WP2: Investigate the medium- and long-term effects of drought on ecosystem structures and processes
Questions and hypotheses:
Do intermittent and perennial stream reaches differ in interstitial water chemistry and surface and hyporheic biofilm densities and activities before drought (spring), during contraction (early summer), during the dry phase (summer) and after flow recovery (late summer/autumn)?
- H1: Intermittent reaches show higher organic matter concentrations, higher interstitial SRP and nitrate concentrations (during drought), and lower biofilm densities and activities than perennial reaches. Differences are larger after the drought than before the drought.
Does partial intermittency (contracted channel with remaining surface water flow) affect hyporheic biofilm structures and processes differently than full intermittency (dry channel)?
- H2: Partial intermittency has smaller effects than full intermittency due to higher sediment moisture.
Method: Field sampling (year 1-2)
In each of the 10-15 study streams, we will select an intermittent and a permanent site where we will install mini-piezometers down to a sediment depth of 30 cm to sample interstitial water (5 transects with 3 piezometers at each site, respectively). Sampling will be done during the first and second project year in spring before the drought (->long-term effects of desiccation), during the contraction phase, during the dry phase, and one month after surface flow has resumed (->medium-term effects). In addition to the water level monitoring, we will recruit citizen scientists to keep us informed about the extent of drought in the different study reaches and document the drying of the sediment surface via photographs. Besides, we will install “Stream Temperature, Intermittency and Conductivity Loggers” (HOBO Pendant Temperature/Light data logger as modified by Chapin et al. 2014) at the sediment surface and in the shallow hyporheic zone to collect detailed information about the cessation of surface flow.
We will analyse the following parameters:
- Hydro-morphology (channel topography, grain size distribution, water velocity, water depth, width of active channel, vertical hydraulic gradients)
- Diurnal oxygen curves via YSI-probes
- Chemistry of surface and interstitial water: DIN (NO3-N, NO2-N, NH4-N), SRP, Ptot, Ntot, DOC
- Sediments (surface and 30 cm depth, 10 reps): Proportion of sediments <2 mm, organic matter (OM) concentrations, C/N, TP (Total P); chl-a concentrations and algal community composition; bacterial abundances; activity of extra-cellular enzymes (alkaline phosphatase, b-glucosidase, leucine aminopeptidase, xylosidase); sediment respiration; primary production
- Physiological state of benthic algae (effective quantum yield, maximum photosynthetic efficiency)
In addition, we will perform in-field slug additions with SRP and NO3-N in the 6(-8) study streams selected for the laboratory experiments during spring, early-summer and autumn in order to compare the potential uptake rates from the lab experiments with in-field data. For that purpose, a solution of nutrients and chloride as conservative tracer is poured into the stream at the head of the reach and water samples are taken at the end of the reach during the entire break-through curve (Covino at al. 2010). Uptake parameters are calculated via a mass balance approach. Besides, we will determine nutrient spiralling metrics based on the TASCC method developed by Covino at al. (2010).