River ecosystems laboratory, EPFL

Introduction to subject/research

High mountain streams are important contributors to carbon processing, yet their metabolic activity remains difficult to quantify because of low nutrient availability, cold temperatures, and dynamic flow conditions. Traditional open water metabolism methods can be uncertain in such environments, particularly due to challenges in estimating atmospheric oxygen gas exchange.

The River Ecosystems Laboratory at EPFL therefore adopted a benthic chamber approach combined with PyroScience FireSting® optical oxygen technology to directly quantify ecosystem respiration and primary production under natural outdoor conditions.

Experimental setup

A set of custom built Plexiglas benthic chambers was deployed directly on the streambed of a high alpine river. Each chamber contained either natural rocks or fine sediments collected from the site. The chambers were connected to low flow recirculation pumps to maintain homogeneous oxygen conditions, while preserving ambient temperature and light. FireSting®-O2 fiber optic probes were positioned facing oxygen sensor spots OXSP5 fixed to the inner chamber walls, using a dedicated 3D printed holder. Dissolved oxygen and temperature data were recorded at high frequency using the FireSting® interface and PyroScience DataLogger software.

FIG 1. Experimental set-up for benthic chamber respiration measurements.

Each chamber was incubated outdoors for three-hour period. The first half of the incubation was conducted under ambient daylight to record net oxygen changes linked to photosynthesis and respiration. The second half occurred in darkness to isolate community respiration. A two-point calibration with sodium sulfite and air saturated water ensured accuracy in field conditions. Oxygen concentrations were logged every 15 seconds to capture fine scale dynamics. This setup allowed direct estimation of aerobic respiration and gross primary production in benthic substrates.

FIG 2. a) DO monitoring of rock and sediment substrates in benthic chambers, using FireSting® interface, during light and dark conditions. b) Estimated daily metabolic rates based on DO monitoring. GPP = gross primary production, CR = community respiration, and NCP = net community production.

Results and Interpretation

The FireSting® sensors provided stable, drift-free measurements that clearly resolved small oxygen changes over short outdoor incubations. Across substrates, respiration rates were consistently detectable and showed comparable metabolic activity across finer and coarse substrates. Gross primary production was measurable during light incubations, confirming that benthic photoautotrophs were active despite low nutrient conditions and cold temperatures. The high temporal resolution of the FireSting® system allowed precise determination of oxygen slopes, essential for low flux alpine environments.

Conclusion from data and application

The benthic chamber approach combined with FireSting® optical oxygen technology enabled robust and reproducible measurements of stream metabolism in a challenging alpine setting. Key advantages included: (1) high sensitivity to small oxygen changes; (2) stable signals with minimal drift during long outdoor deployments; (3) rapid, simple calibration; (4) compatibility with custom chamber designs.

This method offers a practical and reliable solution for quantifying metabolic processes in remote or low productivity freshwater ecosystems, supporting improved understanding of carbon cycling in high mountain environments.