Dynamics in coastal zones

From the Zeeland Delta to the Dunes of Holland, from the island of Texel to the German border and from the North Sea to the Wadden Sea: all in all, the Netherlands has a large coastal zone. Sea, beach, dunes and salt marshes are inextricably linked with our cultural history. They also fulfill all kinds of vital functions: for the natural ecosystems, for our fisheries, shipping, energy supply and recreation, and for the defense of our low-lying land against the sea. All of those functions are changing due to climate change.

Policy and management

“It is important for our country to know exactly what is changing and how changes are related”, says Karen van de Wolfshaar, theoretical ecologist at Wageningen Marine Research. “After all, these changes may have important implications for policy and management. Studying interactions between relationships is exactly what Wageningen Marine Research is good at. Our research is interdisciplinary, and it addresses fundamental processes in the context of practical applications.”

Van de Wolfshaar makes computer models that help to understand the functioning of the natural world, from the growth of individual animals to the dynamics of populations and food webs. “We are already seeing shifts in the occurrence of species due to the warming seawater”, she says. “Cod, for example, a cold-loving species, has almost disappeared from the southern North Sea. Mackerel, on the other hand, is now swimming far above Norway. Sea bass and red mullet, two other species of commercial interest, are increasingly seen in the North Sea.”

Nurseries

Wageningen Marine Research has conducted extensive research into the reproduction of plaice, an economically important flatfish species. “We are already seeing changes in the timing and locations of spawning”, she says. “This means that fewer young plaice may end up in what we call the nurseries: the shallow coastal zones where the conditions are most favourable for young fish to grow up. The young plaice now arrive there earlier, which confirms the earlier timing of spawning.”

This could have long-term consequences for the Dutch plaice fishery, notes the ecologist. “Hence we need to re-examine the way in which we draw up the advice for the catch quota. For example, perhaps we shouldn’t use the spawning stock biomass – which is the amount of fish ready to spawn – as input in our models, but rather the actual amount of offspring.”

If fish populations in the North Sea are indeed changing, adjusted quotas may be necessary: entirely new quotas for fish that are newly colonising the North Sea, or reduced quotas for species whose abundance is decreasing. “Countries may have to divide quotas differently among themselves”, says Van de Wolfshaar. “All of this requires a basis of good scientific data. Our role as Wageningen Marine Research is to contribute to the knowledge base. Then it is up to policymakers to translate this information into practical policy.”

Interactions

Changes in fish populations will also affect the rest of the food web in the North Sea. “The ecological interactions are so complex that we’re not yet able to calculate them accurately”, says Van de Wolfshaar. “But in early 2022 we started a major European project aimed at creating better food web models. Combining those with new climate scenarios will allow us to better predict changes in the North Sea ecosystem.”

More than twenty partners from different European countries are participating in the new project. Van de Wolfshaar is addressing the impact of climate change on fish species and fisheries; partners provide data on factors such as benthic animals, plankton, and physical properties, including currents and temperature, but also, for example, the fishing strategies of fishers.

Many of the researchers use their own models. In conjunction, they aim to make the best possible predictions by combining the different models. “This also allows us to gradually improve the models,” says the ecologist. “We call this ensemble modelling.”

Coastal reinforcement

Van de Wolfshaar also studies the impact of sand extraction on the North Sea ecosystem. Rijkswaterstaat uses North Sea sand to reinforce the Dutch coast. “Such large-scale extraction could have various ecological consequences,” she says. “It could impact for instance the life cycle of plaice, shrimp and other benthic animals. Many of these effects are still unknown."

This also applies to the impact of the subsequent sand nourishment along the coast. “Sand nourishment may effectively bury the food of birds or flatfish,” she says. “On the other hand, it could also provide these species with a free meal, because the extracted sand include large quantities of dead benthic animals. Timing and location are important in this context. And probably also factors such as the grain size of the sand. This differs per sand extraction location.”

Nature as a basis

Van de Wolfshaar works closely together with various colleagues from Wageningen Marine Research. One of them is Martin Baptist. As a marine ecologist with a PhD in civil engineering, he focuses on so-called nature-based solutions. These are engineered solutions for major societal issues, such as coastal defense, based on natural processes. “Examples from nature include the Dutch dunes”, he says. “They form a natural barrier that grows naturally under the influence of wind and sea currents. And salt marshes grow due to the deposition of silt. Such systems can naturally move with sea level rise – as opposed to systems of concrete and steel.”

Along the coast near the city of The Hague, Rijkswaterstaat and other parties, including Wageningen Marine Research, have been working together a large-scale, long-term pilot project: the Sand Motor. In 2011, a total of 21 million cubic metres of sand were supplied to the coast in the shape of an artificial peninsula. Under the influence of wind and currents, this sand is naturally dispersed between Hoek van Holland and Scheveningen, thus contributing to beach and dune formation. “Other institutes are investigating the physical aspects”, Baptist explains, “and we focus on the ecology. For example, how can you minimise damage to nature in these projects – or even increase biodiversity? Bases on this, we advise how, where and when to best carry out sand extraction and suppletion.”

Combinations

The experiences of De Zandmotor have already been applied elsewhere in the Netherlands, including at the Hondsbossche and Pettemer Sea Defense. In recent years, the original concrete dike has been replaced by a dune system. “Other examples include the Prins Hendrik Sand Dike on the island of Texel and the 'Meegroeidijk' in the Ems-Dollard area”, says Baptist. “And in Indonesia we are working on mangrove restoration. Mangroves are not only an important spawning area and nursery for fish and shrimp, but also help to combat coastal erosion.”

Nature-based solutions are not always sufficient, as Baptist emphasises – and therefore by no means the only solution. “In some places you simply need dams and dikes. But the concepts can often be combined. A salt marsh on the seaside of a dike, for instance, can strongly reduce the wave action against the dike. The dike may then be lower and requires less maintenance."

Scaling up

In many of these pilot projects, Wageningen Marine Research works in consortia with partners such as Rijkswaterstaat, dredging and engineering companies, research institutes and nature organisations. “All parties benefit from this collaboration”, says Baptist. “The bottleneck is scaling up pilots to large-scale applications. The lack of space in the Netherlands often proves to be a major limiting factor. Plans for the Oosterschelde estuary quickly clash with mussel farming, for instance. Or if a potential location is a natural habitat for birds, seals or mussels, we have a legal obligation to protect them. This often precludes large-scale interventions in the environment.”

Sand extraction is also increasingly clashing with plans for offshore wind farms, Baptist notes. “Electricity cables run from the parks to the coast in the seabed. This requires very good planning in time and space. Our research contributes to that.”

North Sea Agreement

Oscar Bos, colleague of Van de Wolfshaar and Baptist, is working on nature conservation in the North Sea. “A large part of the Dutch North Sea has been designated as a Natura 2000 area,” he says. “That means that the Netherlands has a duty to protect certain species in those areas. This has been the case since 2005. In practice, however, there is still intensive fishing activity in many of those areas.”

The fishing is done in accordance with the European common fisheries policy. “The new North Sea Agreement includes concrete plans for zones in which no bottom-disturbing fishing is allowed, which will probably come into effect in 2024,” says Bos. “This is a new element – and an important step.”

Indicators for biodiversity

In the meantime, major developments are continuing at full speed, such as the construction of wind farms, the intensification of shipping, and climate change. “Studying what these developments mean for nature, and how to protect it, requires in-depth knowledge of the North Sea and its inhabitants”, says Bos. “This sounds logical, but in fact much of its biodiversity is still unknown.”

Bos investigates biodiversity in various ways, for example with video imaging, seafloor samples and techniques such as environmental DNA. In addition, fisheries surveys provide a lot of data on fish abundancy. Bos: “We use all of these data to develop indicators for biodiversity. Parties such as Rijkswaterstaat can then use these to monitor developments in certain areas, for example in wind farms. That is pioneering work.”

Resilience

In an ecosystem that is changing as quickly as the North Sea, all kinds of ecological shifts are taking place. “It’s hard to contribute specific changes explicitly to climate change”, says Bos. “There is often an interaction of drivers, with climate change as an overarching factor that exacerbates other impacts.”

The best way to counter the impact of climate change is therefore to strengthen our marine ecosystems as much as possible, says Bos, allowing them to be more resilient to human influences. “Establishing fishing-free zones is one fruitful approach”, he notes. “I dare say that fishing in the North Sea is the biggest pressure factor.”

Opportunities for nature

Wind farms may in fact contribute to local biodiversity: after all, they are zones free from fishing. As refuges, they may develop into hot spots for certain fish species. “Also, oyster reefs can develop on the seabed rock that are placed around the windmill bases as scour protection”, says Bos. “We are investigating under which conditions these rocks can be suitable for native oysters – and whether we could attract extra biodiversity by making cavities in the rocks for fish, lobsters and crabs.”

It is not all doom and gloom in the North Sea, as Bos concludes. “There are in fact species groups whose conservation status is improving, such as harbour porpoises and seals. The share of large fish is increasing slightly again. And the construction of wind farms offers new opportunities. Now it is up to us to record and interpret those changes as accurately as possible.”