Seagrasses, including eelgrass of the North Atlantic, are marine flowering plants with long, narrow leaves. They obtain nutrients through photosynthesis, so they are green. They anchor themselves in sand or mud in sheltered coastal waters, forming extensive beds or "meadows." There are some 50 to 60 species of seagrasses worldwide.
Although all seagrasses can clone themselves through spreading horizontal subterranean stems called rhizomes, some species produce seeds and transfer pollen from male to female flowers. There are annual seagrasses that produce seeds that remain dormant for several months.
Seagrass rhizomes can spread at rates of from less than an inch per year for slow-growing, large species to more than five yards per year in small, fast-growing species. After a storm, a meadow of seagrass can regenerate from a few shoots.
Fast-growing species, including Cymodocea, Halophilia and Syringodium, can build a meadow in less than a year. The slow-growing Posidonia oceania can take centuries to develop a meadow.
Seagrasses are able to store oxygen internally in what is called a lacunar system, necessary for growth in an environment that lacks oxygen. Seagrasses with flowers and seeds can pollinate underwater. Their special leaves have evolved to utilize light filtered through water. Deep water varieties can grow 40 yards deep in clear, tropical ocean water.
Extensive meadows along shorelines are especially vulnerable to floods, storms and hurricanes.
Seagrasses can grow in water with a salinity of 10 to 45 percent. They usually grow on muddy or sandy sediments, but they can grow on rocks. Heavy currents can smother them with sediments. Sewage and other organic waste bearing sulfides and other toxic compounds can kill them. Globally, about 10 percent of coastlines have conditions suitable to seagrass growth.
Global Warming and Seagrass Growth
Oceanographers believe that the rising temperatures of seawater caused by global warming will increase carbon dioxide in the water, in turn increasing the rate of photosynthesis in seagrasses. As the levels of sea water rise, beds of seagrass could expand.
While rising temperatures caused by global warming could potentially accelerate the growth of seagrass meadows, they could also push seagrasses to their thermal limit.
Pollution and Seagrass Growth
Herbicides in the runoff of water used for irrigation and the dumping of sewage threaten seagrass habitats. Seagrasses help stabilize sediments that are suspended in the water. If seagrasses are lost, the water turns cloudy and murky.