WP3: Analyse the potential effects of desiccation on the self-purification capacity of the sediment surface and the hyporheic zone and identify factors determining the resilience to desiccation
Questions and hypotheses:
How does desiccation affect the uptake of nitrate, SRP and DOC by the sediment surface and the hyporheic zone in the medium term?
- H3: Desiccated sediments show lower uptake rates than permanently inundated sediments in laboratory addition experiments 1 month after flow recovery.
How do fine sediment content, organic matter concentrations, and biofilm thickness affect the resilience of hyporheic uptake processes to desiccation?
- H4: Increased fine sediment content, organic matter concentrations, and biofilm thickness will reduce the negative effects of desiccation on uptake processes in the hyporheic zone.
Method: Laboratory addition experiments (year 2-3)
We will analyse the potential medium-term effects of desiccation on the self-purification capacity of the sediment surface and the hyporheic zone via experimental flumes and medium-scale sediment perfusion cores. Both facilities have already been used successfully in other projects.
- Sediment perfusion cores:
The facility consists of dark PVC cores (30 cm length, 16 cm diameter) which are installed in a vertical position. Filtered stream water is pumped from storage tanks through the cores from bottom to top via a 10-channel peristaltic pump at a flow rate of approx. 10 cm h-1 (rates will be adapted to in-situ velocities estimated from hydraulic heads and conductivities).
We will take pre-cleaned gravel (up to 5 cm in diameter) from the study sites for the filling of the cores. 20 filled cores will be buried into the sediments of a perennial and an intermittent site of each stream in a horizontal position with both ends open (covered by a coarse mesh) in winter/early spring to enable colonization by organisms and sedimentation of fine particles. Before drying starts (late spring/early summer), 10 cores (5 from each site) will be taken out of the stream, transported to the lab and connected to the perfusion facility to conduct the uptake experiments. Approx. 1 month after flow resumption (late summer/autumn), the remaining 10 cores will be taken out of the stream and performed in the same way.
For the uptake experiments, we will connect the 10 cores with the peristaltic pump in a flow-through mode. Experiments will start after 1 day of adaption to lab conditions (room temperature). For the experiments, we will add SRP, NO3-N and DOC in the form of leaf leachate to the tanks at a target concentration of 3-4 times the background. Cl will be used as conservative tracer at a target concentration of 2 times the background. We will take water samples at the inlet and the outlet of each bioreactor before the addition (ambient uptake) and after full mixing has been achieved (control via conductivity probe; 3 replicates each). Samples will be analysed for DOC, DOM quality, DIN, dissolved organic nitrogen (DON), and SRP. Uptake parameters will be calculated from the difference between input and output via a mass balance approach. After the additions, the sediments will be analysed for the proportion of fine sediments (< 2mm), OM (including macro-invertebrate biomass), chlorophyll-a concentrations (as control for dark conditions), bacterial abundances, bacterial respiration, and the activity of extra-cellular enzymes. Uptake efficiency will be correlated with structural parameters to identify factors which influence resilience to desiccation.
- Experimental flumes:
Our flumes are 40 cm long, 7 cm wide and 15 cm deep and operate in either circulating flow mode or flow-through via submerged aquarium pumps (8 reference and 8 treatment flumes). Light is supplied via day-light lamps in a 12/12 day/night cycle. Flow velocity will be around 5 – 10 cm sec-1.
For the experiments, we will carefully fill the flumes with gravel (stones approx. 5-10 cm in diameter) directly from an intermittent and a permanent section of the selected streams 1 month after re-wetting. This method is preferred over colonization of artificial substrates to ensure that conditions are as close to reality as possible, while disturbance due to handling is kept low.
The flumes will be transported carefully to the lab and incubated in stream water in a circulating flow mode for 1 day to recover from disturbance after sampling. On day 2, we will add SRP, NO3-N and DOC in the same target concentrations as in the perfusion cores. Water samples will be taken in the flumes in exact time intervals after the addition (e.g. 0.5, 1.5, 3, 5 h) and analysed for DOC, DOM quality, DIN, DON, and SRP. Uptake parameters will be calculated via the change in SRP, NO3-N and DOC over time. After the additions, the stones will be analysed for OM, chlorophyll-a concentrations and algal composition, physiological state of algae, primary production, bacterial abundances, bacterial respiration, and the activity of extra-cellular enzymes.