Seagrasses work in conjunction with all tropical ecosystems in a sort of ecosystem symbiosis, contributing to the health of all. Coral reefs exist in an oligotrophic (low nutrient) environment. Too many nutrients can smother and suffocate the corals. Seagrass beds are often found near reefs, trapping the nutrients from the water column and letting the corals flourish and the mangroves form in calm environments. Mangroves trap the runoff and sediment from the land, preventing them from smothering both the seagrasses and the coral.
Seagrasses are often called foundation plant species or ecosystem engineers, literally creating their own suitable habitats. Their leaves slow the flow of water, allowing suspended material to settle to the sea floor where the roots trap and stabilize the sediment. This in turn improves water clarity. It also makes them an important buffer from tropical storms and tsunamis. The slowing of surge and wave action can make a life-and-death difference to coastal villages, a phenomenon we’re increasingly realizing the importance of as seagrass beds are in decline and tropical storm frequencies rise.
They absorb nutrient runoff from land – an ever increasing problem, and they aid in nutrient cycling both directly and indirectly. Epiphytic bacteria living on their leaves can extract nitrogen from the water and make it available to animals. They’ve also been called the “lungs of the sea” because they are so photosynthetically productive. Seagrass beds can literally fizz from all the oxygen bubbles being produced within their cells.
But the benefits don’t end on seagrasses themselves, they spread over every phylum. Within seagrass meadows, you can find small invertebrates like clams, sponges, worms, anemones, and crustaceans sheltered between the blades. Many algae will grow on the seagrass as epiphytes – directly living off of the leaves. Many fish use the meadows as protective nurseries, and many commercial fish and prawn species couldn’t survive without them. Because of all of the small animals taking shelter, this attracts larger animals like large fish and sharks. Marine mammals (dugongs and manatees) along with sea turtles and geese also feed directly on the grasses. A single acre of seagrass can support over 40,000 fish and 50 million small invertebrates.
Even humans have direct uses for seagrass, and have for generations. People traditionally have used them to fertilize fields, stuff mattresses and insulate houses, as bandages, roofs, and furniture. But it is their indirect values that really benefit humanity.
All these benefits are not only extremely valuable to the world ecologically, but also economically. One hectare of seagrass has been estimated to be worth over $19,000 UDS per year – one of the most valuable ecosystems on the planet. Only estuaries and wetlands are estimated to be worth more to humans.
However, as you should be learning to expect by now, it’s not all butterflyfish and rainbow runners. Seagrass beds all over the world are in great peril. Between boating and anchor damage, destructive fishing methods like trawling, dredging, and beach seining, salinity changes due to altered water flow for irrigation, nutrient loading and eutrophication, coastal development, and warming waters and ocean acidification, it’s no wonder seagrasses have decreased by at least 30%. Between 1990 and 2006, 7% of global seagrass meadows were destroyed. When seagrass beds die, they can emit up to 25% more stored carbon that the equivalent area on land. So it is now estimated that seagrass loss accounts for 8-20% of all anthropogenic greenhouse gas emissions.
That’s a lot of figures and stats. But what you need to take away from all this is that seagrasses, while not the most charismatic organisms, are more important than we can actually put a number on. And they deserve much more of a spotlight on their importance to our healthy futures.
So the next time you complain about washed up seagrass on the shore, think about how its very presence is creating that shore you are enjoying. The roots have stabilized the sand, preventing erosion into the sea and the plant matter is stabilizing the sand on the shore, preventing loss of the entire beach. The next time you dive over a monochromatic seagrass bed, think of how it makes the colours and vivacious reef you’re headed towards possible. And the next time you’re about to dismiss a flowing meadow about to be ripped up for a high end resort as boring and unimportant, think about where you’d be without it. To preserve the ocean, seagrass habitats cannot be overlooked.
Seagrass – Part 2 was written by Roya
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