Mangroves and Seagrass

Prop roots dangling from a red mangrove provide support for the tree and a habitat to marine organisms.

Coral reefs are indisputably fascinating, and for many divers exploring the underwater world means heading offshore. But on the ride out to the reef you may be passing through (and over) some astounding marine habitat. Mangrove forests and seagrass meadows make for interesting snorkeling in their own right, and they're essential for healthy coral reefs.

One of the best parts about snorkeling among mangroves and seagrasses is how peaceful these inshore habitats can be. Many inhabitants of these environments are masters of camouflage. It's easy to enjoy a dive on a coral reef without having to look very hard, but if you want to spot that seahorse with its tail wrapped around a blade of grass slowly swaying back and forth in the current or the flatworm sluggishly making its way along a mangrove root to get to its tunicate snack, you'll need to be still and look closely. Just float on your belly and stare. Study single blades of grass, and get up close to the mangrove roots — their beauty can be quite surprising. Remember the smallest of critters can be among the most beautiful or the most interesting to observe. But one day you might turn around to see a big manatee staring you down.
What’s So Special About a Bunch of Grass?
Seagrasses dominate coastal marine waters. They are found around the globe — from polar regions to the tropics — on all continents except Antarctica. This domination is in part due to seagrasses' ability to reproduce asexually through vegetative growth as well as sexually. Seagrass blades grow from a horizontal stem called a rhizome that is buried under the sand or mud. As the rhizome grows, shoots emerge vertically. This is how seagrass plants form small patches that develop into large, continuous meadows.

Careless boating through shallow seagrass beds creates obvious prop
scars that can take years to heal.

Most seagrasses reproduce by pollination while submerged and complete their entire life cycle underwater. Seagrasses produce flowers, fruits and seeds in a salty underwater world; pollination, therefore, is quite different than in terrestrial plants. Turtle grass (Thalassia testudinum), for example, produces buoyant fruits that float on the surface before opening to release seedlings that sink to the bottom. The seedlings have sticky root hairs that help anchor the seedling to the sediment.

Sarah Egner conducts a benthic survey of a seagrass bed.

Seagrasses are not true grasses; they're actually close relatives of lilies. Their name is just a product of the resemblance between their leaves and terrestrial grasses. Seagrasses are true, photosynthesizing vascular plants: angiosperms, specifically — flowering plants. The flowers, usually small and white, can be seen near the base of the leaf clusters. See if you can spot one the next time you're enjoying a seagrass snorkel.
Aren’t Mangroves Just Mosquito-Infested Swamps?
OK, so seagrasses aren't grasses, and they have achieved coastal water domination. But what about mangroves? Although mosquitoes do live among mangroves, these trees are much more than a home for insects. Mangroves, which are woody plants, are much pickier about where they take root than seagrasses are. Mangroves grow in tropical and subtropical latitudes along the land-sea interface in bays, estuaries, lagoons, backwaters and tidal rivers. About two-thirds of the coastline in the tropical areas of the world is lined by mangroves.

Roseate spoonbills feed in the flats by swishing their spoon-shaped bills back and forth in the water to find small invertebrates, fish and crustaceans.

Mangroves are most recognizable by their dense tangle of prop roots that make the trees appear to be standing on stilts above the water. The aerial roots allow the trees to handle the daily rise and fall of tides; most mangroves get flooded at least twice a day. Mangroves do not require saltwater to live, but they are unique in their tolerance of saline environments. Like seagrasses, mangroves make use of a unique set of adaptations to do so.

The recognizable aerial roots not only give the trees stability but also allow the mangroves to use atmospheric oxygen, which is important because the majority of sediment in which mangroves live is anaerobic. If you look closely at the roots you'll notice little bumps, which are called lenticels. It is through these that gas exchange occurs.

Other adaptations to the saline environment include physiological mechanisms that allow the tree to excrete salt. Some of the leaves become covered in visible salt crystals as the tree expels the salt; if you can't see the salt, you may be able to taste it (try it!). There are also reproductive adaptations. Most mangroves are viviparous, which means they give birth to live young. The young trees germinate on the parent tree — in the form of propagules — rather than germinating externally from a seed. Tidal fluctuations distribute these buoyant propagules once they fall into the water.
Connected to Coral Reefs
The brilliant blue water you see on the reef is in part due to mangroves and seagrasses. These inshore habitats improve water quality at the reef. Corals depend on clear water that's low in nutrients. The source of most nutrients and sediment is land; when rivers and streams empty into the ocean, the water tends to be high in nutrients such as nitrogen and phosphorous and is generally very turbid, which means it contains suspended sediment.

Sediment in the water column reduces light penetration and can settle, smothering coral colonies and impairing coral feeding. Seagrass leaves and mangrove roots slow the flow of water, allowing suspended material to settle on the bottom. Consequently, the water that flows through these habitats out to the reef has low turbidity.

Some larger fish, such as the southern stingray, can be seen visiting seagrass beds to forage.

Generally, the water over a coral reef is very low in dissolved nitrogen and phosphorous. When nutrient levels are too high on a reef, faster-growing phytoplankton and seaweeds can outcompete and/or smother corals and eliminate settling habitat for coral larvae. Seagrasses and mangroves are nutrient sinks, buffering nutrient and chemical inputs to the marine environment. Microorganisms that live among the seagrasses and mangroves recycle these nutrients back into the marine ecosystem.

Many of the animals you see while diving on coral reefs spent the initial stages of their lives hiding among blades of seagrass and mangrove roots. These coastal habitats act as nurseries, offering food and safe haven from the predators of the reef. Juvenile mullets, perch, whiting, tailors, bream, snappers and emperors all live among seagrass beds and mangroves. Once these fish grow larger and stronger, they make their way farther offshore.
Vital Ecosystems

A cigar-sized juvenile barracuda enjoys the protection of the mangrove forest.
Coral reefs are not the only beneficiaries of these coastal habitats. Seagrasses and mangroves provide extensive advantages to the overall tropical marine ecosystem. Everything needs to eat, and there are plenty of herbivores that use seagrass blades and mangrove leaves as food. These include vertebrates and invertebrates alike; even residents of the coral reef use these inshore habitats as feeding grounds. While mangrove leaves are on the tree they are food for insects, but when the leaves fall into the water many marine herbivores such as mollusks and crabs feed on them.

These herbivores are the primary-level consumers, which in turn support an array of secondary consumers, including small fish and juveniles of predators such as barracudas. When those juveniles mature, they become third-level consumers. Larger predatory animals such as herons, cormorants, sharks, barramundi, salmon and crocodiles are also attracted to the seagrass meadows by the schools of fish that seek shelter there.

Even decaying mangrove leaves and seagrasses are important to the entire ecosystem as they provide a base for the detrital food web. Bacteria, fungi and other microorganisms colonize and decompose the fallen plant material, creating detritus, a protein-rich food source used by worms, microscopic organisms, nematodes and crustaceans. Small fish eat these detritivores (as well as detritus), and these fish become prey for larger fish, birds and other animals.

Blue-eyed scallops may appear sessile, but they can move using jet propulsion when threatened.

Nearshore reefs protect coastline from wind and waves and help prevent erosion. Likewise, mangrove forests stabilize coastlines, limiting erosion from storm surge, currents, waves and tides. Seagrasses are also important in stabilizing shorelines with their extensive root and rhizome systems.

As mangroves stabilize sediment, islands may eventually form. A mangrove island becomes a habitat for a set of plants and animals entirely different from those that would otherwise inhabit the area. Many wading birds — such as wood storks, herons, egrets, ibises, roseate spoonbills, cormorants and pelicans — nest and roost in mangroves. Mangrove islands are also home to a wide variety of reptile, amphibian and mammal species. The Sunderban mangroves off India and Bangladesh — the largest mangrove forest on Earth — are home to Bengal tigers, spotted deer, saltwater crocodiles, fishing cats and various dolphin species.

Healthy coastal habitats are also important in mitigating global climate change. Mangroves and seagrass beds absorb large quantities of carbon dioxide — a greenhouse gas — from the atmosphere and store it. These habitats, known as carbon sinks, contain large stores of carbon accumulated over hundreds of thousands of years. When these environments are damaged or destroyed, not only is their carbon sequestration capacity lost, but also stored carbon is released, which contributes to increasing levels of greenhouse gases in the atmosphere.
See for Yourself

Kayaking among the mangroves provides a peaceful opportunity for spotting birds, fish and manatees.
All these benefits sound nice, but why would I want to skip my reef dive for a snorkel in the seagrass or mangroves? Seagrass meadows and mangrove forests are amazing places to see all sorts of marine life, from tiny nudibranchs to massive manatees. Substrates provided by seagrasses and mangrove roots are important for sessile organisms such as sponges, tunicates, oysters and anemones. Even immobile organisms can be stunning; sponges growing on mangrove roots add beautiful colors to the habitat.

These environments are home to organisms you probably won't see on the reef. These critters include invertebrates such as horseshoe crabs, sea stars, nudibranchs, flatworms, sea slugs and brittle stars; fish such as tarpons and seahorses; and mammals such as dolphins and manatees. Some protected species such as the dugong (Dugong dugon) and the green sea turtle (Chelonia mydas) feed primarily on seagrass. An adult green sea turtle eats about 5 pounds of seagrass per day, while an adult dugong eats about 60 to 90 pounds per day.

Manatees breathe at the surface through nostrils that have valves that close when they are underwater.

Mangroves are natural wind buffers; use this to your advantage by choosing a snorkel site downwind of a mangrove forest. Visibility can be close to zero in rough seas on the windward side of a mangrove island, while the leeward side is perfectly clear and flat. Seagrass beds are often adjacent to mangrove islands, so you can see both habitats in a single excursion. The bottom can be quite silty and may be stirred up easily when it is windy, but it usually settles fairly quickly. You can almost always find a clear, flat site to snorkel, but don't expect the visibility and crystal blue water found on the reef. The tannins in the mangroves often dye the surrounding waters a reddish or brownish hue.
Take Care
Snorkeling among seagrasses and mangrove roots is quite different from snorkeling on a coral reef. But like reefs, mangroves and seagrasses are fragile environments that should be visited with care.

Don't stand up when snorkeling in mangroves or seagrass beds, and be careful where you kick. If you only have large dive fins, ditch them and snorkel without fins (you shouldn't need them; currents are minimal to nonexistent). Not only is the sediment easily stirred up, but also fragile and/or hazardous organisms such as jellyfish and stingrays live on or near the bottom, and stinging cells tend to settle in the sediment. Repeatedly standing on seagrass blades can kill the seagrass.

A great blue heron (white morph) is one of the many wading birds that depend on mangrove islands for refuge.

Avoid touching the inhabitants, for your benefit as well as that of the marine life. Immobile organisms protect themselves in subtle ways. Sponges, for example, look harmless but have toxins as well as physical defenses in the form of spicules.

Be aware that because mangroves and seagrasses act as filters, they unfortunately tend to collect human debris. If you decide to collect trash for disposal, look closely first. A bottle, jar or aluminum can may be home to a fish or an octopus.

In some topical and subtropical locations, cutting back mangroves is illegal, even if they are in your own backyard. When boating, be aware of the depth — not just for the sake of your prop but for the health of the seagrass as well.
Woven Together
The coral/mangrove/seagrass ecosystem can be thought of as a whole. Some species, such as the gray snapper, use all three during their life cycle. Each habitat is dependent on the others; if one is damaged, the others will be affected, too. To preserve one of these habitats, all three must be protected.

Manatees prefer to live inshore where there is plenty of seagrass to eat.

If you are the type of diver who sees more than just the sharks, turtles and rays when you are on a coral reef, you will really enjoy snorkeling among seagrasses and mangroves. Even if you don't enjoy these habitats first hand, you'll benefit from them indirectly the next time you have an amazing dive on the reef. Seagrass, mangrove and coral-reef habitats are interconnected. If we want to preserve our coral reefs, we must appreciate and protect these inshore habitats.
Kruczynski WL and Fletcher PJ (eds.). Tropical Connections: South Florida's marine environment. (Cambridge, Md.: IAN Press, University of Maryland Center for Environmental Science, 2012).

McKenzie L. Seagrass Educators Handbook. (Cairns, Australia: Seagrass-Watch HQ, February 2008). Available at:
About the Author
Sarah Egner is director of research and development at the Marine Resources Development Foundation.

© Alert Diver — Spring 2014