The role of man-made 'flow-reduced zones" as a sink or a source of water content is unknown for the River Elbe, Germany. We measured and compared: a) the concentration of suspended matter at the inflow and the outflow of one special groyne field ("UFZ groyne field") weekly for a period of two years; and b) the intensity of oxygen metabolism within several groyne fields and the main stream during a Lagrangian survey. Under discharge conditions near or below mean water, we found a significant reduction of suspended particulate matter and particle bound nutrients in the "UFZ groyne field". In contrast, concentrations of most soluble water contents and chlorophyll a did not significantly change between in- and outflow. During the Lagrangian survey, pelagic production and respiration rates developed nearly identically in both the main stream and the adjacent groyne fields but oxygen time curves showed higher amplitudes for the groyne fields compared to the river. This higher net-oxygen production in the groyne fields is due to reduced water depth and reduced stream velocity. It enhances the concentration of oxygen in the main stream. The contrary results show the coexistence of both sink and source functions of "flow-reduced zones" in rivers.