Analysis of phosphorus retention variations in constructed wetlands receiving variable loads from arable land

Seven wetlands, constructed on agricultural land in the south of Sweden, were investigated with respect to phosphorus (P) retention. The overall aim was to increase the understanding of P retention and find possible explanations for the variations in retention that have been observed in previous studies. This was done by i) investigating P retention in wetlands receiving various water and P loads, ii) investigating the effect of variations in water flow on P transport, iii) comparing how well retention estimates based on water quality data agreed with measurements of the amount of P accumulated in the sediment. Results showed that P retention was positive in all wetlands, but it was variable (1–58 kg ha -1 yr -1 ) and months with negative retention were observed in nearly all wetlands. Such monthly negative retention coincided with i) high flow periods, when particulate P was either flushed straight through the wetlands or resuspended from the bottoms, and ii) warm low flow periods, in which case dissolved P was probably released from wetland sediments due to anoxic conditions. The results from the two methods for estimating P retention differed. Based on water quality data, the total P load during four years was 65 kg ha -1 and the mean P retention 2.8 kg ha -1 yr -1 , or 17% of the total P load. In contrast, the amount of P accumulated in the inlet zone alone amounted to 78% of the P load, and the P content in the upper sediment of the whole wetland area exceeded the P load with a factor four. This discrepancy showed the need to add studies of sediment accumulation to inflow-outflow estimates for an improved understanding of wetland P retention.