To celebrate International Day for the Conservation of the Mangrove Ecosystem, we caught up with the Smithsonian’s Steven Canty to learn more about these vital coastal habitats

With the climate crisis reaching new levels of visibility and urgency this summer, ‘blue carbon’ ecosystems like mangroves, salt marshes, seagrass meadows and kelp forests offer humanity – and the oceans – a lifeline. Beyond their immense value to people as coastal storm barriers and to marine life as nurseries and larders, mangrove ecosystems and the diverse range of life within them offer vast carbon capture and storage potential. Mangroves have traditionally been lost to shrimp aquaculture farms, coastal development, road building and felling for firewood – with Vietnam, the Philippines and Thailand losing 50% or more of their mangroves between the 1970s and the early 2000s. With the world now waking up to their value, there are signs the rate of destruction is slowing.

Beyond protecting mangroves, we are also finding some success in restoring them. As a new report published today highlights, “the full return of ‘highly restorable’ areas could restore or stabilize carbon equivalent to over 1.3 gigatons of C02 into the atmosphere – equivalent to over three years of emissions for a country such as Australia, or the avoided burning of three billion barrels of oil. The halting of ongoing losses will similarly produce massive benefits in terms of emissions avoided.” With the protection, preservation and restoration of these vital coastal ecosystems high on the agenda at Parley and globally, we sat down with Steven Canty of the Smithsonian’s Marine Station and Working Land and Seascapes team to learn more.

“When mangroves work in tandem or in triplicate with sea grasses and coral reefs, the productivity of those systems is off the charts.”

– Steven Canty, Smithsonian Institution

Image by Valentina Cucchiara for Parley Mexico.

Q & A

So what defines a ‘mangrove’? Is it the species? Is it the place they grow?

Mangroves are a group of species – there's about 76 species in the world, depending on who you ask, maybe up to 85. They're not that well-related at all – but they’re essentially all plants that have decided to go back to the sea, which for plants is a pretty horrible place. You get drowned for half the day, then you get baked for the other half – you get hit by wind, you get hit by waves, you're terrestrial, but you're marine. It's an awful place to be, but there was an evolutionary opening and these plants came back.

How did mangroves adapt?

There are 26 different families of mangroves, and they all have very specialized root systems. What you've got to be able to do is support yourself in really sloppy, unconsolidated sediments. It's a high energy environment, because there’s nothing stable there. So these root systems bind the sediment together – and they’re also insanely efficient at reverse osmosis, so they can exclude salt from ever entering the plant. So that’s what allows them to fill this niche. Mangroves are never found anywhere else, other than this ecosystem area of the intertidal zone. If you find them outside of that, they're a ‘mangrove associate’ species. The other thing about these guys is a single species of mangrove can form an entire forest – that's what can make them a little bit boring sometimes, as you're walking through the system you might think it's the same tree, it's the same tree, it's the same tree. But the biodiversity that they support is massive – in fact it's unbelievable, because they support both marine and terrestrial species.

How productive are mangroves?

The productivity of these ecosystems is insane. When mangroves work in tandem or in triplicate with sea grasses and coral reefs, the productivity of those systems is off the charts compared to other places – we're seeing double to triple the amount of fish biomass on the reefs when they're associated with healthy mangroves and healthy sea grasses. People call each part an ‘ecosystem’ but now we're trying to say all three are one ecosystem because managing them as a whole is really important. Coral reefs have always been the focus because they have more biodiversity – they're beautiful, people dive in them, they don't smell bad. Whereas mangroves, walking through one species for hours, people get bored. It can smell like rotting eggs, which is a good thing because that's the sulfur, so they’re harder to love – but equally important.

What do mangroves bring to the party?

One of the biggest things, and the easiest to see, is the fisheries. You'll see the juvenile fish are in amongst the mangrove roots and they're either in there for food or they're in there for shelter. Some are in there for both, but you might see during the day, lots of baby fish in there whilst they're hiding from all the predators out on the reefs. During the night, they'll move down to the seagrass and reefs, forage and feed and then swim back before it gets light. So there's this movement through across these three systems. The other contribution is that they can trap a lot of the sediments that come down from rivers and streams. Coral reefs need clear waters, generally, to be highly productive. They also need not to have too many nutrients come down onto them. So mangroves are an important buffer for all of this land pollution or land runoff, whether it's natural or anthropogenic. They ensure the waters are nice and clear and low in nutrients, that way, reefs stay healthy.

Images by Naiara Altuna for Parley Mexico.

It’s pretty easy to visualize mangroves as massive filters for runoff and as barriers for storms, but what about carbon capture? Why are they so good?

When mangrove roots trap and bind of all that sediment, carbon gets accumulated in the soils – and that is where mangroves make their money. So, as trees, they are about the same as any other tree, give or take whatever scientific standard deviations you want to. But it's in the sediments where they can store up to 20 times more than any other terrestrial plant system with the exception of salt marshes – salt marshes, and wetlands in general, are amazing at carbon capture and storage. Mangroves and wetlands create anaerobic conditions in the soil – it's waterlogged, which stops a lot of the normal respiration that you would have of breaking these carbons down for use by other bacteria. So the carbon gets trapped and compacted down. In some places I’ve studied in Belize and Panama and Grand Cayman, some of these deposits go down more than 10 meters. That is thousands of years of carbon that is locked away forever if we do things right.

How can we keep it locked away?

It means protecting not just the plants, but what we call the ‘infaunal benthic community’ – so basically anything living within those soils. This is where animals like crabs come in, because they're basically soil engineers. So they're burrowing and moving the soil around and that's what allows other species, such as worms and insects that move through and get air, because they've put that air into the system. Once you start to lose these ‘bioturbinators’ it becomes a sloppy mess again. It can still bring in and store this carbon, but the soils are loose. So any slight disturbance means that it gets washed away. What you need is these engineers that are compacting it within the root system – and that's the healthy balance.

Once mangroves are gone, how hard is it to replace them? If you replant fairly quickly, can you recover the complex ecosystem you’re describing?

If it's a healthy system and it's had a natural disaster, or even human intervention, and it's left to its own devices, it can come back – it will get recolonized by propagules. So the seeds come in and it can be weeks or even days when you start to see these things settling back. But the time it takes for it to become a true mangrove stand or mangrove forest again – with all of the complexity – will probably be 20 to 30 years. Now, if something horrendous happens to the system and it's majorly unhealthy, that's when you may need to look to restoration, which should take two forms.

Firstly, you should look to see what's caused that problem. One major problem is putting a road through a mangrove and blocking off water flow. They need to be flushed with fresh seawater, so if you block that flow you get this amazingly salty water around them, which means that they can no longer do reverse osmosis. Something as simple as road design that has culverts or holes through drainage systems can help – and we've seen that in Colombia and other places where they brought that back, the systems that repaired themselves more or less. In other places where it's shrimp ponds or other development, that’s had more of an effect on the sediment itself. That's when you need to basically go in and dig it up and turn it around – essentially doing the job of crabs on a large scale. That's a lot of work by people and it's sloppy, heavy soil. So it's very energy intensive and time intensive. But, by doing this and making sure the drainage and everything else is good and there's a healthy mangrove stand nearby, it will start to recolonize itself. Secondly, the direct replanting of propagules or saplings is required when natural recolonization is not occurring, this could be because there are no healthy mangrove populations nearby to assist in natural regeneration.