Heterogeneous landscapes help revitalize streams

River dams cause stream and riverbeds to become homogeneous in Switzerland. © iStock

River dams cause stream and riverbeds to become homogeneous in Switzerland. © iStock

A cross-disciplinary study involving civil and environmental engineers at ENAC has developed concrete recommendations for boosting the resilience of microbial life in Switzerland’s streams.

Three labs at EPFL’s School of Architecture, Civil and Environmental Engineering (ENAC) have teamed up on a study stressing the importance of spatial heterogeneity in streambeds. According to the authors, that’s the best way to help a stream’s microbial life regenerate after a major disruption like a storm or flood. This microbial life is important because it plays a crucial role in streams’ ecosystems: the biofilms of algae and bacteria purify the stream water by recycling carbon and nutrients like nitrogen.

The research findings, published in Biology Letters, give specific recommendations for those in charge of revitalizing streams in Switzerland after major disruptions in order to promote the regeneration and resilience of biofilms. The three ENAC labs involved in the study are the Stream Biofilm and Ecosystem Research Laboratory (SBER), the Laboratory of Ecohydrology (ECHO) and the Hydraulic Constructions Laboratory (LCH).

Increasingly put to the test

“River dams cause stream and riverbeds to become homogeneous because they prevent large sediment from being carried downstream,” says Tom Battin, a full professor at ENAC and head of SBER. “The challenge for us is to figure out how we can restore these streams. Our study shows that one answer is to actively diversify the sediment ‘landscape’ in a targeted manner, in order to help the microbial life regrow and function properly.” Such methods stand to become increasingly important as climate change creates more and more disruption to streambeds by accentuating the swings between droughts and heavy rains.

The study also illustrates how valuable test canals are for experimenting with novel approaches combining microbial ecology and hydraulics. The research team used such a canal at EPFL to test the effects of heterogeneous environments on biofilm dynamics. They employed satellite technology to photograph and map biofilm development and behavior over time, comparing a homogeneous streambed with a heterogeneous one.

Creating refuges

After analyzing the millions of data points they collected, the researchers found that the more varied a streambed’s ‘landscape’ is, the greater the potential for biofilm survival. For instance, microbes can take refuge in large rocks and tiny valleys during floods, and use these areas as safe havens for recolonizing the stream once the flood has passed.

To conduct this research, LCH, in ENAC’s civil engineering Institute, provided hydraulics data on a microbial scale for the different types of homogeneous and heterogeneous landscapes; SBER, in the environmental engineering Institute, provided expertise in biofilm development; and ECHO, also in the environmental engineering Institute, modeled the data to better understand how biofilms regenerate after a flood.

“Our study was possible thanks to the joint effort of all three labs. It shows the advanced fundamental research we can do at ENAC, while keeping an eye on tangible applications and working in a cross-disciplinary manner,” says Battin.