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The state of scientific knowledge on aquatic multiple stressors

July 2, 2015

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

The interactions and impacts of multiple stressors on aquatic ecosystems is one of the key challenges for current environmental research, policy and conservation. Whilst there are many success stories of pollution being reduced on rivers and lakes across the continent, and aquatic life returning (see for example salmon in the River Mersey and River Tyne in England), Europe’s freshwaters are still subject to multiple stresses, many of which are subtle, complex and poorly understood.

New scientific research suggests that these stresses – chemical pollution, drought, floods, habitat destruction amongst many others – can interact in complex and dynamic ‘cocktails’. A key point here is that these interactions may intensify their individual effects on freshwaters: in other words, the combined damage multiple stressors cause to ecosystems may be more than the sum of the individual parts (known as a synergistic effect).

As studies such as this one by Daniel Hering and colleagues from earlier in the year suggest, multiple stressors pose a series of new, complex and non-linear challenges for aquatic ecosystem conservation and, increasingly, restoration. But despite this emerging awareness of the challenges multiple stressors pose to the health of freshwater ecosystems, there are comparatively few scientific studies which provide quantitative evidence on their effects, making it difficult to inform suitable management and mitigation strategies.

Responding to this shortfall in knowledge, a team of MARS scientists led by Peeter Nõges from the Estonian University of Life Sciences, reviewed 219 existing scientific papers, published since 1986, which quantify the prevalence and effects of multiple stresses on river, lake, groundwater and estuary environments. Nõges and colleagues suggest that whilst there is a pressing ongoing need for quantitative evidence on multiple stressors, this is hampered by a lack of suitable and coordinated sampling techniques and analyses amongst researchers.

Publishing in Science of the Total Environment, a key finding of their study is that nutrient stress (e.g. from fertiliser or sewage pollution) was a key element of most (71-98%) multiple stress combinations in surface waters (rivers, lakes and estuaries). Hydrological stress (e.g. water scarcity, flooding) was found to be a key factor in rivers (74% of studies) and groundwater (83%) environments.

Together, combined hydrological and nutrient stresses were found in over half the studied rivers, and around a quarter of lakes. This finding tallies with reports submitted by European Member States under the Water Framework Directive, describing the pressures (a slightly different, and inherently anthropocentric, meaning to stressor) faced by Europe’s freshwaters. Here, again, member states reported diffuse pollution and hydromorphological alteration (i.e. the alteration of river and lake courses and flows).

Across all the biological groups analysed in the reviewed studies, multiple stressors had most impact on lake ecosystems compared to single stressor conditions. However, the effect of multiple stressors was generally lower in estuarine waters.

The review outlines how different retention times for nutrients (i.e. the amount of time spent in an environment) in rivers and lakes, influenced by water movement and flow, causes different effects. Specifically, where in flowing rivers the retention time of nutrient pollution is low, it is much higher in still or slow-moving lake and estuary environments.

This has a couple of important implications for ecosystem health and functioning. First, high levels of nutrient pollution may be carried through river systems without significant impact, yet when they reach the brackish, slow estuary environment (and switch to an increased residence time) become a likely cause of eutrophication. This means that issues of scale must be taken into account when studying the causes and effects of multiple stressors: and that pollutants may be carried many miles before having harmful impacts.

Second, this finding suggests that the environmental impacts of other hydrological stresses which reduce the speed of river flows (e.g. droughts, dam construction and water abstraction) may intensify the effect of nutrient pollution. This is because river environments where nutrients would generally be washed through become slow or still, increasing the nutrient residence time, and potentially causing harmful environmental impacts such as eutrophication and algal blooms.

Another important finding made by Nõges and colleagues is that the response of freshwater species to multiple stressors reported in the 219 studies is largely ambiguous. Across all the varied aquatic conditions in studies analysed in the review, only fish populations were significantly more impacted when the effects of multiple stressors were increased. This is described as being a result of the mobile lifecycle of many fish species, acting as consumers at different levels of the food chain, across a variety of habitats. For the authors of this study, it is this niche diversity that makes fish particularly susceptible to the impacts of multiple stressors, and also, therefore a potentially important group of bioindicators to detect their effects.

Nõges and colleagues conclude by suggesting that their efforts to provide a first comprehensive assessment of existing scientific research on multiple stressors in aquatic environments were complicated by the seemingly unlimited number of potential stressor combinations and numerous sampling strategies and scales in the papers reviewed. Given that increasingly technological and innovative industries across the world are continually developing new chemicals and plastics, a proportion of which are likely to end up in aquatic environments eventually, this almost-unmanageable diversity of potential multiple stressors makes research and management tricky.

However, EU projects like MARS, SOLUTIONS and GLOBAQUA are specifically targeting this shortfall in scientific knowledge on the interactions and impacts of multiple stressors, as a means of helping manage and mitigate their effects on aquatic ecosystems, both now and in the future.

Link to the Nõges et al (2015) paper in Science of the Total Environment.

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