The long, hot summer of 2018 goes down in Dutch history as the warmest in three centuries. But how long will its record stand? NIOO researchers Dedmer van de Waal, Annelein Meisner and Wietse de Boer are preparing for new challenges ahead.


Dedmer van de Waal (NIOO-KNAW)

Even before the summer started breaking weather records, aquatic ecologist Dedmer van de Waal was aware that something was out of the ordinary. Van de Waal's research includes cyanobacteria: ancient, microscopic organisms notorious for their tocix blooms, which pose a risk to animal and human health.

"The drought produced some unexpected effects", says Van de Waal. "There were even cyanobacteria in the river Meuse! Because of low water levels, the river's flow had slowed down and it had turned into a kind of elongated lake where cyanobacteria could thrive. If you study cyanobacteria, that's an interesting observation."

Taste of things to come

It was interesting as a test case, too, says Van de Waal. "I was increasingly aware that a summer such as that of 2018 could well be a taste of what's to come." Sources such as the IPCC leave little doubt: extreme weather will occur more often in the future.

That doesn't mean the impact will be straightforward to predict. "It's tempting to conclude that warm summers mean more cyanobacteria, and we did see more problems in swimming waters last year. But not in all places! A possible explanation is that nutrient run-off may also be affected."

Nutrients from farmlands are swept away by rain or flooding, and end up in surface waters such as lakes where they cause cyanobacteria to proliferate. But if the weather is as dry as it was last summer, that doesn't happen to the same extent. "It's another effect of the drought, one that got less attention. 2018 was a fascinating year in that respect."

Disrupted balance in the soil


Annelein Meisner (NIOO-KNAW)

Microbial ecologists Annelein Meisner (NIOO-KNAW and Lund University, Sweden) and Wietse de Boer (NIOO-KNAW) agree. In a 'perspective article' in the scientific journal Frontiers in Microbiology, they present their outlook on the possible effects of extreme weather for agriculture, including reduced crop yields.

These might not just be the result of the extreme weather itself, they argue, but also of its indirect consequences. One is that the balance between soil-borne pathogens and the microorganisms that suppress them is disrupted.

Microbes interact in soil via a kind of "chemical warfare", according to Meisner. "Microbes interact when they compete for nutrients such as plant litter. But the chemicals they use to suppress their rivals also halt the growth of pathogenic microorganisms, helping to control them in a natural way."

During periods of extreme weather - dry or wet - many species of microorganisms die, changing the overall composition of the microbial community. Interactions between microorganisms may also change, leading to a reduced suppressive capacity of the soil against crop pathogens.

Improving survival chances

To survive extremely wet weather, for instance, microbes need to be able to cope with conditions without oxygen. Some groups of microbes are better at this than others. "Earlier research by De Boer found that the group of microbes that survive such anaerobic conditions do not suppress soil-borne crop pathogens", says Meisner.

Many pathogens survive extreme weather. "If there's a drought, pathogenic fungi can survive because they form survival structures such as spores." So if further research confirms that extreme weather disrupts the balance between pathogens and their suppressors - resulting in more diseased plants - it's important to investigate ways of managing this, says Meisner.

"We should look at potential future avenues to improve the resistance and/or recovery of natural biocontrol. These might include the addition of organic material as a means of protection, or the addition of beneficial microorganisms during the recovery period after extreme weather."

Along broader lines

If one thing is clear, it's that the overall impact of extreme weather on the functioning of ecosystems is in need of further investigation. Dedmer van de Waal, too, finds that the record weather of 2018 has challenged him to explore new avenues. "I've started thinking along broader lines, adjusting the kinds of measurements we do. We're now considering more parameters."

This includes a more prominent role for factors such as the amount of water and the stratification of the water. "Normally, we'd look at the chemical aspect: what nutrients are in the water? Also the biological aspect: what kinds of animals do we find? But now we're focusing increasingly on the physical aspect as well: what happens when there's wind or rain? For the growth of algal blooms, this can be an important determining factor."