Effect of drought on deposition of ferric hydroxides in the HZ of a small upland stream in Northern Bavaria

Global warming forecasts predict an increase in extreme weather events like droughts, heavy rainfalls and floods. Such events act as multiple stressors for streams through extreme discharges, temperature increases and also enhanced erosion of fine sediments into the system. In the last years, Northern Bavaria suffered from severe droughts with extremely dry and hot summers, a situation which is expected to become even worse in the future. We hypothesize that increased periods of drought lead to an increase in oxygen consumption rates in the hyporheic zone (HZ) and subsequent anoxia along with microbial iron reduction and formation of Fe(II). At the redoxcline, in the presence of oxygen, oxidation of Fe(II) and subsequent precipitation of ferric (hydr)oxides occurs. Such processes may have severe effects on stream water ecology causing clogging of interstitial spaces within the riverbed with subsequent habitat loss for benthic organisms as well as clogging of fish gills and trachea, coverage of fish eggs and oxygen depletion.

In this contribution we will present first results from a field study on the understanding of the effects of drought on biogeochemical processes within the hyporheic zone. To these ends we have installed tube bundles into the HZ of a small upland stream in Northern Bavaria, Germany. Stream Mähringsbach (3rd order stream) is located south east of the city Hof and runs through silicate rich areas. Mähringsbach is home to the endangered pearl mussel. Sampling is conducted in the upper stream reach starting at N 50°14.884 E°012°05.743. Eight sampling points are installed including a transect covering a length of about 20 m. To allow sampling at the same spot tube bundles were inserted in HZ and left for the whole sampling campaign. Tube bundles have mesh covered (pore size about 160 x 310 μm) openings at 5, 15, 25 and 40 cm depth. A luer lock-three-way valve combination is attached to the opening at the surface to enable sampling. Water samples are taken using a 60 ml syringe with an attached oxygen flow through cell. This sampling approach allows to investigate the dynamics of dissolved Fe(II), Fe(III) and O₂ in the porewater at different depths of the HZ. For a better depth resolution (2 cm) custom made dialyses samplers are used. A membrane allows only small components < 0,2 μm to enter. To account for the exchange dynamics between surface water and the hyporheic zone, we determine the residence time of water within the HZ using radon as a tracer. First results from filtered (0,45 µm) tube bundle samples indicate high concentrations of Fe(II) up to 216 µmol/l and of Fe(III) up to 7 µmol/l. Peeper samples have Fe(II) concentrations up to 408 µmol/l and Fe(III) concentrations up to 215 µmol/l.