Fishing for herring fry and mysid shrimps in the night on board the research vessel Electra.

This summer there was yet another very strong bloom of cyanobacteria. They often occur when the weather is warm, sunny and calm – just when everyone is looking forward to a refreshing swim. It's no wonder that these organisms are not so popular.

When you also know that they fix more than 400 kilotons of nitrogen per year to the already eutrophic Baltic Sea, are potentially toxic, and sinking blooms  contribute to the severe oxygen deficiency on the seabed, cyanobacteria become even harder to like – even though they were the first organisms to learn photosynthesis and contribute to oxygenating our planet, and still are the only ones that can make atmospheric nitrogen usable for other organisms.

Positive news

That's why it's especially nice to see positive news from research.

 – Our research shows that cyanobacterial blooms can actually have positive effects, even on ecologically and economically important species such as herring, says John Taylor, a doctoral student at the Department of Ecology, Environment and Botany and lead author of the new scientific article.

The researchers wanted to follow the path of nitrogen from its original source in the nitrogen fixation of cyanobacteria through the entire food web from zooplankton to mysid shrimp and herring. They followed nitrogen isotopes in amino acids - the building blocks of proteins - some of which are particularly useful for tracing the origin of nitrogen.

Cyanobacteria save the herring

The study was conducted at two well-studied sites near the Askö Laboratory field station in the Baltic Sea. A eutrophic station in Himmerfjärden where a large sewage treatment plant supplies large amounts of nutrients and a more unaffected site – the Askö B1 station in the open archipelago which served as a reference.

– These sites are investigated within environmental monitoring, and the bloom of cyanobacteria and other phytoplankton is monitored intensively throughout the season, says Agnes Karlsson, a researcher and supervisor to John. In this study, we have taken advantage of this and have been able to follow the development before, during and after the bloom, thus achieving a very high temporal resolution.

Large addition of new nitrogen

The proportion of nitrogen-fixing cyanobacteria was significantly higher throughout the season at the eutrophic station, where they comprised up to 61 percent of the phytoplankton community, compared to only 32 percent at the reference site. The newly fixed nitrogen from the cyanobacteria could then be followed in zooplankton, mysid shrimp and in this year's cohort of juvenile herring.

Tucker trawl in water during sampling.

 – The results showed that at peak levels of the bloom, more than 30 percent of the nitrogen in the herring came from the cyanobacteria, says John. At the reference station, almost 20 percent of the nitrogen in the fish was newly fixed by cyanobacteria.

 – That is a significant proportion, continues John, and this is probably because there is often a shortage of food during the summer when new herring transform out of the larval stage. They then eat zooplankton, but these in turn have a shortage of phytoplankton to eat at that time, when the spring bloom has long since disappeared. This means that cyanobacterial blooms occur at a crucial time, providing food for zooplankton and thus for herring at a critical stage.

Considering that climate change is expected to increase cyanobacterial blooms in both scope and duration, this is at least a small consolation.

At least there will be more fish.

Read the scientific publication:

Incorporation of diazotrophically fixed nitrogen by juvenile fish in a coastal sea Limnology and Oceanography, 2025 

Text: Annika Tidlund