Animals that feed on cyanobacteria

The Baltic Sea is affected by eutrophication and climate change and both the spring phytoplankton bloom and the summer cyanobacteria bloom start earlier today than in the 1980s. Diatoms have decreased while nitrogen-fixing cyanobacteria have increased. How does all this affect food chains in the Baltic Sea?

Photo: Gunnar Aneer/

Cyanobacteria blooms are increasing around the world, as a result of climate change. Research on their role in ecosystems is important to understand the impacts on the environment. Several new studies on the role of cyanobacteria in the Baltic Sea ecosystem have been published at Stockholm University.

Improves the growth of mussels

Blue mussel. Photo: Benutzer-Darkone /Wikimedia Commons

Several studies have shown that invertebrates can utilise nitrogen from cyanobacterial blooms. By looking at gut contents, biomarkers and molecular analyses, it can be concluded that the animals consume cyanobacteria. A new study shows that blue mussels can also synthesise essential fatty acids and obtain nitrogen from cyanobacteria. It was found that while the mussels that were diatom fed in the study had higher levels of essential fatty acids, those fed cyanobacteria had a better growth potential. The study shows that cyanobacteria, at least the non-toxic species used in the experiment, are a good food source for mussels during the summer.

Amphipods thrive on cyanobacteria

The small benthic amphipods Monoporeia affinis fulfil several important functions in the ecosystem, firstly by feeding on algal blooms and in turn being an important prey for fish. Secondly, they live buried in soft bottoms and their burrowing activity oxygenates the sediments.

Monoporeia affinis, a small, benthic amphipod. Photo: Matias Ledesma

It is already known that Monoporeia affinis from the Baltic Sea proper, where cyanobacterial blooms have occurred for thousands of years, can eat cyanobacteria. But the blooms are spreading northwards with climate change. New research shows that the Monoporeia affinis in the northern Bothnian Sea also eat and grow well on cyanobacteria, even the toxic species.

Bottom fauna in Bothnian Sea adapts to new diet

Benthic animals seem to make their own fatty acids

In summer, the nutrient-rich diatoms and dinoflagellates are replaced by much more nutrient-poor cyanobacteria, which lack essential fatty acids. In a study of Monoporeia affinis, Baltic Sea mussels, the priapulid worm Halicryptus spinulosus, and the invasive North American bristle worm, these bottom-dwelling animals appear to be well equipped for the less nutritious spring blooms and nutrient-poor summer blooms of the future. Their nutrient content does not change in the summer and they retain their share of essential fatty acids. They probably have the ability to synthesise the essential fatty acids themselves.

Main food source for zooplankton

Copepod. Photo: Natalia Sukhikh /Wikimedia Commons

When the DNA of phytoplankton found in the stomachs of zooplankton is compared with the DNA available in the water, it is possible to identify the food choices of different zooplankton. Up to 70 per cent of the zooplankton diet is cyanobacteria. In turn, zooplankton are eaten by fish and therefore play a key role in maintaining a large fish population.

However, despite the importance of cyanobacteria as food, fish stocks can be negatively affected by algal blooms. Cyanobacteria can aggravate the effects of eutrophication by binding nitrogen to plants, increasing the growth and amount of organic matter falling to the bottom. This leads to the spread of oxygen-free seabeds and negatively affects benthic animals and fish.

If benthic animals and zooplankton can make a choice, they prefer diatoms. However, they consume a large amount of cyanobacteria when that is the food that is available.

Food for growing Baltic herring

Herring. Photo: Uwe Kils /Wikimedia Commons.

In the summer, when herring have the hardest time finding food, there are also algal blooms with cyanobacteria. A research project is currently underway to investigate whether the summer blooms can provide more food for herring so that they grow faster.

More rapid growth means less time for the herring to accumulate dioxins. Therefore, the ongoing research project asks: "Can cyanobacterial blooms make Baltic Sea fish less toxic?"

Text: Jenny Rosen