Skip to content



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

The Freshwater Blog publishes features, interviews and analyses on freshwater conservation, science and policy, edited by the European Union funded MARS project.

We publish pieces on a wide range of topics, but we particularly focus on the interactions and impacts of multiple stressors – pollution, abstraction, fragmentation, invasive species, climate change and so on – on aquatic environments. This is one of the key research topics for aquatic science, policy and conservation in an increasingly complex and uncertain world.

For comments, ideas and submissions, you can contact us here: info [at]


What is MARS?

MARS (Managing Aquatic Ecosystems And Water Resources Under Multiple Stressors) is a new collaborative European Union Seventh Framework project which aims to identify and understand how different stressors – for example pollution, water abstraction, and habitat fragmentation – impact freshwater environments, both now and in the future.


What are stressors?

Stressors are biological (biotic, e.g. pollution) or non-biological (abiotic e.g. water abstraction) processes that have negative impacts on organisms and communities in an ecosystem.  These can be naturally occurring (e.g. flooding) or man-made (e.g. habitat fragmentation).

European freshwaters are subject to a complex set of stressors resulting from human activity. As MARS partner Steve Ormerod and his colleagues outlined in a 2010 article in Freshwater Biology (‘Multiple Stressors in Freshwater Ecosystems’) human impacts on freshwater ecosystems typically alter more than one environmental stressor.

For example, urbanisation might affect the water quantity and pollution content of runoff into a river; increase the risk of flooding; reduce the amount of habitat available for different organisms; and increase the ability of invasive species (such as Chinese mitten crabs) to disperse.

Freshwaters – lakes, rivers, estuaries and groundwater – can be polluted, abstracted, altered and fragmented by human activity, often with unpredictable results for ecosystem health and function.  In particular, the interaction of multiple stressors is poorly understood and documented.

In a 2010 paper ‘Multiple stressors in coupled river-floodplain ecosystems’, BioFresh leader Klement Tockner states ‘Predicting and understanding the effects of multiple stressors is one of the most important challenges presently facing ecological studies.’


Interactions of multiple stressors

In some cases stressors may cancel each other out (or act ‘antagonistically’).  For example, a 1996 study by Helmut Klapper and colleagues suggests that organic pollution and eutrophication may neutralise acidification from open cast mining in German lake ecosystems.

On the other hand, stressors may interact to worsen their individual effects (in the MARS project, this is termed as the stressors acting ‘synergistically’.)  For example, a 2011 study by Anika Wagenhoff and colleagues suggests that the build up of sediment pollution and nutrients can co-determine the diversity and health of invertebrate and algae communities in New Zealand streams.

Stressors, ecosystem function and ecosystem services

Stressors often negatively impact the organisms living in freshwaters – plants, fish, insects and microorganisms – directly.  These negative impacts on organisms may then affect the functioning of the ecosystem, for example the natural purification of watercourses by photosynthesising plants.  In turn, this change in ecosystem function may alter the provision of ecosystem services to humans, affecting services such as drinking water supply or fish availability for food.  For more information on these interrelations, this 2010 chapter by Rob Haines-Young and Marion Potschin at Nottingham University provides a good introduction

In short, the MARS project is looking to untangle how different stressors interact and impact the biodiversity, function and ecosystem service provision of European freshwaters.  This work will support stronger freshwater conservation and restoration initiatives at the water body, catchment and continental scale, in the context of ongoing climatic and social change.


Providing support for freshwater policy and management at three different scales

MARS aims to yield new and important results for water managers and environmental policy makers across Europe.  In particular this work will contribute to strengthening the Water Framework Directive, the key piece of European legislation implemented in 2000 to protect and improve the quality of Europe’s freshwaters.

As this ten year review of the Water Framework Directive (2010) by MARS co-ordinator Daniel Hering and colleagues states, a key challenge for future water policy is to deal with the impact of emerging stressors caused by climatic and social change.

In particular, Hering and colleagues highlight the need for studies on the ability of freshwater ecosystems to absorb environmental stress (a process known as resilience).  Similarly, the paper suggests new studies on the different trajectories that ecosystems may take in response to stress when restoration management is undertaken. However, at present the Water Framework Directive does not mention the impact of multiple stressors on freshwater ecosystems.

With this shortfall in mind, MARS has been designed to give useful outputs to support freshwater policy at three key scales: individual water bodiesriver basins or catchments and the European continent.

Questions such as ‘what will be the consequences of greening Europe’s agriculture?’ and ‘how does climate change impact on the multiply stressed ecosystems?’ will be investigated at all scales, allowing water managers and policy makers to make decisions over mitigating stressors under a number of different future scenarios.


Experiments at the water body scale

At the smallest scale, experiments will be carried out to simulate multiple stressors and measure the response of an ecosystem.  In lakes, these experiments can be carried out using mesocosms, which provide a small body of water that can be closely controlled and monitored when different stressors are simulated.  In rivers, experiments can be carried out along artificial channels, where variables such as flow volume, flow speed and water temperature can be controlled.  In both sets of experiments, the impact of simulated stressors such as extreme rain or water scarcity can be observed.

Computer modelling at the river basin scale

At the river basin or catchment scale, computer models will be run to simulate the impact of stressors such as climate change or land use change (especially relating to flow alterations and water scarcity ) on variables such as water nutrient levels and temperature .  These results will then be linked to the results on ecosystem functioning at the smaller, experimental scale.  MARS will study 16 river basins across Europe, including the Thames, Ruhr, Elbe and Lower Danube.

Bringing together data at the European scale

At the largest, European scale, MARS will use data gathered from projects such as WISER in the establishment and monitoring of the Water Framework Directive to study and establish large-scale relationships between freshwater biodiversity, ecosystem functioning and ecosystem services to humans.

MARS: providing new information on complex freshwater environments to aid their conservation and restoration

These are complicated issues to which there are no simple answers.  MARS aims to provide new information on how multiple stressors impact freshwater ecosystems in an increasingly complex and uncertain world faced with climatic, environmental and social change.

This information will be provided at the three scales useful to water body and river basin managers and European policy makers through a range of tools and wiki information system.  It is intended that the project will help foster collaboration to promote freshwater ecosystem conservation and restoration at a range of scales, from individual rivers and lakes up to the European continent.

For more information: there is an open-access set of journal articles related to the themes of MARS available online from a special issue of Freshwater Biology, published in January 2010.

Glossary of terms used by MARS.

%d bloggers like this: