The changing face of the North Sea

Around the globe, ocean and sea levels are rising in response to climate change – because more ice is melting at the poles and in alpine regions, and because warmer water expands. The North Sea is no exception and, after roughly 1,000 years of dike construction on its coasts, is tightly hemmed in, with very little room to expand. Since the beginning of satellite observation in 1993, levels have risen more than 4 millimetres per year on average – but with substantial regional differences. This is due in part to currents and wind patterns. Fundamentally speaking, the mudflats can to some extent keep in step with sea-level rise, as the high and low tides constantly deposit sediment on the coast. But if sea-level rise were to exceed a certain rate, roughly three to four millimetres per year, the flats, which dry out regularly, could become permanently water-covered lagoons. Though the impacts could differ greatly on a micro-scale, it could mean a loss of habitat and feeding grounds for many types of mudflat and saltmarsh fauna, including countless seabirds and migratory birds.

Water temperatures of 20 degrees Celsius and higher aren’t just cosy for North Sea beachgoers. At temperatures this high, some types of bacteria grow exponentially – including the dangerous bacteria Vibrio vulnificus. When it finds its way into the human body, hidden in oysters eaten raw, or when people with open wounds go swimming or walk barefoot, the consequences are no laughing matter: food poisoning or even blood poisoning. Since the 1990s, the number of Vibrio wound infections along the coasts of Northern Europe has risen steadily and is particularly high during extended hot periods and at water temperatures above 25 degrees Celsius. Accordingly, the progressive warming of the North Sea will almost certainly worsen this trend.

The Wadden Sea along the coast is undergoing a transformation of its own. For example, there are now more than 100 new species of Pacific oyster. Yet the majority of those species didn’t migrate on their own; they were accidentally introduced by ships hailing from remote regions or intentionally by human beings for breeding purposes. These species also like it hot and feel more and more comfortable as temperatures rise. They interact with native fauna, creating completely new relational networks. As such, the increased biodiversity stemming from climate change represents a massive shift in habitats, a process that is by no means over. In this regard, it remains unclear whether certain fish, bivalve and crustacean species will ultimately disappear from the North Sea entirely, or whether they’ll successfully adapt to the new conditions.

The AWI North Sea Office pursues research into specific environmental topics in the North Sea and provides scientific findings for government offices, political decision-makers, environmental organisations and the general public. The North Sea is a unique habitat, but also a key economic region. Consequently, conflicts over its use emerge and, in connection with global climate change, there is a growing need for management strategies that promote the sustainable use and protection of the marine environment. In this regard, the AWI North Sea Office works to develop new solutions and approaches, drawing on the AWI’s extensive time series gathered at its facilities in Bremerhaven, Helgoland and Sylt for decades, which reflect changes in the North Sea over the past several decades. These resources, together with comprehensive expertise gleaned from countless research projects, are what enable AWI researchers to provide sound advice to national and European committees. Working hand in hand with political decision-makers and environmental protection organisations, the AWI North Sea Office develops strategies for the environmentally responsible use of the changing North Sea, contributing e.g. to the protection of the UNESCO World Heritage site Wadden Sea.

Seagrass is a plant that forms roots and rhizome. This solidly anchors it on the seafloor, allowing the plant to densely cover large areas. It can be found in many shallow and sandy coastal waters, including the North Sea’s Wadden Sea. Seagrass meadows are of vital importance as coastal habitats. Many small organisms like snails and crabs, not to mention young fish, use the dense leaves to hide in. Some fish species attach their eggs to seagrass leaves, using the meadows as spawning grounds. Further, seagrass is a food source for migratory birds, allowing them to gather the energy they need for the rest of their journey, while coastal birds use seagrass meadows for hunting, regularly scanning them for prey. Moreover, seagrass meadows’ dense root structures reinforce the stability of the mudflat soil, while seagrass leaves sift fine sediment from the water, which is then deposited in the meadows. In addition, seagrass filters nutrients out of the water and absorbs carbon dioxide, storing it in the soil for extended periods. For several decades, very little new seagrass grew in the German Wadden Sea; the seagrass was hard-hit by a substantially higher nutrient influx. The higher nutrient load led e.g. large algae to spread so extensively that they covered the seagrass, effectively suffocating it beneath them. The meadows in the northern Wadden Sea have since recovered. One reason is the improved water quality thanks to lower levels of phosphate and nitrogen influx. Unfortunately, the seagrass meadows that have recovered are limited to those located far away from major river mouths. Although the nutrient load has been reduced, it is still too high, which continues to negatively affect seagrass near large river mouths. On a more positive note, this shows that these important coastal habitats can bounce back when local pollution is reduced.

The stony seafloor encircling Helgoland is home to an underwater forest: a one- to two-metre-tall tangle of kelp that offers a habitat, haven and food for more than 200 animal and algae species. These so-called kelp forests differ from more familiar ecosystems like coral reefs: kelp thrives at cooler temperatures, between 5 and 15 degrees Celsius. Now, the warming produced by climate change is threatening these oases of diversity. For example, when the water temperature breaks the 18-degree mark in summer, it damages the kelp’s reproductive cells, reducing its capacity to reproduce. And if it breaks the 20-degree mark, in some cases the kelp doesn’t reproduce at all. At the same time, the topmost parts of the kelp, clearly visible at low tide, wither and die. 
But there’s also been a positive change: because currents are changing, allowing more Atlantic water to reach the North Sea, the water near Helgoland is becoming clearer. This means more sunlight can reach deeper waters, paving the way for kelp forests to expand at greater depths. 
Recent studies indicate that this expansion process is ongoing. However, a broad range of model-based studies also predict that, as the summer warming of seawater worsens, Helgoland’s underwater world as we now know it will eventually be lost.

Around the world, fish stocks are in decline, and fewer and fewer fish are being harvested from the sea. Today, 50 percent of fish and seafood, not to mention 96 percent of algae, stems from aquaculture. In Germany, near-coastal aquaculture in the North Sea is only possible to a limited extent: first of all, the Wadden Sea is a national park and therefore a protected area. Therefore, it is only permissible to breed extractive organisms – those that can acquire their own food and nutrients from the water column and therefore don’t need to be fed. These include mussels, oysters and macroalgae. A good alternative: the shared use of offshore wind parks far out to sea. In these areas, where turbines are used to produce energy, what is known as open ocean aquaculture can be used to cultivate bivalves and algae. Experts from the Alfred Wegner Institute are currently investigating how this type of use could be implemented sustainably – which algae and bivalves are best suited, which technologies are sufficiently robust to withstand the harsh weather conditions produced by currents and wave action, and how much space is needed in order to cultivate each species in an environmentally friendly and economically lucrative way in offshore areas of the North Sea.