Seagrass meadows in the Mediterranean Sea

The most amazing meadows are below the surface

Sooner or later every sailor/diver in the Mediterranean (and around the world) will at some stage encounter their first seagrass meadow. While they are somewhat inconvenient for anchoring and not that exciting for diving (at first sight) there is a lot more to them and they have their well-deserved right to be where they grow!

What makes a Seagrass anyways?

Posidonia oceanica, the smart name for the big Neptune’s grass can grow up to one meter in height and is growing thick mats on the bottom of the sea. For sailors, this clearly means: My anchor will drag. If you look at them from a different perspective it means What an incredible habitat!

Coral Reefs of the Mediterranean Sea

While the tropics grow colourful reefs that host most juvenile organisms with plenty of niches and hiding spots, those reefs are simply missing in the Med. So some other habitat has to fill this gap for us. And the big Seagrass meadows are exactly those organisms. A massive niche system is set up the perfect breedings ground for nearly all local fishes that we enjoy while snorkelling or diving (or in the evenings on a plate). The rough estimate is that on hector of Seagrass meadow can host up to 350 different organisms – quite a big amount of diversity for a small stretch of the meadow!

The rough calculation suggests that each square meter of Seagrass meadow is home to around 30 fishes growing up every year  – what doesn’t sound like a lot on first sight does add up quite quickly. And one square meter could easily be an area that can be destroyed by failing to dig in an anchor just one time.

In case those numbers fail to impress we can also break it down to the entire Mediterranean: The value of meadows in the Med is estimated to about 190 Million Euros: For fishing industry alone! Divers who enjoy visiting the fish stocks, as well as the occasional sailor towing a fishing line, are not yet part of that equation!

The true value of seagrass

So even if the financial value is already something to a seagrass meadow there is much more to those plants (in fact the only plants that found its way back from the surface underwater – algae are setting up their own group of plants with different mechanisms for photosynthesis). 

Seagrass meadows are also the only mechanisms stabilizing usually loose substrate like and fine sediments, that would be washed away by waves and currents.  If you ever get the chance to take a close look while snorkelling or diving: Algae always grows on a solid substrate like rocks, concrete blocks, or even seagrass leaves.  Seagrass itself, however, needs sand: Only on fine sediment can they grow their roots into the bottom and therefore stabilize it.

In some countries of the world the lesson has already been learned the hard way: Where trawlers took out all the seagrass in the past the loose sediment is now getting washed away in rapid rates threatening entire islands and countries now have to invest millions of Euros each year to put sediment back on those islands to keep them alive – happened in many places of the north sea, to name just one example.

Threats for seagrass

But also in other areas of the world seagrass meadows are already threatened. One example is the Mediterranean whereas an example – the coastline of Istria has lost most of its seagrass meadows already – the result of a  combination of unfortunate circumstances. Anchoring is just one of many issues in that case: Too much nutrient influx from land, chemicals, and of course the warming waters are becoming an increasingly big issue for Posidonia and her relatives. 

And especially when talking about warming waters, seagrass does play a key role:

In times where climate change is a common topic and is slowly but surely accepted as a fact rather than a conspiracy theory – people start talking about planting trees to counteract the warming.
Now, what if there was a mechanism that was way more efficient than that? And also with that question in mind, the answer might just be found below the surface:

Posidonia and the climate

As the whole world is still talking about keeping the warming below 1.5 degrees the Mediterranean already cracked the 3 degrees warming a year ago. The good news is: We do have some of the most efficient mechanisms in place to bind huge amounts of carbon out of the atmosphere: Seagrass meadows! Depending on what statistic you choose to trust they can secure somewhere between 3 and 50 times the amount of carbon from a regular forest on land. This adds up to a (rather decent) 1.8 tons per hectare and year!

And another advantage: While carbon that is secured in a tree will be set free again the moment the tree is cut down and eventually burned. Once Carbon is secured in seagrass it will stay below the surface. While the single leaves tend to be thrown off and end up on shore again the vast majority of Carbon is secured in the rhizome – a matte that remains below the surface and usually ends up buried below sediment on the long run.

And just like in every equation: Where ever CO2 is secured something else comes out on the other end: Int his case it a very good thing: Seagrass is producing about twice the amount of oxygen that a rainforest could produce in the same area! The reason is quite obvious ones we think about it: One square meter of Seagrass can grow around 1000 shoots (bundles of leaves) each one of those has again somewhere between 4 and 10 leaves, which in the best case adds up to 10.000 leaves in one square meter. Each one of those again a meter long, so we have around 200 square meters of active surface for photosynthesis! An incredible rate!

And even if the rhizome – the part that ultimately keeps the carbon below the surface – is only growing around 2cm each year it does have a good survival rate.  In parts of Greece, we could find mattes with a thickness of around 5 meters – so they started growing at a time when people on land were still building castles! And judging from the big numbers it is even more impressive: Seagrass meadows have been around the Mediterranean Sea for more than 100.000 years so we should do our very best to keep it that way! After all, they are one of the worlds oldest living systems!

If you want to learn even more about Seagrass meadows make sure to read about our ongoing projects here on the website and also check our YouTube channel for the right videos!

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