What’s your favourite marine habitat? Mangroves are wonderfully cool. And nothing compares to a good dive with great visibility on a healthy coral reef. What about playing on the sandy shore, bathing in the sun? What if I were to tell you that so much of what you love about the ocean is all dependent on something you’ve probably never put much thought into. Something you may even consider gross, slimy, and hope that your resort rips out. Well, without seagrass, the oceans as we know them, the climate we depend on, and even the air we breathe would be unrecognisable.
Very few true plants can survive the harsh conditions of marine life. Most of what we think of as plants under the sea are actually algae instead. Seagrass is the one exceptional flowering plant that can survive such high salinity and thrive where most other organisms would perish. Seagrass can withstand desiccation and extreme temperature fluctuations at low tide, salinity fluctuations near river outflows, and huge storms of surge and wave actions. It seems the only thing it can’t survive is human devastation.
The 72 known species of seagrasses are found from the poles to the tropics with the majority of species found in temperate and tropical shallow seas. They need at least 11% of incident light to photosynthesise, so are usually found in the shallows where the light is plentiful, but some have been recorded in clear water deeper than 50m! They usually need soft sediments like mud or sand, but can sometimes even grow on top of coral or rocky habitats. Seagrasses are truly adaptable organisms.
More closely related to lilies than grasses, all but one species are entirely adapted to life underwater, completing their entire life cycles beneath the surface. Fertilisation underwater provides unique challenges, so seagrasses have evolved a few strategies to overcome them. They produce the longest pollen of any flowering plant. Most terrestrial pollen grains are about .1mm, but seagrasses produce 5mm pollen grains, an increase of 500%. These link together to form a chain that is carried by the waves to a female flower where fertilisation occurs. Additionally, there is some speculation that marine invertebrates have coevolved to act as pollinators similar to insects are to land plants. They feed on the pollen and potentially carry it with them on their bodies to the female flowers. In an entirely different strategy, some species attach large air bubbles to their pollen so it can float to the surface and more easily be carried across the sea with the currents.
Conversely, seagrasses can also reproduce asexually by cloning. The roots (called rhizomes) can spread across the ocean floor with new shoots appearing above the sediment. One clone in the Mediterranean is over 200,000 years old! That one plant has lived through the evolution and extinction of millions of species.
Because seagrasses are so photosynthetically productive, they absorb vast amounts of carbon. In fact, they are the largest carbon sink known across both land and sea, capable of producing 10,000 grams of Carbon per square meter per year, doubling the amount of carbon produced by tropical forests. Unlike forests however, the carbon trapped by the photosynthesis can remain stored indefinitely. This “blue carbon sink” is only released back into the atmosphere when the seagrass beds are disturbed or destroyed. Though they only take up .1% of the ocean floor, they contribute 11% of all carbon trapped in the ocean. This makes seagrass ecosystems a kind of secret weapon in the fight against anthropogenic-led climate change.
Seagrass beds may be mucky, uninviting areas that you actively try to avoid, but they are home to fascinating organisms. Vascular plants may not often be described as such, (especially as this diehard zoologist) but over the geologic periods, seagrasses have more than earned that title. The benefits of seagrasses are too vast for one post, so we’ll dive deeper into how dependent we all are on their contributions to the planet in the next part.
Seagrass – Part 1 was written by Roya
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