They are an ancient species of flowering plants that grow submerged in all of the world’s oceans from the latitudes of northern Alaska to the tip of South America. Seagrasses evolved during the age of the dinosaurs, nearly a hundred million years ago, and are found today in bays and estuaries around the globe. In shallow tropical and subtropical waters these ‘prairies of the sea’ link the offshore coral reefs with coastal mangrove forests.
They provide food and shelter for many marine species that will one day inhabit the reefs. Seagrasses and mangroves stabilize coastal sediments and create buffers against storm surge and flooding. They remove carbon dioxide from the atmosphere, filter contaminants from coastal waters, and add value to local economies.
But seagrasses are in decline globally… and mangroves are being lost to coastal development. These areas are degrading right now, it’s not 50 years from now, or a hundred years from now, but it’s occurring right now. The rate at which we’re losing seagrass habitat globally could result in an ecosystem collapse from the bottom of the food chain all the way up.
Will restoring seagrass and mangrove communities help sustain the ecological and economic values of the ocean’s bays and estuaries? And how will rising sea levels impact their survival in the future? Major funding for this program was provided by The Batchelor Foundation, encouraging people to preserve and protect America’s underwater resources.
Over the last few decades, worldwide human populations have increased dramatically along coastlines. At the same time, seagrasses have declined at a rate of nearly 7% per year – a rate higher than the loss of coral reefs or tropical rain forests. Storm water runoff from coastal communities and human activities along the shoreline contribute to the decline.
Boat groundings and anchor damage add to the problem. Unlike the more recognizable icons of the seas, the value of seagrasses and mangroves can be easily overlooked. If you just casually read newspapers in places that science stories are published you often read about tropical forests, you often read about coral reefs and occasionally you’ll even read about mangroves, but you almost never read about seagrasses.
Seagrasses are the Rodney Dangerfield of coastal marine ecosystems. They really get no respect. But, researchers have discovered that seagrasses and mangroves deserve a great deal of respect for their many values in the ocean.
Seagrasses keep erosion down, they lock sediments into the bottom and they actually remove sediments from the water column. When that happens it maintains a clear water column so a lot of light reaches the bottom. If seagrasses were to disappear then all of a sudden they’re no longer holding these sediments in place, sediments get up in the water and the water becomes very, very turbid.
That’s really important when you’re thinking about areas that are heavily populated, where folks like to recreate where sediments can get stirred up, they root them down and keep them stable. And they provide some hurricane protection which, like mangroves do on a global scale, you’ve got seagrasses and mangroves that break wave action when storms come and that also provide nature’s way of stabilizing the shoreline. We live in an area of the world where we get these catastrophic storm events, hurricanes and tropical storms and where they hit the coast they can definitely create erosion problems.
And what we find is that areas that are protected, or buffered by mangrove forested wetlands tend to do much better. And then we lose a lot of coastline in areas where we removed mangroves and replaced them with things like seawalls. Still, some of the most important values of these resources are less obvious.
It’s been shown that about a hectare of seagrass, which is equivalent to a soccer field, can use the same amount of nitrogen that would come from treated sewage from 800 people in one year. And then if you’re looking at the carbon numbers, seagrass can fix the same amount of carbon per meter squared in a year that you would produce by driving your car 7500 miles. Just the amount of nitrogen they pick up is worth $19,000 dollars a year for that one little spot of seagrass.
And you start to look at everything else that's associated with them, your not talking a monetary amount for fisheries because you’ve got the shrimp industry down here in South Florida, you’ve got all of the sports fishing, as well as most of the fish from either the mangroves or the coral reefs having some sort of connection with seagrass. Seagrasses and mangroves provide the nursery habitat, the foundation of our ecosystem and without having a healthy nursery habitat where fishery industries can survive and our tourism industry thrives because we’ve got this industry that depends on having nice clear water and habitat where fish can grow both for sustainability and sustenance for people to eat and also for our tourism industry where people like to recreate. From the basic part of that food chain up through human consumption you’d have, really, an ecosystem collapse.
It certainly is a crisis. Seagrasses are very valuable for the ecosystem services they provide besides being just hot spots of biodiversity and beautiful places to visit. I’m fascinated by seagrass habitats.
On the surface you look at a seagrass habitat and you see a bunch of green grass essentially that doesn’t look too different from your lawn, for example. But seagrasses, or the habitats are incredibly complex, there are so many different organisms that live there, there are so many different bio-geochemical cycles and processes that are going on in there. If you put your face up close to a seagrass habitat you’ll be amazed at the number of different things that you’ll see on the small scale.
In the sub-tropical waters of Southeast Florida, neighboring metropolitan Miami, Biscayne Bay Aquatic Preserve manages nearly seventy thousand acres of submerged land. Nearly three-quarters of that is covered by seagrass. Mangrove forested islands and shorelines grow adjacent to these seagrass communities.
The most common variety, red mangroves, are found growing at the water’s edge; behind them, black mangroves; and farther upland, white mangroves. Historically, Biscayne Bay received freshwater from the Everglades, but in modern times the hydrology has been altered by drainage canals, channelized rivers, and changes in the flow of ground water into the bay. The diversity of species has also changed but it still remains an estuary where freshwater and saltwater meet.
One of the things that we do is form partnerships with local resource agencies, universities, researchers who are looking at what the resources are in Biscayne Bay so that we can get a better understanding of what’s there now, but also, how have things changed and how are they changing? Marine biologists are monitoring salinity, sediments and organisms that live on seagrasses found in Biscayne Bay with a long term goal of determining what effect a new freshwater canal flowing into the bay will have on the ecosystem. This project is directly related to the Everglades Restoration Project.
And what’s going to be happening is the freshwater regime of South Florida is going to be changing. And everybody that’s involved wants to try to get an idea of what and how these changes are going to be affecting what’s already here. Overall, if you have salinities changing on a much larger scale then you start getting into chemistry a lot more, how warmer waters are going to be acidifying with higher levels of CO2.
That’s going to be affecting how the sediments actually work, or don’t work. It starts to get pretty ugly pretty quick. We don’t know exactly what’s going to happen.
Photo-archival data of the seagrass beds is recorded with a specially designed boat and shallow water positioning system known as SWaPS. We’ve got a glass bottom boat so you can see the stuff that’s out there. We’ve got a digital camera attached to the glass bottom.
We can move that around as we need to. That runs directly through a computer, goes straight to a hard drive. We’ve got GPS so it codes every photo with exactly where it is.
Even if we’re doing a straight line, or just trying to sit in one spot, each one of these is tagged so precisely that we can follow it around because seagrass doesn’t grow all the time as just one big meadow like your yard, there’s going to be little patches of it. This gives us a nice snapshot of the community as it is right up against the mangroves and how that changes as it moves into a little bit deeper water. We’re going out and year after year taking photo quadrants, basically, and so that way in the future we can always come back if we need to check for any specific organisms or any specific changes that we may have missed on our preliminary analysis.
Seagrass and mangrove communities in Biscayne Bay nurture juvenile fish and marine creatures that feed and find shelter here. Some will make their way offshore to the coral reefs others will live out their entire life cycles in the bay. More than a hundred bottlenose dolphins reside in the bay stalking fish around the seagrass beds.
Seagrasses are a primary food source for manatees, but as opportunistic feeders they will consume low hanging mangrove leaves. Seagrasses in tropical and subtropical bays and estuaries around the world have a relationship with mangroves like prairies do to forests on land. A good example of this is along the mangrove fringe down in the southern part of the aquatic preserve near Chicken Key and the Deering Estate.
You’ve got an expansive mangrove forest right adjacent to the shoreline and then out from that for a few miles you’ve got dense seagrass beds. The prop roots of mangrove trees supply food and shelter to some of the same marine organisms and fish that are found in seagrass beds. Because many near shore and offshore species rely on mangroves as a nursery, when mangrove habitat is lost fisheries soon decline.
Human impacts on these ecosystems are a clear challenge for resource managers. In the 1960s, developers acquired a section of shoreline in the northern tributary of the bay and cleared the mangrove forest to dredge and build a marina. Years later, the site was acquired to form a part of the Oleta River State Park.
Today, it’s being restored to its natural condition as a mangrove wetland. This is a red mangrove propagule that has just started to put out some roots and some leaves, as you can see they are about 6 to sometimes 9 inches in length and they come off the tree as a very cigar-shaped, some people say it’s pencil-shaped, very long sort of propagule seed and they float in the system until they find some substrate, and the one end is a little bit heavier than the other end and it tends to take root and get in an upright position. And again you have to remember the water is going up and down as this is all happening.
Then once it starts to establish some root into the substrate it will start to grow and the mangroves grow about 2 feet a year. They differ from one location to the other, but that’s a pretty good average. The red mangroves will grow up to a hundred and fifty feet if they didn’t have impacts as far as storm impacts.
And in this area we see them eighty to a hundred feet tall. Unfortunately, we have our frequency of hurricanes here is great and as a result our mangroves are not as tall as some of the ones in Southeast Asia where they have a lower frequency of storm events. These channel ways feeding into the site were designed minus three feet.
It’s really been exciting in our restoration plans, we not only just have a plane of mangroves but we also have these tidal creeks and channels and tidal pools. And it’s amazing to see the life, the abundance of life that comes in and it succeeds through time. We’ve seen fisheries that have come into areas and just the species richness, or the number of different types of fish or crabs or shrimp have just gone right off the graph as far as the number of individuals and the diversity, the different types that come into an area once it gets established.
My biggest thrill about all of this is pretty much at a stage that we’re at now with this project here where we’re pretty much concluding the construction, we’re starting to plant the site with volunteers, and that’s another big part of this effort is to try to get the community in to obtain a sense of ownership, stewardship with the project, and families come out and can plant these mangroves, and come back and see the abundance of life and what they’ve done and feel a part of the overall restoration. But balancing resource management and restoration with recreational use is a steady challenge particularly for specialists in Biscayne National Park located at the southern end of the bay. It’s a shallow water environment with submerged resources like seagrasses and coral reefs - a popular recreational area, but one that’s frequently impacted by boat groundings and propeller scarring.
We’re trying to put the sediment back in the holes, or the scars themselves. Seagrasses cannot grow well into the water column itself, they need sediment to grow in, so until we replace that, those injuries take years and years and years to recover, if at all. We’re putting in sediment that’s very fine and it can cause a very large turbidity bloom in the water which is not a good thing.
We’re actually required through state water quality standards to control that turbidity very carefully. So we will put up the yellow construction boom around the site that we’re going to be working at and make sure that there is really very little option for leaking. Then we’ll bring the sediment in on barges.
Sometimes we used containerized sediment, for example in a burlap bag. Sometimes we put the sediment that’s loose. We use a crane, some kind of crane to actually put it in the hole.
We have divers in the water feeling out the surface with their feet to figure out where we need to put more so we can place the sediment exactly where it needs to be. And the other thing the divers are doing is collecting water samples for turbidity monitoring. In the final phase of this restoration healthy seagrasses will be transplanted to the site – a method having great success in the Channel Islands National Park off the coast of southern California.
Alright, I’m in. The rich seagrass habitats of the Channel Islands sustain an abundance of marine life - offering food for some and shelter for others. Some of its species are found nowhere else in the world.
Here, resource managers are harvesting healthy seagrasses to transplant in damaged areas. This five-year old restoration project has expanded the seagrass beds up to a mile away adding new habitat for a diversity of species. But, it takes an informed public to share the responsibility for sustaining these ecosystems… And resource managers want to engage people as well as local governments in promoting the best ways to enjoy the bays and estuaries while protecting the resources.
On behalf of Miami-Dade County and this community do hereby proclaim March 2010 as Seagrass Awareness Month. Very good. But will human intervention be enough and in time to make a difference?
One of the things that is at the forefront of everyone’s mind is, we’re managing these areas but what does that mean in twenty five years, or fifty or a hundred years, we’ll be, this will be underwater, this island that we’re sitting on right now. So it’s a quandary. In Southwest Florida’s Ten Thousand Islands, bottle-nose dolphins corral their prey in one of the most productive mangrove estuaries in the world.
From the air, the islands look much the same as they have for thousands of years. But today they are ground zero for researchers trying to solve one of the great scientific problems of the future – predicting sea level rise. At Rookery Bay National Estuarine Research Reserve scientists are trying to determine how changing seas will affect the Ten Thousand Islands’ mangrove forests and low lying coastline.
We know without a doubt that sea level rise is increasing and it’s actually an accelerated increase. That puts communities in Southwest Florida at risk, not just here in Southwest Florida, but I would say particularly so here because people live on Marco Island and Naples and the surrounding areas are living in communities in these built landscapes in a very low lying elevation. A lot of the mangroves we have around Rookery Bay are actually growing on living oyster reef, and if the water quality conditions are good, and in most cases in Rookery Bay that is the case, as the reef slowly expands, then we can also see an expansion of mangroves that occurs at the same time.
Oysters have a tendency to grow up at a certain rate off of the sea floor and if those oyster reefs are growing faster than sea level is going up the oyster reefs become emergent, they come out of the water and then they become areas for recruitment for mangroves. So what was an oyster reef becomes a mangrove forested island and those mangrove forested islands have snaking like shapes, dendritic kinds of patterns, and it’s those islands that give the Ten Thousand Islands their name. Now that sea level rise has accelerated, that rate of sea level rise is faster than oysters can grow, so that balance will shift and has already shifted and it’s quite possible, I would venture to say likely, that the Ten Thousand Islands will disappear given the current rate of sea level rise, which will completely change the configuration of the coast and completely change the estuarine ecology of the coast, because those Ten Thousand Islands also serve as entrapments for fresh water that create the brackish water that serves as the nurseries, serves as the place for seagrass development, serves as a place for other oyster reef development, that’s all likely to change.
Researchers are studying sediment samples from the Ten Thousand Islands to determine changes over time in the mangrove communities as a consequence of sea level rise. These sediments indicate a shoreward retreat of Southwest Florida’s mangrove coastline. They’re also finding evidence of salt water intrusion in the freshwater marshes of the Ten Thousand Islands National Wildlife Refuge.
Arrays of circular sub-tidal ponds have been photographed from the air. Patches of mangroves are taking root where only freshwater plants once grew – another sign of sea level rise. Since humans have become industrialized, since humans have been recording history in history books, we’ve been around during times of modest sea level rise rates.
And we have barrier islands and bays, and Ten Thousand Islands, and Biscayne Bays and Florida Bays. Those are all there because a modest rate of sea level rise has allowed those features to develop. Now, again we see that in the sediments over the last 5,000 years.
Now flash forward to industrialized world and climate change and enrichment of CO2 in the atmosphere, there’s clearly a record in those sediments that sea level is accelerating. Scientists project the retreat of Southwest Florida’s coastline if sea level should rise from its current level one meter at a time up to an additional nine meters. Projections for much, much faster sea rises in sea level over the next hundred, or two hundred years would suggest to us that the area we’re standing in now will soon be under water, so one may ask the question, why bother protecting the seagrasses here if the coast is going to migrate away from here?
One reason is because there always will be a coast and seagrasses will continue to respond and move shoreward as sea level goes up as long as humans don’t armor the coast to such a point that a wall is built around human cities and they continue to go up as sea level comes up. But, how fast will these changes occur? And will seagrasses and mangroves continue to yield their many ecological and economic values with the changing seas?
For today’s researchers, these questions are part of an experiment-in-progress with the answers waiting somewhere in the future. Major funding for this program was provided by The Batchelor Foundation, encouraging people to preserve and protect America’s underwater resources.
