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²ú²â&²Ô²ú²õ±è;​Muriel Brückner

Imagine this: a tidal flat in the Netherlands, your feet several decimetres into the mud, you are surrounded by tidal channels that you don’t want to fall into, bits of saltmarsh clamping onto the ground, and your field of view is limited to 3.5 metres because of the constant drizzle. Your task: finding three times 3000 worms of the specimen Hediste diversicolor for your experiment. And you don’t know what this worm species looks like. I think you get the feeling (if not, look at Fig. 1). So I decided, this is not for me this year! I will go to New Zealand!

Luckily, this little story is not one of mine because I am mainly a modeller. But I did go to such field sites, did not know what Hediste diversicolor was or how to catch it and I fell into the tidal channel. But luckily we had splendid weather, which you can also find in the Netherlands, if you are lucky. So to be honest, this little story was only an excuse to go to New Zealand and study the worms, crabs and shells that live there, the so-called macrobenthic species. 

But hold on for a second, why would we want to study these animals anyway? I mean, after all we are not biologists or ecologists but morphologists (at least I am since I started my PhD). I am happy you asked that question! Biology controls the morphodynamics of coastal systems to a pretty large degree. There is something growing almost everywhere and most certainly has an effect on its environment, as does the macrobenthos. So understanding biology is important if we want to know how coastal systems might change in the future. This means that if, for example, we get a warmer climate, higher sea levels or more frequent or stronger storms, not only will our coastlines change, but so will the animals that live there be affected - they don’t like living in environments that are too rough, or where food gets washed away before they can forage for it. Living organisms might even have to face different predators or competition that they are not adapted to, which could force some to start colonizing new habitats exhibiting more favourable conditions. In short, many factors control where species grow and thrive that can be easily disturbed by external drivers. 

To be clear, even though these relationships are very interesting, this is not what I study. I would, but that would require going to the field and, well, you know what I am going to tell you. So there are actually other scientists with stronger legs that enjoy walking on the mudflats, collecting data and running experiments so we can understand how these species thrive and what affects their growth. Interestingly, they found that hydrodynamics and sediment type together with salinity predict their distribution pretty well. So, each species has their own distribution linked to these three factors, which is pretty straightforward. And exactly what a modeller wants to hear! Give me equations and numbers and off I go! 

So what do I do then, if I just take the work of others and throw it at my model? The reason why I am getting really excited about these little ugly creatures is that they like to dig (Fig. 2). They grovel through the sediment as they eat and mate and they digest the sediment and dump it somewhere else. I think you have already seen plenty of burrows during your Sunday walk at the beach, so you have an idea of what I am talking about. 

And probably you can also already see why this burrowing behaviour can be very important to a morphologist. Of course, if they disturb the sediment bed with their activities AND they move the sediment from A to B they must have an effect on the stability of the sediment and on its composition. And if the stability and the composition changes, more sediment can be transported and redistributed and certainly will affect morphology. And even if you think that one little worm cannot have a large effect on the entire coast, just keep in mind that they can occur at densities of several thousand individuals per square meter! So basically imagine a square meter-sized hair brush that you stick into the sand, creating thousands of holes. This sounds pretty effective to me to loosen up some sediment and is certainly worth looking into with my model. Obviously, their bioturbation efficiency depends on several other things on top of density. For example, size, age and temperature are some of the top variables that we need to consider when modelling their effects. 

So how does that look in reality? The short answer is, we don’t know. Imagine an increase of 0.5 m of the mean global sea level (which is the prediction for moderate global warming). This could result in the macrobenthos moving landwards, bioturbating areas and loosen up the sediment closer to the cities, and lead to increasing erosion. That would have tremendous consequences for our coastlines. Or on the other hand, increasing sea level could lead to reduced efficiency so the macrobenthos might disappear and bioturbation is reduced. We don’t know, yet, what will happen. 

To get a better grip on this question, I want to model some of these scenarios, not only for the Netherlands but also for New Zealand: these two countries have different species, environment, and climate, which makes this comparison is extra interesting. And who knows, there might be similar effects on the morphological development of the coast that we can learn from. By better understanding the role of macrobenthic species on the morphology, we will be able to predict changes of coastlines more accurately and protect ourselves from floods. Now this is exciting, isn’t it? I even dare to say that I would like to see it in real life, so maybe going on fieldwork once in a while does not sound too bad. But maybe this time in New Zealand.

This contribution was originally posted on .