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Multiple stressors in Science of the Total Environment

November 16, 2016

MARS scientists studying multiple stressors in Lake Beyeshir, Turkey. Image: METU Limnology Laboratory

The MARS Project has been undertaking scientific research into the effects of multiple stressors on aquatic environments for nearly three years now, and project scientists are beginning to widely publish their findings.

Six new papers involving MARS research have recently been published online in the journal Science of the Total Environment, some of which are currently available for free through open-access publishing.


Analysing the impact of multiple stressors in aquatic biomonitoring data: A ‘cookbook’ with applications in R

The health and status of Europe’s freshwaters has been closely monitored since 2000, creating detailed environmental and biological datasets covering over 120,000 water bodies. However, this data has – until now – been rarely used to analyse multiple stressor interactions and impacts. This is partly because of the various scales at which stressors are monitored and reported. A MARS team led by Christian Feld has designed an analytical framework that allows for multiple stressor effects to be analysed through this rich biomonitoring dataset, which is outlined in this paper (link).

Effects of hydro- and thermopeaking on benthic macroinvertebrate drift

Hydropower plants – often located on mountain streams and rivers – are commonly designed to operate in response to electricity demand. This means that the timing and flow of the water they release during and after electricity production can be highly variable. This process is known as ‘hydropeaking’, and can have a range of detrimental ecological effects on downstream ecosystems. Working at the HyTEC facility in Austria, a team led by Lisa Schülting used experimental flumes to observe the ‘drift’ (or movement down and across the simulated stream bed) of macroinvertebrate (or aquatic insect) species.

Overall, the team found that hydropeaking significantly increased the rates of macroinvertebrate drift from their original position on the stream bed. However, this pattern was influenced by water temperature: when hydropeaks of water were cold, total drift rates were reduced, although with strong taxon-specific response patterns. The results were also influenced by the time of day that hydropeaks occurred: increased water flows during the night led to significantly higher drift rates than those during daytime (link).


A golden morning on the Danube River in Budapest. Image: Miroslav Petrasko | Flickr Creative Commons

Relative influence of chemical and non-chemical stressors on invertebrate communities: a case study in the Danube River

Understanding the influence of chemical pollution on the health, diversity and status of biological communities in aquatic ecosystems is a key challenge for ecological risk assessments. However, there are a vast number of different chemicals (and new ones developed each year) which can form complex ‘cocktails’ in freshwaters, and can trigger a number of multiple stress effects. In many cases, the toxic effects of chemical pollution on aquatic life is poorly understood.

A collaborative team from the SOLUTIONS and MARS projects led by Andreu Rico used data from a comprehensive ecological survey of the Danube River – from its source in Germany to its mouth at the Black Sea – to analyse the influence of chemical and non-chemical stressors on invertebrates. They found that variations in invertebrate communities along the Danube are influenced more by varying habitat conditions and physico-chemical parameters (e.g. suspended solids, nutrients and dissolved oxygen) than by chemical pollution (link).

Potamodromous fish movements under multiple stressors: Connectivity reduction and oxygen depletion

Potamodromous fish are those which migrate, but only within freshwater environments, such as from a river to a lake for spawning. In Mediterranean rivers, two key stressors are water abstraction – which can reduce the connectivity between ecosystems needed for migration – and diffuse pollution – which can have harmful ecological impacts such as oxygen depletion.

A team led by Paulo Branco studied the impacts of these two stressors on a Mediterranean potamodromous fish species, the Iberian barbel (Luciobarbus bocagei) using experimental flumes. They found that when connectivity was reduced, fish movement was similarly reduced, regardless of oxygen depletion levels. When connectivity was high, fish movements were reduced in response to increasing oxygen depletion. The results suggest that oxygen depletion as a result of diffuse pollution may prove a barrier to fish migration, even when physical connectivity between different parts of a river basin is high (link).

Untangling the effects of multiple human stressors and their impacts on fish assemblages in European running waters

Understanding the impacts and interactions of multiple stresses on aquatic environments is a key research challenge for scientists, both in Europe and across the world. This study, led by Rafaela Schinegger, used data from over 3,000 sampling sites to map the effects of multiple stressor combinations on fish assemblages in European rivers (read an earlier blog post relating to the work here).

Across the sampling sites, 15 different stressor combinations were observed. Rivers were affected by single stressors only at 30% of sites, whilst 42% of sites were affected by multiple stressor combinations, and 28% were un-impacted. The multiple stressor interaction types varied in character: 40% were additive (where the total stress effect is the sum of each stressor), 30% were synergistic (where the total stress effect exceeds the sum of each stressor) and 30% were antagonistic (where total stress effect is less than the sum of each stressor) (read more about multiple stressor interactions here).

Stressor interactions varied with habitat type: antagonistic effects were only observed in headwaters and medium-gradient rivers, whilst synergistic effects increased from headwaters through medium gradient rivers and Mediterranean streams to large lowland rivers. The study is an important step forward in understanding multiple stressor interactions and impacts in European rivers, and will most likely provide valuable information for guiding conservation and restoration management (open-access link).


The Otra River, Norway. Image: Baard Skaaden

Effects of multiple stresses hydropower, acid deposition and climate change on water chemistry and salmon populations in the River Otra, Norway

The Otra River in Southern Norway is impacted by acid deposition, hydropower development (around 40% of the river has been modified for electricity production) and, increasingly, by climate change. The Otra supports populations of both land-locked and migratory (anadromous, moving between rivers and the sea) salmon which have been severely impacted by acidification in the latter part of the 20th century.

Environmental policy and conservation initiatives have prompted a reduction in acid deposition into the Otra since the 1980s, which has caused a partial recovery of both populations of salmon. However, in order to predict and manage the long-term health and status of the Otra’s salmon populations, it is important to consider acidification as part of a multiple stress combination affecting the river alongside hydropower and climate change.

A MARS team led by Raoul-Marie Couture used a set of linked process-oriented models to provide estimates of future water discharge and chemistry and their effects on fish populations in the Otra River. The models were run to 2100 using two Representative Concentration Pathway climate scenarios: RCP4.5, in which global carbon emissions peak at 2040 then decline; and RCP8.5 in which global carbon emissions continue to rise through the 21st century.

The projected changes in climate produced only small – but ecologically positive – changes in the water chemistry of the Otra River. Run-off was predicted to increase by around 30%, largely during winter (as a result of increased precipitation and snowmelt), which, when coupled with projected decreases in acid deposition through the 21st century reduces the possibility of acidification. And, linked to this, the likelihood of river water pH dropping below levels where fish are significantly stressed (5.8 for parr and 6.2 for smolt) is reduced.

The study suggests that future climate change may cause slight improvements the water chemistry conditions for salmon populations in the Otra River through the 21st century. However – as the authors acknowledge – this result addresses only one aspect of climate change and not others such as increased water temperatures, river basin vegetation growth and soil mineralisation. Future river run-off levels will be affected by the continuation of hydropower projects on the river.

As such, the multiple stressor combination of acidification, hydropower and climate change on the Otra River are fundamentally linked, and whilst this study sheds new light on their interactions, their complex, interconnected nature provides ongoing challenges for environmental modelling and management (link).

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