We have just experienced a heat wave, and the weather is getting cooler and wetter, but we will still have to contend with the consequences of recent dry summers. “The drought has been particularly extreme during the last three years,” says Sarian Kosten, aquatic ecologist at Radboud University. “Precipitation increased by 26 percent between 1919 and 2019, but that did not compensate for increasing evaporation due to high temperatures.”
“Water management in the Netherlands has focused on discharging excess water, but attention must now be paid to retaining water.” Kosten points to the damage to forests – “leaves and acorns that are already falling from trees” – and the CO2 emissions from drying peat soils. She is studying the consequences of drought on water quality and aquatic organisms.
Blue-green algae and botulism
“The drying up of aquatic ecosystems such as streams is harmful to the animals that live there, such as tube worms and amphipods that play an important role in the aquatic foodweb.” Aquatic systems which have a more constant water level also suffer from the drought. “In the Netherlands, the water levels in many aquatic systems are kept constant with extra inlet water. In these waters you often also see a deterioration of water quality because the inlet water is often of poorer quality in drier periods. For example, the algae concentration in the Maas can increase enormously, making it more like an elongated lake than a river.”
“The high temperatures also cause pathogenic cyanobacteria, usually called blue-green algae, to bloom.” Botulism poisoning, caused by botulinum toxins, is also a hidden hazard in polluted water. Humans usually do not get seriously ill, but the toxins can be life-threatening for waterfowl, fish and dogs.
The problem will not resolve itself. On the contrary: “Aquatic ecosystems are good at retaining organic carbon. When a body of water dries out, this organic carbon breaks down more quickly and releases greenhouse gases, which contributes to a further rise in temperature.” This applies not only to urban ponds in the Netherlands, but also to areas such as the Amazon rainforest. “We estimate that four petagrams, or 4,000,000,000,000 kilograms, of organic carbon is stored in the lakes and swamps of the Amazon. During decomposition, this is obviously not released as CO2 in one go, but persistent drought can still lead to gigantic emissions.”
Exotic species under control
Thankfully, drought also has positive consequences. “Fish kills in flood plain lakes that temporary dry out, facilitates water plants recovery upon refilling. Amphibians benefit as well because the fish do not eat their eggs. Water managers also use low water levels as a window of opportunity to remove exotic species such as the invasive sunfish from the water to protect the aquatic systems.”
Kosten approves of these interventions, but the situation is still problematic. To maintain the water quantity and quality, much more needs to be done, including policy changes. “But people can start on a small scale. For example, by building a dam to retain water in a wetland. A beaver recently set a good example. Thanks to its huge dam, the Aa river, which had dried up locally in Brabant, is now full of water again.”
Photo: Water in the Ooijpolder. Source: Adriaan Castermans via Wikimedia Commons.
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