One and a half hectares of polder, a large volume of water, and a group of curious researchers from various universities and scientific backgrounds led by ecologist Maarten Schrama. These are the ingredients needed to answer the question: how do water retention areas affect nature, animals, and our health?

Due to climate change, we are experiencing longer periods of higher temperatures and drought, followed by heavy rainfall. Peak storage areas, zones that can be temporarily flooded, are a solution to prevent water nuisance in cities. But could these areas become breeding grounds for disease carriers such as mosquitoes and migratory birds? And what does that mean for the spread of viruses?

To find out, researchers from various universities, research institutes, government bodies, and nature organisations temporarily flooded more than a hectare of the Eendragtspolder in Zevenhuizen this past August. A moment that all involved had waited three years for, Schrama explains just before the inundation-a beautiful word for submersion. 'It is incredibly important to collaborate with all these parties and examine everything from the effects on water quality to the experience of local residents. It is very difficult to determine the societal impact of extreme weather. That can only be done with a diverse coalition of ecologists, virologists, and meteorologists.'

Living laboratory

The Eendragtspolder was designed as a water buffer to protect Rotterdam during extreme rainfall. The area can hold four million cubic meters of water. For their field research, the researchers were able to use part of it as a living laboratory. 'We measure the temperature, the weather, the behavior of birds, the number of mosquitoes - everything is recorded,' explained researcher Reina Sikkema from Erasmus MC, before she and Schrama switched on the water pump. 'We know that climate change is causing more and more viruses to appear, but we don't yet know what effect such a water storage area has in relation to those viruses. Being able to study that here is very special.'

As soon as the water began flowing into the polder, the surroundings changed. Among the grass, there turned out to be far more animals and insects than one might have expected at first glance. A striking number of salamanders, moles, and mice quickly sought out drier places - sometimes helped along by one of the researchers. A week and a half later, many birds had also been drawn to the wet polder, alongside mosquitoes and other insects. 'We notice that it's a place where a lot of activity gathers. We're seeing crows, pigeons, waders, and many other species,' said Schrama. 'We even spotted a Purple Swamphen.'

The arrival of birds is good news for the project. Mosquitoes can't fly far, but birds can, potentially spreading viruses across greater distances. Which species carry which viruses, and how far they travel, are among the questions this research hopes to answer. Smaller animals like salamanders are also part of the picture: they don't carry viruses themselves but feed on mosquitoes, possibly helping limit disease spread.

After the experiment

The experiment lasted just over two weeks, during which the research team lived together in a nearby holiday house. Their days started early and ended late, often spent wading through the flooded fields - sometimes barefoot. The amount of data collected was enormous, and the analysis will take months. But the researchers already consider the experiment a success.

'The knowledge we gain from this gives us the opportunity to act preventively and to design water storage areas effectively. For example, by choosing vegetation that deters certain animal species, or by making areas less attractive to mosquitoes. This way, we can be better prepared for a future in which climate change - and our response to it - continues to shape our living environment', Sikkema concludes.

This experiment is one of the frontrunner projects of the Pandemic and Disaster Preparedness Center (PDPC). The PDPC is a collaboration between Erasmus University Rotterdam, Erasmus MC, and TU Delft, and is one of the five Convergence programs aimed at preparing society for future disasters and pandemics.

Other partners in this project include: KNMI, Naturalis, GGD Hollands Midden, GGD Rotterdam-Rijnmond, Staatsbosbeheer, Natuur- en Vogelwacht Rotta, Recreatieschap Rottemeren, Leiden University, Sovon, the Netherlands Institute of Ecology (NIOO-KNAW), the Schieland and Krimpenerwaard District Water Control Board (HHSK), Blanes Centre for Advanced Studies (CEAB), the Municipality of Rotterdam, the Dutch Youth Association for Nature Study (NJN), and the Municipality of Zuidplas.

Health threats

Erasmus MC's focus on early diagnostics, population screening, and population studies enables us to respond quickly to health threats caused by climate change, globalization, and lifestyle. We study effective measures to prevent and mitigate health effects. As described in Koers28, the Erasmus MC strategy.

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Flooded Polder Helps Fight Mosquito-borne Diseases