Dams, reservoirs, channelisation, water extraction, etc. are just some of the many ways in which human activity exploits water resources. “Human impact has been known for a long time to exert a great impact on river ecosystems; there are numerous pieces of research on the subject,” said Arturo Elosegi-Irurtia, Prof. of Ecology in the Department of Plant Biology and Ecology at the UPV/EHU’s Faculty of Science and Technology. Yet the research conducted on the subject, “is of a partial nature and highly dispersed, which precludes the possibility of gaining an overview of the impact of human activity", he went on.

In order to create that overview, members of 6 research centres and universities in Catalonia, the Netherlands and Portugal, together with the UPV/EHU, have produced a meta-analysis of numerous case studies. The aim of producing a compilation and re-analysis of the information from these individual studies was to find out which components and functions of river ecosystems are affected by human-driven water stress, and the extent to which these ecosystems are altered. “In the initial search we gathered together over a thousand articles but in the end only 44 met the conditions we established for the analysis. In our statistical analysis we worked with a total of 262 cases relating to rivers across the world,” specified Elosegi.

Significant alterations, above all in large reservoirs

Variability is inherent in rivers where periods of flooding and low water levels take place cyclically, but human interventions alter this cycle and affect the ecosystem directly. “For example, one of the clearest consequences we have observed is the excessive growth of algae.  This takes place mainly in the stretches located below reservoirs and water outlets because the water flow is greatly stabilised,” he explained. River invertebrates, for their part, diminish noticeably below the points of stress both in terms of abundance and richness.

Apart from the biological communities, they studied the physical and chemical variables of rivers as well as the functioning of ecosystems, in other words, “how the river ecosystem functions with the structure and community found. Among the structural variables, the increase in the concentration of drugs stood out in the places affected by water stress, such as below the reservoirs or the rivers from which water is extracted".  

As regards how the ecosystem functions, Elosegi stressed that they had been able to see “alterations that have not been evident until now” in zones affected by water stress: firstly, the decomposition of organic matter is reduced, which means that rivers lose their capacity to break down organic matter, and secondly, metabolism is accelerated: “so there is an increase not only in primary production but also in respiration as a result of excessive algae growth”, he added.

The above-mentioned alterations and consequences, despite being general, varied in significance or seriousness depending on the characteristics of the location such as river size, climate or water regime. Likewise, depending on what has caused the water stress, the consequences can be more serious or less serious, and, as they have found, “reservoirs, large ones in particular, are the ones that cause the greatest number of changes or alterations in the structure and function of river ecosystems”. And reservoirs are in fact the most widespread interventions and causes of stress in these ecosystems: over half the cases studied took the alteration caused by reservoirs into consideration.

However, it was not possible to draw clear conclusions with respect to all the variables taken into account in the meta-analysis.  “Sometimes it was due to a lack of data, in other words, because they were explored in few of the studies we had available, and other times because of the great variability that exists in the response of some variables, which has meant we were unable to deduce anything clear in the meta-analysis carried out. But this happened to us with temperature, among other things: below some reservoirs the temperature is higher than it should be, but in others it is lower. In the case of the communities, we saw these fluctuations in the fish, which most likely can be put down to the fact that in each space they are affected by various factors." The research group believes it is essential to continue with these studies so that the gaps can be filled.

Additional information

The research published in Scientific Reports comes within the European Globaqua project. Research personnel from the following seven organisations participated in it: the Catalan Institute for Water Research (ICRA), the University of Girona, the Institute for Environmental Diagnostics and Water Studies (IDAEA-CSIC) and the University of Barcelona, all of which are Catalan organisations; the UPV/EHU’s Department of Plant Biology and Ecology; the Dutch Delft Institute for Water Education, and the University of Coimbra, in Portugal.