- Our results show that rotifers and ciliates feed on spring and summer phytoplankton blooms by grazing. Before it was assumed that only the zooplankton copepod feed on and suppress phytoplankton. With our new method, we are able to show that small zooplankton, often overlooked in ecosystem studies play an important role in the Baltic Sea food web, says Monika Winder, professor at Department of Ecology, Environment and Plant Sciences (DEEP) at Stockholm University.

Center: A large rotifer of the genus Asplanchna, surounded by three cladocerans, Bosmina. Top: A copepodite (juvenile copepode). Right: two rotifers, Keratella. The background is scattered with diatoms. Carbonated water was used as narcotic for the zooplankton. Photo: Andreas Novotny.

With climate warming the phytoplankton community, that are food for zooplankton, will change, and cyanobacteria blooms will increase.

- If we do not know what type of zooplankton eat what type of phytoplankton, we cannot predict how extensive the algal blooms will be and how large the fish stocks will be since the zooplankton themselves are food for fish, says Andreas Novotny, PhD student at Department of Ecology, Environment and Plant Sciences (DEEP) at Stockholm University.

Andreas Novotny out in the field. Photo: Monika Winder.

Zooplankton, have an important role by keeping summer cyanobacteria blooms ‘in check’ since filamentous cyanobacteria are expected to increase with climate warming because they tolerate and grow well at high temperature. Filamentous cyanobacteria are often considered as a nuance because they can be toxic, are of low food quality, bloom forming and make the water slush-green, and are thought to be unpalatable for zooplankton and thus not incorporated in the food web.

Cladocerans, Bosmina. Photo: Andreas Novotny.

A major outcome of this study is therefore method development for assessing the zooplankton’s prey with barcoding of the DNA:

- Typically, zooplankton are grouped into size or phyla and assumed that they feed on the same type of prey. However, we show that there are differences between zooplankton species within the same size and phyla, says Monika Winder, professor at Department of Ecology, Environment and Plant Sciences (DEEP) at Stockholm University. 

One reason for why we need the new method DNA Barcoding is that the animals are so small, that it is almost impossible to observe what they eat:

- With DNA barcodes, we can take any zooplankton directly from the sea and scan all prey species the zooplankton has been eating, says Andreas Novotny, PhD student at Department of Ecology, Environment and Plant Sciences (DEEP) at Stockholm University.

The Cladoceran Evadne being sorted in the lab. Photo: Andreas Novotny.

It has been known for some time that cyanobacteria support productivity in the food web, but it has been unclear how that is possible, since it was thought that zooplankton does not eat them.

- Now we know that several zooplankton, but especially rotifers and ciliates eat cyanobacteria, and that’s likely how cyanobacteria matter enters the food web, says Andreas Novotny, PhD student at Department of Ecology, Environment and Plant Sciences (DEEP) at Stockholm University.

There are many abundant zooplankton that are not copepods:Here two cladoceran genera: Podon and Bosmina, and two rotifer genera: Synchaeta and Keratella. Photo: Andreas Novotny.

Read the scientific publication DNA metabarcoding reveals trophic niche diversity of micro and mesozooplankton species published in Proceedings of the Royal society B here.