Dam construction often causes a range of stresses to the health of river ecosystems and the biodiversity they support. Despite their value to humans in terms of drinking water supply or hydropower generation, dams can change the course, speed and amount of water that flows in a river. As such, dams can exacerbate droughts and flashy floods (see, for example). Dams can also fragment migration routes for fish such as salmon and sea trout, and alter nutrient and sediment flows along the course of a river.
However, the ways in which the multiple stresses caused by dams interact and impact river ecosystems is still not fully understood. In particular, understanding the ways that dam construction affects the functioning of an ecosystem (i.e. all the processes that take place to maintain the health and diversity and the services it provides) is of particular interest to freshwater scientists. What is needed are bioindicators that scientists can use to identify both early (i.e. detecting first signs of alteration) and predictive (i.e. identifying effects on ecosystem functioning and provision of ecosystem services) impacts of ecosystem change as a result of human activity.
In this context, a new study by Fanny Colas from the University of Toulouse and colleagues examines the value of fungal growth from leaf litter breakdown as a bioindicator for changes in ecosystem function in response to multiple stresses from dams. Leaf litter breakdown is a good indicator of ecosystem functioning: partly because it is a key process in rivers and streams, releasing nutrients through food webs; and partly because it is well-studied and understood by scientists.
Leaves that falls into streams and rivers are generally broken down by two types of organisms: microorganisms such as fungi; and aquatic invertebrates. In this study, the team led by Dr Colas focused on a group of fungi called hyphomycetes, which are early colonisers of leaf litter. Hyphomycetes cause the initial breakdown of leaf litter, which in turn makes it more palatable and nutritionally valuable to aquatic invertebrates. So, changes in the growth and activity of fungal hyphomycetes could lead to altered leaf litter breakdown, and changes to the resulting flows of energy and nutrients through the ecosystem.
Publishing in Ecological Indicators, the team investigated the effects of small ‘run of the river‘ dams (lower than 15 metres) on fungal richness and biomass on decaying leaves both above and downstream of the dam wall. They conducted field sampling on dams at nine sites in Eastern France alongside laboratory experiments to assess the response of microbes associated with decaying leaf litter to multiple stressors resulting from dams, and the consequences for the health and functioning of the wider river ecosystem. In the European Union such small dams are not fully addressed by the Water Framework Directive, yet there are many of them: more than 76, 292 small ‘run-of-river’ dams have been identified in France alone by the French National Agency for Water and Aquatic Environments.
The team found that the presence of a small dam reduced the biomass of fungi, both in the upstream reservoir and downstream river. This is thought to be because dam construction restricts the downstream flows of leaf litter, sediment and other detrital resources, and alters the communities of aquatic insects and microorganisms involved in leaf litter breakdown. Interestingly, they also observed a significant decrease in fungal biomass downstream of reservoirs with sediments contaminated by metal pollutants.
Reductions in hyphomycete fungi are likely to have implications for higher trophic levels in freshwater ecosystems, by modifying the effectiveness of both fungi and leaf-shredding invertebrates in processing leaf litter. As such, the authors term describe how reduced fungal growth can have negative ‘cascading effects’ on the wider ecosystem.
Delayed or reduced leaf breakdown is likely to lead to the accumulation of non-rotted organic material in reservoirs above dam walls, reducing water quality and the amount of nutrient and carbon available. It is important, therefore to note that the functioning of the reservoir ecosystem has consequences for the water quality, health and function of downstream ecosystems.
Colas and colleagues’ study highlights the sensitivity of the hyphomycete fungi to the multiple stressors associated with dams and reservoirs. They conclude that fungal indicators could enable scientists to predict changes in ecosystem functioning because of their positive relationships with the effectiveness of fungi and leaf-shredding insects in breaking down leaf litter. These relationships may also be used by environmental managers seeking to understand the vulnerability of ecosystems to the multiple stressors caused by dam construction.
In short, their study suggests an intriguing possibility: that by studying tiny fungal microorganisms, scientists might be able to better understand – and even predict – the ecological effects of large dam engineering projects that fundamentally change the nature of rivers.
The Salmon and Trout Conservation UK charity has this week announced the outcome of its £2,000 “Invisible World’ filmmaking competition. Scottish filmmaker Andrew O’Donnell’s entry was judged the winner by a panel including award-winning wildlife film-makers Hugh Miles, Paul Reddish and Charlie Hamilton James, television personality Matthew Wright and Salmon and Trout Conservation CEO Paul Knight.
The Invisible World competition was launched back in February, to encourage filmmakers to create work that engaged with the invisible beauty and diversity of freshwater ecosystems, and the similarly often unseen threats that they face. Hugh Miles explained why O’Donnell’s film (also titled The Invisible World) came out on top, “It is the most imaginative interpretation of the subject and says all the right things about water. It succeeds really well in linking ‘our’ world with the ‘invisible’, so bravo for the great effort and hard work that went into this production.”
O’Donnell’s film has a sparkling clarity about it, and the aquatic environments he filmed – both above and below the surface – are captured in sharp, perceptive focus. The making of his prize-winning entry was a long and (quite literally) immersive process: “I initially noticed the competition on S&TC UK’s website, whilst doing some research for another project. It appealed to me instantly as I’m very passionate about all the elements of the brief – the beauty, wildlife and threats to the UK’s freshwater environments.”
“The film took around 8 months to make and I travelled all over the UK filming it. My wetsuit was put to use a lot more than usual! I wanted it to show how much people rely on our freshwater environments. We all use them in one way or another. It’s not just people who are involved in aquatic based activities who should be protecting these environments. It’s everyone!”
Free the Snake is a short film about the restoration of the Snake River in Washington State, USA. Featuring interviews with Professor David Montgomery from the University of Washington, Bruce Babbitt, former Secretary of the Interior and novelist David James Duncan, the film focuses on the ecological and hydrological impacts of four major dams along the river.
In particular, the dams fragment migration routes of Pacific salmon upstream to spawning grounds, with damaging consequences for the health and size of their populations. But, all hope is not lost. When discussing the possibility of dam removal, David Montgomery suggests that, “The beautiful thing about salmon, if you look at their history, is that they’re resilient. If you give them half a chance then they can come back.”
We live in an age of widespread ecological restoration, in which centuries of human impacts on the environment are being addressed, under guiding principles such as rewilding, biodiversity conservation and ecosystem service provision. Restoration projects are particularly common on rivers and streams, partly because these ecosystems have often been particularly impacted by pollution, fragmentation and so on; but also because restoration approaches such as weir removal, ‘daylighting’ and riparian zone planting have become widely adopted. However, despite the increasing number of river restoration projects implemented across the world, there is still sparse scientific evidence on the long-term effects of such projects, and the factors affecting their success or failure.
A new study by MARS project leader Daniel Hering and colleagues addresses this shortfall in knowledge by studying the effects of the restoration of river hydromorphology on aquatic habitats and biodiversity. Hydromorphology is the term used to bring together the interactions of hydrology (e.g. water flows), geomorphology (e.g. bedrock) and ecology (e.g. plant and animal communities) in river or stream. Hydromorphological processes include the formation of meanders, riffles and pools: all of which contribute to a diverse set of habitat niches for freshwater life.
As reports such as the 2012 European Environment Agency ‘European Waters: assessment of status and pressures‘ outline, there is increasing evidence that river hydromorphology often has a strong impact on the health and diversity of aquatic ecosystems. However, projects which restore river hydromorphology often have limited effects on freshwater biodiversity. One common explanation for this effect is that river restoration often takes place over short sections of river, which are insufficient to allow ecological communities to develop, and geomorphological processes to take place.
To investigate this phenomenon, Hering and colleagues undertook experiments on rivers in ten regions across Northern Europe, as part of the EU REFORM project. For each region, they studied the impacts of two restoration projects: one on a short section of river; one on a longer section. At each section of restored river, the team sampled habitat composition in the river and its floodplain, three aquatic organism groups (aquatic plants, insects and fish), two floodplain-inhabiting organism groups (floodplain vegetation, ground beetles), and food web composition and land–water interactions. These findings were then compared to samples taken upstream at non-restored sections of river of roughly equal length.
After accounting for regional variations in river size and restoration approaches, the team, publishing in the Journal of Applied Ecology, found that the length of the restored river made no significant difference to its ecological health and diversity, as sampled by the range of indicators above. Instead, they found that what mattered was the substrate composition of the river bed: in other words, the aquatic habitat it provides for organisms.
Where habitat had been created in restoration, populations of fish, aquatic insects, aquatic plants and floodplain vegetation increased, regardless of the length of the restored river section. Substrate habitat restoration might include measures such as increasing the number of boulders or presence of wood (for example tree trunks) to diversify water flows and depths, and habitat niches for different aquatic organisms.
Whilst in-stream habitat restoration was important for aquatic biodiversity, restoration projects also had pronounced impacts on floodplain ecology. The authors suggest that this is because hydromorphological restoration, even at relatively small scales, tends to create habitat types close to the river bank (such as gravel and sand bars), which are often completely lacking in degraded sites. Such habitat types are rapidly colonized by ground beetles and, to a lesser degree, by specialised floodplain vegetation. Both organism groups have comparatively high dispersal abilities and are less affected by barriers such as weirs when compared to aquatic organism groups.
Overall, the study suggests that the ecological success of river restoration for aquatic organisms doesn’t depend on the length of restored river, but rather the quality and diversity of habitats on the river’s bed. However, for floodplain organisms, relatively small-scale restoration projects may yield significant positive effects.
Stressors are environmental changes that place stress on the health and functioning of an ecosystem. There is increasing evidence – largely from marine environments – that multiple stressors may interact to produce unexpected effects on aquatic ecosystems. However, there is a pressing need to better understand the ‘ecological surprises’ caused by multiple stressors in freshwater ecosystems (a point made in papers by MARS scientists Steve Ormerod in 2010 and Daniel Hering and colleagues in 2015).
Existing scientific literature from marine environments show that multiple stressors can have effects that are greater than the sum of those caused by individual stressors. This ‘synergistic’ interaction poses important questions for environmental managers and policy makers. In short, it is difficult enough to manage individual stressors such as pollution, habitat destruction and overfishing, without the unexpected and, as yet, largely unpredictable interactions and effects these stressors might have.
In the light of this uncertainty, a team of researchers from the University of Pretoria in South Africa and the University of Alberta in Canada analysed data from 88 existing scientific studies that show the responses of freshwater ecosystems to pairs of stressors. The team, led by Michelle Jackson from the University of Pretoria, brought together the findings of these studies to investigate the characteristics and effects of different stressor interactions; and the extent to which interactions vary between different stressor pairs and response measurements.
Freshwater stressors may cancel each other out
Recently published in Global Change Biology, the team’s findings are perhaps surprising, at least initially: the environmental effects caused by pairs of stressors in freshwater was most often less than the sum of their single effects. This is known as an antagonistic interaction, where two or more stressors interact to cancel out some or all of their individual effects.
Across the 88 surveyed studies, stressors included acidification, increased water temperatures, ultraviolet radiation, contamination, nutrification, habitat alteration and invasive species. Antagonistic interactions were found in the majority (41%) of surveyed studies, affecting animal abundance, biomass, condition, growth/size and survival and plant diversity. The authors suggest that this widespread antagonism could be due to the effects of the stronger stressor overriding and negating those of the weaker one. Here, environmental managers might seek to rank the effects of the strongest stressor in an antagonistic interaction in order to forecast the cumulative impacts of multiple stress.
Another suggestion is that exposure to one stressor can result in greater tolerance to another. An important point made here is that there may exist a potential for co-adaption within freshwater ecosystems to minimise the net effects of multiple stressors. However, despite the predominantly antagonistic interactions, net effects of multiple stressors were still mostly negative, underlining the widespread threats faced by freshwater ecosystems.
Multiple stressors in freshwaters: the overall picture
Overall, the net effects of stressor pairs were frequently more antagonistic (41%) than synergistic (where the overall effect is more than the sum of individual stressors: 28%), additive (overall effect is equal than the sum of individual stressors: 16%) or reversed (overall effect is opposite to the positive / negative effect of individual stressors: 15%).
Synergistic interactions between multiple stressors such as increasing sea temperatures, species invasions and habitat destruction have been observed in a number of studies of marine ecosystems. Why is the picture different for freshwaters? The authors suggest that this is due to greater inherent environmental variability in smaller aquatic ecosystems, which fosters higher potential for ecological acclimation and co-adaptation to multiple stressors.
Antagonistic interactions were most frequently observed in affecting animal condition; synergies and reversals with plant growth and size; and additive effects with plant diversity. This variability shows that it is important to consider the ecological metrics used to measure the impacts of multiple stressors.
Different effects on ecosystem diversity and function
Interestingly, multiple stressors had different effects on the diversity and function of ecosystems. Multiple stressor interactions affecting species diversity were most often additive. This suggests that the species eliminated by one stressor were often not the same as those eliminated by the second, and additional stress causes additional species loss.
On the other hand, the most common interaction affecting the functional performance of an ecosystem was antagonism. This may be a result of a process known as compensatory species dynamics. Here, the remaining species in the stressed ecosystem may compensate functionally (e.g. in nutrient cycling) for species loss. This in turn points to the idea that an ecosystem’s functional resilience to stress is not simply dependent on biodiversity, but instead determined by species identity and traits. In short, the findings here suggest that freshwater biodiversity is more sensitive than ecosystem function to the impacts of multiple stressors.
The authors identified that 15% of studies showed reversal effects from multiple stressors. These reversals are termed ‘ecological surprises’, in that they reverse the environmental impacts – negative to positive (or vice versa) – caused by the individual stressors. Whilst reversals were the least common type of interaction observed, their existence has potentially important effects for environmental management.
The stressor most commonly associated with reversal interactions was climate warming. For example, a study by Patrick Thompson and colleagues in 2008 found that warming reversed the negative effects of excess nitrogen supply on phytoplankton growth, possibly as a result of increased conversion of nitrates and ammonia by enzymes as a result of increased temperatures. Ecological responses to temperature change are complex, but the evidence here on multiple stressors causing reversals suggests that there may be ever more ‘ecological surprises’ in a warming world.
Lessons for freshwater conservation management
The paper contains potentially important lessons for freshwater conservation management. For multiple stressors that generate additive or synergistic interactions, management that focuses on a single stressor should cause a positive outcome. However, in ecosystems affected by antagonistic stressor interactions, both stressors may need to be removed in order to foster any significant ecological recovery.
The MARS project has now been running for well over a year, and many of the project experiments are beginning to yield results. Today we’re happy to share a short new documentary about the project, which you can watch above.
The film explains the problems posed by multiple stresses such as nutrient pollution and climate change on freshwater ecosystems, and the ways in which the MARS project is implementing innovative scientific research to better understand and manage their effects.
The new documentary features interviews with MARS scientists Anne Lyche Solheim from NIVA in Norway and Steve Ormerod from Cardiff University in Wales, and Anders Iversen, Water Framework Directive Co-ordinator at the Norwegian Environment Protection Agency. It shows fascinating new footage of MARS stream experiments in the Austrian Alps, and deep lake experiments at the IGB LakeLab research station on Lake Stechlin in Northern Germany.
The piece was predominantly filmed on two heavily stressed and modified rivers: the Emscher in Germany and the Calder in West Yorkshire, England. Additional footage was shot on the upland River Hodder in Lancashire, England. The footage of the underwater char was shot by Jack Perks (who we’ve featured on the blog before), and the footage of the blue-green algal bloom was filmed by Scott Nelson (whose Rivière des chutes film will be featured here soon).
We hope you enjoy the film.