Effects of extreme flow changes on periphyton biofilm and carbon cycling in Alpine streams

Project Information

PeriCarb-EFA

Grant agreement ID: 101105996

DOI

10.3030/101105996

Project closed

EC signature date 31 March 2023

Start date 1 May 2023

End date 30 November 2025

Funded under

• Marie Skłodowska-Curie Actions (MSCA)

Total cost

No data

EU contribution

€ 189 687,36

Coordinated by

RHEINLAND-PFALZISCHE TECHNISCHE UNIVERSITAT
Germany

Periodic Reporting for period 1 - PeriCarb-EFA (Effects of extreme flow changes on periphyton biofilm and carbon cycling in Alpine streams)

Reporting period: 2023-05-01 to 2025-11-30

Summary of the context and overall objectives of the project

Climate change is increasingly altering the frequency and intensity of extreme hydrological events, such as droughts and floods, with profound consequences for freshwater ecosystems. In mountain regions, these changes are occurring faster than in most other biomes. Alpine catchments are experiencing reduced snow accumulation, earlier snowmelt, shifts in the timing of peak discharge, and longer periods of low flow. These transformations directly affect streams and rivers, which play a central but still underestimated role in the global carbon cycle by retaining, transforming and releasing large amounts of carbon before it reaches coastal oceans.

Despite growing evidence that inland waters are major sources of carbon dioxide to the atmosphere, a key knowledge gap remains: how climate-driven changes in flow regimes modify the biological processes that control carbon cycling in streams. In particular, the response of benthic biofilms — complex microbial communities that dominate ecosystem metabolism in running waters — to extreme and recurrent flow disturbances is still poorly understood.

The PeriCarb project was designed to address this gap by investigating how extreme hydrological events and their increasing frequency reshape microbial communities and carbon processing in Alpine headwater streams and springs. By combining controlled experiments, long-term field observations and modelling approaches, the project aimed to identify the mechanisms linking flow variability, biodiversity and ecosystem metabolism across spatial scales, from biofilms to individual stream reaches and entire headwater networks.

The overall objective of PeriCarb was to improve the predictability of how climate-driven hydrological change will alter carbon fluxes from headwater systems, with a specific focus on Alpine regions that are highly sensitive to warming and hydrological shifts. The project sought to determine how droughts and floods affect the structure and functioning of microbial communities, how these biological changes translate into altered carbon uptake and release, and how these effects propagate downstream to influence catchment-scale carbon processing.

By providing quantitative and process-based estimates of how extreme events modify carbon cycling, PeriCarb contributes to a more accurate representation of inland waters in climate models and carbon budgets. In the broader political and strategic context of the European Green Deal and international climate mitigation efforts, the project supports the need for science-based assessments of climate feedbacks and for identifying critical thresholds beyond which freshwater ecosystems may shift towards stronger carbon sources.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

PeriCarb implemented an integrated, multi-scale research strategy combining the analysis of controlled experiments, long-term field observations, targeted modelling and global data synthesis to investigate how extreme hydrological events alter carbon cycling in Alpine headwater streams and beyond. The scientific activities were structured around four tightly linked components: the analysis of existing mesocosm experiments at the biofilm scale, reach-scale field measurements in Alpine spring–stream systems, modelling of carbon processing in headwater networks, and the development of a global synthesis on carbon fluxes from springs.

At the biofilm scale, PeriCarb analysed a unique set of experimental stream mesocosms in which flow intensity and the frequency of extreme low-flow events had been manipulated prior to the start of the project. These experimental datasets were comprehensively analysed to address new scientific questions on the links between hydrological disturbance, biodiversity and carbon cycling. Detailed analyses of chlorophyll concentration, organic matter accumulation, primary production and ecosystem respiration were combined with taxonomic characterisation of microbial communities. This work led to the first peer-reviewed publication from the project, which demonstrated how flow variability and macroinvertebrates jointly regulate stream periphyton and ecosystem metabolism based on experimental stream mesocosms.

At the reach scale, PeriCarb carried out year-round monitoring in a network of four near-pristine Alpine spring–stream systems. High-frequency oxygen sensors, repeated carbon dioxide measurements and continuous discharge monitoring produced long-term metabolism time series spanning two full hydrological years. In two of these systems, data were collected also during winter under snow cover, generating rare observations of ecosystem functioning during periods that are typically inaccessible in mountain environments. The project combined high-frequency oxygen data with resazurin tracer experiments to quantify bacterial activity, systematic collection and taxonomic characterisation of periphyton biofilms, and repeated measurements of carbon dioxide concentrations. This integrated dataset enabled the development and application of new coupled oxygen–carbon dioxide modelling approaches, providing a mechanistic quantification of how seasonal hydrological dynamics regulate carbon processing in Alpine headwaters. These data are currently being analysed for publication.

At the headwater network scale, PeriCarb developed and applied modelling frameworks to upscale local biological responses to the scale of Alpine headwater streams. By coupling discharge dynamics, ecosystem metabolism and microbial biomass processes, the project quantified how changes in the frequency and intensity of extreme events propagate through headwater networks to modify organic carbon processing and downstream carbon exports.

During the project, field measurements revealed consistently high CO2 concentrations in several springs. This observation highlighted that groundwater-fed systems represent a previously underappreciated source of atmospheric carbon. It triggered the development of a strong global synthesis work focused on the role of springs in the global carbon budget. Through the creation of an international consortium involving partners from multiple continents, PeriCarb assembled one of the first worldwide databases of CO2 concentrations and fluxes from springs. An international high–impact-factor manuscript based on this global database is currently in preparation and represents a major expected outcome of the project.

Together, these activities delivered a coherent mechanistic and global understanding of how extreme flow events reshape microbial communities, ecosystem metabolism and carbon fluxes in running waters.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

PeriCarb delivered results that extend the current state of the art by linking extreme hydrological variability, ecosystem metabolism and carbon fluxes in Alpine headwater streams, and by uncovering the global relevance of springs for atmospheric carbon dioxide emissions.

A first major advance lies in the causal demonstration that biological community structure actively mediates the response of carbon cycling to hydrological disturbance. While previous studies largely relied on correlative relationships between flow variability and carbon fluxes, PeriCarb provided experimental and field-based evidence that flow variability and macroinvertebrates jointly regulate the balance between autotrophic and heterotrophic processes. This identifies biodiversity as a central control on ecosystem carbon dynamics under hydrological stress.

A second advance is methodological. The project generated long-term, high-frequency metabolism time series in Alpine spring–stream systems, including rare winter observations under snow cover. By integrating discharge dynamics, bacterial activity tracers, biofilm taxonomic characterisation and coupled oxygen–carbon dioxide measurements, PeriCarb developed new modelling approaches that explicitly resolve carbon dynamics under highly variable and extreme hydrological regimes.

A third advance concerns the recognition of springs as a previously underestimated component of the carbon cycle. The identification of consistently high carbon dioxide concentrations in groundwater-fed systems, together with the creation of a worldwide database and an international consortium, provides the first empirical basis for integrating springs into global carbon budgets and climate models.

Together, these results shift the state of the art towards a multi-scale, process-based framework linking hydrological extremes, biodiversity and carbon emissions from biofilms to headwater networks and global carbon assessments.

Fieldwork PeriCarb

Fieldwork PeriCarb