Sunday Sand: Maerl

 
Look at the images below and you would be forgiven for thinking that they were of the gleaming white sands of tropical beaches. But no, this is the west coast of Ireland, County Galway to be exact.

Like many white tropical sands, these beaches are made of biogenic fragments, bits of critters, and, like tropical beaches, they are the local inhabitants. But your credulity might be strained when I tell you that these Irish beaches are made of seaweed. However, it’s true, and they are built of maerl. A Breton word (sometimes written maërl), it refers to loose-lying, normally non-geniculate (i.e. unsegmented because they lack decalcified joints), coralline red algae – seaweed. Or, to quote from the Oceana website:

The term “maerl” describes various species of unattached coralline seaweeds that live on sea beds. Phymatolithon calcareum forms brittle, purple-pink, branched structures that look more like small corals than seaweed. It grows as spherical nodules at sheltered sites, or as twigs or flattened medallions at more exposed sites. In places with some water movement from waves and tides, but not enough to break the maerl nodules, extensive beds can develop. Maerl is as much a habitat as a species, and both the living maerl and the maerl-derived gravel beneath it harbor many small animals. Maerl grows slowly and the beds are vulnerable to damage from bottom trawlers.

Or, since  many of the Atlantic beaches of Scotland are of the same origin, from the Scottish Natural Heritage site:

The beautiful, shining white 'coral' beaches of western Scotland are made, not of coral, but of dead fragments of a strange hard seaweed called maerl, crushed by the waves and bleached by the sun. Living maerl is a beautiful purple-pink, and forms spiky underwater 'carpets' on the seabed.

Here are images of this spiky underwater carpeting material:

 
The algae live on the surface of the hard coralline forms that they continue to  build throughout their life, constructed partly of calcite and partly aragonite (each a different crystal form of calcium carbonate), together with magnesium carbonate and a spicy variety of minor trace elements. They make look like coral, but they are only coralline – they are photosynthesising, like plants, but simpler. They live in clear waters (see the Google Earth image), largely at depths that protect them from all but the strongest waves and tides; some can be hundreds of years old, and their lineage stretches back for hundreds of millions of years. Maerl beds are found along coasts of Atlantic islands, northern and western Europe, the Mediterranean, and the Philippines.

Once they die, they lose their pink-purple colour, and their skeletons form debris that is eventually carried onto the beaches by waves, tides, and currents. The “sands” of these beaches are generally made of somewhat larger grains than the strict definition of sand by size allows – but the grain-size varies, and, besides which, they are dramatic and fascinating.

For centuries, maerl beds have been harvested for a variety of uses, first of all as an agricultural soil conditioner – the white grains can be seen strewn across Atlantic coastal fields of Ireland and France. It is also used as an animal and human food additive (reflecting the trace minerals and elements that it contains), in water purification and filtration, and in cosmetics. But the seaweed grows slowly, and cannot keep pace with the annual extraction of hundreds of thousands of tons from around the coasts of Europe – maerl has become a non-renewable resource.

 

[Thanks again to Carla Lagendijk who sent me a sample of Galway beach maerl, together with her essay on the material that provided much of the background for this post. Image of the living maerl bed from Loch Varron, Western Scotland, copyright Keith Hiscock, published on the MarLIN website and reproduced under their usage terms. For further information on maerl, see, for example, http://www.ukmarinesac.org.uk/communities/maerl/m1_1.htm#a2, http://www.algosophette.com/association/spiruline-news-categoriespiruline-14.html and  http://www.3reef.com/forums/algae/lithothamnion-75146.html, from which other images here have been reproduced.] 

Comments

[Richard Bready]

Amazing, thank you. If they don't undergo debridement, do they form stromatolites?

[Sandglass]

Well, having educated myself as to the meaning of "debridement" (perhaps not a term destined for daily conversational usage), I can only comment that our old friends the stromatolites are constructed by cyanobacteria, and are a somewhat different kettle of fish, so to speak. Their construction method is to use layers of sediment, including, of course, sand.

[Richard Bready]

So the algae build up while the bacteria lay down. Thanks.
If debridement is in a daily conversation, it's a bad day. Lazily, I assumed a connection with debris, but that's wrong. One should check sources.

[Patricia Miller]

This is very cool! Many years ago, when I was visiting County Galway by bicycle, I was stunned to find a beach with this blinding white sand and was curious about what the grains were. I brought a sample back to the States with me, thinking perhaps the stuff was foraminifera tests. After examining it under the scope, I found that was not it, but, not knowing much about algae, I had no idea of the sand's real origin. Some years later I attended a conference presentation in which the speaker mentioned disarticulated pieces of the coralline algae - I described my sample to him and finally found the answer to my mystery.

I just happened to think of the material again because I'm preparing a workshop for high school teachers on the topic of biominerals - fascinating to find your post about the maerl!

[Sandglass]

Patricia - many thanks for taking the time to leave this comment. It's a great pleasure to hear from you and to know that, in some small way, the blog is contributing to educational projects.

One of my very early posts on some of the work of Robert Hazen might also be of interest for this: http://throughthesandglass.typepad.com/through_the_sandglass/2008/12/evolution---org.html