A sea hare is not a kind of rabbit, but a marine slug. A sea mouse is also not a mammal, but a worm. A sea lion is not the Lord of the Jungle, but a large seal. A sea horse cannot gallop, because it is a rather strange-looking fish — so too is a sea robin. Sea pigs aren’t pork, but sea cucumbers, which aren’t vegetables but a strange elongated relative of the sea urchin — which isn’t a hedgehog ("urchin" is an old word for hedgehog). We do seem to like to name marine things after something more familiar above the waves.
One thing that is correctly named, though, is seagrass. At low tide in Otago Harbour, you can see extensive greenish patches in various bays, and they are actually grass. Seagrasses do everything normal grasses do: they grow from seeds, they photosynthesise in the sun, they produce pollen which fertilises a flower, which becomes a fruit, which makes seeds. Seagrasses are connected by roots and rhizomes under the sand, forming a dense mat that is just like a meadow.
Seagrasses need a lot of light, so they only live in shallow waters. They are tolerant of a range of temperatures, of a certain degree of freshwater input, and some brief exposure to air. Seagrass meadows are among the most productive ecosystems in the world, in part because their photosynthesis process is unusually flexible: they can use CO2 from the air, CO2 from the water, and bicarbonate (HCO3-) from the water. Most plants and seaweeds can only use one of these sources of carbon.
The blades of seagrasses provide oxygen, shelter and food for a wide range of creatures. Sea turtles, sea cows, sea urchins, and many fish and invertebrates eat seagrasses. Other animals eat the tiny bugs and mats that grow on the seagrass blades. The meadow itself offers shelter to baby fish, sharks and rays, scallops, crabs, and shrimp.
Seagrasses are also baffling! Meaning, they slow down water currents, which makes suspended sand and mud fall down and build up the seafloor. Then the roots and rhizomes help to bind the sediment and keep it from washing away. It’s very common for seagrass beds to be raised above the surrounding bare mud — they stop erosion and stabilise the seafloor.
Seagrasses are called "ecosystem engineers" — they create the environment around them. They provide important coastal habitats for many other creatures, and clean up the ocean while they are doing it. But they are vulnerable to anything that interferes with their light source, such as muddy waters, and to fishing that disturbs the bottom. It is estimated that seagrass beds are being lost worldwide at about 1.5% of their area per year. We are really fortunate to have thriving and functioning seagrass beds in Otago Harbour — they show us that the water is reasonably clear and clean. But even they are decreasing, particularly in years when the water gets very warm.
It is possible to extend seagrass beds by cutting divots out of an existing bed and planting them nearby. While Earth Sciences New Zealand has been experimenting with putting out plastic fronds that mimic seagrasses, they clearly are not as good as having real ones. They provide shelter but not food. Awareness of seagrasses is also important — did you know that World Seagrass Day is March 1 each year?
What seagrasses need is water clarity, to ensure their access to light. We can take more care of what we allow to flow into our coastal waters. (Incidentally, that approach will help a whole range of other coastal photosynthesisers, such as kelp.) Seagrasses are doing their bit to remove CO2 from the atmosphere and lock it into sediments, but they need us to do our part to reduce global warming. Next time you are near the harbour at low tide, look for these underwater meadows, where the grass really is greener on the other side.
Abby Smith is a former professor of marine science at the University of Otago. Each week in this column writers address issues of sustainability.
