Every half hour, when the Marsdiep is crossed, everything turns on the Texelstroom ferry. At ‘t Horntje, vacationers give way to those returning home on the way to the mainland, and vice versa in Den Helder. On the Teso ferry bridge, the faces turn in the opposite direction. And at the bottom of the bow, which was just the stern of the ship, a measuring instrument from the Netherlands Institute for Sea Research (Nioz) replaces its counterpart at the new stern and begins collecting data about the current the ship encounters during transit.
The latter has been happening since 1999, and the predecessors of Texelstroom also had such a measuring tool. This led to a startling conclusion: the depth of Mars itself is also mirrored.
“On Mars, the current sometimes goes one way, sometimes the other, depending on the tides,” says News researcher Johan van der Meulen. “But in the long term, there is a little bit of a ‘residual flow’, which is going in a certain direction. In the past it was outward, towards the North Sea, and we see in the measurements that this has been slowly decreasing, and it is now kind of around zero. If you expand this direction to the future, it will reverse.”
Echoes of animals in the water
Van der Meulen presented his findings last week at the main annual conference of the European Geosciences Union (EGU) in Vienna. At the end of last year, he and his colleagues also published it in the trade journal Oceanography.
Measurements are made using an acoustic Doppler current meter, a device that emits sound waves and picks up echoes of small particles and animals in the water. Shifts in the pitch of the received sound reveal whether the water is flowing toward or away from the device. The current scale transmits and looks up and down, forward and sideways, thus forming an image of the current in and out of Mars.
Data sent to the News River every evening after the last crossing shows how, at high tide, roughly the same amount of North Sea water enters the Wadden Sea that it leaves each day. It also applies to all tides in the Wadden Sea. But over a full year on average, there was a definite pattern in Marsdiep, says Van der Molen: “There’s a net inflow on the Den Helder side, and a net outflow on the Texel side. So there’s a kind of circulatory cell residual.”
The Wadden Sea does not empty
And if you subtract those two residual outflows from each other, you have a net outflow in recent years. Not that the Wadden Sea was emptying, these waters were, again, on average, drawn from the North Sea by a different route: via the Vlie, the tidal inlet between Vlieland and Terschelling, by which the balance must have been reversed.
In fact, around 2009, about 32 billion cubic meters per year are circulating clockwise around Vlieland and Texel. The reason for this dance, as the first computer models of currents in the area have shown, is that there is a slight difference in time and height between the tidal passage of the Vlie and Marsdiep.
“What we suspect now is that something is changing,” van der Meulen says. A recent German study showed that there are changes in tidal amplitudes in the North Sea, and that this is related 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.”
The arrival of the tide
Whether the Earth gets warmer, the Sun and the Moon get worse: it really doesn’t change the tidal rhythm. But it affects the temperature of the sea water, and therefore the arrival of tides on the Dutch coast.
“Tides on Earth are actually generated in the ocean around Antarctica,” van de Meulen explains. “Because there are no continents on the way there, a tidal wave can follow the Earth’s rotation relative to the sun and moon, all around the world. From there it spreads across the Atlantic Ocean until it reaches the North Sea. The gravitational pull of the sun and moon plays a role in this, but it is also separate By and large, most of the energy comes from the south.”
By the way, the tidal wave enters the North Sea from the north. It rolls first along the English coast and then from the south along the coast of the French Channel to Belgium and the Netherlands. In this shallow sea, she had to deal with summer stratification: a layer of water warmed by the sun floated on top of a layer of cold water.
Finely mixed water column
This makes a difference. Van de Meulen: “The speed at which a tidal wave propagates depends directly on the depth of the water. The deeper it is, the faster it propagates. This applies when the water column is well mixed in the winter. In the summer, when there is stratification, you see a kind of What is a split in the propagation of that wave. Because it has to deal with two water layers of different thicknesses, then you get two more or less independent waves that start to move. You can imagine that if something changes in that split, it becomes stronger or the depth of the layers changes, which It affects the spread of the tides. And that German study shows that this is already happening.”
Whether this also explains the stagnation and possible reversal of average flow across Marsdiep has not yet been established by analysis of tidal heights tracked by the Rijkswaterstaat for many years, and with the help of computer models.
Effect on sweet-salt balance
These models should also help answer the question of what effect the inversion could have on the Wadden Sea. Van der Meulen: “What you can imagine is an effect on the fresh salt balance. Fresh water is drained from the IJsselmeer at two locations on the Afsluitdijk, at Den Oever and Kornwerderzand, and that fresh water has to go somewhere. What is happening now on average is It eventually flows through Marsdiep into the North Sea.You can imagine that if this residual current were reversed, it would change and the distribution of fresh salt in the Wadden Sea would look somewhat different.
“At the same time, it can also affect the transfer of eggs and larvae of marine organisms, but whether this is a strong enough effect compared to the tides that are already there, you have to take a closer look at that. At the same time as fresh salt is being distributed, there is Of course also nutrients in the water, phosphates and nitrates, which help algae grow, I can imagine something is changing in those beds. And maybe affecting the introduction of sediment into the Wadden Sea and where it ends up.”
These are all residual current effects, which can only be gauged by sailing back and forth with Texelstroom every so often. So it won’t be dramatic. Then we’d already seen it. But it may be a subtle implication that in the long run may mean that things will turn out differently in the Wadden Sea.”
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What does the Vlieland masked seal do?
Everything is constantly lost. Last week I reported a hoopoe that had swum too far north on its trip (it?), and immediately after that, a hooded hood swam too far.
