfalls, excess groundwater enters those pipes, picks up iron bacteria in the soil, and exits to the surface along the road. The red-stained curbs are evidence that iron is common in the local soil.

When touched, the sheen produced by iron bacteria will fracture. This is an easy way to differentiate it from actual oil. Photo credit: City of Kirkland, Washington

While it’s possible someone could dump oil in a backwoods area (and if you do ever see that, report it to FDEP), it is much more likely that you are seeing the natural aftereffects of iron-oxidizing bacteria. To determine the difference between iron bacteria and actual oil, one simple test is to touch the water and its oily film with a stick. If the sheen fractures into small pieces, it’s iron bacteria. If it oozes back to an intact slick (and smells like petroleum), it could very well be oil.

“They Call Me… Karenia brevis”: the story of red tide

“They Call Me… Karenia brevis”: the story of red tide

At the time of this writing, red tide is still lingering off the Pensacola coast.  By the time this is posted it may or may not be.  I have had a few questions about red tide while this has been occurring here, and some misconceptions about it – so, now is a good time to try and set the story straight.

The dinoflagellate Karenia brevis.
Photo: Smithsonian Marine Station-Ft. Pierce FL


Red tide is actually caused by a group of small, single-celled marine plants.  The one responsible for the red tide in the Gulf of Mexico is called Karenia brevisKarenia is a naturally occurring dinoflagellate.  If I were to pull a water sample off of Pensacola Beach right now I would find it there – albeit in small concentrations – say 300-500 cells in a liter of water.  At these concentrations there are no problems.  When we say problems, we mean respiratory problems or fish kills.  See, Karenia is a dinoflagellate that when irritated or disturbed, will release a toxin – brevotoxin.  This toxin is a neurotoxin that is known to kill fish, sea turtles, and marine mammals at high concentrations – greater than 1,000,000 cells / liter.  For humans the issue is more of respiratory and eye irritation.  Though consuming filter feeding shellfish, such as oysters and scallops, during a red tide can cause serious gastrointestinal problems and possibly hospitalization in humans.  This is why the state closes shellfish harvesting when Karenia concentrations reach 5,000 cells / liter.


What causes Karenia concentrations to increase from 500 cells to 5,000 cells, or even 1,000,000 cells / liter?


The same thing that causes all plants to grow – sunlight and nutrients.

Here is where the first misconception arises.

“Red tides are caused by the increase of nutrients in the ocean due to human activity”.

Not exactly correct.  Red tides have occurred in the Gulf of Mexico since the colonial period, and the colonists certainly did not discharge enough nutrients to spawn a red tide bloom.  No, these blooms occur naturally.  Most form off the coast of southwest Florida.  There the continental shelf extends about 200 miles offshore before reaching the slope to the deep sea.  At this slope there are upwelling currents bringing nutrients from the seafloor to bath these phytoplankton in the warm Florida sun.  This combination, along with some other water chemistry needs, fuel the growth of phytoplankton from a few hundred cells / liter to a few thousand, hundred thousand, or even a million cells / liter – an algal bloom.  At concentrations of 1,000,000 cells or more the water actually changes color to reddish – hence the name “red tide”.



Today humans ARE discharging large amounts of organic and inorganic nutrients into local waterways.  These eventually make their way to the Gulf and can enhance a natural bloom from say 10,000 cells / liter to over 1,000,000 – we can make the situation worse.  This typically happens when offshore winds blow the naturally occurring red tides closer to shore to meet our “cocktail of nutrients” and wa-la – an enhanced bloom with enhanced problems.

Dead fish line the beaches of the Florida Panhandle after a coast wide red tide event in October of 2015.
Photo: Randy Robinson

Here in the northern Gulf the conditions to spawn naturally occurring red tides do not typically exist.  What we usually see are the blooms generated in southwest Florida pushed northward but weather patterns.  At the time of this writing, Escambia County is experiencing a red tide offshore at background/very low concentrations (0-10,000 cells/liter).  Though are no reports of fish kills or respiratory issues in humans, but these are happening to our east in Okaloosa, Walton, Bay, and Franklin counties.


The state is aware of the not only the red tide situation, but other harmful algal blooms occurring around the state and has a task force to try and address these.  We, of course, can help by reducing the amount of nutrients (fertilizers) we discharge into our local waterways.  This would include not only commercial fertilizers, but any plant and animal waste.




Red Tide Current Status.  2021.  Florida Fish and Wildlife Conservation Commission.