The island Stora Karlsö southwest of Gotland is known for its huge colony of common murres (Uria aalge), or common guillemots as they are also called. All the research that has been conducted there since the beginning of the 20th century has given us important knowledge on the Baltic Sea foodweb and ecosystem. Jonas Hentati-Sundberg, at SLU Aqua, is one of the researchers who has studied the seabirds a lot. He was involved when an 11 meter tall research shelf was installed on the cliff edge in 2008 and has since then led several projects with technology at the forefront.

Jonas Hentati-Sundberg
Jonas Hentati-Sundberg is an expert on the links between seabirds and fish. Photo: Aron Hejdström.

- This guillemot colony is by far the largest in the Baltic Sea and up to 100 000 different birds live on Stora Karlsö, says Jonas. To analyse this enormous amount of data we have had great use of both artificial intelligence and citizen science.

Curiosity and cooperation…

But on the island, where nature has been protected for a long time, not only seabirds thrive. Almost ten years ago, Jonas and a colleague talked about the unusually large number of a certain type of swallow; house martins (Delichon urbicum), that lived on the lighthouse just above the nesting guillemots. Could there be a connection between the house martins and the seabirds? Does the house martin's favorite food; Chironomids, have anything to do with it?

… led to results

Jonas then managed to get a few more young colleagues interested in cracking the bird-mystery. In 2017 they were granted time on the brand new research vessel R/V Electra af Askö to investigate possible connections between the common house martins and the guillemots. 

Carro på Electra
Caroline Raymond, researcher at DEEP, Stockholm University, sieves through sediments in search for chironomid larvae on R/V Electra. Photo: Jonas Hentati-Sundberg. 

- It was probably thanks to our combined expertise that we were able to write a good application and receive a research grant from Stockholm University's strategic Baltic Sea initiative, says Jonas. With the help of Electra's sonar we investigated the sediments around the island. It turned out that they were low in oxygen and empty of chironomid larvae but we discovered a lot of larvae in the seaweed near the rocky shores!

Connecting the sediments and the sky

Through looking at stable isotopes in the guillemot residue and in the chironomids the researchers found that they were connected. The istopes matched which ment that the house martin's food - the chironomids - live on the guillemot residue. Europe's largest colony of house martins thus, by extention, depend on nutrients from the sea and need healthy fish stocks. These results have just been published in a scientific paper and according to Jonas, they are of value in conservation work:

Sillgrissla med fisk
A guillemot has catched a fish. Photo: Jonas Hentati-Sundberg. 

- These type of connections or links between species are important to take into account when developing plans to protect the environment, he explains. We now have a clear example of how natural values in one place can depend on a seemingly completely separate ecosystem or other species far away. 

Enhanced local eutrophication

As a part of this study the team also looked closer into the nutrient transport in the guillemot ecosystem. They found that the daily amount of nutrients, especially phosphorus, that reach the water and sediment below the colony is of the same magnitude as the nutrients released from one of Sweden's largest wastewater treatment plants.

- The new results clearly show how a biological process, such as a colony of seabirds, affects the nutrient circulation in the sea, says Bo Gustafsson, head of the Baltic Nest Institute at the Baltic Sea Center and co-author of the study.

Another author of the study; Stefano Bonaglia assistant professor at University of Gothenburg agrees:

- Everything is so interconnected! Our study essentially suggests that the bird residues sooner or later reach the sediments, which under hypoxic conditions - low or no oxygen - may slowly act as a source of nitrogen and phosphorus to the water, he explains. 

Even if the guillemots do not add new nutrients to the system, this knowledge is important for everyone who works with measures against eutrophication.

- In this specific case, with a colony of guillemots living on a island at sea, nutrient transport can be positive for the ecosystem. It maintains the populations of insects and birds! But with an equally large seabird colony in a more protected archipelago environment, we would probably see negative eutrophication effects, Bo concludes.

Nutrients from isheating seabirds to the seabed and up in the sky
Conceptual illustration from the scientific article. Illustration: Fredrik Saarkoppel. Click below to view full size. 


"Fueling of a marine‐terrestrial ecosystem by a major seabird colony"


Jonas Hentati-Sundberg, Associate Professor, Department of Aquatic Resources, Swedish University of Agricultural Sciences,

Stefano Bonaglia, Assistant Professor, Department of Marine Sciences, University of Gothenburg,

Bo Gustafsson, head of the Baltic Nest Institute at the Baltic Sea Center,