Whether through agriculture or wastewater – human activities often have a negative impact on the chemical status and biological diversity in streams and rivers. But that’s not all: A new meta-study now shows that human influences also impair the ability of watercourses to clean themselves and other ecosystem services more than previously thought. As a result, the rivers lose some important ecosystem functions.
Running waters are the lifelines of our earth, hotspots of biodiversity and an indispensable basis of life for humans: They provide drinking water and are used to irrigate agricultural land. Natural, unstraightened rivers also have a valuable position in flood protection, as they are good at absorbing sudden rainfall. But humans have been influencing the water systems for a long time: the rivers are diverted, straightened, made faster and thus adapted to human needs. In addition, the water bodies are often heavily polluted with nutrients from over-fertilized fields or human waste water.
When determining the pollution of the rivers, scientists usually use the presence of certain indicator species in addition to chemical analyzes to assess the ecological status. . But that doesn’t cover everything, says Mario Brauns from the Helmholtz Center for Environmental Research (UFZ) in Magdeburg: “The vast majority of studies deal with the effects on biodiversity, which from our point of view only covers part of the problem. Because a loss of biological diversity can indicate that something is wrong in a body of water, but whether and to what extent its ecological functions have been affected remains unanswered. Braun and his colleagues therefore examined 125 studies from 30 countries that dealt with the human influence on running water and evaluated the data according to new criteria.
Impaired self-cleaning
The evaluation showed that the ecosystem services of the rivers suffer more from human stressors than assumed according to the usual classification. What is particularly affected is the ability of rivers to clean themselves, i.e. the decomposition of substances by microorganisms living in water: The efficiency with which watercourses can retain nitrate, for example, is almost five times lower in streams that flow through agricultural land in streams with natural surroundings. “That’s really enormous,” says Brauns and explains: “Agricultural watercourses are so heavily polluted by high nutrient concentrations and a damaged water body structure that they can no longer adequately fulfill their natural ecological retention function and thus lose a large part of their purification capacity.” Their capacities for that are exhausted.
According to Braun and his colleagues, the reduced absorption capacity of running waters for nitrate is a major problem, since nitrate, along with phosphorus, is primarily responsible for the accelerated growth of harmful algae and the formation of bottom sludge. But other substances also have a serious impact on the ecosystem function of water bodies: “Modern sewage treatment plants retain nutrients efficiently, but the wastewater can contain significant amounts of toxic compounds such as pesticides and pharmaceuticals that can reduce the decomposition of leaf litter,” explain Braun and sein Team. As a result, the river ecosystem lacks important organic components that are released when microbes break down leaves.
Sewage on the inglorious number one
Another important result of the meta-study is the comparative evaluation of the different human influences. Which stressor has the strongest impact across all ecological functions? Accordingly, the discharge of sewage is by far the most polluting factor for rivers. This is followed by agriculture in second place and urbanization in third place. According to the scientists, other factors are the loss of habitat, the accumulation of nutrients and the regulation of river flow. But the first three factors in particular give an indication of where urgent action needs to be taken when it comes to water protection in the future.
Thus, the ecological functions of watercourses are good indicators for assessing water hazard and for initiating appropriate measures, explains Braun. “We hope that in the future there will be more study approaches that focus on the ecological functions of running water. And best of all on a broad level worldwide – because there is an urgent need for action around the world”.
Source: Helmholtz Center for Environmental Research; Specialist article: Global Change Biology, doi: 10.1111/gcb.16210