>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.
>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.
>What’s So Special About a Bunch of Grass?
>scars that can take years to heal.
>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.
>Aren’t Mangroves Just Mosquito-Infested Swamps?
>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.
>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.
>Connected to Coral Reefs
>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.
>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.
>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.
>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.
>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.
>Kayaking among the mangroves provides a peaceful opportunity for spotting birds, fish and manatees.
>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.
>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.
>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.
>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.
>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: Seagrasswatch.org/education.
>Sarah Egner is director of research and development at the Marine Resources Development Foundation.
>About the Author
>© Alert Diver — Spring 2014