To understand why cyanobacterial populations change, the researchers calculated the daily increase or decrease in biomass – the so-called net growth rates. Using a recently developed analytical method, they quantified the extent to which a particular factor altered the temperature and nutrient conditions necessary for cyanobacterial growth. The results reveal a highly complex system. While classic environmental factors such as temperature or phosphate do have a significant influence on cyanobacteria, “But on a daily timescale, biological factors proved to be even more important,” says Pinelopi Ntetsika.

Certain groups of zooplankton and other algae had a particularly significant impact. For example, so-called rotifers – microscopic aquatic animals that take their name from a ring of cilia on their head that moves like rotating wheels – have an inhibitory effect on cyanobacteria. High rotifer densities increased the water temperature required for accumulation of Dolichospermum biomass by around 13 degrees Celsius. “Rotifers clearly feed on the cyanobacteria and act as regulators,” says Pinelopi Ntetsika. “That was surprising. Until now, it was assumed that water fleas, known as Daphnia, were the cyanobacteria’s main predators.”

Other organisms had the opposite effect. In particular, chrysophytes, a group of mostly golden-brown single-celled organisms, appear to promote the growth of cyanobacteria. In their presence, Dolichospermum populations grew even at water temperatures nearly nine degrees lower and under extremely low phosphorus concentrations.

New predictive possibilities

How such synergistic interactions arise has hardly been investigated to date. It could be, for example, that one species excretes certain metabolic products that another species requires as a nutrient source, says Pinelopi Ntetsika. “Or: a plankton-feeding organism prefers a green alga because it is easier to digest than a blue-green alga. If both occur side by side, the blue-green algae benefit because the grazing pressure on them decreases.”

According to the study, favourable conditions for an algal bloom therefore arise in dynamic processes through the interaction of numerous species within the ecosystem. A temperature that does not trigger a bloom in one year can suddenly become highly problematic under changed biological conditions. Conversely, not every warm summer in a nutrient-rich lake necessarily leads to a massive proliferation of blue-green algae.

The findings also have practical implications for water management. Rather than focusing solely on temperature, phosphorus or nitrogen, the researchers suggest that in future, greater attention should be paid to monitoring specific plankton groups. The abundance of rotifers, water fleas or chrysophytes could provide indications of whether a cyanobacterial bloom is developing in a lake.

However, the interactions between species are extremely complex and site-dependent. “Our results apply to Lake Greifensee – in other lakes, different species and interactions may be important for cyanobacterial growth,” says Pinelopi Ntetsika. To investigate such differences, more research is needed – and more automated underwater microscopes, such as those used by the team in Lake Greifensee. Francesco Pomati and Pinelopi Ntetsika are convinced that this is how the secrets of cyanobacterial blooms can be unravelled step by step.