The large ship drags a cloud of brown sludge behind it as it rotates in the blue-green seawater. The mass doesn’t quickly sink to the bottom but spreads visibly in the water and remains on the surface for a long time.
The ship is engaged in dumping. It essentially means that the ship is dumping tons of sludge dredged up from the seabed, which is potentially filled with environmentally harmful substances.
Dumping is carried out on a large scale in Denmark, and according to several experts, it further degrades an already pressured marine environment along the coasts.
»Nearly all of our marine areas are in poor chemical condition, so it is unacceptable to dump harbor sludge filled with toxins into the marine environment«, says Stiig Markager, professor and marine biologist at Aarhus University.
A new analysis from the think tank Ocean Institute, which has not yet been published, calculates that between 2015 and 2023, 18.4 million cubic meters of dredged seabed have been dumped around the Danish marine environments. This corresponds to the contents of 87,000 trucks every year during the period.
»We know that 98 percent of our marine environment is in poor chemical condition, so we have an already pressured marine environment that dumping further stresses«, says Henriette Selck, professor at RUC, senior consultant at the Ocean Institute and one of the authors behind the new analysis.
According to her, even more of Denmark’s marine environment risks becoming like deserts with virtually no life if the pollution contributed by dumping is not addressed.
»If we don’t do something and get the seas into good chemical condition but continue to pollute, we risk the fish completely disappearing. We’ve seen in some of our fjords that life has virtually vanished«, she says.
Toxic starfish
There are several reasons why dumping is carried out in Denmark, but it most often occurs in connection with the maintenance of shipping channels and the dredging of harbors. The latter can be particularly harmful to the marine environment.
»There is a lot of pollution in harbors that has accumulated over time. Much of it is currently well buried, but when you start dredging and dumping elsewhere, it ends up on the surface of the seabed«, says Henriette Selck.
These are very toxic substances that are either difficult to break down or are forever chemicals, such as mercury and TBT, which have accumulated as pollution in the harbors over time. TBT was previously used in, for example, ship bottom paint but is now banned because it is so toxic. These substances can be spread from one place to another when seabed from harbors is dumped into the sea.
It is a condition that harbors gradually fill up with sludge and that it is full of toxins, says Stiig Markager, but instead of keeping it concentrated in one place, it is spread throughout the marine environment by dumping it.
»It’s like throwing your household waste in the nearest forest instead of collecting it in a trash can. It’s unacceptable in 2025«, he says.
There haven’t been many large studies on how dumping affects the marine environment chemically. A study from 2003 that examined the dumping of seabed from the port of Rotterdam showed that the concentration of toxic forever chemicals like mercury was two to three times higher where the seabed was dumped compared to an area without dumping.
Additionally, starfish from the dumping site contained twice as much mercury, zinc, and PCB, which is a banned toxic substance, compared to starfish from other areas where there had been no dumping.
Stiig Markager believes that the only solution to the problem is to completely stop dumping harbor sludge. He points out that instead, a treatment plant should be built somewhere in Denmark that can clean the polluted harbor sludge from all Danish harbors.
»But of course, it’s always cheaper just to throw your waste out in the yard or in the forest, which is what dumping equates to«, he says.
read next
