The state of health of intermittent rivers

More than 50% of the world’s fluvial network is made up of temporary or intermittent rivers: those that, during a time of the year, mainly the summer, present completely dry channels or with some isolated rafts. These rivers present a high variability, both spatial and temporal, which means that the same tools used to gauge the state of health of permanent rivers cannot be applied to them.

A study led by researchers from the University of Barcelona (UB) has identified new biological indicators —organisms such as insects or other aquatic invertebrates— that will be used to assess the human impact on the water quality of these rivers, which are home to a very important fraction both terrestrial and aquatic biodiversity.

The study, based on data from 33 rivers in Catalonia, has analyzed the aquatic invertebrates present in intermittent rivers and has classified them according to their resistance to the duration and frequency of dry phases. These results could allow managers to adapt some of the indices currently used that are not adapted to this type of intermittent rivers.

The work, led by Professor Isabel Muñoz, has the participation of researchers from the Department of Evolutionary Biology, Ecology and Environmental Sciences of the Faculty of Biology of the UB Rebeca Arias-Real —first author of the study— and Margarita Menéndez. Cayetano Gutiérrez-Cánovas, a researcher at the Doñana Biological Station (EBD) of the Higher Council for Scientific Research (CSIC), in Spain, has also been part of the research.

The results of the study will improve the current tools to detect the human impact on the water quality of intermittent rivers. (Photo: Rebeca Arias-Real / University of Barcelona. CC BY)

Despite the extension and importance of intermittent rivers, these ecosystems have been excluded from almost all conservation and evaluation programs, because it is very difficult to establish reference conditions, due to the different aquatic and dry phases they go through. “Current biomonitoring tools are based on the species that live in permanent rivers and, therefore, are not efficient if we apply them, without adaptations, to intermittent rivers,” explains Isabel Muñoz.

The organisms that these intermittent rivers harbor present adaptations that allow them to live in conditions of change between aquatic and terrestrial phases. Therefore, the objective of the study has been to find out which species can live in intermittent rivers, or under what hydrological conditions they are capable of doing so, in order to adapt and modify the current indices for evaluating the ecological status of rivers. “With our approach, we could identify taxa and indicators that respond to anthropogenic impacts and not to the stress of the dry or terrestrial phase,” explains Rebeca Arias-Real. “In other words, the fact that a species does not appear in an intermittent river does not necessarily mean that it is due to anthropogenic contamination, perhaps it is because that species does not have the necessary adaptations to survive in these highly fluctuating ecosystems.”

To this end, researchers have measured intermittent river hydrological variables, such as the exact number of days rivers were “dry” or how many times they dried up in a year. “In recent years, the use of sensors capable of measuring the temperature or water level in situ has made us increasingly have more quantitative approximations that help us better understand the effect of intermittent flow on biodiversity,” highlights Rebeca Arias-Real.

From these variables and from the detailed study of the characteristics and the abundance and density of aquatic invertebrates in the different hydrological phases, the researchers have been able to construct the hydrological niches of these organisms, that is, their limits of resistance to desiccation. Based on the results, four different groups of invertebrates were established: one sensitive to desiccation and three with different levels of resistance. Thus, species present in niches resistant to the dry phase could serve to develop or adapt current biomonitoring indices and establish reference conditions for intermittent rivers, while species present in sensitive niches “should be excluded from the indices, since their absence is not due to anthropic impacts but to the impossibility of their presence”, points out Rebeca Arias-Real.

«For example —he continues— we identified taxa sensitive to contamination with tolerance to partial desiccation, such as Lepidostoma; moderate, like Corduliidae; or high, like Nemoura, which could serve as potential bioindicators for intermittent rivers.”

These results open the door to implement this methodology in other regions and use it with other species in order to improve the conservation of these ecosystems, which is expected to increase in the coming years. “Due to climate change (increase in temperature and decrease in rainfall), it is expected that many rivers that are now permanent will become intermittent and that the already intermittent ones will increase the frequency and duration of their dry phases,” warns Rebeca Arias-Real.

Therefore, for the researcher, it is very important “to continue working to understand how biodiversity responds to the continuous cycles of aquatic and dry phases and how this affects its functioning in order to advance in the conservation of these unique ecosystems”, concludes the researcher. .

The study is entitled “Drying niches of aquatic macroinvertebrates identify potential biomonitoring indicators in intermittent and ephemeral streams.” And it has recently been published in the academic journal Ecological Indicators. (Source: UB)