Starving for Seagrass

Thalassia, or turtle grass, in Puerto Rico.  This species also grows in the Indian River Lagoon (IRL).  (NOAA Photo Library) 

We have wanted to create more amusing, light-hearted posts that feature more photos and less text.  But sometimes, a topic just grabs you by the throat and demands attention.  This is one of them.  It's the story of water pollution, nuisance algal blooms, seagrass, and manatees.  It's about the life, and potential death, of an ecosystem. 

S/V Fulmar spent over a month on the Indian River Lagoon (IRL).  The IRL is an enormous estuary; a 156 mile-long embayment paralleling Florida's Atlantic coast, where salty seawater mixes with freshwater.  It's a very productive ecosystem and is one of the most biodiverse estuaries in the world.  But after completing a lap around nearly the entire lagoon, the thing that struck us most forcefully wasn't anything we saw or heard, it's something that was missing: seagrass.  And that's a problem.  Until recently, the IRL had lush underwater meadows of seagrass, which were the foundation of the ecosystem.   

The Indian River Lagoon on Florida's east coast, circled in red.  Lake Okeechobee lies inland of the Lagoon's southern end.  

The IRL runs roughly from New Smyrna Beach to West Palm Beach, and is separated from the ocean by a narrow chain of barrier islands.  The IRL system includes the Indian River, Banana River, and Mosquito Lagoon. 

Seagrasses are a collection of flowering, vascular plant species that all descended from terrestrial plants and that have adapted to live fully submerged in seawater.  Worldwide, there are over 70 species of seagrass, seven of which are native to the IRL.  One, called Johnson grass, is found nowhere else in the world besides southeast Florida.  Because of its limited distribution and recent conservation concerns, Johnson grass has been categorized as threatened under the Endangered Species Act.  

Seagrasses can form large beds, or meadows, that provide many ecosystem services:

• Provide food and habitat for countless other species 
• Clarify the water
• Absorb and sequester carbon dioxide.

A healthy meadow of Syringodium, also called manatee grass.  None of the images of seagrasses shown here were taken in the IRL because we couldn't find any seagrass there to photograph.  (NOAA Photo Library).

Some of the familiar species that depend on seagrass include manatees, green sea turtles, bay scallops, spotted seatrout, and pinfish.  The young of many species, including a lot of species found on the menu at your favorite seafood restaurant, use seagrass beds as nursery habitat.  Seagrass beds help clarify the water and maintain water quality in two ways.  First, the dense tangle of grass blades slows the flow of water, causing sediment particles suspended in the water to sink to the seafloor.  Second, the network of fibrous roots and rhyzomes holds the bottom sediments in place, preventing the fine particles from becoming resuspended in the water.  Finally, similar to mangroves and salt marsh plants, seagrasses "suck up" and sequester climate-warming carbon dioxide.  Because these aquatic plants are so good at taking up carbon dioxide and locking it away for centuries, they are collectively referred to as "blue carbon."  Because of all of these ecosystem services that they provide, seagrasses are economically important.   

Just like the grass on your lawn or the plants in your flower pots, seagrasses use sunlight, carbon dioxide, and nutrients (molecules containing nitrogen and phosphorus) to make their own food to support their growth.  Seagrasses are extremely efficient, being able to grow where nutrients are limited.  To a large extent, light availability determines where seagrass can grow.  If the water is clear, which allows the light to penetrate easily, seagrass will grow at depths greater than 10 feet (about 3 m).  On the other hand, if light availability is limited by cloudy water, seagrass will be restricted to very shallow depths, close to shore.  Because seagrass health is so dependent on water quality, it's often used as an indicator species signifying the health of coastal ecosystems.

Turtle grass in St. John, USVI.  This is what the IRL used to look like, not that long ago.  (NOAA Photo Library).

    

The shallow waters of the IRL should host, or at least used to host, enormous seagrass meadows.  However, starting in 2011, a series of algal blooms has caused a catastrophic die-off of seagrass in the IRL, particularly in its northern portion.  Blooms of microscopic algae, that drift with the currents and cloud the water, greatly limit the depth to which sunlight can penetrate.  Because seagrass is rooted to the bottom, all that cloudy water above shades it.   Without sufficient light, the seagrass cannot grow.  Between 2011 and 2013, the biomass of seagrass in the Banana River (northeast portion of the IRL complex) plummeted by approximately 91%.  In the northern IRL, the proportion of the river bottom that was covered in seagrass declined from 35% in 2011 to 0% in 2018 and 2019.  

Conditions seem to be as they were in 2019...bad.  We didn't perform a standardized survey of seagrass when we were in the IRL, but we've spent enough time on the waters of Florida to know how it's supposed to look.  We saw no seagrass at all in the northern IRL and very little further to the south (around Vero Beach and Ft. Pierce).  We didn't see much seagrass but on the first island that we explored in the IRL, we found the carcass of a dead manatee.  And these two things are connected.  Manatees are the only existing marine mammals in North America that are herbivores.  They depend on seagrass.  The IRL, with its formerly vast seagrass meadows, was a hot spot for manatees.  The disappearance of seagrass has caused mass starvation and unprecedented mortality of manatees.  The situation is so bad that in 2021, the U.S. Fish & Wildlife Service formally designated this manatee die-off as an "Unusual Mortality Event" (UME) under the Marine Mammal Protection Act.  A UME is a die-off that has the potential to affect the persistence of a marine mammal population.  Prior to the start of the die-off, there was a total of about 7,500 manatees in all of Florida.  During each of the past 7 years, annual mortality has been in the range of 520 -1,100, with the highest mortality rates being in the IRL during 2021.  Imagine living in a town of 7,500 people and having almost 4,000 of those people die in a 5 year period.  This is exactly what is happening with manatees in Florida, and it is shocking.  

The situation is so dire that, as a stopgap measure, the U.S. Fish & Wildlife Service and Florida Fish & Wildlife Conservation Commission have provided food--tons and tons of lettuce--to wild manatees in the IRL during the past two winters.  This is a controversial step, one that wasn't taken lightly, and it's clearly not a long-term solution.  The supplemental feeding has been taking place in the winter for two reasons: (1) manatees are very susceptible to hypothermia and starvation makes them even more vulnerable, and (2) the availability of seagrass naturally declines in winter (think about how your lawn goes dormant in the winter).  Manatees are "warm blooded" mammals but are unable to maintain their core body temperatures in waters that are cooler than about 68 deg F (20 deg C). To avoid colder water, manatees migrate to South Florida in winter.  Even here, the water can occasionally dip below that critical 68-degree threshold.  When it does, they retreat to warmwater refuges: natural warm springs and electrical power plants that discharge large amounts of warm water.  During cold snaps, manatees congregate at these warmwater refuges.  They stay in the warm water for as long as possible, until they absolutely need to eat a meal.  Then they find the nearest seagrass bed, eat their fill of succulent grass blades, and return to the warm water as quickly as possible.  Since seagrass in the IRL is becoming increasingly scarce, they have to travel further and further away from the warmwater refuges to find sufficient food.  This means they burn many more calories trying to stay warm while searching for seagrass but they are also eating fewer calories.  

Manatees congregating in DeSoto Canal during cold weather.  This canal is a warmwater refuge in Brevard County, just off the IRL.  (USFWS Digital Library)

So, what are the causes and the solutions?  The algae blooms are the proximate cause of the seagrass decline.  But what's causing these blooms?  Increasing concentrations of nutrients in the water, coming from sewage effluent, private septic systems, lawn fertilizer, stormwater runoff, agriculture, and boats.  In short: humans and the things we do.  Six Florida counties border the IRL.  Their combined population is 3.35 million, which has increased by a factor of 4 since the 1970s.  A booming tourism industry adds even more people to the number of permanent residents.  Thirty percent of these people live in homes with private septic systems, which have a high rate of failure and a working life expectancy of about 20 years (most were installed more than 20 years ago).  But the pollution in the IRL originates over a much broader area than just the communities that border it. Some of the IRL's pollution problem comes from Lake Okeechobee, in the center of the South Florida peninsula.  Before humans re-plumbed the natural waterways of South Florida, water from Okeechobee flowed south to Florida Bay via the Everglades.  The Everglades is actually a very broad, shallow, slow-flowing river, sometimes called the "River of Grass."  Historically, the waters of the Okeechobee were low in nutrients and the Everglades acted like a filter, removing even more nutrients before the water reached the coast.  As the water flowed slowly through the 'Glades, the aquatic plants removed the nutrients, so that it arrived at Florida Bay fresh and clear (Florida Bay is the area off the tip of south Florida and west of the Florida Keys).

Beginning in the 1800s, humans started messing around with South Florida's natural plumbing to improve navigation and agriculture, and to control floods.  Okeechobee's water has been cut off from the Everglades and is now directed eastward down the St. Lucie River to the Indian River Lagoon and westward, down the Caloosahatchee River to the Gulf of Mexico.  The water in the Lake is high in nutrients from the sources described above and it is discharged down these two rivers at high flow rates, meaning that relatively few nutrients are taken up before it reaches the coastal estuaries.  As a result of this aquatic engineering, the Indian River Lagoon and Caloosahatchee Estuary have both been severely degraded by nutrient pollution and upper Florida Bay has been degraded due to the extreme reduction of freshwater flowing to it.  

So why can't we just reverse engineer it and restore Okeechobee's flow to the Everglades?  Well, we are doing just that...sort of.  For over 20 years, there has been a very large public works program aimed at restoring the Everglades and Florida Bay (and the IRL and Caloosahatchee by extension).  It's actually one of the largest, most expensive public works projects in our country's history.  One of the problems, however, is that towns have grown up around Lake Okeechobee since the water was redirected and the lake's water level was maintained at an artificially low level. If we were to restore the system entirely, the lake level would rise and flood these towns.  So it's a balancing act; we want to restore some flow of water from Okeechobee to the Everglades but we also need to periodically "dump" lake water into the Caloosahatchee and St. Lucie Rivers to prevent Lake Okeechobee from flooding the towns along its shores.  

Now, you're probably thinking this situation is hopeless.  But we've been here before and found solutions.  Fifty years ago, our country's waterways were plagued by pollution.  Seagrass was disappearing and marine mammals were dying at high rates as a result of human activities.  The public grew tired of the situation and demanded change.  In the early 1970s, a series of groundbreaking environmental laws were passed by Congress and signed by a Republican president, Richard Nixon.  Among these laws were the Clean Water, Marine Mammal Protection, and Endangered Species Acts.  These laws have been remarkably effective.  People born after 1980 may not even realize how far we've come in improving environmental quality and preserving species.  Manatees are a good example.  They were among the original group of organisms designated as Endangered under the Endangered Species Act in 1973.  Thanks to the Clean Water Act, water quality improved and seagrass recovered over the period from the 1970s to about 2010.  And conservation efforts required by the Endangered Species Act and Marine Mammal Protection Act enabled the manatee population to grow and therefore they were "down-listed" from Endangered to the less critical status of Threatened in 2017.  Of course, now we know what has happened since 2017 and there have been calls to place the Florida manatee back on the Endangered Species list.  

But the point is that for four decades, both seagrass and manatees were recovering strongly thanks to public outrage and a responsive government.  But the IRL is facing many more environmental challenges because of the influx of new residents.  Today, scientists know much more about the complex ecology of systems like the IRL; we understand the problems and know how to solve them, and we have been able to solve them in the past.  Many of the conservation measures that would make a difference are pretty mundane, such as building new sewage treatment plants and engineering better stormwater management systems.  We just need to show resolve and adapt those laws from the 1970s that worked so well for four decades to solve the conservation problems of today.