Linking hyporheic community respiration and inorganic nitrogen transformations in the River Lahn (Germany)

Abstract

Hyporheic community respiration was measured in a riffle-pool-riffle sequence located up- and downstream from a wastewater treatment plant of the River Lahn, Germany. Sediment filled tubes were incubated for several months in the hyporheic zone and then transferred to the laboratory where the oxygen consumption by the hyporheic community colonizing the tubes was measured in a recirculating system. Oxygen consumption in 64 tubes from four sites averaged 0.69 mg O 2 dm -3 h -1 with surface water from upstream of the wastewater treatment plant. After the successive amendment of nitrite and ammonium, community respiration increased to 1.14 and 1.36mg O 2 dm -3 h -1 , respectively. Nitrate production during the experiments increased from 0.024 mg NO 3 -N dm -3 h -1 to 0.219 and 0.220 mg NO 3 -N dm -3 h -1 with the amendment of nitrite and ammonium, respectively, indicating nitrogen limitation in the hyporheic zone. Respiration and nitrate production correlated strongly with the amount of organic matter accumulated in the tubes during the several months of sediment incubation. Significantly higher respiration and nitrate production after amendment of ammonium were observed for the two sites located downstream of the wastewater treatment plant effluent in comparison to those upstream of the WWTP. This increase indicates a possible impact of wastewater supply to rivers, especially on the hyporheic zone where an important part of organic matter break-down and nutrient recycling occurs in mesotrophic rivers.