Scientists: ponds don't save the landscape from drought — their water overheats and turns foul

A four-year study by researchers from Masaryk University and the Water Management Research Institute has found that ponds are not the best way to hold water in the landscape. Scientists examined life in ponds and in the streams below them, discovering that water flowing out of ponds is overheated and polluted, unable to sustain aquatic organisms. The cause lies in climate warming combined with current land-management practices.

Over the past year, many ponds have looked equally grim: dead fish, blooms of blue-green algae, and low water levels. "In ponds with a small inflow that aren't fed by large rivers, the water is so scarce that these bodies of water dry up completely," confirmed Radek Ondra, a water management specialist at Rybníkářství Pohořelice. One example is the pond in Litobratřice in the Znojmo region — just last year it was full of water and carp, and six tonnes of fish were caught there.

At a nine-hectare pond near Suchohrdly by Miroslav, fishermen had to carry out an early catch two weeks ago, relocating all the fish before they could die. "If hydrological conditions in the landscape don't improve, we're considering agricultural use of the ponds — for example, planting crops," Ondra said.

Drought is worse below a pond, due to evaporation

According to scientists, ponds have changed significantly over recent decades. As part of a four-year project funded by the Technology Agency of the Czech Republic, researchers tracked key groups of aquatic organisms — fish, invertebrates, algae — and analyzed how the chemical and physical properties of water change downstream of ponds. The key question was whether ponds compensate for reduced flow in drying streams during drought.

"We originally thought a pond would help maintain flow during drought, but it turned out to be exactly the opposite. Drought is much more severe below a pond than in the stream feeding into it. That's because of evaporation," explained Michal Straka, a hydrobiologist at the Faculty of Science of Masaryk University.

Hydrobiologists estimate that during high temperatures, a ten-hectare pond can lose up to eight liters of water per second to evaporation. "Water quality really does deteriorate sharply. It gets hot, and there's a mass proliferation of algae," Straka noted.

Oxygen levels drop

Because the water in a pond overheats, the concentration of oxygen needed for most aquatic organisms to survive drops sharply. Scientists also recorded elevated concentrations of nutrients — phosphorus above all — below ponds, which fuels algae and blue-green bacteria growth.

Researchers argue that anti-drought measures need to be far more comprehensive and cover the entire landscape. Retaining water in reservoirs, whether large or small, only addresses the symptoms of the problem, not its cause.

The state is offering new subsidies

The Ministry of Agriculture has been providing subsidies for small watercourses and reservoirs for a decade now. A new round of applications for municipalities has just opened, worth 300 million crowns — the third phase of the program. In the previous two phases, more than 1,400 projects received support totaling over two billion crowns.

Municipalities can use the funds to reconstruct, build, restore, repair and desilt non-commercial fishing ponds and small reservoirs. The goal is to strengthen water retention and accumulation in the landscape, boost utility water reserves for towns and villages, and build up reserves in case of fires.

Subsidy funds are planned for repairing a reservoir in Bohutice, where water quality worsens every summer due to weak inflow — the repair is expected to cost around two million crowns. In Bykovice in the Blansko region, authorities want to restore a reservoir on the village square, which will later serve as a fire-fighting reserve. "We decided to support this mainly because subsidy programs like this are received very positively by municipalities," explained Marek Otava, a spokesperson for the Ministry of Agriculture.

Restoration is already underway on the pond in Kotvrdovice, where Václav Zoubek came to watch the progress — he used to swim there as a child. "By the dam we always used to do swallow dives, dive in, and race to see who could swim furthest underwater," he recalled. The pond's dam was leaking and the reservoir could not hold water — the region also contributed funding for the repair.

Why reservoirs matter

"Reservoirs are an important element of the landscape, helping to maintain the local microclimate and a stable groundwater level, and serving as a habitat for a variety of animal and plant species. They're also important for recreation and create a water reserve in case of drought or fire," the Ministry's statement reads.

Many reservoirs in municipalities have been neglected. "Many are silted up, and structures such as emergency spillways need repair, dams need reinforcing where they leak, and banks need shoring up where erosion has caused damage," said Tomáš Havlíček, a water management specialist at Atelier Fontes.

Municipalities can submit applications through the Ministry of Finance's unified subsidy portal. Applications are open from September 7 to October 7, or until the full amount is allocated. The program will run until 2028.

According to the Czech Statistical Office, water surfaces cover just over two percent of the country's territory — more than 1,700 square kilometers. Most reservoirs are found in South Bohemia, and the fewest in the Karlovy Vary region. The Czech Republic has 177 dams, more than 1,100 natural lakes and nearly 1,500 artificial ones — for example, flooded quarries and sand pits. Ponds and fish farming pools are the most numerous, at almost 38,000.

Read also: 8 best swimming spots near Prague: lakes, quarries and beaches within an hour of the capital

Around Prague, based on reporting by ct24.ceskatelevize.cz