Baltic Breakfast: New DNA methods reveal surprising role of cyanobacteria in Baltic food webs

”If you really want to understand ecosystem functioning and the human impact on ecosystem functioning, you need to account also for the biodiversity food webs”, Monika Winder, Professor at the Department of Ecology, Environment and Plant Sciences at Stockholm University, opens the seminar.

Marine food webs are very diverse, also in the Baltic Sea, in particular at its base – where the primary producers are found, Monika Winder explains. These phytoplankton come in many different shapes and sizes. The round and chain-forming diatoms and the filamentous species like cyanobacteria all play very important and different functions in the ecosystem.

Equally diverse is the next level in the food web – the zooplankton. Dominating are the copepods and the soft bodied organisms like rotifers, as well as cladocerans.

“Zooplankton are important because they graze on the phytoplankton and therefore keep the phytoplankton blooms in check, but they also transfer energy from phytoplankton to a higher trophic level”, says Monika Winder.

One level further up is the small zooplankton feeding fish. This group is less diverse – the Baltic Sea is home to three dominant species; sprat, herring and stickleback.

“All these species interact in very different and complex ways, forming networks of interactions – what we call food webs – that transfer energy, carbon and nutrients to upper trophic levels”, says Monika Winder. “The food webs are the engines of healthy, resilient and biodiverse oceans.”

Rapid ongoing changes

However, the food webs in the Baltic Sea are changing rapidly at the moment, due to climate change and human activities. The spring bloom occurs earlier than before and shows an increase in dinoflagellates, and there is also an increase in the summer cyanobacteria blooms.

“Interestingly it’s mainly the small-sized cyanobacteria that have been increasing the recent decades”, says Monika Winder.

The composition of zooplankton is also undergoing change, with a decline in the key copepod species and an increase in rotifers. Further up there is a decline in herring and sprat and an increase in stickleback.

“All these changes reorganise how species interact. So if we would want to predict how ecosystems respond to this changing biodiversity, we need to know how food webs are built and how they work.”

The food web models used today generally fail to account for the biodiversity, especially at lower trophic levels, says Monika Winder. The models typically assume that zooplankton feed only on dinoflagellates and diatoms, and that cyanobacteria are not eaten at all. They also ignore feeding selectivity and seasonality – which is increasingly important as the timing of species appearances is shifting.

New methods reveal surprising results

To address these knowledge gaps, Monika Winder and her colleagues are using new DNA methods, so called metabarcoding. Extensive sampling was followed by DNA analysis of identified species of zooplankton, that was compared to the content of the water. This enabled them to draw conclusions on who has been eating whom and also on selectivity.

“Using this approach really changed our view of plankton interaction”, Monika Winder tells us. “Most interestingly, we see that cyanobacteria are really important for supporting secondary production.”

The results show that large copepods feed on pico-cyanobacteria and that filamentous cyanobacteria are grazed by diverse zooplankton, whereas the proportion of diatoms and dinoflagellates in their diet was relatively low. In total, 70 percent of what the zooplankton ate was cyanobacteria.

The results also show clear seasonal patterns. From winter through spring, energy flows mainly through diatoms and dinoflagellates, while cyanobacteria dominate in summer and early autumn. By late autumn, diatoms and dinoflagellates become more important again.

“We really need a revision of our understanding of cyanobacteria’s role in the food web”, concludes Monika Winder.

Moderator Ellen Bruno asks whether zooplankton eat cyanobacteria because they prefer them or simply because they are abundant. The answer, says Monika Winder, is largely a matter of timing. Copepods actually prefer diatoms and dinoflagellates, but these peak in early spring when copepod numbers are still low. By summer, when copepods are abundant, cyanobacteria dominate. "So basically, the copepods may not have any other choice," says Monika Winder.

Fish and rising competition

The DNA metabarcoding method was also applied to the gut content of pelagic fish, revealing that herring, sprat and stickleback surprisingly occupy distinct niches – contrary to what is commonly assumed in models.

However, rising temperatures appear to be changing that. A reconstruction of energy fluxes over 16 years, presented by Kinlan Jan, PhD from Stockholm University, who is moving to Queen's University Belfast, shows that as temperatures have increased, the diets of the three species have become more similar. This appears to be linked to a decline in Pseudocalanus – a copepod that sprat and herring previously relied on.

The dietary overlap between the three species is greatest in spring, when prey diversity is at its lowest and predation pressure on zooplankton at its highest.

"Spring may act as a bottleneck, because resources are limiting for fish production," says Kinlan Jan. "And if we add competition on top of that, it could be quite bad for the fish."

DNA metabarcoding allows the researchers to account for trophic diversity, but also highlights that diversity of trophic interactions contribute to ecosystem functioning and resilience.

”If a species decline another, maybe another species can fulfil its function because we have alternative pathways,” explains Kinlan Jan.

Implications for management

The knowledge revealed by DNA metabarcoding could be important for achieving an ecosystem-based fisheries management, says Monika Winder.

"I think why fisheries management has failed to preserve herring and sprat stocks is because of uncertainty about how ecosystems respond to environmental change. Knowing how species contribute and where the bottlenecks are can help us to be better prepared when ecosystems change."

Text: Lisa Bergqvist

Watch a recording of the seminar