The temperature of the North Sea is increasing with climate change. Good news for immigrant species - bad news for cold-loving residents...

A stiff wind, beach grass waving  amongst the sand dunes and black  mussels shimmering in the sand. These are the features normally associated with the North Sea coast. But here, too, climate change is in evidence - especially under the surface of the water. The North Sea is warming. And this is drawing in a  lot of new species, which previously kept to warmer regions.

One of these species is the Pacific Oyster. The North Sea used to be too cold for it. "Today it practically dominates the whole Wadden Sea," says marine ecologist Christian Buschbaum. Stationed on the North Sea island of Sylt for the Alfred-Wegener-Institute, he monitors organisms that live very near the surface in the Wadden Sea. He explains that to reproduce, the Pacific Oyster requires a few weeks where sea temperatures consistently reach at least 18 degrees Celsius. And for the past few years, the North Sea easily provided those. According to the German federal agency for maritime shipping and hydrography (BSH), water temperatures in the North Sea reached an average of 11 degrees Celsius in 2016, which was the second highest temperature since 1969. Only 2014 saw a higher temperature of 11.4 degrees. 

As a consequence of this, former mussel beds are steadily turning into oyster reefs. But the oysters are not pushing the mussels out, Buschbaum told  DW:  “The mussels hide between the oysters, especially in the lower areas – this offers them more protection against predators,” he says. Of course some competition for food  is to be expected, though this is not overly dramatic for the survival of either species. 

Miesmuschel - Mytilus trossulus (Imago/Bluegreen Pictures)

Mussels galore - good neighbours for the oysters

"Ecological Sleepers"

Sardines and anchovies are also taking increasingly well to the North Sea. As early as in the 1970s and 1980s, they could be  found sporadically in the North Sea. Since then, the sardine has integrated itself exceedingly well here, and  young anchovy populations are also drawn northward more and more frequently. 

"Ecological sleepers",  is the name marine ecologist Buschbaum gives to these creatures that “wake up” when temperatures rise in the North Sea and they can reproduce better. "Long frost seasons used to keep populations of many such warmth-loving, invasive species small," he says. But over the course of the last few decades, such intense periods of frost have decreased tremendously. This has improved conditions for some species. 

This is also true for the australiasian barnacle, introduced in the 1950s, and the common slipper shell, introduced in the 1930s. Harsh winters have regularly caused their populations to collapse. This has now changed, and they are visibly on the increase. "It cannot be ruled out that more 'ecological sleepers' exist," Buschmann told DW. Which these are and when they will reveal themselves depends on how the temperatures in the North Sea  develop. The warmer it gets, the higher the possibility that new species will appear.

Australische Seepocke, Neuseelaendische Seepocke, Australseepocke, Austral-Seepocke, Elminius modestus, Modest barnacle (picture alliance/dpa/blickwinkel/F. Hecker)

The common slipper shell also feels comfortable in the North Sea

Cod at a disadvantage

But not every inhabitant of the North Sea finds the increase in temperature to its liking. Cod is feeling the pressure from two sides. On the one hand, it is fished excessively; on the other hand, the North Sea is becoming much too warm for this fish species, which prefers cold water. "Cod is definitelybeing negatively affected by the warming of the North Sea," says scientist Anne Sell. She has been doing research for the Thünen-Institute since 2005 and has repeatedly monitored the North Sea with scientific research vessels for several weeks during summer and winter.

The North Sea is the southern border for the cod range. Nevertheless, the increase in temperature does not only have drawbacks. The Barents Sea, which borders on the Arctic Ocean, is also warming. This means the  habitat for cod is expanding towards the North. 

New predators

So what will it mean for the North Sea, if temperatures  continue to increase and normally resident species have to share their habitat with new ones? "There will be changes in relations between the species," says Buschmann.

This could, for example, affect predation. For birds and crabs, for instance, mussels were high up on the menu. But mussels are now better protected – thanks to the oysters. Buschmann and other researchers have observed that fewer of them are being eaten. "The birds and crabs will adapt," says Buschbaum – how they will adapt is not clear yet. "You can be sure there will be changes, though."

  • Date 21.02.2017
  • Author Hanna Pütz


original story HERE


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