Every half hour, once the Marsdiep is crossed, everything turns around on the Texelstroom ferry. In ‘t Horntje holidaymakers give way to those returning home en route to the mainland, and vice versa in Den Helder. On the Teso ferry deck, faces turn the other way. And at the bottom of the bow, which was only the stern, a measuring instrument from the Netherlands Institute for Marine Research (Nioz) replaces the counterpart of the new stern and begins to collect data on the current encountered by the vessel during the crossing.
The latter has been occurring since 1999, the predecessors of the Texelstroom also had such a measuring instrument. And that led to a striking conclusion: the Mars Deep itself is also reversing.
“In the Marsdiep, the current sometimes goes in one direction and sometimes in the other, depending on the tide”, explains Johan van der Molen, researcher at Nioz. “But in the long term, there is a bit of ‘residual flow’, which goes in a certain direction. It used to be outwards, towards the North Sea, and you can see in the measurements that it has slowly decreased , it is now around a kind of zero point, if you extend this trend in the future, it will reverse.
Animal echoes in water
Van der Molen presented his findings last week at the major annual conference of the European Geosciences Union (EGU) in Vienna. At the end of last year, he and his colleagues also published them in the specialist journal ocean science.
Measurements are made with an acoustic doppler current meter, a device that emits sound waves and picks up echoes of small particles and animals in the water. Changes in pitch of the received sound reveal whether water is flowing towards or away from the instrument. The current meter transmits and looks up and down, forward and sideways, forming a picture of the current flowing in and out of Mars Deep.
The data that is sent to Nioz each evening after the last crossing shows that at high tide approximately the same amount of water from the North Sea enters and leaves the Wadden Sea each day. As applies to every tidal inlet of the Wadden Sea. But over an entire year on average, there was a specific pattern at Marsdiep, explains Van der Molen: “There is a clear inflow on the Den Helder side and a clear outflow on the Texel side. So there is a kind of residual circulation cell.
The Wadden Sea does not empty
And if you subtracted those two residual flows from each other, you would end up with a net outflow in recent years. Not that the Wadden Sea is emptying, that the water is, once again on average, drawn from the North Sea by a different route: via the Vlie, the arm of the tide between Vlieland and Terschelling, whose balance had to reverse.
In fact, around 2009, around 32 billion cubic meters per year rotated clockwise around Vlieland and Texel. The cause of this dance, as early computer models of the currents in the region showed, is that there is only a slight difference in time and height between the passage from the ebb and flow to the Vlie and at the Marsdiep.
“And what we suspect now is that something is changing,” says Van der Molen. “A recent German study showed that there are changes in the tidal range in the North Sea, and that this is linked to climate change. We think that’s the mechanism, but if you really want to be sure, you have to get to the bottom of it.”
Arrival of the ebb and flow
If the earth gets warmer, the sun and the moon will be the worst: it really doesn’t change the rhythm of the tides. But this influences the temperature of the sea water, and therefore the arrival of the ebb and flow on the Dutch coast.
“The tide on Earth is actually generated in the ocean around Antarctica,” says Van de Molen. “Because there are no continents in the way, the tidal wave can follow the rotation of the earth relative to the sun and the moon, all around the globe. From there, it spreads across the the Atlantic Ocean until it reaches the North Sea. The attraction of the sun and the moon plays a role in this, but it is also largely decoupled, with most of the energy coming from the south.
Incidentally, the tidal wave enters the North Sea from the north. It rolls first along the English coast and then south along the French Channel coast to Belgium and the Netherlands. In this shallow sea, it has to deal with stratification in summer: a layer of water warmed by the sun floats above a layer of cold water.
Thoroughly mixed water column
And that makes a difference. Van de Molen: “The speed at which the tidal wave propagates directly depends on the depth of the water. The deeper it is, the faster it can reproduce. This applies when the water column is well mixed, in winter. In summer, when there is stratification, you see a kind of division in the propagation of this wave. Because it has to deal with two layers of water of different thickness, and then you get two somewhat independent waves that start moving. You can imagine that if something changes in this stratification, it becomes stronger or the depth of the stratification changes, it has an effect on the propagation of the tide. And this German study shows that this is indeed what is happening.
Whether this also explains the stagnation and probable reversal of the mean flow through the Marsdiep has not yet been demonstrated by analyzing the tidal heights that Rijkswaterstaat has tracked for many years, and using computer models.
Influence on the sweet-salty balance
These models should also help answer the question of what effect the inversion might have on the Wadden Sea. Van der Molen: “What you can imagine is an influence on the balance of fresh salt. Fresh water is discharged from the IJsselmeer at two places on the Afsluitdijk, at Den Oever and Kornwerderzand, and this fresh water has to go somewhere. What happens now on average is that it eventually crosses the Marsdiep to the North Sea. You can imagine that if this residual flow reverses, it will shift and the distribution of fresh salt in the Wadden Sea will be somewhat different.
“At the same time, it could also affect the transport of eggs and larvae of marine organisms, but if it’s a strong enough effect compared to the tide that’s already there, you should take a closer look. And in along with a distribution of fresh salt there are of course also nutrients in the water, phosphates and nitrates, which help algae growth, I imagine something is changing in this And maybe it affects the input of sediment into the Wadden Sea and where it ends up.
These are all the effects of a residual current, which can only be measured by going back and forth with the Texelstroom very often. They will therefore not be dramatic. “So we would have seen it already. But it could be more subtle effects that, in the longer term, could make things happen differently in the Wadden Sea.
What does the hooded seal do on Vlieland?
Everything keeps getting lost. Last week I reported a hoopoe that swam too far north on its hike, and immediately after that a hooded cap swam too far.
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