Human-induced water stress modifies ecological functioning of watercourses

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In many parts of the world, especially where water is scarce, watercourses are being intensively developed. The need to provide water for agricultural, municipal and industrial purposes involves the construction of weirs, dams, river regulation, groundwater and surface water abstractions. This, in turn, leads to changes in the flow regime and, consequently, to a decline in water quality and biodiversity [1,2].

A situation in which there is not enough water of sufficient quality to meet the needs of people and the environment is called water stress. As the European Environment Agency’s 2021 Water Stress Report indicates, such a condition is already being observed in many parts of Europe. Droughts are no longer rare or extreme phenomena in Europe, and on average about 20% of the continent’s territory and 30% of its population are affected by water shortages throughout the year. Climate change is expected to exacerbate the problem as the frequency, scale and impacts of drought increase.

These trends are of particular concern in southern and southwestern Europe, where, under a 3°C temperature increase scenario, summer river flows could drop by as much as 40%. And this, in addition to the obvious negative socio-economic effects, will also have the effect of worsening the ecological conditions of the rivers.

Human water stress (HWDS)

Water stress can be caused by natural phenomena (e.g., droughts), those resulting from human activity (e.g., unsustainable water abstraction, deterioration of water quality), or a combination of these (climate change). This human-driven water stress(HDWS) is a different phenomenon from naturally occurring water shortages, such as in intermittent rivers and ephemeral streams ( IRES). While in IRES flow restrictions or inhibition are due to climatic conditions, are characterized by a certain regularity and often occur at specific times of the year, HWDS are unexpected changes in the flow regime, occurring at any point in the hydrological cycle as a result of human management decisions, at locations associated with water facilities.

That is, unlike IRES, where ecological processes follow characteristic spatial and temporal patterns that depend on the intensity and length of the drought period, HDWS occurs at times and places where communities of organisms are completely unsuited to it.

Meta-analysis of the impact of HDWS on ecosystem components

The phenomenon of the impact of HDWS on rivers was looked at by a group of scientists led by Professor Sergei Sabater, who reviewed more than 1,000 publications and identified 44 publications and 262 BACI-type impact comparisons (before-after-control-impact), enabling assessment of the significance, magnitude and direction of the impact of anthropogenic water stress on water quality (nutrient and micropollutant concentrations), abundance, biomass and diversity of biological communities (bacteria, algae, benthic invertebrates and fish) and river ecosystem functions (organic matter decomposition, gross primary production and respiration). The authors also attempted to identify factors that may affect the magnitude and direction of HDWS effects on the analyzed components of the aquatic environment. The results of their meta-analysis were published inScientific Reportsin July 2018. [3].

The authors analyzed the magnitude of the effect of HDWS on the components of the aquatic environment on the basis of theresponse ratio, which expresses the ratio of the studied variable under disturbed (subjected to pressure) and undisturbed (control, reference) conditions, where a value of R = 1 indicated the absence of influence of water stress, R < 1 for inhibition or decline, and R > 1 for stimulation or growth of the analyzed trait due to water stress.

Biology more, metabolism less

And what does it turn out to be? Of all the variables analyzed, the greatest positive (stimulating) effect of water stress was found for algae, and the greatest negative (inhibiting) effect was found for benthic macroinvertebrates.

Water stress primarily promoted an increase in algal biomass, which was mainly due to the slowing of water flow due to damming or abstraction, reducing drift and creating conditions favorable for the proliferation of populations. Under water stress conditions, algal biomass increased by 1.5 to 10 times, and the strength of the response depended on climate (stronger under continental than temperate climate conditions), the time of year (stronger in autumn than at other times of the year), river size (stronger in larger systems than in smaller ones), the type of water stress (stronger in dam-regulated rivers than in those with diversions), and nutrient availability (stronger in nutrient-rich sites than in poor ones).

The opposite trends were shown by benthic macroinvertebrate communities, whose abundance, density and taxonomic richness significantly decreased under water stress conditions. Invertebrates in general are a relatively sensitive group to the stability of hydrological conditions. Declines in species richness were observed in almost all types of water scarcity, both in regulated rivers and those transformed by water abstraction, especially in regions with arid climates. Flow regulation due to the presence of dams and dams has had a particularly negative impact on filterers, scrapers and shredders, while relatively little on predators. This selective influence on trophic guilds was reflected in the reduced participation of representatives of the families Ephemeroptera (mayflies), Plecoptera (forkbeards) and Trichoptera (caddisflies), the taxa most sensitive to changing physical conditions.

Although to a lesser extent than on biocenoses, HDWS was not indifferent to the lake’s metabolism, expressed by the ratio of gross primary production and respiration. Under slower flow conditions, a significant (average 3-fold) increase in river metabolism was observed in the lower reaches, likely due to increased accumulation of organic matter. The water deficit reduced water distribution by an average of 31%, with the reduction being greater in areas with a continental climate, during the autumn-winter season and in medium-sized rivers, mainly under nutrient-poor conditions. The slowing down of organic matter decomposition due to water stress was mainly contributed to by the reduced physical fragmentation of particles under slowed flow, as well as a decrease in the biomass and abundance of reductivores (mainly saprotrophic bacteria and fungi) and detritusophages.

Quite surprisingly, water stress had only a minor effect on chemical indicators of water quality. The meta-analysis showed a decrease in phosphate concentrations (by 27%) and an increase in pharmaceuticals (more than 8-fold on average, but here very high variability was observed, likely due to the availability of only 6 studies in this area). In contrast, there was no significant effect of HDWS on nitrate, ammonia, organic nitrogen or total phosphorus concentrations. The increase in the concentration of pharmaceuticals can be explained by a simple thickening due to a decrease in the amount of water of substances that are not subject or are subject only to limited biological degradation. In the case of phosphate, on the other hand, extended retention times and increased photolysis may promote more efficient utilization by organisms and ultimately contribute to lower concentrations.

Climate change will exacerbate the effects of HDWS

A meta-analysis by S. Sabater’s team showed that HDWS has a number of effects on the structure and function of river ecosystems, other than those that occur naturally, such as in intermittent rivers and ephemeral streams. These effects varied in intensity depending on the type of water stress, with the strongest observed in the case of cross-barriers (dams, weirs), slightly weaker in situations of water flow regulation and embankment construction, and the effects of groundwater abstractions were relatively least severe.

As the EEA report cited earlier points out, in order to minimize the impact of water scarcity on people and the environment, we need to move away from crisismanagement torisk management, i.e., focus on actions directed at strengthening ecosystem resilience and using water more efficiently. The progressive effects of climate change are certain to exacerbate the negative impact of anthropogenic water stress on our rivers. When considering how to reduce the risk of undesirable phenomena in our waters (including toxic blooms of certain golden haptophytes), keep in mind the stimulating effect of HDWS on algal biomass.

In the article, I used, among others. From the works:

[1] Nilsson, C., Reidy, C. A., Dynesius, M. & Revenga, C. (2005). Fragmentation and flow regulation of the world’s large river systems. Science 308, 405-408.

[2] Veldkamp, T.I.E., Wada, Y., Aerts, J.C.J.H., Doll, P., Gosling, S.N., Liu, J., Masaki, Y., Oki T., Ostberg, S., Pokhrel, Y., Satoh, Y., Kim, H. & Ward, P.J. (2017). Water scarcity hotspots travel downstream due to human interventions in the 20th and 21st centuries. Nature Communications 8, 15697.

[3] Sabater, S., Bregoli, F., Acuña, V., Barceló D., Elosegi, A., Ginebreda, A., Marcé, R., Muñoz, I., Sabater-Liesa L. & Ferreira V. (2021). Effects of human-driven water stress on river ecosystems: a meta-analysis. Scientific Reports 8:11462, DOI:10.1038/s41598-018-29807-7.