Small wetlands designed for phosphorus retention in Swedish agricultural areas

Abstract: This thesis examined retention of particles and phosphorus (P) losses from agricultural clay soils in small wetlands with an initial deep area followed by a shallow area with emergent plants. Two long, narrow wetlands (Bergaholm and Nybble) specially designed for P retention (P-wetlands) were constructed, and their efficiency in sediment accumulation were compared with those of six existing wetlands with a different design. Monthly area-specific P retention was positively correlated with P load in the Ber P-wetland, though outflow P concentrations were occasionally higher than inflow concentrations during extreme flow events or low flow periods with ice cover. However, seasonal sediment deposition was not correlated with hydraulic load (HL), particle load or concentrations, as deposition was mostly higher than particle load in spring-summer. Annual P accumulation was positively correlated with HL (up to a possible maximum) and negatively with an index for fast flow variations (FFI). Similar annual P retention (mean 89 kg P ha⁻¹ yr⁻¹) in Bergaholm P-wetland was estimated with two independent methods. In contrast, inflow-outflow studies indicated P and particle release in the Nybble P-wetland, while results from sediment plates indicated major sediment accumulation. Measurements of water quality at a point after the deep section indicated that the discrepancy may be attributed to erosion and resuspension in that section. However, further studies are needed to confirm this, and to determine whether it was a temporary phenomenon that will disappear as the wetland bottom stabilises. In three wetlands with different depth sections, sediment accumulation was higher in deep than in shallow areas. Annual sediment and P accumulation was generally higher in the two P-wetlands than in the other six studied wetlands, and was positively correlated with wetland length to width ratio. Overall, the results suggest that P-wetlands can efficiently retain P lost from agricultural clay soils, provided that HL is not too high or peak-based.