Climate change reduces the size and survival of aquatic insects

The low availability of oxygen in warm waters due to high temperatures associated with climate change may be causing a decrease in the size of aquatic insects, compromising their survival and biological performance, according to an international study led by the Doñana Biological Station-CSIC.

As a result, there is an increased risk of deterioration of aquatic ecosystems, which would translate into a decrease in fish availability due to significant impacts on the food web. This study, which involved collaboration between the Slovak Academy of Sciences and the University of Granada, has been published in the journal Ecological Entomology.

Rising global temperatures are leading to a further warming of marine and inland waters, which, among other effects, causes a decrease in oxygen solubility. The lower concentration of oxygen in water affects fish, aquatic insects, and microorganisms that need it to breathe.

"The decline in size is another aspect of the general decline of insects, associated with increasing temperature increases. Not only are there fewer insects, but they could also be smaller," says Viktor Baranov, a researcher at the Doñana Biological Station and first author of the study. "This dual effect on the size and number of aquatic insects could be reducing their ability to maintain crucial ecosystem functions."

To conduct this study, the team designed an experiment to evaluate the relationships between water temperature, oxygen concentration, and the size of Chironomus riparius, a species of fly of the genus Chironomidae, whose larvae are aquatic. The size and survival of the insects were compared in six different scenarios: three at a temperature of 20°C, with high, medium, and low oxygen concentrations, and three at 30°C, with the same three oxygen concentrations.

The results were robust and join those obtained in another study published in 2021, in which Viktor Baranov also participated, in which they found that the size of flies of this genus and temperature were related.

Chironomus riparius, commonly known as non-biting midges or chironomid midges, are so named for their resemblance to common mosquitoes (Culicidae), but they have important differences, especially the fact that they neither bite nor feed on blood. Their genome has been sequenced and is used as a model to assess environmental stress and the impact of pollutants in aquatic ecosystems.

In this new study, chironomid larvae that developed in warm waters with low oxygen concentrations were 10% smaller than those that developed in the other five scenarios. These conditions also led to faster growth and higher mortality in the insects.

"Since climate change is causing temperatures to rise, and oxygen is less soluble in warmer waters, the larvae of these animals are having trouble growing. This is because respiration is essentially a driver of animal growth," the researcher explains.

Aquatic insects are essential for the purification of aquatic pollution and the assessment of water quality, as well as for the proper functioning of the food web, as they are food for many other species. Not only that, as adults, these insects also perform other functions, such as crop pollination. Some of them also perform recreational functions for humans, as is the case with dragonflies, which have an important aesthetic and cultural component. "Due to their prominent role, the negative effects that climate change has on these organisms are extremely significant," emphasizes Viktor Baranov.

This work is a starting point for a broader study of the effects of climate change on animal functional traits, such as size, and is combined with the widely documented decline in insect numbers. It also opens the door to investigating how animal size declines affect the ecosystems they serve.