The study site was located in the experimental base of the
Research Center for Pond Ecological Engineering, Chinese Academy
of Fishery Sciences, Jingzhou, Hubei Province, China. The RAS consisted
of purification units (i.e., a primary biological pond, two
parallel horizontal subsurface flow CWs [30 m in length, 8.5 m in
width for each] and an ecological ditch) and 4–5 series-connected
culture ponds (Fig. 1 a). The frame of the CWs was built using bricks
and mortar, whereas the bottom was reinforced with concrete as
an impermeable liner fixed to a slope of 0.5%. Each wetland cell
was partitioned into three subareas along the direction of the flow,
an inlet area, a wetland bed (A and B) and an outlet area, which
were all filled with different sizes of gravel to a depth of 0.6 m