PHOSPHORUS RETENTION IN CONSTRUCTED FRESH-WATER RIPARIAN MARSHES
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abstract
Four constructed freshwater riparian marshes in northeastern Illinois, USA, each experimentally subjected to either high or low loadings of turbid river water over a 3yr period, were investigated for phosphorus retention and fate. Average phosphorus concentrations decreased by 6492% in lowflow wetlands (to 1140 mg P/L) and by 5390% (to 1257 mg P/L) in highflow wetlands. Intensive sampling at 4h intervals during spring and summer of the final year showed phosphorus mass decreases of 8174% for low and highflow wetlands, respectively. During this frequent sampling, inflow averaged 176 mg P/L, while outflows were 34 mg P/L for the lowflow wetland and 45 mg P/L for the highflow wetland. Spatial patterns of isopleths showed clear gradients in highflow wetlands and nearhomogeneous concentrations in lowflow wetlands. Annual budgets illustrated that most inflowing phosphorus was retained through sedimentation with some capacity for phosphorus retention by macrophytes and a lesser amount by microbial (periphyton and planktonic) communities. A simple Vollenweidertype model predicted well the retention of phosphorus in these wetlands for low and highflow wetlands with 3 yr of data, i.e., the model was calibrated and verified with six independent data sets. A mean deviation of only 9.3% between measured and simulated retention was found when the retention coefficient was k = 6.51 wk1. Adding a factor for seasonal temperature fluctuation did not improve the fit, whereas using a model where the retention is a function of flow provided the best overall fit to the data (mean deviation of 6.3% between data and predictions). These constructed wetlands retained about the same amount of phosphorus per unit area (0.53 g Pm2yr1) as have several other natural and constructed wetlands receiving similar concentrations of phosphorus.
