One and a half hectares of polder, a large volume of water, and a group of curious researchers with diverse scientific backgrounds from various universities. These are the ingredients needed to answer the question: how do temporary water buffers affect nature, animals, and our health?

Due to climate change, we are facing longer periods of higher temperatures and drought, followed by heavy rainfall. Peak storage areas – land that can be temporarily flooded – are a solution to prevent flooding in cities. But rising temperatures, extreme rainfall, salt water intrusion and water buffers possibly influence ecology and behaviour of disease carriers – such as mosquitoes and migratory birds. Can water storage areas become breeding grounds for these disease carriers? And what does that mean for the spread of viruses?

To address these pressing questions, a team of atmospheric scientists, virologists, mosquito experts, hydrologists, bird watchers, foresters, and water managers temporarily flooded more than a hectare of the Eendragtspolder in Zevenhuizen in August 2025. “For everyone involved, this was the moment they had been waiting on for three years,” said Maarten Schrama from Leiden University and the PDPC  just before the inundation – a fancy word for submerging land under water. “It is crucial to collaborate with all these parties, and to examine the impacts ranging from water quality to the experience of local residents.”

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 the field study, the researchers were able to utilize a section of it as a living laboratory. “We measure temperature, weather, bird behavior, the number of mosquitoes, virus presence – everything is being documented,” explained researcher Reina Sikkema of Erasmus MC and the PDPC. “We know that climate change is one of the drivers causing more and more mosquito-borne viruses – such as Usutu or West Nile virus – to emerge here in the Netherlands, but we don’t know what effect such a large scale water storage area has on those viruses. Being able to study that over here is really special.”

Researchers have started to study the area already a year before the inundation started, to serve as a baseline. For example, volunteers of the bird watch organisation Rotta spent the last two years monitoring which bird species have been visiting the area. From this baseline, researchers can more accurately determine the specific effects of the flooding.

As soon as the water was pumped onto the polder, researchers saw the area come to life. Hidden in the grass were far more animals and insects than met the eye at first. An unexpectedly high number of salamanders, moles, and mice quickly sought out a drier place, sometimes helped along by one of the researchers.

“After a few days, the mosquito larvae are proliferating fast,” says Jordy van der Beek, PhD researcher in mosquito ecology at Leiden University, Erasmus MC, Naturalis and the PDPC, who catches, identifies and counts the mosquitoes in the newly created marsh area. A week and a half later, besides mosquitoes and other insects, many new bird species had also come to the wet polder. “Mosquitoes cannot fly very far, but birds can,” says Tijmen Hartung, PhD researcher in bird virology at Erasmus MC and the PDPC. “Which bird species carry which viruses, and how far they spread them, remains to be investigated. The birds drawn to the wet polder can help answer those questions.” Smaller animals such as salamanders are also being studied. They do not carry diseases but do eat mosquitoes and can thus play a role in reducing the spread of viruses.

In addition, the study examines the impact of flooding on people. Sille Pelser, PhD researcher in Public Health at the Municipal Health Services (GGD) Rotterdam-Rijnmond and Erasmus MC, explains: “We measure the number of visitors, and record the time of day they visit: for example, are people coming in the early morning or at dusk – the time that mosquitoes are most active?” The experiences of citizens visiting the Eendragtspolder are also captured through questionnaires and the MosquitoAlert app, helping to assess whether local residents perceive an increase in mosquito-related nuisance.

The local climate

One crucial parameter for the development of mosquitoes is water temperature. Because of this, climate researchers closely monitored the local climate in and around the floodwater using several measuring stations. This data will help ecologists and virologists understand which circumstances most favour virus spread. Besides this, the research serves an additional goal. “The local weather and stagnant water pools have a big influence on each other, but the problem is that this is currently difficult to simulate on the computer,” says Maarten Boonekamp, PhD researcher at TU Delft and the PDPC. By using real-world data as input for models, it will be possible to more accurately predict what the influence of shallow water is on weather and climate. “We can use this opportunity to make weather forecast models – used by for example the KNMI – much more accurate.”

From polder to the lab

The research in the flooded part of the polder lasted more than four months. During that time, the researchers stayed in a nearby holiday cottage. From early in the morning until late in the evening they were busy with fieldwork, in waders or barefoot. The amount of data they gathered is enormous: in the end, a total of 397 rodents and over 8000 adult mosquitoes were caught. Collected samples of mosquitoes, birds and small animals in the environment were sent to the lab to be identified and analysed.

Kiki Streng, postdoctoral researcher at Erasmus MC and the PDPC, is analysing the data to determine how the inundation changed the overall ecology, and if it increased the occurrence of viruses that pose a risk to human and animal health. She found that the inundation indeed dramatically increased the amount of mosquitoes around the flooded area. It also changed which bird species were present – after the inundation, researchers observed more carrion crows, common snipes and canada goose, and caught more blackcaps, common snipes and common reed buntings. Finally, two mosquito-borne viruses of interest, Usutu and West Nile virus, were also found.

“The insights we gain from this research will allow us to take preventive measures and to redesign water storage areas ” explains Sikkema. Water boards, local authorities, forestry commission Staatsbosbeheer and other policy makers, both national and international, can use this information to make decisions that will keep our living environment safe.

The research on the Eendragtspolder flooding comes at exactly the right time. As of yet, we do not face large-scale outbreaks of mosquito-borne viruses in the Netherlands, giving us a crucial window to act. Because of its unique research design and involvement of stakeholders, the study serves as a catalyst for collaboration and knowledge exchange. Moreover, the project is already attracting international attention. In doing so, it not only advances scientific understanding, but also lays the groundwork for more resilient and proactive approaches to managing environmental health risks worldwide.