In the North Sea, sand and silt have long been dredged from harbor basins and shipping channels so that ports and shipping routes remain usable. However, material washes up on beaches. New analyzes now show how much material is being relocated in the North Sea overall – as well as where this material ends up and what consequences this has for the Wadden Sea, ecosystems and climate.
Sand and mud are natural components of the seabed, but differ significantly in their composition. Sand consists of comparatively coarse grains, while silt is a very fine, mud-like sediment made up of tiny clay and silt particles. It occurs when fine material sinks from rivers or seawater and is deposited on the bottom, especially in calm waters such as the Wadden Sea, in river mouths or in harbor basins.
Washed up here, dredged there
Humans are now heavily interfering with the natural distribution of sand and silt in the sea. Every year, special ships remove large quantities of sand from the North Sea, which is used to build new port facilities or for coastal protection. For example, sand is regularly washed up off the islands of Sylt and Wangerooge to compensate for beach losses caused by winter storms and strong surf. In contrast, silt is primarily deposited in harbor basins and shipping channels.
Without regular dredging, ports and shipping routes would flatten and become impassable for shipping. The dredged silt is then transported by ship to areas of the North Sea and reintroduced into the sea. This so-called “dumping” moves the material from the harbor into the open coastal sea, where currents and tides distribute it further. After a while, the material often settles back into harbors or shipping channels – which then have to be dredged again.
Millions of tons of material with a potential climate effect
Researchers led by Lucas Porz from the Helmholtz Center Hereon in Geesthacht have now, for the first time, examined in more detail the total amounts of sand, gravel and silt that are moved by dredging and dumping in the North Sea. For their analysis, the researchers evaluated dredging activities over a period of 30 years. According to this, around 200 million tonnes of sediment are relocated every year – as much as is transported naturally by North Sea currents and all neighboring rivers combined. Model calculations also show that some of this material from the dumping areas is also transported into the Wadden Sea over time and is deposited there in the long term.
The rearrangements of seafloor material not only have consequences for local ecosystems, they also affect the carbon balance and climate. Because along with the sand and silt, dredging and flushing also stirs up organic remains from the seabed – remnants of dead algae and dead marine animals. These release carbon, which reacts with the oxygen in the water – resulting in the greenhouse gas carbon dioxide.
Porz and his team have also determined how much this is. Accordingly, the amount of carbon released worldwide is up to 500 million tons annually. This is significantly more than the construction of pipelines or offshore wind turbines. “This data is important in order to better estimate in the future how much carbon dioxide will be released in total in coastal regions,” says Porz. “So far, these human activities have not been taken into account in carbon budgets.”
Help for the Wadden Sea?
However, the redistribution of silt and sand could also have positive effects – for example on the Wadden Sea. Many tidal flats no longer naturally deposit enough sediment to keep pace with rising sea levels. Displaced sand and silt could counteract this deficit: “If you relocate the dredged material strategically, it could reach the affected areas from there and allow the mudflats to grow again,” explains Porz. He and his colleagues are currently working out in detail which areas are suitable for this. However, since silt from ports in particular can be chemically contaminated, the effects on the marine environment would have to be carefully examined beforehand.
Source: Helmholtz Center Hereon; Specialist article: Nature Communications, doi:10.1038/s41467-025-68105-5