Animal Impact: How red tide affects marine life
By Kathy Worley | Conservancy Environmental Science Director
Florida Red Tide: How can such a tiny microscopic dinoflagellate known as Karenia brevis (K. brevis) cause so much misery?
Florida red tides are nothing new. As early as 1530, the Spanish explorer Alvar Nuñez Cabeza de Vaca noted fish kills along our coast when the water appeared red, hence the term “red tide”. The first scientific investigations of the phenomena occurred in 1946–1947. Researchers at the time speculated on what conditions could have led to the bloom, which ranged from an upwelling of nutrient salts from the Gulf to high rains the year prior. Unfortunately no smoking gun was discovered then or even now — it is really a complex mix of factors that could result in a bloom and the exact recipe (if one even exists) is still a mystery.
So what is Karenia brevis?
Well, simply put it’s a very hardy little dinoflagellate with adaptations that allow it to start blooming offshore on the West Florida Shelf. There the dinoflagellates often lie in wait for favorable conditions before becoming vegetative cells that can reproduce either sexually or asexually. At this stage K. brevis can either flourish and bloom or wait around until conditions improve. The West Florida Shelf is “oligotrophic” which means it is typically low in nutrients such as phosphorus and nitrogen. Only hardy species thrive in these conditions and K. brevis is one of them. K. brevis is what we call nitrogen-limited that means it needs nitrogen. Out on the West Florida Shelf, far from land, many strategies can be used to get the nitrogen they need to initiate a bloom along the Shelf as shown in the diagram below:
Blooms have four stages including initiation, growth, maintenance and termination. Unfortunately, no one source appears conclusively to be the primary contributor to prolonged bloom maintenance. There are likely multiple sources and the mechanisms of a cause and effect relationship have yet to be fully demonstrated. Once a bloom gets going, the prevailing winds and currents do the rest and sometimes push the lovely little devils in our direction. The danger to wildlife arises when K. brevis is ingested, since they produce a nasty neurotoxin that can bioaccumulate up the food chain. The toxin can also become aerosolized, or air-borne, when the cells break apart or lyse from wave action.
Since historically Florida red tide blooms break up once a cold front occurs, it is likely that this particular bloom could remain active for a while longer. Just because we don’t feel or see the effect does not mean that the bloom has subsided, it can continually move at the whim of the wind and currents and could be just offshore.
What happens to marine life when they encounter the toxin produced by K. brevis?
Unfortunately, sea life can be adversely affected by Florida Red Tide, whether through ingestion or inhalation of neurotoxins, called “brevetoxins”. These brevetoxins can cause immense fish kills, shell fish contamination, death of sea turtles and marine mammals and even nasty respiratory and gastrointestinal issues in humans.
K. brevis vs Sea Turtles
Sea turtles, large air breathing reptiles that have been around for 100 million years, are in trouble mainly due to both direct and indirect exploitation by humans. We once (and some people still do) considered sea turtles a delicacy, we have destroyed much of their nesting and foraging habitats, polluted their home, killed them in nets as bycatch, and placed many other pressures on their wellbeing. Add to all this is K. brevis toxicity.
Once exposed, sea turtles act confused, they lose their coordination, often swim in circles and can exhibit aberrant behaviors such as head bobbing, twitching and extreme listlessness. Exposed turtles are often unable to dive and evade predators, and many die from the resulting paralysis and inability to lift their heads above the water to breathe. If a turtle is found early enough in the exposure they can recover.
On the flip side, the effects of chronic annual exposure to brevetoxins are unknown. Sea turtles live a long life and tend to have a slow metabolism, which means they can accumulate toxins within their bodies. Long-term effects of exposure are unknown. Additionally, recent findings suggest that brevetoxins can be transferred from an affected mother to their hatchlings. These toxins when metabolized leave the turtle struggling with toxic effects long after the red tide has disappeared from the environment. However, it is believed that most of the sea turtles die of red tide poisoning through ingestion of prey items contaminated with brevetoxins.
Sea turtle species that have borne the brunt of red tide effects this year and during the last large outbreak in 2005–2006, include loggerheads, Kemp’s ridleys, and occasionally a green turtle. Over 300 sea turtle strandings (dead or injured turtles that have washed ashore) occurred during the 2005–2006 bloom in comparison to over 400 (386 dead) and counting during this bloom. In Collier County alone, as of August 6, 2018, 95 sea turtles have washed ashore on our beaches. To put this in perspective, we usually see an average of around 30 strandings in Collier County on an annual basis, so the number has already more than tripled. These numbers represent only a fraction of the actual mortality since most of the sea turtles that die will sink in the Gulf and never wash up on our beaches. While all strandings cannot be attributed to K. brevis toxicity, it is probable that many are.
Conservancy Research Manager Dr. Jeff Schmid is attempting to collaborate with regional rehabilitation facilities (CROW and Mote) to try to get a better understanding of how diet may influence Kemp’s ridley red tide exposure.
K. brevis vs Fish
Florida red tide can negatively impact all stages of fish development. Signs of fish toxicity often manifest as coordination problems that are expressed in twisting motions or swimming in a spiral pattern. Fish that are exposed can exhibit regurgitation, fin paralysis, and convulsions. Death is usually a result of suffocation due to loss of gill function. Similar to this year’s bloom, an unusually persistent red tide event in 2005 concerned scientists and angler’s alike. Researchers discovered that during that red tide event and shortly thereafter, there were changes within fish assemblages (abundance and variety of fish species within a waterbody). A short term assemblage shift is not unexpected following such a large scale red tide event. However, what is important is that within a year the fish assemblages had returned to normal, suggesting that these fish communities are adapted and resilient to red tide events. The problem lies in separating out whether any declines following a red tide event are just the result of normal fluctuations in the population, or other environmental fluctuations, species habitat preferences and/or different tolerances to red tide effects. Interestingly, researchers found that larger-bodied fish may be resilient to these events in the short term, provided that there is sufficient stock remaining to repopulate each affected species. More research is needed to substantiate the long-term effects of red tide on fish stocks.
K. brevis vs Marine Mammals
It has long been suspected that brevetoxins are responsible for higher marine mammal mortality during incidents of Florida red tide, either directly through ingestion or indirectly through inhalation and/or bioaccumulation. It has been hypothesized that brevetoxins could have a delayed response in marine mammals that results in mortality even when K. brevis is no longer in the vicinity, due to the bioaccumulation buildup tendencies of these nasty toxins in their food sources! Signs of toxicity in mammals include nervous system disorders, lethargy, equilibrium issues and seizures.
So what can we say about Florida Red Tide?
The nature, cause and maintenance of blooms are still under investigation. We do not fully understand the organism, its ecology or oceanography. Intensity and duration of blooms are highly variable. Some are local and some cover hundreds of miles. Some occur at the same time and place each year and some seem totally random. Some people blame it on a global problem linked to pollution and anthropogenic changes to coastal ecosystems, while others argue that the expansion is a result of improved detection techniques or proliferation of aquaculture. Still others speculate that climate change and increased storm activity over the Gulf could spur bloom formation. To date at least 24 different hypotheses exist to explain the inception, growth and maintenance of a large bloom. Yet we still cannot answer how a low light adapted, slow growing dinoflagellate can outcompete diatoms with triple K. brevis’s growth rate! We do know that K. brevis is an opportunistic feeder that can derive energy from a variety of natural sources both offshore and near shore such as Trichodesmium species (nitrogen fixers), zooplankton grazing and excretion, decomposing fish, etc. Additionally, land based anthropogenic nutrient sources such as stormwater and fertilizer runoff may contribute to bloom perpetuation. Again, the science has not, as yet, been able to develop a working theory on exactly how these blooms are maintained.
As far back as the 1970’s the use of ozone ((O3) created when oxygen (O2) is exposed to an electrical charge) has been known to mitigate K. brevis in laboratories and seawater aquariums. Ozone acts like a disinfectant causing the dinoflagellate cell structure to rupture. While this sounded great it also presented many challenges that needed to be overcome. While we would like to be able to control K. brevis we don’t want to do this at the expense of other marine life. At the same time we need to keep trying to nail down the recipe that causes blooms to become large in the first place. Mote Marine is currently investigating ozonation for use in dead-end canals or small embayments and while this method will not eliminate a large-scale bloom that is up and down the coast, if it works without adverse effects, it could mitigate affects in small localized areas.
Karenia brevis has long been associated with mortality of Florida’s sea life. As the science of harmful algal blooms (HAB’s) are still in development and as such in its relative infancy. Although K. brevis is responsible for most, if not all, of the red tide events that we feel on southwest Florida beaches, it is important not to have tunnel vision and as so many other phytoplankton can cause similar ramifications. Previous studies have shown that harmful algae other than K. brevis are present in the Gulf of Mexico. In 2002, researchers documented 17 different species of toxic dinoflagellates like K. brevis in the Gulf of Mexico, so it might be ill advised to overlook them.
