DEFORESTATION OF THE AMAZON basin has garnered attention as concerns rise about climate change. The carbon dioxide the tropical rainforest ecosystem stores has earned its nickname as the lungs of the planet. Collectively, the carbon absorbed and stored through photosynthesis by all types of terrestrial forests is referred to as green carbon. This has led to calls to plant trees to mitigate greenhouse gases. However, there are three ecosystems that store as much or more carbon by the acre than any forest on land. Yet they have received less attention in the fight against climate change. All three of these ecosystems occur in Florida: mangrove forests, seagrass meadows and salt marshes. “The reason people can walk into the Amazon Forest and appreciate that it’s carbon-dense is there are trees that are 80 to 100 feet tall and five feet around that are just made out of carbon — there’s the carbon — it’s above my head,” said James Fourqurean, Ph.D., the associate director of the Institute of Environment at Florida International University, where he heads the Fourqurean Laboratory for Seagrass Ecosystems Research. He studies the carbon dynamics of coastal ecosystems and is a principal investigator for the National Science Foundation’s Florida Coastal Everglades Long- Term Ecological Research (LTER) site, and a lead scientist in the International Blue Carbon Working Group.
“But in a tropical rainforest, there’s not a lot of carbon stored in the soil,” Mr. Fourqurean continued. “On the other hand, mangroves are places where there’s a forest, so you can see and appreciate that carbon, but the soil underneath them has much more carbon than is in the trees because there’s no oxygen in that waterlogged soil. And seagrasses don’t have nearly the amount of carbon in (living) biomass — they don’t have trees sticking up — but they have a tremendous amount of carbon in those waterlogged soils that they sit on top of. So, seagrasses are small compared to trees, but the Amazon Forest soils contain very low carbon compared to seagrass soils.”
Some 83% of the global carbon cycle circulates through the world’s oceans. This carbon dioxide absorbed and stored by the oceans is referred to as blue carbon, and 50% of it is stored in coastal regions in these three ecosystems that cover only 2% of total oceanic area. Mangroves occur in southern Florida, salt marshes in northern Florida and seagrasses in both regions. These ecosystems are better at sequestering carbon than the terrestrial forests that have become the focus of climate change activities. This is because microbes that would otherwise break down the carbon stashed below their roots are slowed by the anaerobic environment of the waterlogged muck soil. Likewise, when these marine ecosystems become degraded and the plants die, the stored blue carbon that was beneath their roots is released into the atmosphere.
“Where most of the carbon gets stored is below the root mass, in soils we call peats,” said Michael Savarese, Ph.D., a professor in the Department of Marine & Earth Sciences at Florida Gulf Coast University’s Water School. He studies the history of environmental change in coastal settings, particularly in response to human development, climate change and sea-level rise.
“It’s the peats that sequester the carbon, and if they remain unoxidized — if the carbon isn’t released into the atmosphere — that carbon gets buried,” Mr. Savarese continued. “Virtually all the coal that society around the planet uses is essentially burning old peats. The carbon stays there because, as they get buried over time, they’re subjected to a very oxygen-poor environment, so there isn’t a lot of bacterial degradation to release the carbon as carbon dioxide. If you can create and then maintain the peats in intertidal wetlands like mangrove forests that remain productive and continue to fix carbon within the peats and then bury those peats without re-oxidizing them, then that becomes a carbon sink, and all is good.”
While seagrass meadows cover less than 0.2% of the oceans’ floors globally, they store about 10% of the carbon buried in the oceans each year, with 95% of their carbon stored into the soils they hold in place. The black muck soils beneath the coastal ecosystems are classified as peat soils, which contain over 40% carbon. The mangroves on the western end of Florida Bay sit atop 10 to 12 feet of peaty muck soil that the forest has accumulated over the last 5,000 years. Elsewhere in Florida, three-foot-thick muck is estimated to be 1,500 years old. By comparison, scientists estimate the carbon stored in rainforests has only been there for centuries, not millennia. Researchers estimate that the carbon held in the living trees plus first meter (3 feet) of soil in one hectare (about 2.5 acres) of mangrove forest is the equivalent of the carbon in three to five hectares of tropical rainforest. One hectare of a seagrass meadow’s soil is estimated to hold about the same amount of carbon as the living biomass in one hectare of rainforest, and seagrass meadows are the most common of the three coastal blue carbon ecosystems because they cover more acreage and grow in more parts of the world than either mangrove forests or salt marshes.
“The rate per year at which seagrasses and mangroves, and even forests, fix new carbon and store it out of the atmosphere is relatively modest,” Dr. Fourqurean said. “But they’ve been collecting it into the soil for thousands of years, so it’s like contributing to your IRA, where you put a little bit of money in for your entire working life. It’s a very slow input, but it grows and accumulates. When we destroy the ecosystems and allow oxygen to get to the soils, then all that carbon accumulated over 5,000 years can go to the atmosphere in a few years. Now you’re talking about hundreds of tons per year — per hectare — going to the atmosphere in a short amount of time.”
One study found that 50% of the carbon stored in the sediment below a mangrove forest that was cut in Panama returned to the atmosphere in a mere eight years. When a mangrove forest or a seagrass meadow dies, the carbon below starts releasing because of a combination of factors. First, there’s no longer plant cover to protect the sediments against agitation by wave action. The agitation mixes the soils into the water, where they’re exposed to significantly more oxygen than when they were buried on the bottom.
“The thing about wet dirt, as compared to soil on land, is that in a wetland or a seagrass bed the soil is completely waterlogged,” said James Douglass, Ph.D., associate professor of marine science in the Water School at FGCU. He studies living organisms in benthic (sea bottom) habitats, such as seagrass, and researches how ecosystems respond to man-made environmental stressors.
“Oxygen doesn’t penetrate far into waterlogged soil because in typical soil the oxygen diffuses through the gaps in the soil,” Mr. Douglass continued. “But when those gaps are filled with water, only a very thin layer on the surface of the sediment has any oxygen. Below that, it’s the sort of black mud where decomposition is slow because there’s no oxygen.”
Aerobic microbes that break down carbon-containing materials flourish in the oxygenated waters. Additionally, the lack of plant cover causes the water to become warmer. Since microbes love warmth, this further exacerbates the breakdown and release of carbon dioxide into the atmosphere.
“The respiratory rates of the soils accelerate because of the temperature of the water that is on them and the temperature of the soils themselves,” said Brian Bovard, Ph.D., program coordinator and assistant professor in the Department of Ecology & Environmental Studies at FGCU’s Water School. He studies the role forest ecosystems play in carbon storage, including in mangrove forests.
“This increases the microbial decomposition of the carbon compounds that are there, so that will have consequences associated with it,” Mr. Bovard said. “The mangrove trees are keeping the soils cool because they’re blocking the radiation load on those systems, so if you cut them down, it heats up.”
Beyond the long-term climate implications of quickly releasing carbon that’s been sequestered for millennia, the degradation of coastal ecosystems shows up with immediate impacts of water quality diminished by turbidity as well as increased algae growth. These impacts affect Florida’s economy from tourism to fishing to real estate values.
“In addition to carbon, seagrasses sequester sediments and nutrients,” Mr. Douglass said. “The silts and mud get stirred back up into the water when they’re no longer held down by the seagrass, and it creates a chocolate milk-looking plume of gross water because even the slightest wave, once the seagrass is gone, can stir up hundreds of years of silt from the bottom. The other unfortunate thing that happens is, all the nutrients that are stored in the soil beneath the seagrass bed are exposed, such as the nitrogen and phosphorus the seagrass absorbed while it was alive and released into the water where they can cause harmful algae blooms.”
Scientists estimate that seagrass meadows have already been lost in 30% of their historical range worldwide, and loss continues at 1.5% annually. Seagrass suffers from dredging, prop scars and improper anchoring, but degraded water quality poses an even greater threat because it turns into a vicious cycle for seagrass meadows. Despite their name, the plants aren’t part of the grass family; they’re flowering plants more closely related to the lily family. As flowering plants, they require a great deal of light, which means they need shallow, clear water for the sunlight to reach them. Sediment runoff from development activity on land clouds the water, as does algae overgrowth fed by nutrient runoff from fertilizer and septic sources. As seagrass beds die, it leaves the silt, nutrients, and carbon in the soil beneath them free and vulnerable to wave action.
“It can become a runaway feedback effect because the resuspended silt then darkens the water further, which leads to more seagrass death,” Mr. Douglass said. “That leads to more exposed sediments, which leads to more resuspension. And you get this ugly scenario where the seagrass beds can rapidly deteriorate — and our water quality, too — because of the sediment resuspension.”
Florida is seeing the impacts of seagrass die-off happening right now and right in Florida Weekly’s publishing areas. As recently as a decade ago, seagrass meadows covered as much as 77,000 acres of the Indian River Lagoon’s bottom. Then a series of harmful phytoplankton blooms that made the water look like a chocolate milkshake shaded out the seagrass, killing 50,000 acres by 2019. Excess nitrogen from sources such as wastewater and septic tanks fed the harmful blooms. Although some seagrass beds still grow in the lagoon, that doesn’t mean they’re growing well.
“In terms of overall coverage — the size of the beds and the density — we now see much sparser grass than we used to,” said Dennis Hanisak, Ph.D., research professor at Florida Atlantic University. He serves as the director of the Marine Ecosystem Health and the Indian River Lagoon Observatory programs at the university’s Harbor Branch Oceanographic Institute, where he studies the health of the Indian River Lagoon ecosystem.
“It’s more than just the total acreage,” Mr. Hanisak continued. “It’s also how dense the total biomass is. We used to have thick seagrass beds, but now they’re sparse — very little there compared to what it was — so, it’s a huge change. You lose the seagrasses, and that’s a hit on their ability to continue to absorb carbon dioxide to use it or sequester it. In the Indian River Lagoon, the loss of seagrasses is going the wrong way for blue carbon.”
The most attention-grabbing symptom of the Indian River Lagoon seagrass die-off has been the unusual numbers of manatees dying, specifically from starvation. Since manatees rely on seagrass as their primary food, to the tune of 150 pounds of seagrass per day for an adult manatee, some might wonder if the manatee population recovery under endangered species protection has caused the seagrass collapse. Since scientists conducted the first manatee census in the 1970s, the pre-settlement manatee population is unknown. However, Florida undoubtedly had a higher manatee population then, when the seagrass meadows were healthy.
“In a healthy ecosystem, the manatees graze on the seagrass and move on, and the plants keep producing new shoots,” Mr. Hanisak said. “They don’t destroy the whole bed. Manatees weren’t responsible for the big die-off. The phytoplankton blooms were so severe that they kept the light very low. Seagrasses have a high light requirement, so that’s what triggered the die-off.”
Internationally, many mangrove forests grow on the shorelines of developing countries. Scientists estimate that mangroves have already been lost in up to 67% of their historical range worldwide, and deforestation of the remaining mangroves continues at 2% annually. At the current pace, nearly all unprotected mangroves could be cut in the next 100 years. A major reason developers cut mangrove forests in developing countries is to build shrimp farms. The aquaculture operations only use the ponds for a few years before they become too contaminated to raise shrimp, so then they clear another swath of mangroves for new ponds. To help protect mangroves, it matters to read the label on shrimp at the store to learn if they are American-origin wild-caught shrimp or imported farmed shrimp.
“With the appetite Americans have for shrimp, and the fact that most of those shrimp were farmed in areas where mangroves were removed to build shrimp farms, it’s our decisions as consumers that are causing mangrove deforestation in Central America and Southeast Asia,” Mr. Bovard said. “I think people in the United States don’t know that the decision they make when they purchase shrimp is important because of their impact on mangroves in other parts of the world. When you buy shrimp at the grocery, really think about where it’s coming from because your decision has consequences on other places.”
Florida has laws to protect its mangroves. However, it’s a myth that no mangrove in the state can ever be trimmed or removed. The Florida Department of Environmental Protection provides a list of rules about how much, and under what conditions, mangroves may be trimmed to accommodate property owners’ use of their properties. Generally, the rules allow for trimming if the stand of mangroves is less than 50 feet wide, and the trees cannot be cut shorter than 6 feet. The state provides some provisions for alteration permits if a mangrove has grown to block the useability of a boat dock. Despite the laws, fines and enforcement may be too weak to deter violations. Typically, the charges filed are civil, not criminal, and the fines could simply represent an affordable nuisance tax to people well-off enough to afford waterfront homes.
“Here in Florida, we shouldn’t be losing mangroves very rapidly because we have measures in place to protect them,” Mr. Bovard said. “Yet it seems like every year or two we see in the news where somebody’s gotten a slap on the wrist for cutting down mangroves so that they can see the water again.”
Developers can obtain permits to remove mangrove trees entirely, as has happened recently in Lee County where a developer is building a subdivision of homes near Bunche Beach. To obtain the permit to clear-cut, the developer purchased credits in a wetlands mitigation bank on Little Pine Island, about 10 miles away from the development.
“We have what is referred to as a ‘no net loss of wetlands’ systems policy in the United States,” Mr. Bovard said. “That policy is short-sighted. The idea is you’re going to cut down a wetland system in one place, and then you either restore or create wetlands somewhere else to offset those impacts. The problem is that, created and restored wetlands oftentimes take many years to get back to a similar state that the cut wetland was operating at. There’s recovery time for the functions and services that we benefit from. If you cut those mangroves down, we’ve lost coastal protection in that location. So, there’s a negative impact there, and if you’re restoring mangroves somewhere else to offset it, you may not reap the benefits of those mangroves for 10 to 20 or maybe even 50 years.”
The problem is more than the fact that replanted baby trees or reseeded seagrass aren’t working at the same capacity as mature plants in a naturally occurring ecosystem. The no-net-loss policy does nothing to address the problem of the carbon-rich sediments exposed when the government-permitted developers destroy the mature ecosystems. Replacing old plants by replanting new plants somewhere else doesn’t hold down the carbon the old plants sequestered. Restoration work is also expensive; the most recent estimate is that it costs a quarter-million dollars per acre to restore seagrass beds.
“A recently published study analyzed the potential for seagrass restoration to sequester excess carbon to help with climate change, and they found that seagrass on a per-acre basis has a major positive effect on carbon storage,” Mr. Douglass said. “However, this big analysis found that, if you accounted for the success of all seagrass restoration projects worldwide, it was a pittance compared to the rates of seagrass lost every year. The conclusion was that trying to restore seagrass is slow and cost-inefficient compared to saving the seagrass beds we have because, when seagrass is destroyed, it releases its carbon into the atmosphere just like when a rainforest is destroyed. The existing seagrass beds are so much more effective at storing carbon and have already stored so much carbon that our priorities should be on protecting and saving what we’ve got.”
This past legislative session, a state bill that proposed creating seagrass mitigation banks, modeled after the wetlands mitigation banks system, was proposed but failed. However, such legislation doesn’t fix the water quality issues endangering seagrass. It also doesn’t address the problem of the carbon-rich soil that would be released when developers remove the seagrass.
“If seagrass could be growing somewhere, it’s already growing there,” Mr. Douglass said. “It’s not like any part of the oceans haven’t been reached by seagrass seeds. The problem is that there are very few places seagrass can grow. It needs to be near-shore — somewhere like an estuary or bay — and most of the things that we do that harm seagrass are happening because humans live on and around the shore, so it’s the near-shore waters we want to disrupt. If people are thinking to just plant (mitigation banks) away somewhere, like out in the Gulf, that won’t work because seagrass needs a ton of light and very shallow, clean water protected from waves to grow.”
We already knew that protecting mangroves and seagrass was important because they buffer against hurricane storm surges and provide habitat for the seafood and sport fishing we enjoy. Their role in blue carbon storage makes it even more critical to protect them.
“Probably the worst fossil fuel greenhouse gas producer is coal — particularly very dirty coal — and essentially dirty coals are old peats,” Mr. Savarese said. “One could argue that we’re our own worst enemy. Not only are we prohibiting new carbon sequestration by reducing the amount of peat soils that are being produced, but at the same time, we’re burning peat soils that are hundreds of millions of years old. We’re liberating the carbon dioxide from those old peats, so that just always strikes me as being a twisted irony for our current problems.” ¦
In the KNOW
Help protect seagrass
Avoid prop scarring seagrass beds: Use only a pole or electric trolling motor over seagrass beds, and don’t anchor in seagrass.
Don’t fertilize your lawn during rainy season.
Use mechanical methods to clear plants out of water instead of spraying herbicides.
Certain plants and fishes can be introduced into ponds to keep them clean instead of using chemicals.
Talk to your HOA about implementing these more environmentally friendly procedures to protect seagrass.
Check your septic tank for leaks.
Pick up your dog’s poop.
Contact legislators to oppose seagrass mitigation bank legislation.
Help protect mangroves
Don’t purchase imported farmed shrimp at the grocery.
Don’t improperly or illegally prune mangroves on your property.