If you dive in the tropics, you’re probably quite familiar with coral reefs. You know your hard corals from your soft corals and your parrotfish from your wrasse. Some of you may even know your pleurobranchs from your nudibranchs. But you may not know much about the beautiful stretches of seagrass that you sail over to reach those reefs. In reality, we can’t ignore the importance of seagrass meadows when it comes to ocean health, and we shouldn’t ignore these environments as divers.
Mostly inhabiting depths of 3 to 10 feet (1 to 3 m), seagrass meadows rarely top a diver’s list of must-visit destinations. But take some time to explore these unique marine environments and you may be pleasantly surprised. You may even want to swap your tank for a snorkel. From decorator crabs to manatees, seagrass meadows all over the world not only support thousands of mesmerizing marine species, but are also responsible for the crystal-clear waters of your favorite coral reefs. Just as on the reef though, be careful not to touch anything because one stray kick could instantly stir up decades of ocean deposits, completely ruining the visibility.
Seagrass vs. Seaweed
Peering down over the side of your dive boat, you could easily mistake that swaying green mass of seagrass for a patch of seaweed. But there are some essential differences between the two. For starters, seagrasses are plants and seaweeds are macroalgae. Next time you’re close enough to some seaweed, look for its roots and leaves. Technically, you won’t find them. Instead of true roots, most seaweeds have holdfasts that cling onto the seabed, just like a feather star. Instead of leaves they have blades. Seaweeds still get most of their energy from the sun through photosynthesis, but they don’t flower and they lack a specific internal structure to transport nutrients.
Seagrasses, on the other hand, are a bit more advanced. Much like terrestrial plants and grasses, they flower, photosynthesize and have roots that burrow into the seabed to find nutrients. These nutrients pass between plants via complex root structures and the plants transport them through veins. This allows seagrasses to cascade across seabeds as they grow, creating vast underwater meadows. Whether grass or weed, plant or algae, both can form ecosystems that support your favorite marine life.
A Little House on the Prairie
If you’re a macro-lover, you’ve probably already realized that all your favorite marine animals just want a little house on the prairie. Seagrass meadows are ideal habitats for seahorses, pipefish, shrimps, crabs, octopuses and cuttlefish, just to name a few. The long stretches of interlaced leaves provide shelter and safety from the open ocean’s bigger predators.
Aside from the permanent residents, seagrass beds can also provide numerous coral-reef species with protection for their young. Once hatched, many fish species start their lives navigating the tiny spaces of twisting mangrove roots. When these little fish start to outgrow their mangrove nurseries, they’ll venture out into the seagrass fields next door. They can still duck for cover inside the grass when a barracuda or a tiger shark swims by, but they can also start to explore more interesting prey and playmates. Seagrasses are the essential middle school for many species before they graduate onto the coral reef. Without these meadows, many juvenile snappers, parrotfish and jacks would have nowhere safe to grow up. The best coral-reef dive sites in the world are usually right next door to extensive seagrass fields and mangrove forests.
As well as supporting reef-fish populations, seagrasses also help protect coral reefs from the rising impacts of global climate change. When we release greenhouse gases, such as carbon dioxide (CO2) and methane (CH4), into the atmosphere, they insulate the planet. This contributes to rising sea temperatures and ocean acidification. Coral reefs are very sensitive, slow-growing ecosystems that depend on stable ocean environments to survive. Seagrass meadows help maintain ocean conditions by counteracting climate change in a few ways.
First, seagrass beds reduce the amount of CO2 in the atmosphere by photosynthesizing and turning CO2 and water (H2O) into oxygen (O2) and glucose (C6H12O6). Sometimes you can even see the oxygen bubbles escaping the seagrass. Second, they sequester carbon, removing it from the ocean by using it to build their leaves and roots. Finally, they trap the carbon from dead and decaying organisms in their complicated root systems, burying it in the seafloor. Plant systems like forests that do this on land are known as green-carbon reserves. The world’s seagrass meadows, therefore, are called blue-carbon reserves. Scientists have estimated that they capture 27.4 million tons of carbon each year.
From Turtles to Tiger Sharks
If you’re more interested in diving with turtles and tiger sharks than seahorses and shrimp, seagrass beds should still rate as one of your favorite marine habitats. There are 72 different species of seagrass, but you’ll probably be most interested in the aptly named manatee and turtle grasses. Grazers like dugongs and green turtles depend on these seagrasses to survive. The average adult dugong eats 64 to 88 pounds (28 to 40 kg) of seagrass every day. If grazers had their way, they would mow these grasses down into non-existence. Luckily, top predators like tiger sharks keep their grazing activities in check. When sharks sweep over shallow seagrass beds looking for prey, they temporarily force turtles and dugongs away from their meadows. This limits the amount of seagrass they can eat and keeps the meadow healthy.
The seagrass community needs a delicate balance to survive. If meadows are removed, thousands of species could be left starving, homeless, or both. If we remove too many sharks, turtle populations could rise until they eat entire meadows into extinction. These impacts won’t only affect seagrass ecosystems, but will also damage the coral reefs so intrinsically tied to their survival. Unfortunately, global seagrass coverage is decreasing by 1.5 percent each year. This equates to the destruction of roughly two football fields of seagrass every hour.
Large-scale threats to seagrass survival include global climate change, storms and mass removal for coastal development. The diving and snorkeling tourism industry can also pose a significant risk. Operators often ground or anchor boats in shallow seagrass areas for loading and unloading. Divers and snorkelers also often accessed sites via a long trudge from shore over this delicate habitat. This leaves large scars in the seagrass, removing essential habitat and food for many species. If this disturbance is repeated regularly, the meadow may never recover.
Do your part to protect these important ecosystems by choosing to dive and snorkel with sustainable operators that do their best to minimize their impacts on the marine environment. Choose dive centers that always use mooring buoys and plan their shore entries over sand where the water quickly becomes deep enough to descend. Choose dive guides who want to share their knowledge on local habitats and the marine species that live there.
If you’re looking for a green dive center, start by checking the Green Fins website. Over 400 dive and snorkel centers across Southeast Asia and the Indian Ocean have signed up to be Green Fins members, meaning that they have agreed to improve their environmental practices above and below the water by following the Green Fins code of conduct. Use the website to find active Green Fins members at your desired destination.
Drop them an email to let them know how you found them. Ask them about their environmental policies for anchoring and shore diving. Ask them if you can help with any of their conservation initiatives, such as seahorse monitoring or underwater clean-ups. And next time you’re on vacation, ask if there are any seagrass beds nearby that you can easily dive or snorkel, and start a new underwater adventure.
By Charlie Wiseman, Project Coordinator, The Reef-World Foundation