Danish waters are filled with plankton, not microplastics

In Danish waters, there is very little micro plastics so small that copepods can eat them. If microplastic particles enter their mouths, they usually spit them out. This is shown by new research from DTU. The researchers are pleased with the findings because they allay the fear that microplastics accumulate in the food chain via copepods.

Copepods glued to human hair.
Researchers at DTU have filmed copepods that were glued to human hair. Suspending the copepods like this allowed the researchers to examine how much plankton and how many microplastic particles from the surrounding water the copepods would ingest and reject respectively. Photo: Jiayi Xu

Plastic particles are few and far between

The researchers found between 25 and 100 microplastic particles per cubic metre of water collected. In the samples with the highest measured concentration, this corresponds to one plastic particle per 10 litres of water. By contrast, the samples contained about 100,000 times more plankton than microplastics.

In total, 88 per cent of the microplastic particles captured by the fine-meshed nets are so small that they would escape the conventional Manta nets. According to PhD student Gunaalan Kuddithamby, not many studies have used this method—both because it is difficult to collect the samples and because it is expensive and time-consuming to analyse them.

DTU Aqua have conducted several studies that examined the smallest fraction of microplastic. These studies were collected in the waters around Greenland, and on an expedition from Denmark to the Caribbean that sailed through the ‘North Atlantic Garbage Patch’. Here, too, the researchers found smaller amounts of microplastics than they expected. Even the samples with the highest concentrations contained less than one plastic particle per litre of water.

As the presence of microplastics in the oceans will increase, partly because plastic waste there breaks down into much smaller pieces, Gunaalan Kuddithamby stresses the importance of repeating the measurements with these more precise methods to follow the development.

Female reseacher in DTU's algae room examining samples.
Rocio Rodriguez Torres DTU’s algae room checking the status of the algae culture that was used to feed the copepods in experiments aimed at determining their ability to distinguish between plankton and microplastic. Photo: Rocio Rodriguez Torres

Tiny, fussy gluttons

The good news about the low microplastic concentrations is backed up by experiments at DTU that investigate what happens if copepods encounter the particles while they are eating. They show that the copepods are like fussy children who can detect pieces of celery in the spaghetti sauce and spit them out.

Video footage from the laboratory experiments shows that in four out of five cases, the copepods spit out the plastic particles.

“Even though they catch thousands of particles in their tiny mouthparts, they can tell that they’re not food, either because of the structure or taste of the particles. They taste hundreds of particles a minute, but when a plastic particle goes in, they spit it out,” explains Torkel Gissel Nielsen.

“If they do eat the microplastic particles, we’ve shown in other experiments that they excrete them—just like kids who’ve accidentally eaten small beads,” he says. 

This also means that the microplastics don’t bioaccumulate when the copepods become meals for larger organisms, which in turn are eaten by larger animals, and so on.

Using a high speed camera, DTU researchers have been able to study the behaviour of copecopds when they feed. Video: Jiayi Xu

Plastis in the oceans is a problem

Torkel Gissel Nielsen stresses that plastic waste is part of humanity’s impact on nature and clearly poses a serious environmental problem: 

“It’s frightening that plastic has been found in the penguin colonies in Antarctica, where no people live. There’s plastic in the ocean north of Greenland. There’s plastic at the bottom of the Mariana Trench. There’s plastic at the top of Mount Everest. On any tiny island you can sail to where there have never been people, you’ll find plastic.”

For the professor, however, it’s important to turn our attention to where the problem is most serious. Based on the data he has helped collect on microplastics, he believes that larger pieces of plastic (so-called macroplastics) in our oceans pose a much bigger problem for marine life:

“Thousands of animals die because they get caught in the many so-called ghost nets floating around in the oceans. These are animals such as tuna, sea turtles, dolphins, and albatrosses. It’s either because they get caught up in these nets or because they eat them.”

And as he points out: removing macroplastics from the oceans won’t just protect the large vulnerable marine animals but will also remove one of the main sources of microplastics.


  • The samples were collected in Denmark on an expedition on the marine research vessel Dana between 20 October and 1 November 2020.
  • Water samples of approximately 1 m3 were collected from 14 locations in coastal areas: 12 in the Kattegat and 2 in the Skagerrak.
  • The differences in the concentration of microplastics between collection points are not significant.
  • Microplastics are defined in the study as particles smaller than 300 micrometres.
  • The samples have been analysed in cooperation with Aalborg University, which is one of the few places in the world with state-of-the-art equipment that can analyse such small microplastic particles.
  • 17 different types of microplastics have been found, but most often polyester (46 per cent).
  • The work was carried out with funding from the Velux Foundation and an additional grant from the JPI Oceans project RESPONSE.


Copepods, are small crustaceans measuring 1-5 mm. They are the most abundant multicellular organism in the world’s oceans.

They filter the water to eat the phytoplankton found in it.

Despite their small size, copepods play an important role in the ocean. Not only are they a significant part of the base of the marine food chain, but they also eat phytoplankton, which absorbs a considerable part of the CO2 on the ocean surface. When copepods eat phytoplankton, they produce excrement that falls to the bottom of the ocean, and in this way, they contribute to removing CO2 from the surface.


Torkel Gissel Nielsen

Torkel Gissel Nielsen Professor National Institute of Aquatic Resources Phone: +45 35883494

Gunaalan Kuddithamby

Gunaalan Kuddithamby PhD student National Institute of Aquatic Resources