Using microscopic fungi to understand the impact of dams on rivers

The River Bienne in the Jura Mountains, Eastern France: the region in which this study was undertaken. Image: KlausFoehl | Wikipedia

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.

Microscopic image of Clavariopsis aquatica, an aquatic hyphomycete which breaks down leaf litter. Image: Wikipedia

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.

Decomposing leaves are an important source of nutrients and energy to river ecosystems. Image: Wikipedia

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.