The Importance of Hyporheic Zone Processes on Ecological Functioning and Solute Transport of Streams and Rivers

The influence of hyporheic exchange on the transport and transformation of solutes occurs in the environments where hydrologic and biogeochemical processes are dynamic and highly heterogeneous. We present three examples to specify the importance of hyporheic zone processes on different spatial scales ranging from small reach scale to whole river sections. We investigate the (i) impact of physical and biological clogging on the functional significance of the hyporheic zone, use (ii) small scale numerical studies analysing factors controlling advective exchange and solute transport and transformation and reveal (iii) spatial variation of nitrogen removal in river networks. Using a river in a pristine environment in central Mongolia we demonstrate that biological clogging shows seasonal effects on the hydrologic connectivity whereas biogeochemical regulation and habitat seemed to be affected less. Physical clogging revealed to have long-term impacts on the hydrologic connectivity, biogeochemical regulation and habitat. The simulation study of a lower mountain range river in central Germany shows that the location of highest hydraulic gradient is the location of the highest water exchange and nitrogen transformation. In spite of uncertainties involved in the process-based models valuable conclusions can be made towards focused theoretical and experimental studies for new process understating. The large scale denitrification study indicates a decreasing denitrification rate with increasing river length, but river morphology may modulate this general trend considerably. Furthermore nitrate concentration affects the nitrogen removal significantly. The associated longitudinal pattern of nitrogen removal can be assumed to be typical in highly eutrophic low order rives of central Europe.