Tara and Scott SCUBA along the Wall at Runway Ten, 15 m deep, San Salvador, Bahamas.
The Wonders of Seagrass!
Seagrass is found in both tropical and temperate coastal waters around the world and is typically located near the shallow and sheltered coastal environments (Seddon, 167). In addition, seagrass is usually found in close physical proximity to mangroves and coral reefs. These three ecosystems have a special interconnected relationship and depend on each other to exist (Davidson, 64-67). One will not often encounter a single blade of seagrass, but rather extensive meadows or fields of the grass. In temperate waters the seagrass meadows are often mono-specific, but in tropical waters multiple species co-exist. For example, in the Philippines one seagrass bed contained 13 different species of seagrass. Seagrass is a general term used to describe flowering plants from four genera, Posidoniaceae, Zosteraceae, Hydrocharitaceae, and Cymodoceaceae that occur in saline environments (www.wikipedia.com). They are commonly referred to as seagrass because they superficially resemble terrestrial grasses. There are nine species that persist in the Caribbean Sea, which is more than colder environments like the North Atlantic, but still fewer species than found in regions throughout the Pacific Ocean.
One of the most common species found in the Caribbean Sea is Thalassia testudinum, or turtle grass and the other two species found in abundance are manatee grass and shoal grass. Thalassia testudinum is referred to as turtle grass because it is a popular habitat for sea turtles (Davidson, 64). Turtle grass has existed for an extremely long time, and somewhere around the Cretaceous period this plant traveled onto land and then decided to travel back to sea. Seagrasses are the only land plant to successfully journey back to the sea (Davidson, 65). One reason that seagrass is able to persist in such a harsh saline environment is that the mature leaves are saltwater tolerant. However, the young leaves when exposed to saltwater die, but the plant has created a unique adaptation to help it survive (Davidson, 66). The young seagrass leaves grow inside a protective sheath where they are protected and surrounded by fresh water. The mature leaves remove the salt from the water to help the juvenile leaves grow and once the leaf is old enough to withstand the harsh saline waters it will break out of the protective sheath. These unique plants reproduce in two ways, sexually and asexually. The first way these plants reproduce is sexually through flowering and producing seedlings (Davidson, 66). The second method of reproduction involves a rhizome, or underground stem, that grows horizontally (Davidson, 66). The leaves grow relatively fast and can sprout up to three inches a week. In addition, they usually grow close together and in one square yard there can be thousands of leaves.
Seagrass beds are important for a variety of reasons, including habitat, sediment and erosion control, produce high amounts of energy, and remove unwanted nutrients before they reach fragile coral reef ecosystems. The first reason seagrass beds are important is they provide home, food and shelter for a variety of animals, and are particularly important as a nursery habitat for coral reef fishes. This is because they help the fish to avoid predators and contain an abundant food supply. Seagrass beds in the Caribbean are particularly important to coral reef fishes, one study suggested that seventy percent of species found in seagrass beds also inhabit coral reefs, however this percentage is not as high in other regions like the west pacific (Nakamura, 795). One explanation for why seagrass beds in the Caribbean are more vital to the existence of coral reef fishes is that the tidal range in the Caribbean is very small. The seagrass beds are never exposed to the open air, but remain covered by water. However, in the West pacific during low tide the seagrass beds are exposed to the open air and therefore are not consistently suitable habitats for fish (Nakamura, 796). Numerous animals live in seagrass beds not merely for protection, but also for the abundance of food, including the blades of grass themselves. Although the leaves are not nutrient rich a variety of animals consume these leafy blades, including, manatees, fish, crabs, geese, and swans (www.wikipedia.com). Another important function of seagrass beds, similar to mangroves, is they are excellent at binding sediments and many islands would not exist without the seagrass beds that surround them (Davidson, 66-67). When seagrass beds are removed, erosion occurs and the water is murkier and the overall water quality decreases. Lastly, when seagrass beds are removed from coastal environments, nutrients from agriculture, sewage, and urban run-off flow directly out to the coral reefs (Seddon, 167). Phosphates and nitrates are common nutrients that occur in animal waste and fertilizers and also aid in algal growth. When seagrass beds do not remove these nutrients from the water excess algal growth can occur on coral reefs. Too many algal blooms can block the sun and do not allow the plants to photosynthesize and therefore coral reefs die. There is a unique and interconnected relationship between mangroves, seagrass, and coral reefs and they rely on each other to endure.
There are both natural and human induced threats to seagrass beds. Some of the natural threats to seagrass beds are hurricanes, strong wind and waves; these powerful forces can destroy seagrass, just like coral reefs. Human presence is becoming a larger threat to seagrass, since humans like to reside and vacation in coastal regions. Some seagrass beds have been removed for coastal development. In addition, changes in water temperature and salinity can also kill seagrass (Seddon, 167). In certain regions fresh water outlets have been rerouted and increased saline levels that killed entire seagrass beds.
Currently, an array of scientific projects and research are being conducted to help restore seagrass beds. Planting by hand is very time consuming and not cost effective, therefore mechanical techniques are being developed to enable large sods of seagrass to be extracted from donor fields and transplanted to new regions (Seddon, 170). There is a concern that donor fields might become damaged by these processes, and the scope of the projects should be limited to small areas. One new technique that is being developed is planting seagrass seeds that have been treated with a root growth hormone. One study conducted in the Bahamas using turtle grass seedlings showed that after nine months over two-thirds of the planted seeds were still thriving (Seddon, 173). More research about the impacts of human development on seagrass and possible restoration projects need to be conducted to ensure the continuation of this unique plant species.
Davidson, Osha Gray. The Enchanted Braid: Coming to Terms with Nature on the Coral Reef. New York: John Wiley & Sons, Inc., 1998.
Duffy, J. Emmett, J. Paul Richardson, and Kristin e. France. “Ecosystem Consequences of Diversity Depend on Food Chain Length in Estuarine Vegetation”. Ecology Letter; 2005, p301-309.
Kricher, John. A Neotropical Companion. Princeton: Princeton University Press, 1997.
Nakamura, Yohei and Mitsuhiko Sano. “Overlaps in Habitat use of Fishes between a Seagrass Bed and Adjacent Coral and Sand Areas at Amitori Bay, Iriomote Island, Japan: Importance of the Seagrass Bed as Juvenile Habitat”. Fisheries Science; 2004, p788-803.
Seddon, Stephanie. “Going with the Flow: Facilitating Seagrass Rehabilitation”. Ecological Management & Restoration; Dec. 2004, Vol. 5, p167-176.
Welsh, David T. “Nitrogen Fixation in Seagrass Meadows: Regulation, Plant-Bacteria Interactions and Significance to Primary Productivity”. Ecology Letters; 2000, p58-71.
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