Spatial decoupling of in-stream nitrogen cycling observed in an open air stream mesocosm.

Human wastewater emissions can cause amongst other impacts a nutrient surplus in the connected river systems. Nutrient uptake in the river system is driven by the interaction of hydraulic, ecological, and biogeochemical conditions and processes. Hence, information about these complex interactions would allow better predicting the metabolism of fluvial environments. 

Within this study, we attempt to quantify in-stream nitrogen transformation processes with regard to hydraulic system characteristics as well as ecological characteristics such as vegetation cover, water temperature, dissolved oxygen concentrations and solar radiation. From 07 - 09/2019, four nutrient-addition-experiments were carried out in a continuous flow open air stream-mesocosm, comprising a 32.5 m highly aerated stream section (rifle) with a mean slope of 23 %, where low water levels and fast flow velocities characterize the hydraulic boundary conditions leading into a 9 m³ slowly flowing pool section (pool) with a mean depth of about 0.3 m and a spur dike increasing the residence time. The circulation of the system is driven by an electrical pumping system at the downstream end of the pool covering a flow range of 1 - 3.5 l/s. Floating algae and saturated oxygen conditions characterize the rifle section while the pool section is partly vegetated by algae, phragmites, typha and others and shows diurnal cycles of dissolved oxygen concentrations remaining most of the time below the oxygen saturation concentration. The system as a whole is decoupled from the underground with a tarp that is covered with a gravel-layer of about 3 - 8 cm depth.  Additionally, the ground of the pool section is covered by an organic litter layer of about 5 cm depth. Depending on the flow rates, the residence time in the rifle section varies between 5 - 15 min while the residence time in the pool changes from 25 – 75 min, accordingly. After nutrient-additions (Ammonium chloride and Monobasic potassium phosphate) at 10:00 water samples were taken at the downstream end of both sub-systems, with an increasing frequency of 30 min to 3 hours for the next five days. Interpolating the outlet concentrations of each system as input concentrations for the next system continuous changes in ammonium, nitrate and phosphate concentrations were identified for each system separately.

The results show that the combined ecosystems promote different types of reactions and processes in different parts of the system. The rifle induced highly aerated oxic conditions, promoting biological oxidation of ammonium consistently. On the other hand, the pool section produced limited oxic environments and longer residence times where denitrification occured, reaching the highest rates when the vegetation cover increased. Throughout the complete experimental period, phosphate transformation presented a stable behavior regardless of the environmental conditions. Therefore, spatial decoupling allowed us to  demonstrate that in-stream nitrogen cycling depends on the enduring variation and combination of local ecological and hydrological factors which occur in natural streams frequently.