Can science relieve a harmful algal bloom hotspot?

A man in a purple jacket and beige cap smiles at a woman in a sweatshirt who is holding water samples. They are both sitting on the ledge of a boat.

SCCF marine laboratory director Eric Milbrandt (left) and research assistant Sierra Greene (right) collect water samples to help identify the drivers of harmful algal blooms in southwest Florida. (Sarah Anderson/MEDILL)

Related Topics:

By Sarah Anderson

There’s no bathroom on the boat, subjecting the researchers to an eight-hour dilemma between hydration, bladder comfort and a quick dip in the water. They grab bites of sandwich during rare lulls in activity, one almost losing their lunch as the boat rocks in another’s wake. The crew proceeds to the next sampling site, their chatter fading under the forceful wind and the roar of the engine as the boat gains speed. The sun blazes down from the clear, mid-afternoon sky. 

Suddenly, a pack of dolphins surfaces alongside the boat, appearing to defend their territory. Little do the dolphins know, the people onboard have the same goal –– to defend the territory against invading algae.  

Each month, researchers at the Sanibel-Captiva Conservation Foundation (SCCF) Marine Laboratory collect water samples from the Gulf of Mexico and Caloosahatchee River and Estuary, following a route reaching from Sanibel Island to Lake Okeechobee. The project, referred to as CLEW, an acronym for “coastal, lake, estuary and watershed”, is a collaboration between scientists at SCCF, the University of Florida and North Carolina State University. The researchers are systematically analyzing water quality, microscopic algae material and discharges from water control structures to identify the driving forces behind the region’s harmful algal blooms. In doing so, they hope to inform water management policies and other solutions that will help control the blooms that have devastated southwest Florida’s critical ecosystems.

A grey-scale map with red stars indicating CLEW sites.
The CLEW team samples from 14 sites. Nine are accessed by boat the first day of the sampling trip, while five are reached by truck the second day. (Image courtesy of SCCF)

An Upstream Battle

Microscopic algae called phytoplankton rely on nutrients like nitrogen and phosphorous to proliferate. Too much growth creates an algal bloom, which is harmful as a plethora of algae can clog fish and invertebrate gills, use up the oxygen in the water when decomposed by bacteria and block the light that reaches submerged aquatic plants. Additionally, some phytoplankton produce toxic compounds that pose serious health risks to marine organisms.

Fertilizer, wastewater and other anthropogenic sources of nutrients that enter water bodies can help algae quickly multiply, contributing to harmful algal blooms. This connection has been controversial for red tide, a toxic algal bloom of the marine phytoplankton Karenia brevis. Red tide was documented in the Gulf of Mexico as far back as the 1700s, according to the Mote Marine Laboratory and Aquarium. It is believed to originate offshore, when iron-rich dust from the Sahara Desert blows across the Atlantic Ocean and settles in the Gulf of Mexico. The bacteria Trichodesmium consumes the dust’s iron, creating a useable form of nitrogen for Karenia brevis. The bloom may be further fueled by a current that causes deep ocean water loaded with nutrients from plant material on the seafloor to rise to the surface. 

But red tide’s long history and natural underlying processes doesn’t mean humans aren’t part of the equation. In a 2022 study, researchers used advanced data analysis techniques to demonstrate a causal relationship between the amount of nitrogen and Karenia brevis in the water along a path reaching over 100 miles into the interior of Florida. Their results reveal that nitrogen pollution, even from far upstream, can intensify red tide blooms that travel to the coast. 

Red tide is caused by an interplay between oceanographic phenomena and human influence, said Miles Medina, an environmental scientist at the University of Florida Center for Coastal Solutions and lead author of the study. “Sometimes, it gets characterized as: It’s either natural or it’s manmade,” he said. “It gets its start naturally, but we can make it worse.” 

And we have. “The frequency of very intense blooms over the past decade is higher,” said Eric Milbrandt, the director of the SCCF Marine Laboratory and co-author of the study, citing serious red tide blooms in the region in 2018, 2019 and 2020. The expanding population and development of south Florida has increased the runoff of nutrients that fuel blooms, and warming water allows the algae to grow more rapidly, he said. 

Major nutrient sources include the Caloosahatchee River and Lake Okeechobee, which are classified as impaired for nutrient enrichment by state standards. Discharges can transport nutrients from the lake and river to the water bodies surrounding Sanibel, several of which were recently added to the list of impaired waterways. And any freshwater blue-green algal blooms in these bodies of water can travel downstream. Although these algae will dissipate in the salty conditions of the ocean, their toxins can linger, and the nutrients they leave behind can help fuel blooms of Karenia brevis and other marine phytoplankton.

But Sanibel depends on this freshwater to hydrate wetlands and maintain the delicate salinity balance of critical food sources and habitats like tape grass and oyster reefs during the dry season. “We’ve often advocated for the water from the lake during that time because we need the freshwater, regardless of how polluted it is,” Milbrandt said.

The salinity at various points in the estuary system versus the optimal range for oysters and tape grass. (Image courtesy of SCCF)

Advocating doesn’t guarantee access amid the competing agricultural, domestic and environmental demand for south Florida’s finite freshwater resources. During two droughts in the early 2000s, the estuary didn’t get the freshwater it needed to maintain the salinity gradient, leading to mass tape grass die-offs, said Rick Bartleson, a research scientist at the SCCF Marine Laboratory. The grass never fully recovered and still cannot support more than a few manatees, which can eat 100 pounds of grass per day, he said.

Just as everyone is scrambling for enough water when it’s dry, no one can handle too much when it’s wet. But an overflowing lake could have dire consequences. In 1928, a hurricane caused Lake Okeechobee to flood, killing thousands of people, according to the National Weather Service. “When the lake gets too high, the water has to go somewhere,” said Paul Julian, a hydrologic modeler at SCCF. “We’re kind of the release valve for the lake because we’re the biggest estuary out of the two.” Excess water discharged to the Caloosahatchee Estuary delivers a one-two punch of salinity disruption and pollution. The fact that the region’s precipitation is becoming concentrated into fewer, heavier rainfall events exacerbates water storage issues and increases runoff, Julian said. When it rains, it pours. 

A perfect storm of conditions led to a catastrophic red tide outbreak on the Florida Gulf Coast in 2018. The previous season’s Hurricane Irma threatened to overflow Lake Okeechobee, where freshwater Microcystis phytoplankton —which produce the liver toxin microcystin — were blooming. “There was panic, the gates were opened and the water levels were lowered,” Milbrandt said. “When they released that bloom, the conditions were perfect. In the Caloosahatchee, the water was warm, it hadn’t started raining yet to dilute it, and it ended up getting transported all the way down to Sanibel.”

Once the bloom reached and dissipated in the Gulf of Mexico, its nutrients helped fuel a patch of red tide that had moved near shore. “It was really bad conditions in the freshwater portion and really bad conditions in the marine portion,” Julian said. “And as discharges were happening, the red tide was essentially being fed by the nutrients.” 

A man in a cap and plaid shirt reaches his hand into a white pool filled with green water and vegatation.
SCCF marine laboratory research scientist Rick Bartleson checks on the tape grass he grows to conduct experiments and plant in the estuary to promote regrowth. (Sarah Anderson/MEDILL)

The interaction between freshwater and saltwater algal blooms spells disaster for the island’s shorebirds, turtles and other animals. “When those two feed off of each other, it creates this entire habitat of death,” said Breanna Frankel, the wildlife rehabilitation manager at Sanibel’s Clinic for the Rehabilitation of Wildlife (CROW). “2018 was a devastating year for us.” 

Red tide’s acute deadliness is due to brevetoxin, a toxic compound produced by Karenia brevis that affects the nervous system of many aquatic creatures as it passes through the food chain. Shorebirds impacted by red tide experience a range of symptoms, including twitching and tremors, disorientation and loss of function in their legs, sometimes stumbling like “a drunk kid walking home from the bar,” Frankel said. Similarly, sea turtles exposed to brevetoxin can suffer seizures and may struggle to right themselves in the water, causing them to drown, said Kelly Sloan, the coastal wildlife director and sea turtle program coordinator at SCCF.

About 250 sick or dead turtles were documented during the 2018 red tide outbreak — a dramatic increase from the approximately 35 seen in a normal year, Sloan said. What’s worse, the toll of this bloom may still be rising. By analyzing the blood of nesting sea turtles and unhatched eggs or unsuccessful hatchlings, Sloan has found that adult turtles can transfer brevetoxin compounds to their offspring.

Red tide could be a tipping point for sea turtles and other species in the region facing a myriad of threats. “When you add one more on top of that, we could very realistically start seeing declines in an already stressed population,” Sloan said.

Gathering CLEWs

The CLEW research project aims to unravel the complex network of relationships between lake discharges, water quality and algal blooms in this interconnected system. “We are so often in a response mode when the blooms happen,” Milbrandt says. “Having the conditions of the water before, during and after an event is what we need to understand the drivers of the event.” 

At each sampling site, the researchers operate like a well-oiled machine, handing off equipment, logging data and storing samples with assembly line efficiency. They submerge a sensor that reads the salinity, temperature, pH, oxygen content and turbidity, or clarity, of the water and measures its level of chlorophyll, a photosynthetic pigment found in plants, and dissolved substances from organic matter. The team also deploys an instrument to measure the transmission of light through the water.

SCCF marine laboratory manager A.J. Martignette services a RECON sensor at Redfish Pass. (Sarah Anderson/MEDILL)

Some of the sampling sites feature a continuous sensor that measures and transmits water quality parameters hourly as part of SCCF’s River, Estuary and Coastal Observing Network (RECON) system. RECON generates much more information than can be acquired through physical sampling, providing necessary context to noteworthy snapshots. It also allows for near real-time responses to changes in water quality. For example, if the RECON data indicates the beginnings of a bloom, the researchers can initiate large-scale, event-driven sampling — “what we do for the monthly trips, but on steroids,” Milbrandt said.

The team collects water samples that are sent to a National Environmental Laboratory Accreditation Program (NELAP)-certified laboratory that measures nitrogen and phosphorous in various forms. Other samples are returned to the SCCF marine laboratory, where research associate Mark Thompson validates the sensor’s chlorophyll readings — which serve as a measure of the amount of algae material in the water — using a more rigorous technique. 

Research assistant Sierra Greene analyzes the samples using a FlowCam instrument that takes a picture of anything in the right size range and with the same light transmission properties as phytoplankton. Using this data, she can estimate the distribution of different types of phytoplankton in the water, including cyanobacteria, dinoflagellates and diatoms. Greene is building libraries of each category of algae to train the computer to automatically recognize and sort the images. CLEW researcher Ed Phlips, a professor of algal physiology and ecology at the University of Florida, is using a more painstaking but precise microscope-based method to quantify the species composition of the phytoplankton in the water samples.

A young woman in a long-sleeve shirt sits on the edge of a boat and holds a blue instrument, a sensor, in her hands.
SCCF research assistant Sierra Greene records water quality parameters from a CLEW sampling site using a submerged sensor. (Sarah Anderson/MEDILL)

CLEW researcher Natalie Nelson, a professor of biological and agricultural engineering at North Carolina State University, is using these data inputs to develop a statistical model that identifies correlations between water quality parameters, lake discharges and phytoplankton species distribution. “What we’re trying to understand is: Are there certain types of environmental conditions that encourage the growth of certain types of algae?” Nelson said. In addition to providing insight into the different variables promoting algal blooms, the model may be able to help predict the effect of an upcoming discharge on algae populations, she said.  

A lengthy step in the sampling assembly line is passing water through a filter to collect the particulate organic matter (POM) — solid particles suspended in the water. CLEW researcher Elise Morrison, a professor of environmental engineering sciences at the University of Florida, analyzes certain chemical characteristics of the POM to understand its source. The ratio of a heavier and lighter form of carbon is a signature of different photosynthetic pathways, helping to distinguish phytoplankton from other types of plant matter in the water. “From that value, we can say that it looks like a certain source material. So it looks like it might be phytoplankton or it looks like it might be from terrestrial plants that are in the area,” Morrison said. “It’s a way of fingerprinting where the carbon came from.”

Similarly, the ratio of a heavier and lighter version of nitrogen serves as a mark of various environmental processes, shedding light on the origin and journey of the nitrogen in the samples. “It can give you an idea of whether it looks like the nitrogen came from a septic system versus an inorganic fertilizer versus a Lake Okeechobee discharge,” Morrison said. Additionally, by measuring the carbon and nitrogen ratios in the amino acids in the POM, she can track how these elements are cycled between phytoplankton as they decompose and their nutrients are consumed, providing insight into the phytoplankton dynamics that can fuel algal blooms.  

Collectively, the data will help water management agencies identify important targets for mitigating algal blooms. “This will give them information that can help guide them to manage the parts of the system that would be most effective in terms of reducing the potential for blooms,” Phlips said.

Since the first sampling trip in November, the team has observed the highest nitrogen and phosphorous concentrations in the upper Caloosahatchee River, suggesting that the nutrients came primarily from the land bordering the river rather than lake inputs. “What it tells us is that there’s a lot of work to do in the watershed, and that for a long time, we ignored that part of the system,” Milbrandt said. 

The project may also help inform a rate of discharge that could relieve a brimming lake while minimizing blooms and other ecological damage. Phlips’ research has shown that the longer the water stews in the Caloosahatchee River and Estuary, the more time there is for algae to grow and form blooms. A fast, high-volume discharge could limit blooms upstream, assuming there is no bloom in the lake that could be released, but the nutrients in the water could feed marine phytoplankton in the Gulf of Mexico. Additionally, a burst from a hose instead of a trickle from a tap may affect the estuary’s salinity balance, and a rush of water from the nearest water control structure could displace the larvae of spawning fish and oysters in the estuary to less suitable downstream environments. “All these relationships are so complex that without doing sophisticated modeling, it’s too difficult to come up with reasonable suggestions,” Phlips said. “That’s why this research is going on right now: to define the ideal discharge rates under all different kinds of scenarios.”  

A man in a laboratory leans over a counter to look at a blue piece of equipment.
SCCF marine laboratory research associate Mark Thompson measures the chlorophyll levels in the water samples. “Chlorophyll is like a symptom of poor water quality,” he said. (Sarah Anderson/MEDILL)

Progress in Bloom

Other efforts to manage how water is dispersed throughout the region are underway. One is the construction of a holding reservoir for excess water that can be tapped to maintain the salinity gradient in the Caloosahatchee Estuary during the dry season. The water that enters the reservoir will be treated with aluminum sulfate that binds to and removes some nutrients. “The water that’s coming out of the reservoir will be cleaner than when it came in,” said Leah Reidenbach, a research and policy associate at SCCF. “But even though it’s really big, it doesn’t even account for half of our water storage needs in the estuary. So water storage needs in our watershed are an important issue for the future.”

A woman in a gray t-shirt stands in front of a monitor in a lab with her hand on the mouse.
Sierra Greene peruses the data captured by the FlowCam instrument. The water samples collected farther upstream tend to contain more phytoplankton, with 1 milliliter of water generating tens of thousands of images, she says. (Sarah Anderson/MEDILL)

Another major development is the Lake Okeechobee System Operating Manual (LOSOM), an intricate plan to reengineer how the lake’s water is distributed throughout the region. The goal is to regulate discharges in a way that addresses the needs of all parties, including supplying more freshwater to the Everglades once it has passed through natural water treatment areas that soak up nutrients to prevent further degradation and maintaining only desired flows to the Caloosahatchee Estuary. “What we’re really trying to do through the LOSOM process and while working with our partners is establish more of a balance,” said Matt DePaolis, the environmental policy director at SCCF. “Because essentially, the way it’s set up now, we are the dumping ground for everything, regardless of how the health of the estuary is affected.” 

“We need to share the adversity,” Milbrandt said. “It can’t always be the environment that has to be the sacrificial lamb.”

In addition to water management, reducing nutrient loading is critical for south Florida’s freshwater to benefit all who need it. The conversion of septic tanks to sewage systems prevents leakage of nutrient-packed waste into waterways — an issue that has been exacerbated by sea level rise, DePaolis said. It’s even better if treated sewage, which still contains nutrients, can be used on land rather than sent directly into the water. The Caloosahatchee Connect is a pipeline that will send reclaimed wastewater from Fort Myers to Cape Coral, where it will be used for irrigation and fire protection, instead of releasing it into the Caloosahatchee River.

Three people sit on the bow of a boat using a collection of testing instruments.
Eric Milbrandt (right) and Morrison group researchers Amanda Chappel (center) and Shin-Ah Lee (left) prepare samples to be delivered to the various laboratories for analysis. “Being a Florida native, I have seen the amount of degradation we can have on our systems,” Chappel says. “But there’s an amazing opportunity, if you catch it at the right moment, to restore and preserve and conserve these systems.” (Sarah Anderson/MEDILL) 

Throughout southwest Florida, ordinances that help combat nutrient enrichment include restrictions on the timing and location of fertilizer application, protection of native plants, which do not require fertilizer because they have adapted to thrive in local conditions, and limits on the amount of impervious surfaces, like concrete, that contribute to stormwater runoff. 

Political agendas are at the heart of many such measures to protect the environment — and their demise. Years into the development of a science-backed LOSOM plan, a section of a new bill introduced a slew of red tape, requiring legislative approval for funding for water management projects. After the bill passed through the Senate, SCCF provided a platform to send an email opposing the bill to the governor, who vetoed it. SCCF offers action alerts that inform subscribers of other opportunities to advocate for the needs of coastal ecosystems. 

“I think the biggest thing that you can do as an individual is to vote. You’ve got to vote for local legislators that believe in water quality and that you can trust,” Reidenbach said. “Stay active; stay involved. Use your voice when you can. And let your legislators know how you feel about water quality. Because, sometimes, they listen.”

How do you move the planet forward?
Submit Story
clean water, nutrient pollution, toxic

Get the Newsletter

Get inspiring stories to move the planet forward in your inbox!

Success! You have been added to the Planet FWD newsletter. Inspiring stories will be coming to your inbox soon.