Seagrasses, much more than "ugly weeds" stinking up your beach

still from Océans

 I couldn't let World Ocean's Day go by without trying to put out a more "scientific" post about the Oceans... and while I was writing up my review of the movie Océans I remembered joking a bit with my friends about a certain scene where a dugong in the Red Sea is eating seagrass. What tickled us was the idea of the poor fellow nuzzling around the sand for these very small leaves which didn't seem to be all that abundant, when he could just hang out (if such a thing were possible) off-shore from Alicante and enjoy a diet of proper Posidonia oceanica, a much larger species of seagrass which forms meadows in the shallow sandy waters of the Mediterranean Sea. Wishful thinking... I'd love to run into one of these guys while diving off the Cabo de las Huertas, but I'm afraid even the Med isn't warm enough for them. :o(

For World Oceans Day the Ocean Project suggests we pick a favourite animal and protect it. Well those of you who've been reading me for a while know I'm not great at following instructions so I'm breaking out and choosing a PLANT!!! Ha! Anyone familiar with it will have guessed from the previous paragraph that I'm about to talk about a species of seagrass known as Posidonia oceanica. Why a seagrass? Well for one thing hopefully to change the minds of anyone who might harbour negative feelings towards this plant. I know I did as a child. It clutters the beach, makes it "icky" to swim in certain areas... and it stinks! At least that's how I felt as a child, now the smell just reminds me of the Mediterranean...

First things first: seagrasses are not seaweed. They're real plants, they're not algae! Unlike the latter they have roots, they flower (a very rare event to witness!), have seeds (known as sea olives) and a vascular system. In fact they're the only flowering plant that can grow under water.

adapted from European Seagrasses

Posidonia oceanica, washed up on an Alicante beach. The "branches" are the lateral growth or "rhizomes"

Like terrestrial plants they get their energy from the sun, thanks to photosynthesis, so they can't be found in deep waters where the sunlight wouldn't reach them. This means they have a limited distribution along shallow coastlines which is sadly the most highly impacted area of the seas (due to human interaction). The fact that they have roots allows them to colonise the sandy bottoms (as opposed to most algae who prefer hard surfaces) frequently forming vast meadows which form a unique habitat for a large number of fish and invertebrate species.

School of Salps (Sarpa salpa) swimming over a P.oceanica meadow off the Cabo de las Huertas in Alicante

There are approximately 60 species of seagrasses (sometimes called eelgrass or turtlegrass) found in the coastal waters around the world. Here in the Mediterranean Sea four species are present: Zostera marina, Zostera noltii, Cymodocea nodosa and last but not least the queen: Posidonia oceanica, pictured above. 

Unlike the other three P.oceanica is endemic to the Mediterranean which means it can't be found (naturally) anywhere else. In spite of being very slow-growing, P.oceanica can form vast meadows and reefs of slowly compacted sand due to a higher sedimentation rate in the presence of this plant. These meadows can often been detected from the coast, and traces are always found along the beaches, either rhizomes with leaves dredged up during a storm like the first photo above, or maybe just compact balls of the coarse rhizome material (that my poor dog always confuses with rocks, much to her disappointment!):

compact nodule of P.oceanica rhizome fragments on an Alicante beach

So why are seagrasses, and Posidonia in particular, so important?

Well first and foremost they form a unique ecosystem that is home to a myriad of vertebrate and invertebrate species. Many smaller fish hide from predators among the leaves of a Posidonia meadow, while bigger fish lay their eggs and hope for this same protection as a nursery for their young. There is even a whole epifauna (tiny invertebrates) and epiphytes (algae) that can be found living on a single Posidonia leaf! These are therefore areas of high biological productivity and diversity.

Not enough for you? Want something more useful? Here are a few facts:

• While seagrass beds are only responsible for 1% of the oceanic primary production, they do take care of 12% of the carbon stored in oceanic sediments and play a significant role in the global carbon cycle. Something to keep in mind during this Global Warming / carbon crisis.
• Seagrass meadows are key habitats for many organisms, including younger stages of commercial species.
• Seagrasses as filters improve water quality and increase transparency and the passage of light to the seabed thereby increasing the primary production of both the seagrasses and photosynthetic algae.
• Seagrass meadows are an effective protective element for the coasts. The leaves, rhizomes and
  roots fix and stabilize the sands they're growing on, and reduce the resuspension of sediments by currents and waves. This means that seagrasses can reduce the erosion of the coastline. Detached leaves often accumulate along the beaches and further protect them from erosion by dissipating wave energy and directly protecting beach sediments from the impact of waves.

accumulation of P.oceanica fragments along an Alicante beach

So all that "icky seaweed" I used to complain about as a kid was actually protecting the beach! And this protection is currently sadly lacking as the Posidonia meadows here (and in many other places) are in poorer health each year, threatened by trawling (no longer allowed, but anchoring boats is also a problem), dredging (for sand to feed the beach ironically!), poorer water quality (pollution, increased water turbidity)... add to that the daily "cleaning" of the beach by City Hall crews and the result is seen in a yearly reduction of the amount of sand on our beaches and the regular "re-building" of beaches with sand dug up from the seabed.

The public tends to respond very well to conservation efforts for "flagship species" (frequently charismatic megafauna, a.k.a. any "cute" animal that can elicit an emotional response), but in some areas it is actually more effective to protect an entire habitat, particularly one as crucial as the seagrass beds, because so many species depend on it for their survival. The needs of the many and all that.

What can be done to protect seagrass beds? Well, forbid certain types of fishing (anything involving the seabed) and aquaculture in those areas, warn boaters about the risks when they anchor, but especially improve the water quality by treating waste waters (particularly those of agricultural origin) before they are dumped in the sea to reduce excess nutrient and organic matter loads which increase water turbidity (darker waters mean less photosynthesis for plants). At a more personal level we can stop complaining about the presence of seagrass litter on our beaches and appreciate that its presence is a protective element against erosion!

Wow, I can't believe I finally got this done! I'm glad I was able to find the time to add another entry to the Oceanic Blog-A-Thon for World Oceans day, remember there's still time for you to add yours as well! Just send me the link and I'll add it to the post.

Sources:

Photos: mine except for the underwater scene which was taken by my sister last summer here in the Cabo de las Huertas in Alicante.

www.wikipedia.com

www.seagrasswatch.org

European seagrasses: an introduction to monitoring and management. Editors: J Borum, CM Duarte, D Krause-Jensen and TM Greve. Pdf freely available at www.seagrasses.org

more seagrass websites can be found here.