Improving in-stream nutrient retention potential through factitious manipulation: the stepping stone structures of flying-geese pattern and their reinforcement structures.

Small streams are essential parts of water ecosystems, such as rivers, lakes, and reservoirs, performing vital functions in the attenuation of nutrient pollution. As eutrophication becomes an increasingly severe problem in waters, it is necessary to investigate how to improve nutrient retention potential in streams. In this study, the effect of artificial manipulation was examined on transient storage and nutrient uptake in streams by setting up the stepping stone structures of flying-geese pattern (SG) and the combination mode of SG and bilaterally staggered spur dikes (SG+SD) in the channel. The tracer experiments were performed to confirm the effectiveness of SG and SG+SD in two headwater streams, which are tributaries of the Chaohu Lake basin. Additionally, the transient storage and nutrient uptake potential were assessed by the OTIS (one-dimensional transport with inflow and storage) model and the nutrient spiraling theory. Compared with the control, the implementation of SG in the Banqiao River increased the retention of ammonium (NH) and phosphate (PO). Furthermore, the transient storage capacity and nutrient uptake potential in the Ershibu River were strengthened with the addition of bilaterally staggered spur dikes based on SG. These results highlight the importance of manipulating the geomorphology of the streambed to enhance the nutrient retention potential in streams.